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Articles in press have been peer-reviewed and accepted, which are not yet edited and assigned to volumes/issues, but are citable by Digital Object Identifier (DOI).
Lu Zhen-quan, Wu Chu-guo, Wu Neng-you, Lu Hai-long, Wang Ting, Xiao Rui, Liu Hui, Wu Xin-he. 2022. Change trend of natural gas hydrates in permafrost on the Qinghai-Tibet Plateau (1960‒2050) under the background of global warming and their impacts on carbon emissions. China Geology, 5(3), 475‒509. doi: 10.31035/cg2022034.
Citation: Lu Zhen-quan, Wu Chu-guo, Wu Neng-you, Lu Hai-long, Wang Ting, Xiao Rui, Liu Hui, Wu Xin-he. 2022. Change trend of natural gas hydrates in permafrost on the Qinghai-Tibet Plateau (1960‒2050) under the background of global warming and their impacts on carbon emissions. China Geology, 5(3), 475‒509. doi: 10.31035/cg2022034.

Change trend of natural gas hydrates in permafrost on the Qinghai-Tibet Plateau (1960‒2050) under the background of global warming and their impacts on carbon emissions

  • Corresponding author: Zhen-quan Lu, luzhq@vip.sina.com
  • Received Date: 18 December 2021
  • Accepted Date: 23 June 2022
  • Available Online: 04 August 2022
  • Global warming and the response to it have become a topic of concern in today’s society and are also a research focus in the global scientific community. As the world’s third pole, the global warming amplifier, and the starting region of China’s climate change, the Qinghai-Tibet Plateau is extremely sensitive to climate change. The permafrost on the Qinghai-Tibet Plateau is rich in natural gas hydrates (NGHs) resources. Under the background of global warming, whether the NGHs will be disassociated and enter the atmosphere as the air temperature rises has become a major concern of both the public and the scientific community. Given this, this study reviewed the trend of global warming and accordingly summarized the characteristics of the temperature increase in the Qinghai-Tibet Plateau. Based on this as well as the distribution characteristics of the NGHs in the permafrost on the Qinghai-Tibet Plateau, this study investigated the changes in the response of the NGHs to global warming, aiming to clarify the impacts of global warming on the NGHs in the permafrost of the plateau. A noticeable response to global warming has been observed in the Qinghai-Tibet Plateau. Over the past decades, the increase in the mean annual air temperature of the plateau was increasingly high and more recently. Specifically, the mean annual air temperature of the plateau changed at a rate of approximately 0.308‒0.420°C/10a and increased by approximately 1.54‒2.10°C in the past decades. Moreover, the annual mean ground temperature of the shallow permafrost on the plateau increased by approximately 1.155‒1.575°C and the permafrost area decreased by approximately 0.34×106 km2 from about 1.4×106 km2 to 1.06×106 km2 in the past decades. As indicated by simulated calculation results, the thickness of the NGH-bearing permafrost on the Qinghai-Tibet Plateau has decreased by 29‒39 m in the past 50 years, with the equivalent of (1.69‒2.27)×1010‒(1.12‒1.51)×1012 m3 of methane (CH4) being released due to NGHs dissociation. It is predicted that the thickness of the NGH-bearing permafrost will decrease by 23 m and 27 m, and dissociated and released NGHs will be the equivalent of (1.34‒88.8)×1010 m3 and (1.57‒104)×1010 m3 of CH4, respectively by 2030 and 2050. Considering the positive feedback mechanism of NGHs on global warming and the fact that CH4 has a higher greenhouse effect than carbon dioxide, the NGHs in the permafrost on the Qinghai-Tibet Plateau will emit more CH4 into the atmosphere, which is an important trend of NGHs under the background of global warming. Therefore, the NGHs are destructive as a time bomb and may lead to a waste of efforts that mankind has made in carbon emission reduction and carbon neutrality. Accordingly, this study suggests that human beings should make more efforts to conduct the exploration and exploitation of the NGHs in the permafrost of the Qinghai-Tibet Plateau, accelerate research on the techniques and equipment for NGHs extraction, storage, and transportation, and exploit the permafrost-associated NGHs while thawing them. The purpose is to reduce carbon emissions into the atmosphere and mitigate the atmospheric greenhouse effect, thus contributing to the global goal of peak carbon dioxide emissions and carbon neutrality.
  • Since the mid-19th century or the beginning of the 20th century, the global temperature (referring to the air temperature of two meters above the land surface or the temperature of the ocean surface) has increased significantly (IPCC, 2013), namely global warming, which is the cause and research focus of global climate change (Ren GY et al., 2021). It is widely believed that global warming is caused by an increase in carbon dioxide (CO2) and CH4 in the atmosphere. Greenhouse gases produced by human factors, such as CO2 and CH4, accelerate the process of global warming (Cheng C et al., 2020; Wang R and Liu ZF, 2020). Owing to the increase in greenhouse gas emissions, the global mean temperature (GMT) increased by 0.85°C from 1880 to 2012 (IPCC, 2014). The warming rate of global temperature in the past 50 years was twice that in the past 100 years, and the Northern Hemisphere may be the warmest since 1983 in the past 1400 years (Shen YP and Wang GY, 2013). According to the State of the Global Climate 2020 issued by the World Meteorological Organization (WMO), the decade from 2011 to 2020 was the warmest decade on record, among which 2020 is one of the three hottest years on record, and the GMT in 2020 was about 1.2°C above the pre-industrial level. The mean annual temperature in China increased by 0.24°C every decade from 1951 to 2018, with a warming rate significantly higher than the global average level during the same period (China Meteorological Administration, 2019).

    As a global concern and a significant feature of modern climate change, climate warming has become one of the core issues affecting the sustainable development of a country (Su C and Zhang QY, 2021). At present, most of the governments in the world have accepted the United Nations Framework Convention on Climate Change (UNFCCC), setting the maximum limit for the GMT to 2°C above the pre-industrial level. The Chinese government attaches great importance to addressing climate change and has set the goal of achieving peak carbon dioxide emissions by 2030 and carbon neutralization by 2060. Global warming has become an extensive focus of the international community, while climate change and its impacts have become a hot topic in global scientific research. Human beings are moving toward an ever-increasing unstable climate in which record levels of rising global temperatures and extreme heat and heatwaves, wildfires, tornadoes, hurricanes, droughts, floods, and food shortages will become the new norm (Letcher TM, 2021). Global climate change leads to increasingly unstable weather and climate systems, causing a series of extreme weather and climate events (Shi C et al., 2020). Studies reveal that the impacts of climate change on yields of crops (rice, maize, and wheat) in China is mainly reflected in temperature rise, with crop yields decreasing by 2.6%‒12.7% for every 1°C increase in the temperature, and the crops in northeastern and northwestern China are the most significantly affected by the temperature increase in the country (Liu Y et al., 2021). Permafrost and glaciers are also vulnerable to global warming (Fig. 1).

    Figure  1.  Global distribution of permafrost areas (after Zhao SM et al., 2022).

    A recent study shows that the greenhouse gas emissions from agriculture, forestry, and other land use sectors account for 23% of the total anthropogenic greenhouse gas emissions globally from 2007 to 2016 (Waisman H et al., 2019). The greenhouse gas concentrations in Earth’s atmosphere are directly related to the Earth’s GMT (Glasby GP, 2003; Fig. 2), and the greenhouse gas concentrations and the GMT have increased steadily since the Industrial Revolution. As the most abundant greenhouse gas, accounting for about two-thirds of the total greenhouse gases, CO2 is largely the product of burning fossil fuels (Letcher TM, 2021). It is estimated that more than two trillion tons of CO2 have been added to the atmosphere since the Industrial Revolution. In 2019, human activities contributed 36.8×109 t of CO2 through burning coal and other fossil fuels, cement production, deforestation, and land landscape changes. Human activities emit 60 or more times the amount of CO2 released by volcanoes each year, and the current CO2 concentration in the atmosphere exceeds the natural equilibrium of dissolved CO2 in the oceans and the CO2 uptake by biota on land (Letcher TM, 2021). For example, the global CO2 concentration was 316×10−6 in 1960 and increased to 417×10−6 in 2020 mainly due to human activities (such as burning fossil fuels). Even if human beings stopped burning fossil fuels, it will take a long time for CO2 levels to decrease as the lifetime of CO2 in the atmosphere is hundreds of years. For example, the lockdown over the COVID-19 pandemic has not reduced the CO2 concentration in the atmosphere. On the contrary, the CO2 concentration hit an all-time high of 417×10−6 according to the latest measurements taken at Mauna Loa, Hawaii in June 2020. This is because the CO2 concentration in the atmosphere does not decrease once human beings reduce emitting CO2 into the atmosphere, moreover, feedback mechanisms such as the oceans releasing CO2 continue as the atmosphere heats up (Letcher TM, 2021). According to Letcher TM (2021), the Earth’s climate has been in a state of equilibrium with the concentration of atmospheric CO2 at approximately 280×10−6 for thousands of years (probably more than one million years). The atmospheric CO2 has kept the Earth relatively warm, and the GMT would be been much cooler than it is today without the atmospheric carbon (Letcher TM, 2021).

    Figure  2.  Curves of atmospheric CO2 and CH4 concentrations reconstructed from Antarctic ice cores and synthetic temperature curve (after Beeman JC et al., 2019).

    In addition, atmospheric CH4 concentrations have roughly doubled over the past 150 years from 850×10−9 to approximately 1750×10−9 at present (Cicerone RJ and Oremland RS, 1988). Hansen J et al. (2000) considered that the anthropogenic sources of CH4 mainly include rice cultivation, emissions from ruminants, pipeline leakage, and diffusion from landfills, as well as CH4 escape from coal mines, oil well sites, and treatment stations of anaerobic waste. Reshetnikov AI et al. (2000) estimated that the total CH4 emissions from the fossil fuel industry are about 100 ×106 t per year. In 2017, agriculture contributed 42% of the total global CH4 emissions (Malyan SK et al., 2021). Among the CH4 currently emitted into the atmosphere, 43% is natural (e.g., CH4 from CH4-producing bacteria and termites) and 57% is anthropogenic (e.g., the fossil fuel leaks and excretions of domestic ruminants; Wuebbles DJ and Hayhoe K, 2002). As revealed by the data from Etheridge DM et al. (1998) and Wuebbles DJ and Hayhoe K (2002), the atmospheric CH4 concentration has increased from about 700×10−9 in 1800 to the present more than 1700×10−9 as a result of industrialization. The increase was the equivalent of approximately 3.7×109 t of methane carbon into the atmosphere since the atmospheric mass is 5×1018 kg (Wayne RP, 1985). It is estimated that the annual methane carbon emissions from the 1980s to the 1990s were 420×106 t (Etheridge DM et al., 1998). Lelieveld J et al. (1998) discovered that approximately 450×106 t of methane carbon were emitted into the atmosphere in 1992 (Crutzen PJ and Lelieveld J, 2001), including 80×106 t from energy consumption and 7×106 t from natural CH4 releases (e.g., decomposition of unstable NGHs and permafrost thawing).

    An important source of atmospheric CH4 is definitely related to NGHs (Chazelas B et al., 2006). The current five climate change mechanisms related to greenhouse gases (i.e., the water vapor feedback mechanism, the melting of ice, the release of CO2 from the oceans, the peat bogs, and permafrost regions, and NGHs feedback; Letcher TM., 2021) include NGHs feedback. For example, huge NGHs resources have been found in seafloor sediments, and rising ocean temperatures may cause the NGHs to decompose and suddenly release large amounts of CH4, thus leading to runaway global warming events. It is estimated that the amount of CH4 stored in NGHs is 3000 times that of atmospheric CH4 (Kvenvolden KA, 1993, 2000). The global huge amounts of NGHs can not only form potential energy (Koh CA et al., 2012) but also play a role in the global carbon cycle and affect the Earth’s climate (Nisbet EG, 1990; Dickens GR et al., 1995; Kvenvolden KA, 2002; Kennett JP et al., 2003; Archer D, 2007; Archer D et al., 2009; Ruppel CD, 2011; Ruppel CD and Kessler JD, 2017). Climate warming caused by human activities may make NGHs unstable and release CH4, leading to a significant increase in the global greenhouse effect and bringing great uncertainty to human society (Chazelas B et al., 2006). Once released in huge quantities, CH4 would cause disastrous consequences for the global climate (Desa E, 2001).

    The Qinghai-Tibet Plateau at a high altitude has alpine permafrost at the middle latitudes of the Northern Hemisphere (Fig. 3) and holds abundant NGHs resources (Chen DF et al., 2005; Zhu YH et al., 2009; Lu Z et al., 2011). Known as the world’s third pole (Qiu J, 2008), the driver and amplifier of global climate change, and the starting region of China’s climate change (Pan BT and Li JJ, 1996), the Qinghai-Tibet Plateau is extremely sensitive to climate change. The recent warming in the Qinghai-Tibet Plateau was largely caused by intensified greenhouse gas emissions from human activities, which may have more significant impacts on the climate of the Qinghai-Tibet Plateau than that of the rest of the world (Chen BD et al., 2001; Duan AM et al., 2006a). It is yet to clarify whether the rising temperature caused permafrost-associated NGHs to decompose to further aggravate the greenhouse gas emissions in the Qinghai-Tibet Plateau. There is an urgent need to determine many major scientific concerns about the huge amounts of NGHs resources in the permafrost of the Qinghai-Tibet Plateau under the background of global warming, including their changes, their response to global warming, their dissociation degree, and resulting CH4 emissions into the atmosphere, and their positive feedback degree on the global warming.

    Figure  3.  Sketch map showing the distribution of the permafrost on the Qinghai-Tibet Plateau (modified from Zhou YW et al., 2000).

    Given this, this study reviewed the global warming trend, the temperature increase of the Qinghai-Tibet Plateau, the distribution characteristics of NGHs in the permafrost of the Qinghai-Tibet Plateau, and the positive feedback mechanisms of NGHs on climate warming. Based on this as well as the fact that CH4 has a higher greenhouse effect than CO2, this study comprehensively explored the response degree of permafrost-associated NGHs on the Qinghai-Tibet Plateau to global warming, estimated the CH4 emissions from the NGHs in the past 50 years, and predicted the CH4 emissions in the next 10 and 30 years. The purpose of this study is to prepare in advance and contribute to reducing atmospheric carbon emissions, mitigating the atmospheric greenhouse effect, and achieving the goal of global peak CO2 emissions and carbon neutrality.

    Modern climate change reflects not only the state of the atmosphere itself but also the overall state of the complex system composed of the oceans, land, cryosphere, and biosphere (Zhang YJ et al., 2004). In some sense, the changes in modern climate are continuations of paleoclimatic changes. Therefore, the study of the changes in historical climate and paleoclimatic changes can be used as references for predicting future climate change (Ren GY et al., 2021). As a basic manifestation of climate change, the global temperature change is not only affected by solar radiation, the orbital parameters of the Earth, and the tectonic movements of the Earth (e.g., continental drift and volcanic eruption) but also closely related to the multi-scale complex feedback processes among the Earth’s surface hydrosphere, cryosphere, biosphere, and atmosphere. Based on this, scientists summarized and classified the climatic evolution cycles and their main driving mechanisms on the tectonic, orbital, millennium-century, and century-multi-decade scales (Ren GY et al., 2021).

    Regarding paleoclimatic change characteristics, it is still controversial whether the temperature in the early and middle Holocene was higher than that of the pre-Industrial Revolution (Marcott SA et al., 2013; Kaufman D et al., 2020; Bova S et al., 2021). However, there was possibly no sustained period with a temperate 1.0°C above the present temperature in the whole Holocene (Ren GY et al., 2021).

    During the Quaternary glacial and interglacial periods, the Earth’s climate change cycle was approximately 100 Ka in the past 400 Ka and was approximately 40 Ka in other periods. The Holocene interglacial period was the latest episode of the climate change cycle of approximately 100 Ka. During the last interglacial period at approximately 130‒116 Ka, the global temperature might be 0‒2.0°C higher than that of today on average, making this period the warmest interglacial period in recent hundreds of thousands of years (Turney CSM et al., 2020; Hoffman JS et al., 2017; Berger A et al., 2016; Thomas ZA et al., 2020). The Last Glacial Maximum occurred at approximately 20 Ka, followed by deglaciation till the beginning of the Holocene. During the deglaciation, a series of cold and warm events on a time scale ranging from decades to millennia occurred in the North Atlantic, including the Younger Dryas event at approximately 11.6 Ka. During the Quaternary glacial and interglacial periods, besides changes in continental ice sheets, terrestrial vegetation, sea level, and seawater acidity, the Earth’s climate system had a significant change that the greenhouse gas concentrations in the atmosphere fluctuated as the global temperature changed (Jouzel J et al., 2007; Wolff EW et al., 2010; Lüthi D et al., 2008). For example, the atmospheric CO2 concentration was the lowest (generally approximately 190×10−6) in the glacial period and was the highest (up to 280×10−6) in the interglacial period (Jouzel J et al., 2007; Wolff EW et al., 2010; Lüthi D et al., 2008). Another significant change during the periods was that the contents of water vapor and dust aerosol in the atmosphere also fluctuated greatly as the global temperature varied. For example, the dust content in the atmosphere was very low in the interglacial period but was extremely high in the glacial period (Ren GY et al., 2021).

    At 100‒3 Ma, the Earth’s temperature was significantly higher than that of any climate period on a tectonic or orbital scale in the Quaternary and was 4.0°C higher than that of today mostly (Wang PX et al., 2018). The latitudinal and relative positions of continental plates, as well as changes in atmospheric greenhouse gas concentrations, may be the main driving factors of the anomalous high temperature of global land, the continuous cooling throughout the whole Tertiary, and the development of Antarctic continental ice sheets (Ren GY al., 2021). Studies have shown that the glacial cycle in Gondwana began during the Middle Carboniferous (approximately 320 Ma; Crowell JC, 1995). Evidence suggests that the presence of polar ice sheets in the Middle Carboniferous and the sharp drop in sea level during that period indicate that NGHs may have formed under the frozen ocean and the permafrost during that period (Fielding CR et al., 2008a, 2008b).

    Studies have shown that glacial activities were recorded in the Middle Permian sediments in eastern Australia (Fielding CR et al., 2008a). Evidence of the Middle Permian climate cooling has also been observed in the Arctic Circle in Canada at the margin of the northwestern continent of the Pangea (Beauchamp B and Baud A, 2002; Beauchamp B and Grasby SE, 2012). Paleosoils of the early stage of the Late Permian permafrost have also been recorded in high latitudes of the Southern Hemisphere (Retallack GJ and Krull ES, 1999). The Middle-Late Permian underwent a rapid change from a cryochamber to a greenhouse, followed by a thermal chamber from the end of the Permian to the Early Triassic (Kidder DL and Worsley TR, 2004). Kvenvolden KA (1988) first realized the role of NGHs in the global carbon cycle and the Earth’s climate feedback. The rapid release of NGHs was considered the cause of the rapid global warming during the Latest Permian Extinction (LPE), and the NGHs dissociation led to ocean hypoxia and the mass extinction of marine biotas at the end of the Permian (Matsumoto R, 1995). During the global warming of the LPE, volcanic eruptions produced a high CO2 concentration (Kidder DL and Worsley TR, 2004; Wignall PB and Hallam A, 1993), which led to global warming and further induced NGHs dissociation and CH4 release (Dorritie D, 2002; Erwin DH et al., 2002; Kidder DL and Worsley TR, 2004; Racki G and Wignall PB, 2005; Winguth AME and Maier-Reimer E, 2005), which further contributed to the global warming. In this manner, a positive feedback mechanism was formed (Kennett JP et al., 2000). This mechanism was considered one important factor causing the LPE and global warming at the end of Permian (Krull ES and Retallack GJ, 2000; Krull ES et al., 2000). The simulation performed by Majorowicz JSE et al. (2014) showed that the potential NGHs released during the LPE affected the greenhouse effect of escaped gases indeed. This positive feedback mechanism was also considered the factor driving climate warming, such as the Last Glaciation (Kennett JP et al., 2000) and the end-Triassic mass extinction (Ruhl M et al., 2011).

    Increasing pieces of evidence have shown that many major geological events were associated with NGHs dissociation, whose products served as a major source of CH4 (an atmospheric greenhouse gas). These events include: (1) The Oceanic Anoxic Event (OAE) during the Early Toarcian of the Jurassic at approximately 183 Ma (Beerling DJ et al., 2002; Hesselbo SP et al., 2000; Padden M et al., 2001); (2) the Aptian OAE during the Cretaceous at about 120 Ma (Beerling DJ et al., 2002; Hesselbo SP et al., 2000); (3) the Cretaceous/Tertiary boundary (Max MD et al., 1999); (4) the Latest Palaeocene Thermal Maximum (LPTM) at about 55.5 Ma (Bains S et al., 1999, 2000; Dickens GR, 2001; Dickens GR et al., 1995, 1997; Katz ME et al., 1999; Matsumoto R, 1995); (5) the Paleocene-Eocene Thermal Maximum (PETM) from the Late Paleocene to the Early Eocene, during which the NGHs dissociation and the release of large amounts of CH4 (Clift P and Bice K, 2002; Hudson TL and Magoon LB, 2002; Dickens GR, 1999; Katz ME et al., 1999) may have led to the extensive mass extinctions during the PETM at 55 Ma (Dickens GR et al., 1995), and (6) at least four interglacial periods during the Quaternary (Dickens GR, 2003; Kennett JP et al., 2000, 2002). Records indicated that the dissociation of a large amount of NGHs may have occurred during the LPTM (Dickens GR et al., 1995,1997; Matsumoto R, 1995). The thermal dissociation of marine NGHs and the release of large amounts of CH4 into the exogenous carbon cycle are the only reasonable explanation for the negative perturbation of carbon-isotope geochemistry within approximately 10 Ka at that time (Crouch EM et al., 2001). Particularly for the event of the Santa Barbara Basin during the Quaternary, Kennett JP et al. (2000) believed that warmer sea water at a moderate depth entered the basin at the beginning of each interglacial period, resulting in a 2‒3.5°C rise in the bottom water temperature. Since the sea level was still lower (80 m lower than the current sea level) at that time, the rising bottom water temperature was considered to be sufficient to induce NGHs dissociation. As a result, the sediments in the basin collapsed, and the NGHs accordingly lost their top load, causing large amounts of CH4 to be directly released into water columns and the atmosphere.

    Mienert J et al. (2005) discovered that the paleo-bottom water temperature in the Storegga Slide area at the continental margin of central Norway rose at a relatively fast rate after the Younger Dryas, i.e., at about 12.5‒10 Ka, and then maintained a stable and high level thereafter. Despite sea level rise, warm water flows significantly reduced the thickness of the NGH stability zone on the upper continental slope at the continental margin in central Norway. Although the main stage of the NGH dissociation predated the Storegga Slide event at 8.2 Ka, the increasingly unstable NGHs may have contributed to the collapse of seafloor sediments. The existence of bottom simulating reflectors in the slide complex indicates that the NGHs seem to have nearly adapted to the new equilibrium conditions and thus can reflect the dynamic influence of environmental changes (e.g., climate change and geologic hazards) on the stability of NGHs in epicontinental sediments.

    Since the second half of the past century, most regions in the world have undergone significant warming processes, and the global mean surface temperature increased by 0.89°C (0.69‒1.08°C) from 1901 to 2012 (IPCC, 2013). The WMO announced that the global temperature increased by up to 1.1°C in 2019—the second warmest year on record (WMO, 2020), and the global mean annual temperature from 2015 to 2019 was the highest since 1850 (WMO, 2018, 2019). According to independent analyses by the National Aeronautics and Space Administration (NASA) and National Oceanic and Atmospheric Administration (NOAA), Earth’s mean global surface temperature in 2019 was the second highest since modern recordkeeping began in 1880, and globally, the mean temperature in 2019 was second only to that in 2016 and continued the Earth’s long-term warming trend, that is, the past five years have been the warmest in the past 140 years. According to scientists at NASA’s Goddard Institute for Space Studies (GISS) in New York, the mean temperature in 2019 was 0.98°C higher than that from 1951 to 1980 (Letcher TM, 2021).

    The Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) confirms that the climate system has warmed significantly. The globally averaged combined land and ocean surface temperature, i.e., the GMT, increased by about 0.85°C (0.65‒1.06°C) from 1880 to 2012 (IPCC, 2013). The IPCC also pointed out that the GMT has increased by 0.45‒0.74°C in the past 100 years, and by 0.30‒0.92°C per 100 years, indicating that the rate of global temperature rise has been increasing (Zhao ZC et al., 2007). The Special Report on Global Warming of 1.5°C issued by IPCC in 2018 claimed that limiting global warming to 1.5°C, compared to 2°C could “reduce the number of people exposed to climate-related risks and susceptible to poverty by several hundred million by 2050” (IPCC, 2018).

    Since its founding in 1988, the IPCC has compiled five comprehensive climate change assessment reports and 14 special reports (Wang Q and Zhai PM, 2021). The assessment reports show that global warming is unequivocal, the rate of warming is increasing, and there are increasing pieces of evidence that human activities are contributing to global warming (Table 1). The fourth assessment report on climate change (AR4) of the IPCC pointed out that human behavior may have played an important role in the rise of surface temperatures on all continents (except Antarctica) since the mid-20th century (IPCC, 2007). The AR5 of the IPCC further pointed out that the impacts of human activities are increasingly noticeable, have been detected throughout the climate system, and are extremely likely (probability: 95%‒100%) to have been the dominant cause of the observed warming since the mid-20th century (IPCC, 2013, 2014). The AR5 also pointed out that the globally average combined land and ocean surface temperature showed a warming trend of about 0.85°C (0.65‒1.06°C) over the period 1880‒2012 (IPCC, 2013). The sixth assessment report on climate change (AR6) of the IPCC will be completed in 2022, and its three special reports have been released. The latest study results of the Special Report on Global Warming of 1.5°C (SR1.5) show that since the industrial period, human activities are estimated to have caused approximately 1°C of global warming above pre-industrial levels (IPCC, 2018).

    Table  1.  Impacts of human activities on climate change pointed out in previous IPCC reports.
    IPCC reportObservation period
    (reference period)
    Mean surface
    temperature change /°C
    Impacts of human activities
    FAR (1990)1861‒1989
    (1951‒1980)
    0.45 (0.3‒0.6)Little observed evidence can prove the impacts of human activities on climate
    SAR (1995)1861‒1994
    (1961‒1990)
    0.45 (0.3‒0.6)Various evidence shows that human activities have discernible impacts on the global climate
    TAR (2001)1861‒2000
    (1961‒1990)
    0.60 (0.4‒0.8)New and convincing evidence shows that the majority of the warming observed over the past 50 years can be attributed to human activities.
    AR4 (2007)1906‒2005
    (1961‒1990)
    0.74 (0.56‒0.92)Majority of the observed increase in GMT since the mid-20th century is likely to be caused by the observed increases in anthropogenic greenhouse gas concentrations
    AR5 (2013/2014)1880‒2012
    (1961‒1990)
    0.85 (0.65‒1.06)The impacts of human activities have been detected in the whole climate system, and they are likely the main reason for the observed warming since the mid-20th century
    AR6: SR1.5 (2018)1850‒2015
    (1850‒1900)
    1.0 (0.8‒1.2)It is estimated that human activities have caused global warming by about 1oC above the pre-industrial level
     | Show Table
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    In particular, climate change in the European Alps during the 20th century was characterized by increases in minimum temperature of about 2°C and a more modest increase in maximum temperatures through to the mid-1980s (Haeberli W and Beniston M, 1998). The increase in the temperature in the European Alps has been the most intense in the 1940s, followed by the 1980s. Since the early 1980s, the warming in the European Alps, while synchronous with global warming, is of far greater amplitude, reaching up to 2°C for some individual sites though close to 1°C for this ensemble average (Haeberli W and Beniston M, 1998). These changes caused pronounced effects on the glacial and periglacial belts. Since the middle of the past century–the end of the Little Ice Age, the glaciers in the European Alps have lost approximately 30%‒40% in surface area and around half their original volume; the estimated total glacier volume in the European Alps was some 130 km2 in the mid-1970s, but strongly negative mass balances have caused an additional loss of approximately 10%‒20% of this remaining ice volume since 1980 (Haeberli W and Beniston M, 1998).

    Some studies have also pointed out that the global warming process has shown a downward trend since 1998 (Easterling DR and Wehner MF, 2009; Knight J et al., 2009). Since 1909, the surface air temperature (SAT) has increased by 0.9‒1.5°C in China, higher than the global mean warming level. Over the period 1913‒2012, the temperature increase in China’s land areas was up to 0.90‒1.52°C, with the rate of warming far exceeding the global average [Editorial Board of the National Assessment Report of Climate Change (III), 2015]. The simulated results also show that assuming global warming of 1.5°C and 2°C, the mean temperature in China will be 1.7‒2.0°C and 2.4‒2.7°C above the pre-industrial levels, respectively, especially in northwest China (Jiang D et al., 2009; Lang X and Sui Y, 2013; Fu Y et al., 2018; Shi C et al., 2018; Sun C et al., 2019; Yin SY et al., 2020). As shown by the simulated results of Zhang GW et al. (2020), under the background of global warming of 1.5°C and 2.0°C, the temperature increase in China will be approximately 1.63°C and 2.24°C, respectively, exceeding the global average, and the temperatures of extreme heat events will increase by approximately 0.7‒1.7°C and 1.4‒2.6°C, respectively compared to the current period (1976‒2005) across China.

    The North and South poles and the Qinghai-Tibet Plateau are the northernmost and southernmost points and the highest plateau of the Earth, respectively. They are collectively known as the Earth’s three poles and are the key areas sensitive to global climate change (Li F et al., 2021). Under the background of global climate change, the temperature increases in the Arctic and the Qinghai-Tibet Plateau are significantly higher than the global average over the same period. Specifically, the Arctic sea ice and the snow cover on the Qinghai-Tibet Plateau continue to shrink, the meltwater of the Qinghai-Tibet Plateau increases year by year, and lakes on the plateau accordingly expand. Despite slight warming in the Antarctic, the rate of warming of the deep water in the Antarctic Ocean is significantly higher than the global average. This has induced an obvious collapse and contraction trend of the ice shelf in the west of the Antarctic and further drives the climate change of the Antarctic Ocean. The changes in the cryosphere (e.g., sea ice, snow cover, and glaciers) of the Earth’s three poles influence the atmospheric and ocean circulations on regional and global scales through various feedback mechanisms, causing extreme climatic events, such as cold winter in Eurasia, summer floods in China, and summer droughts in south Australia in recent years (Li F et al., 2021).

    The Arctic is one of the regions that are affected by global warming the most significantly, with a rate of warming more than twice the average rate of global warming. Such polar amplification phenomenon and the rapid melting of sea ice in the Arctic not only cause drastic changes in the local environment but also have a profound impact on the climate system in middle latitudes (Chen XL and Wang P, 2021). The Arctic is warming fast, and frozen soils are also starting to thaw, often for the first time in thousands of years. Around 20% of frozen lands have features that increase the likelihood of abrupt thawing. Across the Arctic and Boreal regions, permafrost is collapsing suddenly as pockets of ice within it melts. As the temperature of the ground rises above freezing, microorganisms break down organic matter in the soil. Greenhouse gases, including CO2, CH4, and nitrous oxide, are released into the atmosphere, accelerating global warming. Predictions suggest that abrupt permafrost thawing increases permafrost carbon release by approximately 50%. That is, permafrost collapse is accelerating carbon release by about 50%. In other words, the sudden collapse of thawing soils in the Arctic might double the warming from greenhouse gases released from the tundra (Turetsky MR et al., 2019). Therefore, the impacts of thawing permafrost on Earth’s climate could be twice that expected from current models. Stabilizing the climate at 1.5°C of warming requires massive cuts in carbon emissions from human activities; extra carbon emissions from the thawing Arctic make that even more urgent.

    As revealed by Chen XL and Wang P (2021), the land surface temperature in a region with a latitude greater than 60° N in the Arctic showed an overall upward trend from 1979 to 2017, with a changing rate of 0.57°C/10a. They also found that the coastal regions of the Arctic Ocean had the highest rate of warming during that period, which exceeded 1.0°C/10a locally. These results are roughly consistent with previous study results. Meredith M et al. (2019) analyzed that the land surface temperature in the region with a latitude greater than 60° N in the Arctic over the period 1980‒2019 increased at a rate of approximately 0.56°C/10a, making this region one of the regions with the most significant global warming. Under the influence of global warming, the permafrost zone in the Arctic has been warming at a rate of (0.4–0.9)°C/10a since 2000 (Blunden J and Arndt DS, 2019). Qi W et al. (2021) also found that the Arctic is extremely sensitive to global warming. Based on research on temperature sequences, they found that the North Atlantic, Alaska, and Siberia sectors showed the temperature change trends of stable stability - decrease - sharp rise, decrease - slow rise - decrease - sharp rise, and stability - sharp rise, respectively in the past 200 years. In other words, the climate in the Arctic showed a trend of rapid warming. However, some scientists considered that the CMIP6 model tends to overestimate the trend of the global mean surface temperature from 1981 to 2011, meaning that the Arctic experienced weaker warming than the median values predicted using the CMIP6 model (Hu XM et al., 2021).

    Despite this, the Arctic experienced several significant warming and cooling periods during the 20th century and the early 21st century, i.e., a cooling period from the 1900s to the 1910s, a continuation of the cooling period starting at end of the 19th century (Hartmann DL et al., 2013); a notable warming period from the 1920s to the 1940s in the early 20th century (ETCW; Yamanouchi T, 2011); a cooling period from the 1960s to the 1970s (Bekryaev RV et al., 2010), and the current warming period from the 1980s to the 21st century (Rahmstorf S et al., 2017).

    Latonin MM et al. (2021) pointed out that the multi-model ensemble mean of global climate models fails to reproduce the ETCW in the Arctic. They thought that the Arctic climate system was very sensitive to external disturbances, which led to faster changes in the SAT in the Arctic than in lower latitudes. For example, regarding the increasing amplification effects in the Arctic in the warming period from the 1920s to the 1940s, the predicted results using the CMIP5 model are not consistent with the actual observations, indicating that the ETCW was unlikely due to external factors and that there is no consistent response to the warming in the Arctic. Bengtsson L et al. (2004) suggested that the Arctic warming under the North Atlantic negative oscillation (NAO) was likely due to the regional positive feedback associated with sea ice loss in the Barents Sea, which created suitable conditions for intensifying the northward heat transfer from the atmosphere and ocean to the Barents Sea. These findings are consistent with the phenomenon that the warming was limited to northern high latitudes (i.e., low latitudes did not undergo warming or cooling). However, this mechanism alone cannot explain the warming occurring in other areas in the Arctic. According to the results of Suo L et al. (2013) obtained using a model, the ETCW was mainly caused by combined natural forcings (low volcanic activities and high solar activities) and anthropogenic forcings. The results of Suo L et al. (2013) seem credible since the model used in this study successfully recovered the ETCW event in the Arctic. However, these results may be highly sensitive to the model design and therefore are not reliable (Bentsen M et al., 2013). Another recent study was conducted using a coupled climate model and revealed that the Pacific Decadal Oscillation (PDO) and the Aleutian Low (AL) mechanisms played a dominant role in the ETCW event (Svendsen L et al., 2018). It is generally accepted that the ETCW was the result of combined different mechanisms caused by factors, such as natural and anthropogenic forcings, regional feedback, and internal climate change (Yamanouchi T, 2011; Hegerl GC et al., 2018).

    The Arctic cooling from the 1960s to the 1970s has been widely attributed to an increase in the quantity of sulfate aerosols (Myhre G et al., 2001; Boucher O and Pham M, 2002). Besides, there are also other important factors. For instance, critical factors possibly include the sudden cooling of the northern North Atlantic and the change in atmospheric circulation during the deceleration of the Atlantic meridional overturning circulation (Hodson DLR et al., 2014). Besides, natural forcings may originate from increased volcanic activities after a long period of absence of volcanic eruption and decreased solar activities (Sato M et al., 1993).

    The current Arctic warming began in the 1980s. It occurs globally, with the Northern Hemisphere having a higher rate of warming. Moreover, it features significant Arctic amplification but weak Antarctic amplification (Allen MR et al., 2018), marking the beginning of an era in which the climate system is increasingly driven by human activities. Therefore, the current Arctic warming is generally considered the result of greenhouse gas emissions (Dai A et al., 2019). However, the Arctic amplification has a strong inherent feature of the global climate system, which was found in the study of paleoclimate proxies (Masson-Delmotte V et al., 2006), which are observed now and can be used for future predictions (Collins M et al., 2013). In the past, natural physical mechanisms were the only driving force behind Arctic amplification. In other words, Arctic amplification was the result of the interactions of climate feedback mechanisms in the nonlinear climate system, as well as the changes in the meridional transport of energy in the atmosphere and the oceans (Goosse H et al., 2018). The current positive phase of the Arctic amplification is associated with the loss of the Arctic sea ice. For example, the sea ice area of the Arctic-Pacific region has significantly decreased at the end of the thawing season since the beginning of this century. This decrease is associated with the increase in the meridional atmospheric moisture transport from the North Pacific in summer. In addition, stratospheric moisture transport is induced by the subsequent effects of downward long-wave radiation to form positive feedback, reducing the formation rate of sea ice (Lee HJ et al., 2017). The changes in the mechanism similar to sea ice have been examined using the PDO. The results show that anomalous meridional moisture transport has existed in the recent period (or even since the end of the 20th century or even since the end of the 20th century; Kim H et al., 2020). In both cases, the sea surface temperature (SST) anomalies in the North Pacific, which can be used to define different phases of the PDO, are believed to be the source of a sudden regional increase in poleward atmospheric moisture transport. At present, the research is yet to be enhanced to understand Arctic amplification.

    As shown by the 2000‒2019 air temperature data from China’s Zhongshan and Great Wall stations in the Antarctic and the 2000‒2017 SST data inverted from the NOAA data, the air temperature, and the SST increased year by year in the Emery and Larsen ice shelf regions. The rise of SST in the two ice shelf regions was the miniature of the rise in the SST throughout the Antarctic, which further promoted the melting of glaciers in areas sensitive to SST and accelerated the rise in the sea level (Wang H et al., 2020). Scientists found that the mean surface temperature in the Antarctic increased by 2.5°C from the 1950s to the end of the 20th century, which was consistent with global warming (Yan QD, 2008). Ice shelves are both the most significant mass exchange area and the most active characteristic area at the margin of the Antarctic ice sheet. Their dynamic changes have a profound impact on the Antarctic and even on the world. The meltwater from the ice shelf surface flows through the ice sheet to form glacial lakes, which may trigger the disintegration of ice shelves (Bell RE et al., 2017). Under climate warming, the subsequent increase in glacial lakes accelerates the future ice loss in the Antarctic (Kingslake J et al., 2017).

    The remarkable differences in bivalves’ δ18O profiles between the Antarctic Peninsula and East Antarctica may reflect different responses to regional warming over the past few decades. As shown in the δ18O profiles, the oxygen isotope values are more negative than the predicted equilibrium values, indicating that the oxygen isotope depletion results from the reduced ocean salinity caused by glacier melting. That is, the evidence for global warming effects recorded by the Antarctic bivalves suggests that glaciers in the Antarctic are melting in a pulsating pattern (Woo KS et al., 2019). In particular, the Antarctic Peninsula experienced gradual warming during the 20th century (Jones PD, 1990; Jones PD et al., 1993; Barrand NE et al., 2013; Turner J et al., 2014; Wouters B et al., 2015). In the past 50 years, about 87% of the glacier fronts on the Antarctic Peninsula and its associated islands have receded, and the rate of recession has been increasing since the early 21st century (Cook AJ et al., 2005). The Maxwell Bay, located at the northern tip of the Antarctic Peninsula, shows a similar trend, with the glacial front of the Marian Cove receding by more than 683 m over the period 1956‒1994 (Park BK et al., 1998) and by more than 1100 m from 1956 to 2013 (Moon HW et al., 2015).

    American scientists Mann ME et al. (1999) established the GMT curves of the past thousand years and believed that the 1990s was the warmest decade in the past thousand years and 1998 was the warmest year. The past 2000 years are especially significant in global climate change (Fig. 4). During this period, the driving force of the global climate change transitioned from nature to combined nature and human activities, in which human activities played an increasingly important role in driving the Earth’s climate system to change. Especially since the start of the Industrial Revolution (around 1750), the massive use of fossil fuels has resulted in a continuous increase in anthropogenic CO2 emissions. As a result, the global atmospheric CO2 concentration increased from about 280×10−6 to 414×10−6 in 2020, with the increased amplitude exceeding its maximum amplitude of natural fluctuations in the past 830 Ka, thus exacerbating the Earth’s greenhouse effect and causing the global temperature to shift from a slow downward trend before the 19th century to the rapid increase in the 20th century (IPCC, 2013). These findings indicate that the Anthropocene in which human beings affect the Earth’s natural evolution has arrived (Subramanian M, 2019; Hamilton C, 2016; Monastersky R, 2015).

    Figure  4.  Global mean temperature anomalies in the past 2000 years reconstructed based on proxy data (after PAGES 2K Consortium et al., 2019).
    Note: The color curves represent the median values of the 30-year low-pass filter integrated using different reconstruction methods. Reconstruction methods: PCR‒principal component regression; PAI‒pairwise comparison; BHM‒Bayesian hierarchical model; CPS‒composite-plus-scaling; M08‒ regularized errors in variables; OIE‒optimal information extraction; DA‒offline data assimilation. The black curve represents the anomaly sequence of instrumental temperature from 1850 to 2017, and the anomaly is the difference from the averages from 1961 to 1990. The red dotted line is the zero-value line of temperature anomaly.

    Zheng JY et al. (2021) sorted out the progress in the integrated studies of global temperature change over the past 2000 years, obtaining the following results. Although the GMT of the past 2000 years reconstructed using different methods had different ranges, all the reconstructed data showed that it was warmer in the first one thousand years than in the second thousand years (except the 20th century). There was a significant cooling trend before 1850, followed by rapid warming. The warmest decade, 30 years, and 50 years in the past 2000 years all occurred in the second half of the 20th century. The GMT in the 20th century was 0.3°C higher than that in the previous five centuries and was approximately 0.1°C higher than that in the previous warmest century (403‒502 A.D.). Meanwhile, the GMT from 1971 to 2000 was more than 0.2°C higher than that of the previous warmest 30 years (746‒775 A.D.;). These results present many new characteristics of global temperature change in the past 2000 years, especially including: (1) There were significant inter-continental differences in temperature fluctuations on a decade to century scale before the 20th century, while the climate was nearly synchronous warming in all continents during the 20th century; (2) before the 20th century, continents such as Arctic, Europe, North America, South America, and Antarctica (except for Asia and Australia,) all experienced periods warmer than the warmest 30 years of the 20th century; (3) on a millennium scale, the temperature of all continents showed a similar long-term downward trend before the late 19th century and then showed an increasing trend. In other words, the GMT decreased at a rate of about 0.03°C/100a from 1‒1850 A.D. and then increased at a rate of approximately 0.43°C/100a from 1851‒2000 A.D. The 20th century is the warmest or nearly warmest century for all continents (except for Antarctica), with the mean temperature of the six continents approximately 0.4°C above than that of the previous five centuries (PAGES2K Consortium, 2013).

    Previous studies generally divided the global climate in the past 2000 years into several cold-warm stages (Alverson K et al., 2003; Christiansen B and Ljungqvist FC, 2017), including the Little Ice Age (about 1450‒1850 A.D.; IPCC, 2013), the Medieval Climatic Anomaly (approximately 950‒1250 A.D.; IPCC, 2013), which is also referred to as the Medieval Warm Period (Li YY et al., 2019; Li KK et al., 2018), the Dark Ages Cold Period (approximately 300‒800 A.D.; Lamb HH, 1977; Ljungqvist FC, 2010), and the Roman Warm Period (approximately 0‒300 A.D.; Lamb HH, 1977; Ljungqvist FC, 2010). Global warming since the mid-19th century just occurred following the ending process of the Little Ice Age (Ren GY et al., 2021).

    It should be noteworthy that, apart from the globally synchronized warming in the 20th century, the start/stop time of all cold and warm periods around the world was not synchronous before the Industrial Revolution. This finding indicates that the forced anomalies of the climate system before the Industrial Revolution are not sufficient to cause the extreme synchronous anomalies of the temperatures on a multi-decade - century scale in various regions of the world. However, the 20th century was the warmest period in the past 2000 years for more than 98% of regions in the world (except for local parts of the Antarctic; Neukom R et al., 2019). This result indicates that the global warming in the 20th century caused by human activities has unprecedented spatial consistency.

    It is worrying that some scientists predict that the GMT in the 21st century may rise by greater than 4°C (Betts RA et al., 2011; Sanderson MG et al., 2011; World Bank, 2012; James R and Washington R, 2013). In a warmer world, extremely cold weather will decrease, extremely warm weather will increase, and the former will be more sensitive to global warming than the latter (Wang X et al., 2017). Lewis SC et al. (2019) also considered that the hot spot regions with extreme air temperatures generally respond faster to the GMT increase than the rest regions, that is, hot spot regions with extreme air temperatures will be warmer. The droughts in the Hu Huanyong Line in northwest China are predicted to be extremely severe and complex in the future (Zeng P et al., 2021).

    The Qinghai-Tibet Plateau covers an area of about 2.5×106 km2(Deli GJ, 2019) and is the highest plateau in the world, with an average altitude of above 4000 m and a unique geographical environment. The climate in the plateau has been changing greatly since the Pliocene. The mean annual air temperature (MAAT) of Plio-Pleistocene lakes on the plateau has been reconstructed (Cheng F et al., 2022). The results show that the mean summer lake surface temperature (SLST) was approximately 15.0°C at 4.3‒2.7 Ma and decreased rapidly to approximately 8.1°C at 2.7‒2.6 Ma, with an average of approximately 9.0°C at 2.7‒0.8 Ma (Cheng F et al., 2022; Fig. 5). Based on a published relationship between SLST and MAAT, it can be estimated that local MAAT was approximately 13‒17°C lower than SLST at 4.3‒0.8 Ma. Overall, the reconstructed MAAT shows the same stepwise change pattern as SLST over the past 4.3 Ma (Cheng F et al., 2022; Fig. 5). The average MAAT was 1.7°C at 4.3‒2.7 Ma, sharply decreased to −7.7°C at 2.7–2.6 Ma, and was −6.4°C at 2.7–0.8 Ma (Cheng F et al., 2022; Fig. 5), which was similar to the local modern MAAT (−6°C to −7°C) in the plateau.

    Figure  5.  Paleoclimate records of the Kunlun Pass (KP) section of northern Qinghai-Tibet Plateau and global proxies (after Cheng F et al., 2022).
    a‒lacustrine δ18Oc record from the KP section; b‒lacustrine δ13Cc record from the KP section; c‒carbonate content from the KP section; d‒surface lake summer temperature (SLST) from the KP section based on clumped isotope thermometry; e‒mean annual air temperature (MAAT) from the KP section estimated using a transfer function relating SLST and MAAT; f‒δ18Ow (SMOW) record from the KP section. The dashed lines in a‒f are the bootstrap plots (1 Myr loess regression) derived from various detailed records. The error bars in d‒f represent the 1σ error of SLST, MAAT, and δ18Ow of each sample; g‒normalized plots showing trends in paleoclimate proxy records from the KP section. These trends are bootstrap plots (1 Myr loess regression) derived from detailed records shown in a‒f; h‒magnetic susceptibility (MS) record from the Chinese Loess Plateau (CLP), showing aridification at 2.7‒2.6 Ma due to intensified winter monsoon associated with global cooling; i‒MAAT record from the Weihe basin; j‒biogenic opal mass accumulation rate (MAR) at ODP Site 882; k‒Sea Surface Temperature reconstructed for ODP Site 982; l‒global benthic δ18O stack. The cooling event at 2.7–2.6 Ma on the northern Qinghai-Tibet Plateau was simultaneous with the intensification of the Northern Hemisphere Glaciation (blue shaded area). The orange shaded area demarcates the mPWP (3.3–3.0 Ma). The numbers marked in a–g represent the averages of various proxies at 4.3–2.7 Ma and 2.7–0 Ma, respectively.

    As an indicator and amplifier of global climate change, the Qinghai-Tibet Plateau is very sensitive to climate change (Wu TH et al., 2017), and its warming trend is more significant than that of the Northern Hemisphere (Fig. 6). Ding YH et al. (2006) pointed out that under the background of global warming, the increase in the annual average land surface temperature in China was slightly higher than the global warming in the past century, especially in the Qinghai-Tibet Plateau—a climate-sensitive area. The air temperature of the Qinghai-Tibet Plateau has increased significantly in recent years (Du J, 2001; Tan CP et al., 2010; Liu GF and Lu HL, 2010; Ma ZZ et al., 2019), with the temperature increase much higher than the global warming level. Before the mid-late 1990s, the global warming trend was significant, while the air temperature of the Qinghai-Tibet Plateau changed more gently. Global warming has slowed down since the mid-late 1990s, while the temperature on the Qinghai-Tibet Plateau has increased at an increasingly high rate since the 21st century (Zheng R et al., 2015). The temperature of the Qinghai-Tibet Plateau increased significantly in 1997 and 1998 and peaked in 2010, with the highest value exceeding the temperature of the global warmest year (1998) in nearly 100 years before the 21st century. Moreover, in the past 41 years, the Qinghai-Tibet Plateau was the warmest during the first decade of the 21st century (Zheng R et al., 2015).

    Figure  6.  Correlation between the warming trends of the Qinghai-Tibet Plateau and the Northern Hemisphere (after Yao TD et al., 2006).

    Based on the 1961‒2015 temperature data from meteorological stations on the Qinghai-Tibet Plateau, Ji Q et al. (2020) found that the temperature in the plateau significantly increased from 1961 to 2015. Spatially, the temperature in northwestern Yunnan rapidly increased and fluctuated; the temperature in the Qaidam Basin, the basins of the Yarlung Zangbo, Nianchu, and Lhasa rivers, and the West Sichuan Plateau experienced a trend of first slow rise, then rapid fluctuations (or rapid rise), and then slow fluctuations, and the temperature in the Naqu Plateau and the Yellow River source region showed a stable trend of rapid rise accompanied by weakened fluctuations. Li HD et al. (2010) revealed that the Yarlung Zangbo River source region on the Qinghai-Tibet Plateau had the most significant high temperature in China in the same period. In the past decades, the climate elements such as air temperature have changed significantly on the Qinghai-Tibet Plateau, causing a series of problems, such as glacier recession (Yang JP et al., 2015; Cui ZY et al., 2014), permafrost degradation (Jiao SH et al., 2016, 2012), and ecosystem degradation (Bai W et al., 2011; Yang JP et al., 2007). Since the 1980s, the climate of the Qinghai-Tibet Plateau has changed significantly, generally showing a warm and humid trend. Moreover, the temperature of the Qinghai-Tibet Plateau increased at a higher rate than that of any other area with the same latitude in the Northern Hemisphere in the past 50 years, causing severe environmental changes, such as accelerated melting of glaciers and permafrost (Yao TD et al., 2017). For example, Yan LJ (2020) found that the climate in Tibet experienced a warm and humid trend from 1981 to 2017, mainly manifested by increasing temperature and rainfall and decreasing evaporation. As a result, the lakes in Tibet continuously expanded during that period. The Namjagbarwa Peak in the eastern Himalayas lies at the intersection of the eastern Himalayas, the Nyainqentanglha Mountain, and the Hengduan Mountains. It is the highest peak in the eastern Himalayas, with an altitude of 7782 m. Glaciers in the peak area are very sensitive to the increase in temperature and have generally shrunk due to temperature rise. From 1980 to 2015, the glacier area in the Namjagbarwa Peak area continuously decreased at an increasing rate. As a result, it decreased by approximately 75.23 km2 totally in the 35 years, which accounted for approximately 25.2% of the total glacier area in 1980, with an average annual decrease rate of approximately 0.73% (Wu KP et al., 2020). From 1981 to 2021, the thickness of the active layer in the permafrost area along the Qinghai-Tibet Highway showed a significant increase trend, with an average thickness of 19.6 cm every 10 years; from 2004 to 2021, the temperature at the bottom of the active layer (the upper limit of the permafrost) showed a significant upward trend. In 2021, the average thickness of the active layer in the permafrost area along the Qinghai-Tibet Highway will be 250 cm, the highest value ever recorded (Fig. 7; National Climate Change Centre, China Meteorological Administration, 2022).

    Figure  7.  Thickness of active layer in permafrost area along Qinghai-Tibet Highway and the temperature change at the bottom of the active layer (after National Climate Change Centre, China Meteorological Administration, 2022).

    The Qinghai-Tibet Plateau covers a wide area. Although studies with different time ranges and time scales yielded different characteristics of temperature variations on the Qinghai-Tibet Plateau, they commonly revealed that abrupt temperature changes occurred on the plateau (Fig. 8). Xu LJ et al. (2019) analyzed the characteristics of climate change in the Qinghai-Tibet Plateau and its surrounding areas based on the 1961‒2010 temperature data from 158 meteorological stations, and the results showed that the main body of the plateau became increasingly warm and humid during that period. The hinterland of the Qinghai-Tibet Plateau is the most sensitive to the mean, minimum, and maximum temperatures. Yongzhu ZG et al. (2017) made statistics of the 1958‒2005 temperature data from 15 meteorological stations on the Qinghai-Tibet Plateau and concluded that the mean annual temperature on the Qinghai-Tibet Plateau remained low at first and then fluctuated to a high level in that period, showing an upward trend overall. Deli GJ (2019) further studied the 1984‒2014 temperature data from 56 meteorological stations on the Qinghai-Tibet Plateau, obtaining the following results. The interannual variations in the mean temperature of the plateau in that period were distributed in a high-low-high form in the latitudinal direction, with the temperature showing an upward trend. Overall, the mean annual temperature was commonly low before the 21st century and then rose to a high level. It is considered that the mean annual temperature has increased significantly since 1997 and showed a significant warming trend since 2005, with abrupt changes occurring since 2003. As an important part of the source region of the Yellow, Yangtze, and Lancang rivers (collectively referred to as the Three-River Headwaters region), the Yellow River source region on the Qinghai-Tibet plateau has an ecological vulnerability. In particular, the Three-River Headwaters region is located in the hinterland of the Qinghai-Tibet plateau and is known as the “Chinese water tower” (Yao TD, et al., 2019). According to research, the mean annual temperature and the annual maximum and minimum temperatures in the Yellow River source region all showed a consistent warming trend from 1960 to 2019, with an increase in temperature higher in the east than in the west. Among them, the mean annual temperature changed abruptly around 2000 and in recent 10 years it rose to the highest level in the past 60 years, during which the Yellow River source region was the warmest and most humid overall. Owing to the continuous warm and humid trend, the mean extreme temperature threshold from 1960 to 2019 showed a significant upward trend, which may bring risks and challenges to the ecological protection of the Yellow River source region and the high-quality development of the whole Yellow River reaches (Liu CH et al., 2021).

    Figure  8.  Characteristics of the mean temperature before and after the abrupt change of the Qinghai-Tibet Plateau from 1961 to 2010 (Xu LJ et al., 2019).

    Based on the temperature data of the Qinghai-Tibet Plateau from 1961 to 2010, Wu CQ and Tang DY (2017) investigated the variation trend of temperature on the plateau and found that the mean annual temperature on the plateau was rather low during 1961‒1993 but was rather high during 1994‒2010, with an abrupt change occurring in 1996. Zhu WQ et al. (2001) collected the 1951‒1998 temperature data from 217 ground observation stations on the Qinghai-Tibet Plateau and its adjacent areas and analyzed the characteristics of modern climate change on the plateau. Based on the meteorological data of the Lhasa station from 1935, they considered that the temperature on the Qinghai-Tibet Plateau was the highest in the 1940s, then dropped until the mid-1960s, and then it rose until the 1990s, during which, however, the mean annual temperature did not reach the level in the 1940s. These results are consistent with the climate change in other areas of China, except that the second warming occurred in the mid-1960s in the plateau but occurred in the 1970s in the rest of China. Li L et al. (2019) also considered that the abrupt temperature change in the northeastern Qinghai-Tibet Plateau in the past 56 years occurred around 1994. After studying the changes in the temperature of the Qinghai-Tibet Plateau in recent 41 years based on the 1971‒2011 temperature data from 81 meteorological stations on the plateau, Zheng R et al. (2015) concluded that the temperature of the Qinghai-Tibet Plateau had a downward trend from the mid-1970s to the mid-1980s, then an upward trend in the late 1980s. The temperature rise trend became significant in the late 1990s, especially in 1997, when the temperature changed abruptly. That is, abrupt warming change on the Qinghai-Tibet Plateau occurred in the mid-late 1990s, while that in the rest of China occurred earlier in 1994. For example, the abrupt warming change on the plateau lagged behind that in North China (1989) and Northeast China (1987) for 8‒10 years and lagged behind that in Southwest China and South China (1996) as well as East China and Northwest China (1994) for 1‒3 years. The temperature fluctuation pattern of the Qinghai-Tibet Plateau is similar to the fluctuation pattern of the global temperature in the same period, and there is a close correlation between them. However, compared with the global gentle warming trend in the past 15 years, the Qinghai-Tibet Plateau showed a more significant increase in temperature after the abrupt change. Li CL et al. (2006) summarized the study results of the climate change of different periods in the Qinghai-Tibet Plateau in recent years, concluding that the climate change in the Qinghai-Tibet Plateau generally occurred earlier and was more significant than that in the rest of China on various scales. By contrast, based on the 1961‒2012 temperature data from 88 meteorological stations on the Qinghai-Tibet Plateau, Li L et al. (2018) believed that while maintaining a general upward trend in that period, the temperature on the Qinghai-Tibet Plateau increased more gently after 2006, with the warming on the plateau lagging behind the global climate change for approximately eight years.

    Other researchers believed that the abrupt temperature change in the Qinghai-Tibet Plateau occurred in the mid-1980s and that the mean annual temperature of the plateau showed a rising trend with fluctuations. The Qinghai-Tibet Plateau was warm in the 1950s and experienced a relatively cold period from the 1960s to the 1970s. The temperature of the plateau began to rise rapidly in the mid-1980s and even more rapidly in the 1990s, reaching its maximum value in the past 50 years in 1998. In other words, the change in the temperature on the Qinghai-Tibet Plateau in the past 50 years can be divided into three periods, namely a warm period in the early 1960s, a cold period from the mid-1960s to the early 1980s (Cai Y et al., 2003), and a period of continuous temperature rise since the mid-late 1980s (Wu SH et al., 2005), showing an overall rising trend with fluctuations in the early and late stages (Yao L and Wu QM, 2002). Song C et al. (2012) also pointed out that the mean annual temperatures in the vast majority of the Qinghai-Tibet Plateau all had significant interdecadal changes in the second half of the past century. Since 1960, the temperature in the Qinghai-Tibet plateau has risen remarkably, and the plateau has shifted from a cold period (from the mid-1960s to the early 1980s) to a warm period (since the 1980s). The abrupt temperature change occurred in the 1980s, and the temperature on the plateau showed a rising trend with fluctuations overall since 1960. Wang N et al. (2010) also concluded that the temperature variations on the Qinghai-Tibet Plateau showed significant interdecadal characteristics. Regarding the change in the temperature in the Qinghai-Tibet Plateau, the past century can be divided into two cold and two warm periods, during which three abrupt changes occurred. In other words, the temperature on the plateau was low before the 1920s, rose from the 1920s to the 1950s, dropped from the 1950s to the 1980s, and remained high afterward. Moreover, the abrupt temperature changes on the Qinghai-Tibet Plateau occurred earlier than those in the rest of China. Zhu WQ et al. (2001) also believed that the Qinghai-Tibet Plateau was the warmest in the 1930s and 1940s, with an average temperature of about 8.55˚C. Afterward, the plateau became cold in the 1960s, warmed up again after the 1970s, and entered another high-temperature period in the mid-late 1980s. The warming in the plateau was more significant in the 1990s, which was the warmest decade in the 20th century, and 1998 was the warmest year of the 20th century (Editorial Board of the National Assessment Report of Climate Change, 2007). Li SC et al. (2006) pointed out that the temperature in the Qinghai-Tibet Plateau decreased gradually from the 1970s to the early 1980s and then increased significantly from the mid-1980s to the early 21st century (especially in the late 1990s). Tang MC et al. (1998) also pointed out that the climate variation trend in the Qinghai-Tibet Plateau was consistent with that in the rest of China and in the Northern Hemisphere, with abrupt warming changes in these areas all occurring in the early 1980s. This result is roughly consistent with the findings of previous studies (Lin ZY and Zhao XY, 1996; Cai Y et al., 2003; Wang Y et al., 2004). Based on the high-resolution surface temperature grid data from a British university, Ren YL et al. (2012) found that the Qinghai-Tibet Plateau experienced four cold and warm alternating periods in the past century: (1) A cold period from 1901 to 1930; (2) a warm period from 1931 to 1950, during which the temperature increased significantly; (3) a period from 1950 to around 1980, in which the temperature changed steadily on the whole; (4) a warm period since 1980, in which the temperature has been rising rapidly. Li L et al. (2010) analyzed the relevant data of the Qinghai-Tibet Plateau from 1961 to 2007 and concluded that the abrupt temperature change on the plateau occurred in 1987. The air temperature in the Gonghe Basin, Qinghai Province took a significant upward trend from 1961 to 2018. The mean annual temperature of the basin rose rapidly from the beginning of the 1990s to 2018. It has increased by 1.2˚C since the 21st century and by approximately 2.5 ˚C compared with the low temperature in the 1960s (Yan R and An GH, 2021). These results are consistent with the fact that the temperature in China began to rise in the late 1980s and rose rapidly in the 1990s (Qin DH, 2004). However, Lin ZY and Zhao XY (1996) and Li CL and Kang SC (2006) believed that the warming of the Qinghai-Tibet Plateau occurred earlier than that in the rest of China. For example, Tang MC et al. (1984) considered that climate change in the Qinghai-Tibet Plateau occurred 15 years earlier than that of East China on average. Feng S et al. (1998) argued that the temperature change in the Qinghai-Tibet Plateau occurred 10‒60 years earlier than that in East China on a scale of 100 years. However, by comparing the abrupt temperature changes in the Qinghai-Tibet Plateau and the rest of China, Ding YH and Zhang L (2008) held that the abrupt temperature change in the Qinghai-Tibet Plateau in the mid-1990s was more significant than that in the 1980s and that the significantly rapid climate change on the plateau occurred later than in the rest of China.

    These climatic features have also been confirmed by changes in the geochemical composition of polycyclic aromatic hydrocarbons (PAHs) in sediment cores. Wang AT et al. (2021) analyzed the concentration and composition of PAHs in sediment core samples from different lakes on the Qinghai-Tibet Plateau and found that the total concentrations of 16 PAHs were relatively low in four historical periods of thermal anomalies (1973‒1975, 1988‒1989, 1998‒1999, and 2006‒2007), meaning that high temperatures may have inhibited atmospheric PAHs from being deposited and precipitating in lake sediments. It should be noteworthy that PAHs, which are typically persistent organic pollutants, are not only emitted by natural processes (e.g., volcanic eruptions, forest fires, and biological production) but also by human activities (e.g., internal combustion in engine transportation and industrial production). Among them, human emissions are considered the most important source of PAHs, including oil leakage and the incomplete combustion or pyrolysis products of organic materials such as various biomass (e.g., dung cakes, grass, and wood) and fossil fuels (e.g., coal, gasoline, and diesel; Wang AT et al., 2021).

    The abrupt temperature change characteristics of the Qinghai-Tibet Plateau can also be seen on a longer time scale. Based on the analysis of the records of four ice cores covering more than 1000 years taken from the Qinghai-Tibet Plateau, Yao TD et al. (2006) found that the temperature of the Qinghai-Tibet Plateau gradually increased with cold and warm fluctuations in the recent 1000 years. In other words, the medieval warm period (MWP) lasted until the 13th century (during which three warm periods and cold periods occurred) in the Qinghai-Tibet Plateau, followed by the cold periods in the 14th and 16th centuries and warm periods in the 15th and 17th centuries. The temperature fluctuated frequently from the end of the 17th century to 1920. Since then, the temperature has risen rapidly until now. In particular, the climate became warm sharply after a short-lived temperature fall in the early 1950s, and the present is in the warmest period of the past 1000 years. Overall, the Qinghai-Tibet Plateau experienced a gradual warming trend in the past 1000 years. But strictly speaking, the temperature has increased while fluctuating since the medieval temperature fall event, and the increase in temperature since the 1980s is the highest in the past 1000 years (Yao TD et al., 2006). The overall variation trend of temperature on the Qinghai-Tibet Plateau in the past 1000 years is roughly consistent with that in the Northern Hemisphere. In other words, the temperature of the plateau showed a linearly decreasing trend with fluctuations from the medieval warm period to the end of the 17th century and has risen at the highest rate since the beginning of the 18th century compared to the temperature in the past 1000 years (Yao TD et al., 2006).

    Based on environmental records (e.g., tree rings, ice cores, and sediments) with a high resolution of ten years, Hou GL and Xu CJ (2016) found warm and cold periods in the Qinghai-Tibet Plateau on a longer time scale of the past 2000 years, with the warm periods mainly including 0‒235 A.D., 775‒1285 A.D., and 1845‒2000 A.D. and cold periods mainly including 245‒765 A.D., 1045‒1145 A.D., and 1275‒1835 A.D. The Little Ice Age significantly occurred in the Qinghai-Tibet Plateau, with the period 1635‒1675 A.D. serving as the coldest period of the plateau. The temperature began to rise in the 1820s and rose rapidly in the 20th century, roughly showing a linear trend. The temperature in the late 20th century exceeded the extreme values of the past 2000 years. The temperature change in the past 2000 years had significant impact on human society. The period 245‒765 A.D. was the longest cold period in the past 2000 years. During this period, China was in the Wei, Jin, and Southern and Northern Dynasties, when the monsoon weakened, the monsoon boundary moved southward, and the vegetation belt moved southward. As a result, the Tufa and Tuyuhun branches of Xianbei tribes, who had always lived by water and grass in the original monsoon boundary, migrated southward. Therefore, temperature changes were closely related to wars and meteorological disasters in history (Hou GL and Xu CJ, 2016).

    Based on the data on tree rings, observed meteorological data, historical records, and pollen analysis, Wu XD and Lin ZY (1981) deduced the basic pattern of climate change in the Qinghai-Tibet Plateau over the past several thousands of years and identified five periods, namely a climatically optimum period (7000‒3000 BC), the New Ice Age (1000 BC to the 5th century A.D.), a warm period (the 6th century to the 12th century), the modern Little Ice Age (late 12th century to early 19th century), and the recent warming period (mid-19th century to present).

    As indicated by observed data, the Qinghai-Tibet Plateau is one of the areas that are the most sensitive to global warming. As a regional response to global warming, the temperature in the Qinghai-Tibet Plateau has changed significantly in recent decades, with higher altitudes on the plateau corresponding to more significant temperature changes (Zhang YH et al., 2011). Studies have shown that the Qinghai-Tibet Plateau had a rate of warming of 0.16°C/10a during 1955‒1996 (Liu XD and Chen BD, 2000). Zhang YH et al. (2011) analyzed the characteristics of climate change in the Qinghai-Tibet Plateau since the 1960s using the 1961‒2001 temperature data from meteorological stations on the plateau, obtaining the following results. The Qinghai-Tibet Plateau has responded significantly to modern global warming since the 1960s, and the temperature of the plateau has increased at an average rate of about 0.16°C/10a, which is greater than the national warming rate. Moreover, climate change has caused a series of ecological effects on the Qinghai-Tibet Plateau, including rapid permafrost degradation, glacier shrinkage, vegetation belt migration, the expansion of land desertification, and reduction of surface runoff in the Three-River Headwaters region, lake shrinkage, and wetland degradation. From 1961 to 2006, the average temperature of the Qinghai-Tibet Plateau increased at a rate of about 0.34°C/10a (Liu XD and Chen BD, 2000; Chen XG et al., 2009), while the rate of change in the national temperature was only about 0.28°C/10a during the same period (Editorial Board of the National Assessment Report of Climate Change, 2007). The World Wide Fund for Nature (WWF) has reported that the rate of warming in the Qinghai-Tibet Plateau has also increased with the acceleration of global warming. The mean annual temperature has increased significantly since the 1950s, with an increasing rate of 0.37°C/10a from 1961 to 2007 and a higher rate in cold seasons than in warm seasons (Li ZF, 2018). Li L et al. (2010) also believed that the temperature on the Qinghai-Tibet Plateau increased at a rate of up to 0.37°C/10a from 1961 to 2007. Based on the 1961‒2010 temperature data of the Qinghai-Tibet Plateau, Wu CQ and Tang DY (2017) concluded that the mean annual temperature increased at a rate of 0.228°C/10a from 1994 to 2010. Xu LJ et al. (2019) found that the average temperature of the Qinghai-Tibet Plateau was 6.0°C, changed at a rate of 0.28°C/10a in the past 50 years, and increased by 1.4°C from 1961 to 2010. Ji Q et al. (2020) discovered that the mean annual temperature of the Qinghai-Tibet Plateau increased significantly at a rate of 0.33°C/10a from 1961 to 2015 with a sharp acceleration since 1994, which is consistent with the trend of global temperature change except for the rate of temperature change, which was much higher in the plateau than in the rest of the world (IPCC, 2013) and the rest of China (Shi PJ et al., 2014), including geographic units such as the Loess Plateau (Gu CJ et al., 2017) and the Yunnan-Guizhou Plateau (Gu F et al., 2018). This finding confirmed the sensitivity of the Qinghai-Tibet Plateau to climate change. By analyzing the 1969‒2008 temperature data from 15 meteorological stations in the Qinghai-Tibet Plateau, Yao L and Wu QM (2002) concluded that the mean annual temperature in the Qinghai-Tibet Plateau increased at a rate of 0.167°C/10a during 1969‒2008. The warming occurred earlier in the Qinghai-Tibet Plateau than in the Northern Hemisphere, and the temperature rose at a higher rate in the Qinghai-Tibet Plateau than in the Northern Hemisphere and other regions at the same latitude (Liu XD and Chen BD, 2000). Li SC et al. (2006) concluded that the mean annual temperature of the Qinghai-Tibet Plateau experienced a rising trend from 1971 to 2004, with an increasing rate of 0.28°C/10a. Tan CP et al. (2010) revealed that the average annual temperature of the Qinghai-Tibet Plateau increased at a rate of 0.33°C/10a from 1971 to 2007 with values up to 1.14°C/10a during 1998‒2007. The mean annual temperature of the Qinghai-Tibet Plateau from 1971 to 2011 showed a significant upward trend at a rate of 0.39°C/10a (Zheng R et al., 2015), which is slightly higher than the increasing rate of 0.37°C/10a from 1961 to 2007 calculated by Li L et al. (2010) but is significantly higher than the warming rate of China in the past century (0.08°C/10a) and in the past half-century (0.25°C/10a; Fu CB and Wang Q, 1992). This result indicates that the warming rate of the Qinghai-Tibet Plateau is much larger than that of other regions. Wang PL et al. (2012) pointed out that the mean annual temperature of the Qinghai-Tibet Plateau increased at a rate of 0.40°C/10a from 1981 to 2010. Overall, the temperature of the Qinghai-Tibet Plateau increased at a rate of 0.37°C/10a in the past 50 years, which is much higher than the national warming level (0.16°C/10a). Moreover, the temperature of the plateau changed at a higher rate more recently, indicating that the plateau underwent more significant warming in recent years (Kang XC, 1996; Liu XD and Hou P, 1998; Cai Y et al., 2003; Li L et al., 2010). Wang B et al. (2008) pointed out that the surface temperature of the Qinghai-Tibet Plateau has increased by 1.8°C in the past 50 years. Based on the high-resolution surface temperature grid data from the British University, Ren YL et al. (2012) found that the temperature of the Qinghai-Tibet Plateau has increased at a rate of about 0.07°C/10a in the past 100 years overall. In addition, Hou GL and Xu CJ (2016) found that the temperature of the Qinghai-Tibet Plateau increased by more than 1°C from 1800‒2000 A.D. based on environmental records (e.g., tree rings, ice cores, and sediments) with high resolution of ten years. Different warming rates and increases in temperature of the Qinghai-Tibet Plateau have been obtained by different studies, as listed in Table 2.

    Table  2.  Mean temperature increase of the Qinghai Tibet Plateau over the past years.
    No.YearChanging rate of air temperature/(°C/10a)Increase in temperature/°CReferences
    11955‒19960.16Liu XD and Chen BD, 2000
    21961‒20010.16Zhang YH et al., 2011
    31961‒20060.34Liu XD and Chen BD, 2000; Chen XG et al., 2009
    41961‒20070.37Li ZP, 2018
    51961‒20070.37Li L et al., 2010
    61994‒20100.228Wu CQ and Tang DY, 2017
    71961‒20100.281.4Xu LJ et al., 2019
    81961‒20150.33Ji Q et al., 2020
    91969‒20080.167Yao L et al., 2002
    101971‒20040.28Li SC et al., 2006
    111971‒20070.33Tan CP et al., 2010
    121971‒20110.39Zheng R et al., 2015
    131981‒20100.40Wang PL et al., 2012
    141998‒20071.14Tan CP et al., 2010
    15Recent 50 years0.37Kang XC, 1996; Liu XD and Hou P, 1998; Cai Y et al., 2003; Li L et al., 2010
    16Recent 50 years0.361.8Wang B et al., 2008
    17Recent 100 years0.07Ren YL et al., 2012
    181800‒2000>1.0Hou GL and Xu CJ, 2016
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    The increase in the mean annual temperature varies slightly in different regions of the Qinghai-Tibet Plateau, as shown in Table 3. For example, Wang PL et al. (2012) pointed out that the Qinghai-Tibet Plateau continued to warm up from 1981 to 2010, especially in eastern, central, and western Tibet and northern Qinghai. According to the monitoring data, from 1960 to 2010, the mean annual temperature in the south and north of the Qinghai-Tibet Plateau increased by about 1.4°C and 2.0°C, respectively (Da S, 2011; Dan Z et al., 2018). According to the statistics of Han GJ et al. (2011), the temperature data for the past 50 years from most meteorological stations on the Qinghai-Tibet Plateau show an increasing trend. Except for the Henan station, whose temperature data showed a trend of cooling at a rate of −0.24°C/10a, the Lhasa and Nagqu areas in central Tibet, the Dingri area in southern Tibet, and the Golmud, Delingha, and Mangya areas in northwestern Qinghai show temperature rise at rates of up to (0.5‒0.9)°C/10a; the Pali, Jiali, and Qamdo areas in southeastern Tibet, the Minhe area in eastern Qinghai, and the Lenghu area in northwestern Qinghai exhibited temperature rise at low rates of (0‒0.3)°C/10a; and the Shiquanhe, Zhiduo, Qamdo, and Qingshuihe areas in northern Tibet, the Wudaoliang-Zaduo area, the Qilian, and Gangcha areas in northern Qinghai, and the Jiuzhi and Yushu areas in southeastern Qinghai witnessed temperature rise at rates of (0.3‒0.4)°C/10a. Ren YL et al. (2012) discovered that the climate has changed at different rates in different portions of the Qinghai-Tibet Plateau in the past 100 years. Specifically, the climate has varied at a rate of 0.065°C/10a in the central portion, at the highest rate of 0.128°C/10a in the west, and at a rate of 0.108°C/10a in the north, 0.015°C/10a in the south, and 0.022°C/10a in the east.

    Table  3.  Rise in the mean annual temperature in different areas of the Qinghai-Tibet Plateau in the past years.
    No.AreaYearWarming
    rate/(°C/10a)
    Increase in
    temperature/°C
    References
    1Southern Qinghai-Tibet Plateau1960‒20100.281.4Da S, 2011; Dan Z et al., 2018
    2Northern Qinghai-Tibet Plateau1960‒20100.42.0Da S, 2011; Dan Z et al., 2018
    3Henan Station in QinghaiPast 50 years−0.24Han GJ et al., 2011
    4Central and southern portions of Tibet, northwestern QinghaiPast 50 years0.5‒0.9Han GJ et al., 2011
    5Southeastern Tibet, eastern and northwestern QinghaiPast 50 years0‒0.3Han GJ et al., 2011
    6Western and northern Tibet, southern, northern, and southeastern QinghaiPast 50 years0.3‒0.4Han GJ et al., 2011
    7Central Qinghai-Tibet PlateauPast 100 years0.065Ren YL et al., 2012
    8Western Qinghai-Tibet PlateauPast 100 years0.128Ren YL et al., 2012
    9Northern Qinghai-Tibet PlateauPast 100 years0.108Ren YL et al., 2012
    10Southern Qinghai-Tibet PlateauPast 100 years0.015Ren YL et al., 2012
    11Eastern Qinghai-Tibet PlateauPast 100 years0.022Ren YL et al., 2012
    12Siling Co1979‒20170.49Wang KX et al., 2020
    13Three-River Headwaters regionPast 60 years0.37Jin Z et al., 2020
    14Yellow River source region1960‒20190.37Liu CH et al., 2021
    15Upper reaches of the Yellow River1980‒20180.23Ye PL et al., 2020
    16Wudaoliang1961‒20190.34Wu SG et al., 2020
    17Northeastern Qinghai-Tibet PlateauRecent 560.39Li L et al., 2019
    18Qilian Mountains Nature Reserve1995‒20141.1156.61‒8.84 oCWang PP et al., 2020
    19Central Qilian Mountains1960‒20170.39Cheng P et al., 2020
    20Hala Lake1986‒20150.4Li DS et al., 2021
    21Southern margin of Qaidam1961‒20190.321.9Zeng GY et al., 2020
    22Gonghe in Qinghai1961‒20180.51Ding YH et al., 2006
    23Tibet1961‒20080.32Wang QC et al., 2007
    24Qinghai1961‒20060.36Wang QC et al., 2007
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    The research results of the Siling Co area on the Qinghai-Tibet Plateau also showed that the mean annual temperature of the plateau was −1.8°C and increased significantly at a rate of 0.49°C/10a from 1979 to 2017, during which the area had a significant warm and humid climate (Wang KX et al., 2020). The Three-River Headwaters region is facing climate change toward a warm and humid climate and is an area with significant climate change and sensitive to it. Based on the temperature and other data of 20 ground stations run by the China Meteorological Administration located in the Three-River Headwaters region, Jin Z et al. (2020) revealed that this region has had an average rate of warming of 0.37°C/10a in the past 60 years, which is more than twice the average rate of global warming (0.16°C/10a) and is significantly higher than that of the same latitudes in the world (0.19°C/10a) and China (0.28°C/10a) in the same period. They also found that most of the proxies for the extreme climate in the Three-River Headwaters region have increased under the background of global warming. Specifically, the minimum nighttime temperature has increased the most significantly (0.55°C/10a), extreme high-temperature events have occurred more frequently, the regional daily temperature difference has decreased, and the temperature changes have become more extreme. Liu CH et al. (2021) discovered that the temperature in the Yellow River source region increased at a rate of about 0.37°C/10a from 1960 to 2019. Ye PL et al. (2020) pointed out that the upper reaches of the Yellow River showed a consistently warm and humid trend from 1980 to 2018, with the temperature increasing at a rate of about 0.23°C/10a. The meteorological data of the Wudaoliang area from 1961 to 2019 show that the mean, maximum, and minimum temperatures in the area had a very significant rising trend in recent years, with the mean annual temperature increasing at a rate of 0.34°C/10a in the 59 years (Wu SG et al., 2020). Li L et al. (2019) also concluded that the mean annual temperature in the northeastern Qinghai-Tibet Plateau has increased at a rate of up to 0.39°C/10a in the past 56 years.

    The mean annual temperature in the Qilian Mountains Nature Reserve rose from 6.61°C to 8.84°C from 1995 to 2014 (Wang PP et al., 2020). The mean annual temperature in the central Qilian Mountain also showed an overall rising trend from 1960 to 2017, with an increasing rate of 0.39°C/10a (Cheng P et al., 2020). The mean annual temperature in the Hala Lake area on the Qinghai-Tibet Plateau showed a rising trend with fluctuations from 1986 to 2015. It rose from −2.82°C to −1.39°C at a rate of about 0.04°C/a (Li DS et al., 2021), which is similar to that of the Delingha area (Ma JF, 2014), higher than that of the Qinghai-Tibet Plateau (0.02°C/a; Wu CQ and Tang DY, 2017), and slightly higher than that of the Qinghai Lake-Hexi Corridor (0.03°C/a; Li SS et al., 2014; Yang XL et al., 2011). The mean annual temperature at the southern margin of the Qaidam Basin was 4.6°C from 1961 to 2019, showing an increasing rate of 0.32°C/10a, and it was 3.5°C in the 1960s and has been 5.4°C (increased by 1.9°C) since the 21st century (Zeng GY et al., 2020). The temperature in the Gonghe Basin in Qinghai increased at an average rate of 0.51°C/10a from 1961 to 2018, which was significantly higher than the national warming level (0.16°C/10a; Ding YH et al., 2006).

    According to Wang QC et al. (2007), the mean annual temperature in Tibet increased at a rate of 0.32°C/10a from 1961 to 2008, and that in Qinghai also showed a significant upward trend from 1961 to 2006, with an increasing rate of 0.36°C/10a. Both increasing rates are higher than the national average warming rate, which was about 0.28°C in the same period (Editorial Board of the National Assessment Report of Climate Change, 2007). The Tibet meteorological data from 2011 to 2017 (Table 4) show that the mean annual temperature in Tibet was 4.62°C. The data also reveal that the temperature in Tibet showed an upward trend with fluctuations in the past 40 years, with an average increasing rate of 0.336°C/10a.

    Table  4.  Temperatures recorded at different meteorological stations in Tibet from 1981 to 2017 (Yan LJ, 2020).
    No.StationMean annual air temperature/°C
    1981‒
    1990
    1991‒
    2000
    2001‒
    2010
    2011‒
    2017
    Warming rate /(°C/10a)
    1Shiquanhe0.471.192.012.240.69
    2Bange−0.310.060.730.900.48
    3Naqu−0.84−0.480.340.630.59
    4Shenzha0.400.801.251.240.36
    5Rikaze6.967.127.817.860.39
    6Lahsa8.208.629.559.710.61
    7Dingri3.053.604.264.070.44
    8Longzi5.845.976.396.230.21
    9Pali0.480.371.231.130.31
    10Suoxian2.132.273.163.310.47
    11Dingqing3.803.804.484.490.32
    12Changdu7.827.868.458.500.29
    13Linzhi8.859.329.819.740.36
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    Although the Qinghai-Tibet Plateau shows an obvious warming trend, some studies (Li WJ, 2012; Li DL et al., 2007) have found that the cooling effect brought about by the decrease in ENSO events and solar radiation has counteracted the warming caused by human activities to a certain extent, thus mitigating the warming on the plateau. However, the extent of future climate change is still unclear. According to the long-term goal of the 2015 Paris Agreement, i.e., holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels (UNFCC, 2015), scientists have carried out simulated calculations of the future warming trend of the Qinghai-Tibet Plateau under different climate scenarios, obtaining the following results (Table 5).

    Table  5.  Future climate change trends of the Qinghai-Tibet Plateau (Li L et al., 2018).
    ScenarioΔT /°CΔR
    2011‒20302031‒20502011‒20502011‒20302031‒20502011‒2050
    RCP2.61.200.781.464.82%5.13%4.98%
    RCP4.51.21.011.583.62%5.92%4.77%
    RCP8.51.281.372.423.94%6.52%5.23%
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    Shi C et al. (2018) also assessed the extreme temperature change in China based on the temperature rise of 1.5°C and 2°C from 1861 to 1900, achieving the following results. The increase in the mean temperature in China (relative to the mean temperature from 1986 to 2005) was predicted to be higher than the change in the GMT. Moreover, the increase in the mean temperature of the Qinghai-Tibet Plateau was predicted to be more sensitive to the increase in global temperature, with an increase in temperature higher in the west and northwest of the plateau than in the rest of the plateau. Shi C et al. (2020) also concluded that under the global warming scenarios of 1.5°C (RCP2.6) and 2.0°C (RCP4.5), regions such as the Qinghai-Tibet Plateau are at a greater risk of experiencing extremely high temperatures. According to Yin SY et al. (2020), compared with the period from 1986 to 2005, the national average midsummer land surface temperature would increase by 1.1°C and 2.0°C, respectively under the global warming scenarios RCP2.6 and RCP4.5. They also concluded that the warming rate in some areas of the Qinghai-Tibet Plateau would decrease, while the surface temperature in some areas on the plateau (e.g., Qinghai) would rise sharply. Under global warming scenarios RCP2.6, RCP4.5, and RCP8.5 (3°C), You Q et al. (2020) conducted simulations using the Multi-model Ensemble Mean (MMEM) mode of the CMIP5 model, and the results showed that the Qinghai-Tibet Plateau can significantly amplify future temperature changes and that the temperature increase in the plateau would be greater than that of the rest of China and the Northern Hemisphere and even the global average level. Under global warming scenarios RCP2.6, RCP4.5, and RCP8.5, the amplification effect of the Qinghai-Tibet Plateau will also affect the wider climate system in its surrounding areas (Kang SC et al., 2019; You Q et al., 2020). As for the future climate change trend of the Qinghai-Tibet Plateau, Liu XD et al. (2009) concluded that compared with the temperature in most areas from 1980 to 1999, the average annual land surface temperature would rise by 1.4‒2.2°C in the Qinghai-Tibet Plateau from 2030 to 2049, with warming generally more significant at higher altitudes.

    Using global warming models RCP4.5 and RCP8.5, Dixit A et al. (2021) also simulated and analyzed the impacts of the current (1980‒2015) and future (2006‒2100) climate on glacier dynamics in the Beas basin situated in the northwestern Himalayas, obtaining the following results. Compared to 1980, 50% of the total glaciers in the basin disappeared by 2011. The predictions obtained using the RCP4.5 model showed that the basin would lose 75% of its glaciers by around 2040 and 90% of its glaciers by around 2094. The predictions obtained using the RCP8.5 model showed that the basin would lose 75% of its glaciers by around 2040 and lose about 90% by around 2084. Observed data showed that the glaciers in the basin had an initial volume of about 42 km3 (during 1980), which further decreased to about 28 km3 in early 2000, with average reduction rates of 0.69 km3/a (1980‒2000) and 0.61 km3/a (2001‒2015). The data also showed that the glacier area in the basin changed at rates of 4.3 km2/a (1980‒2000) and 8 km2/a (2001‒2015). The prediction results obtained using the RCP4.5 model showed that the mean annual temperature of the basin would be about 1.2°C, about 2.1°C, and about 3.0°C, respectively when the glacier volume decreases by 50%, 75%, and 90%. The prediction results obtained using the RCP8.5 model showed that the mean annual temperature of the basin would be about 1.1°C, about 2.0°C, and about 4.6°C, respectively when the glacier volume decreases by 50%, 75%, and 90%.

    NGHs, commonly known as flammable ice, are crystalline solids composed of water and light-weight gas molecules (e.g., CH4, ethane, propane, isobutane, hydrogen sulfide, and CO2) formed under low temperature (generally around 273.15K), high pressure (generally above 3‒5 MPa), and sufficient gas supply (Sloan ED and Koh CA, 2008). NGHs are usually distributed in submarine sediments at a water depth greater than 300 m (Kvenvolden KA, 1993) or in permafrost 130 m below the surface (Shi D and Zheng JW, 1999). China discovered NGHs on the northern slope of the South China Sea during drilling and conducted production tests (Wu NY et al., 2009; Li JF et al., 2018; Ye JL et al., 2018). The Qinghai-Tibet Plateau is an important NGH-bearing area within onshore permafrost in China (Lu ZQ et al., 2009). The Qiangtang Basin is the region on the plateau that bears the most favorable conditions for the formation of NGHs and the greatest prospects for NGH exploration (Fig. 9), followed by the Qilian Mountains area, the Fenghuoshan-Wuli area, the Kunlun Pass Basin, the Tanggula Mountain-Tumen area, the Karakoram region, and the Western Kunlun Mountain - Hoh Xil Basin area. (Zhu YH et al., 2011; Lu ZQ et al., 2007).

    Figure  9.  Map showing the distribution of NGH prospect areas in China (modified from Zhu YH et al., 2011).

    In 2008, the China Geological Survey first obtained NGHs samples (Fig. 10) from the Qilian Mountains alpine permafrost region of the Qinghai-Tibet Plateau through drilling (Fig. 11), thus confirming the existence of NGHs in the permafrost on the Qinghai-Tibet Plateau (Zhu YH et al., 2009; Lu Z et al., 2011). This finding is the first breakthrough in the NGHs survey in onshore permafrost regions in China (Lu ZQ et al., 2018), and the NGHs in the Qinghai-Tibet Plateau represent the hydrates in mid-latitudes permafrost regions at high altitudes (Lu ZQ et al., 2020; Fig. 12). Wu QB et al. (2015) also found evidence for NGHs occurrence in the Kunlun Pass Basin on the Qinghai-Tibet Plateau through drilling, geophysical logging, and gas geochemistry. It is estimated that the permafrost on the Qinghai-Tibet Plateau has natural gas resources contained in NGHs of about 1.2×1011‒2.4×1014 m3 (Chen DF et al., 2005). Moreover, the natural gas in the NGHs is mainly thermogenic (Fig. 13). These results indicate that the NGHs in the permafrost of the Qinghai-Tibet Plateau can provide new clean energy for national energy resource security (Wu CG et al., 2011). However, the NGHs in the plateau may also become a new source of greenhouse gas emissions, thus accelerating climate change under the background of global warming.

    Figure  10.  Map showing the distribution of drilled wells that encountered natural gas hydrates and the location of a production test in the Qilian Mountains permafrost region in the Qinghai-Tibet Plateau (after Lu ZQ et al., 2020).
    Figure  11.  Natural gas hydrates collected from wells drilled in the Qinghai-Tibet Plateau (after Lu ZQ et al., 2011).
    Figure  12.  Histograms showing the horizons of the natural gas hydrates and permafrost in the Qilian Mountains permafrost region on the Qinghai-Tibet Plateau (after Lu ZQ et al., 2020).
    Figure  13.  Gas hydrate genetic diagrams of C1/(C2+C3) vs. δ13C1 (left) and δDC1 vs. δ13C1 (right) (after Lu Z et al., 2011).

    As mentioned above, the atmospheric mass and density are 5×1018 kg (Wayne RP, 1985) and 1.205 kg/m3, respectively. Accordingly, the atmospheric volume can be calculated to be approximately 4.15×1018 m3. The monitoring data show that the global CO2 concentration was 417×10−6 in 2020 (Letcher TM, 2021), and the current atmospheric CH4 concentration is more than 1700×10−9 (Etheridge DM et al., 1998; Wuebbles DJ and Hayhoe K, 2002). Therefore, the natural gas resources of 1.2×1011 ‒ 2.4×1014 m3 (Chen DF et al., 2005) contained in the permafrost-associated NGHs on the Qinghai-Tibet Plateau account for approximately 0.007%‒14% of the current atmospheric CO2 volume and are equivalent to 0.017‒34 times the current atmospheric CH4 volume.

    Under the background of global warming, the permafrost on the Qinghai-Tibet Plateau will inevitably thin, the scope of the NGHs stability zone on the plateau will decrease, and most of the gases dissociated from the NGHs will be released into the atmosphere (Fig. 14). The permafrost-associated NGHs on the Qinghai-Tibet Plateau serve as a large natural carbon pool, and their dissociation will be sufficient to warm the Qinghai-Tibet Plateau. Greenhouse gases such as CO2 and CH4 in the atmosphere can transmit solar short-wave radiation but block surface long-wave radiation, thus warming the atmosphere. Therefore, global warming is increasingly considered to be related to the emissions of CO2 and CH4. CH4 has a short lifetime in the atmosphere (Cicerone RJ and Oremland RS, 1988), and its peak concentration in the atmosphere can be not maintained for long time frames because it will be oxidized into CO2 after a residence of approximately 10 years. The efficiency of CH4 for absorbing heat from the atmosphere is over 20 times that of CO2 (Archer D, 2007; Archer D et al., 2009). Moreover, the global warming potential (GWP) of CH4 is approximately 21 times that of CO2 with the same mass (Cicerone RJ and Oremland RS, 1988). Based on the current CH4 and CO2 levels in the atmosphere, the GWP of a single molecule of CH4 is approximately 62 times that of CO2 on a 20-year time scale (IPCC, 2007), and the cumulative global greenhouse effect of CH4 is approximately 84 times that of CO2 on this time scale. On a 100-year time scale, the cumulative global greenhouse effect of CH4 is 28.5 times that of CO2 (Babatunde DE et al., 2020), and even the cumulative global greenhouse effect of C2H6 is 1.46 times that of CO2 (Babatunde DE et al., 2020). Presently, CH4 and CO2 contribute approximately 15% and 55% to the greenhouse effect, respectively (Demirbas A, 2010).

    Figure  14.  Effects of permafrost changes on the NGHs of the Qinghai-Tibet Plateau under the background of global warming (modified from Wang PK et al., 2014).

    As reported in the 2010 Greenhouse Gas Bulletin issued by the World Meteorological Organization (WMO), CO2, CH4, and other greenhouse gases have contributed 64%, 18%, and 18%, respectively to the total effects of increased greenhouse gases on global warming since 1750. The China Global Atmosphere Watch Baseline Observatory (CGAWBO), located in the Waliguan area in the hinterland of the Qinghai-Tibet Plateau, is a member of the Global Atmosphere Watch (GAW). The observed data at the CGAWBO can effectively reflect the atmospheric greenhouse gas concentrations in the Qinghai-Tibet Plateau and their changes (Lean JL and Rind DH, 2009). Li L et al. (2018) concluded that the 1991‒2011 CO2 and CH4 concentrations from the CGAWBO showed a significant upward trend. The mean annual concentrations of CO2 and CH4 increased from 355.73 mL/m3 and 1782.72 μL/m3 in 1991 to 392.25 mL/m3 (annual increasing rate: 1.83 mL/m3) and 1862.88 μL/m3 (annual increasing rate: 4.01 μL/m3), respectively in 2011. It is noteworthy that the warming in the Qinghai-Tibet Plateau was mitigated from 2006 to 2011. However, the mean annual concentrations of CO2 and CH4 increased at a rate of up to 2.02 mL/m3 and 6.04 μL/m3, respectively in the five years. Both rates were significantly higher than the mean growth rates of the two gases in the past 20 years, indicating that the greenhouse gas emissions on the Qinghai-Tibet Plateau are accelerating (Li L et al. 2018), which may be caused by the dissociation and release of permafrost-associated NGHs on the Qinghai-Tibet Plateau.

    Climate change on the Qinghai-Tibet Plateau may be related to the earth’s rotation rate, the duration of the sunspot cycle, the mean cloud amount, and the snow cover effect (DeLi GJ, 2019). However, it is generally believed that the climate warming of the Qinghai-Tibet Plateau in recent decades is likely caused by intensified greenhouse gas emissions related to human activities. Moreover, the increased greenhouse gas emissions may have more significant impacts on climate change in the Qinghai-Tibet Plateau than in other portions of the world. By contrast, the increased atmospheric aerosols caused insignificant warming and even cooling of the Qinghai-Tibet Plateau in winter (Duan AM et al., 2006b; Li XZ and Liu XD, 2009). The warming-induced dissociation and release of permafrost-associated NGHs on the Qinghai-Tibet Plateau may also contribute a certain amount of greenhouse gas emissions.

    The stability of permafrost-associated NGHs on the Qinghai-Tibet Plateau mainly depends on the ambient temperature and pressure. In other words, the NGHs stability is mainly controlled by the geothermal gradients in and under the permafrost, and the permafrost thickness is affected by the land surface temperature. On the phase diagrams, the NGHs occurrence status in strata is jointly determined by the intersections of the temperature-pressure curve of NGHs and the lines of geothermal gradients in and under the permafrost with different thicknesses (Fig. 15). As shown in Fig. 15, in the case of thick permafrost, line 3-3’ and curve A intersect, indicating a large occurrence thickness of NGHs; in the case of moderately thick permafrost, line 2-2’and curve A intersect, indicating a small occurrence thickness of NGHs; and in the case of thin permafrost, line 1-1’ and curve A intersect, indicating almost zero occurrence thickness of NGHs. The last case means that NGHs are difficult to form or exist. These results show that permafrost thins as the land surface temperature rises. As a result, the NGHs in permafrost become unstable and then decompose and release greenhouse gases such as CH4. Based on these results, the climate warming-induced changes in permafrost-associated NGHs on the Qinghai-Tibet Plateau in the past decades can be estimated through simulations.

    Figure  15.  Changes in NGHs formation conditions under different permafrost thicknesses (left) and the influence of temperature and pressure on NGHs (right) (after Lu ZQ et al., 2008).
    A‒the temperature-pressure curve of NGH formation; 1, 2, 3‒lines of geothermal gradients in the permafrost under different permafrost thicknesses; 1', 2', 3'‒lines of geothermal gradients under the permafrost in the case of different permafrost thicknesses.

    This study used the NGHs formation condition model established by Sultan N et al. (2004). This model, based on the thermodynamical theoretical model of NGHs formation, is essentially consistent with other NGHs models such as CSMHYD (Sloan ED, 1998), and both were developed from the van der Waals model. In practice, the NGHs formation conditions calculated using the two models have very small differences, especially when the temperature is less than 295.15 K (Lu Z and Sultan N, 2008). The model established by Sultan N et al. (2004) was applied to the comparative study of the NGHs stability zone in the Qilian Mountain permafrost region on the Qinghai-Tibet Plateau. In other words, the top and bottom depths of the NGHs stability zone in the region were simulated based on the surveyed data including the NGHs gas composition obtained from drilling and the temperature of drilling muds. Afterward, the simulated results were compared with the NGHs occurrence depth revealed in the drilling, exhibiting high consistency (Jin CS et al., 2011). For instance, the simulated results showed that the top depth, bottom depth, and thickness of the NGHs stability zone were calculated to be 148.8‒122.7 m, 324.6‒354.8 m, and 175.8‒232.2 m, respectively, while the drilling revealed that the NGHs and their anomalies occur in intervals at a depth of 133‒396 m. Therefore, the simulated results were consistent with the drilling results, indicating that the simulation method can be used to effectively predict the top and bottom depths of the NGHs stability zone.

    As revealed by previous simulation (Lu ZQ et al., 2008), under the formation conditions of pure NGHs (i.e., on the temperature-pressure stability curve of NGHs; Fig. 15), when CH4 and water form NGHs at point A, the temperature and pressure are 278.88 K (5.73°C) and 5000 kPa, respectively (approximately equivalent to a water depth of 500 m or a formation thickness of 182 m); when CH4 and water form NGHs at point B, the temperature and pressure are 286.02 K (12.87°C) and 10000 kPa, respectively (approximately equivalent to a water depth of 1000 m or a formation thickness of 363 m). It can be inferred that the pressure required to form NGHs increases by approximately 700 kPa for every 1°C increase in temperature (equivalent to approximately seven atmospheres, a water depth of 70 m, or a formation thickness of 25 m). Therefore, the thickness of permafrost-associated NGHs on the Qinghai-Tibet Plateau will decrease by 25 m for every 1°C increase in temperature under the influence of warming.

    It is generally accepted that human activities may have caused approximately 1°C of global warming above the pre-industrial level (IPCC, 2018). Regarding the response of the Qinghai-Tibet Plateau to global warming, data on the increase in the mean annual temperature of the plateau in the past periods (Table 2) show that the mean annual temperature increased at a higher rate more recently. For example, the temperature of the Qinghai-Tibet Plateau changed at a rate of 1.14°C/10a, 0.16‒0.4°C/10a (mean: 0.308°C/10a), 0.07°C/10a, and slightly greater than 0.005°C/10a, respectively in the past 10, 50, 100, and 200 years. This characteristic also exists in different portions of the Qinghai-Tibet Plateau. As shown in Table 3, the temperature changed at a rate of 1.115°C/10a, 0.375°C/10a (mean), and 0.0676°C/10a, respectively in the past 20, 50, and 100 years. The 1981‒2017 temperature data from limited meteorological stations in Tibet (Table 4) indicate that the mean annual temperature in Tibet rose at a rate of 0.42°C/10a, which is higher than that of the Qinghai-Tibet Plateau in the past 50 years. According to the predicted data in Table 5, the temperature of the Qinghai-Tibet Plateau will increase by 1.20°C and 1.46°C, respectively by 2030 and 2050 when the global temperature increases by 1.5°C.

    Regarding the heat conduction effect of air temperature on ground temperature, the study results show that there is a positive correlation between soil temperature and air temperature, and the mean temperature of the active layer increases by approximately 0.78°C for every 1°C increase in the air temperature; however, the temperature rise gradually decreases with increasing soil depth (Xu HL et al., 2021). Zeng GY et al. (2020) found that the mean annual temperature of the shallow layers at depths of 0 cm, 10 cm, and 20 cm at the southern margin of the Qaidam Basin increased at a rate of 0.43 °C/10a, 0.32 °C/10a, and 0.33 °C/10a, respectively from 1961 to 2019, showing a significant upward trend, which is generally consistent with the air temperature change. Compared to the 1960s, the annual mean ground temperatures have increased by 1.3‒1.8°C since the 21st century. Moreover, there has been a positive correlation between the mean annual air temperature and the mean ground temperatures of these shallow layers since the 21st century. For every 1°C increase in the mean annual air temperature, the mean annual temperatures of the shallow layers at depths of 0 cm, 10 cm, and 20 cm have increased by 0.98°C, 0.74°C, and 0.75°C, respectively.

    Suppose that Φ is the changing rate of the mean annual temperature, °C/10a; Y is the time, 10a; ΔT1 is the increase in the mean annual air temperature, °C; and ΔT2 is the increase in the mean annual ground temperature in shallow layers, °C. Assuming that the annual mean temperature of the shallow layers increases by 0.75°C for every 1°C increase in the mean annual temperature of the Qinghai-Tibet Plateau, ΔT1 = Y × Φ and ΔT2 = 0.75 × ΔT1. Therefore, it can be roughly concluded that the air temperature in the Qinghai-Tibet Plateau increased by 1.54°C, 1.875°C, and 2.1°C, respectively in the past 50 years under the background of global warming when the mean annual air temperature of the plateau change at a rate of 0.308°C/10a, 0.375°C/10a, and 0.42°C/10a in the same period. Under the influence of the increase in the air temperature, the mean temperature of the shallow layers on the plateau increased by 1.155°C, 1.406°C, and 1.575°C, respectively in the past 50 years and is predicted to increase by 0.90°C and 1.095°C, respectively by 2030 and by 2050.

    Suppose that ΔH is the reduced thickness of NGHs caused by temperature increase, m. Assuming that thickness H of NGHs will decrease by 25 m for every 1°C increase in the annual mean ground temperature, ΔH = 25 × ΔT2. In this case, it can be calculated that ΔH on the Qinghai-Tibet Plateau was 29 m, 35 m, and 39 m in the past 50 years and will be 23 m and 27 m, respectively by 2030 and 2050 (Fig. 16).

    Figure  16.  Prediction of the air temperature, reduced thickness of hydrates and released CH4 on the Qinghai-Tibet Plateau in the next 30 years.

    Under the influence of global warming, the permafrost on the Qinghai-Tibet Plateau is gradually degrading, and the permafrost area and thickness are gradually decreasing. Accordingly, the active layer is thickening, and the lower boundary of the permafrost continues to rise. The permafrost area of the Qinghai-Tibet Plateau is approximately 1.06×106 km2 at present (Zou DF et al., 2017) and was previously reported to be approximately 1.4×106 km2 (Xu XZ et al., 1999), thus having decreased by approximately 0.34×106 km2. Based on a permafrost area of the plateau of approximately 1.4×106 km2, Chen DF et al. (2005) calculated that the NGHs stability zone in the permafrost on the plateau had a thickness of approximately 50‒1510 m, and permafrost-associated NGHs resources of approximately 1.2×1011‒2.4×1014 m3.

    Suppose that S1 is the permafrost area of the Qinghai-Tibet Plateau at time t1 and is also the NGHs distribution area at time t1 on the plateau, unit: km2, value: S1 = 1.4 × 106 km2; H1 is the NGHs thickness on the Qinghai-Tibet Plateau at time t1, unit: m, value: H1 = 50‒1510 m; and Q1 is the NGHs resources on the Qinghai-Tibet Plateau at time t1, unit: m3, value: Q1 = 1.2 × 1011‒2.4 × 1014 m3. Similarly, suppose that S2 is the permafrost area of the Qinghai-Tibet Plateau at time t2 and is also the distribution area of NGHs on the plateau at time t2, unit: km2, value: S2 = 1.06 × 106 km2; H2 is the NGHs thickness on the Qinghai-Tibet Plateau at time t2, m; and Q2 is the NGHs resources on the Qinghai-Tibet Plateau at time t2, m3. Moreover, suppose that ΔS is the reduced NGHs area due to temperature increase, m2 and ΔQ is the reduced NGHs resources due to temperature increase, m3. In this case, there is an approximate formula: ΔQ/Q1 = (ΔS × ΔH) /(S1 × H1). However, as the permafrost area or NGHs distribution area on the Qinghai-Tibet Plateau from 2030 to 2050 is unknown, it is assumed to be 1.06×106 km2.

    It can be inferred from the abovementioned data that under the influence of global warming, the dissociated or reduced NGHs in the permafrost on the Qinghai-Tibet Plateau in the past 50 years may be the equivalent of approximately (1.69‒2.27)×1010 m3 of CH4 at least and the equivalent of approximately (1.12‒1.51)×1012 m3 of CH4 at maximum. The permafrost-associated NGHs on the plateau will continue to decompose, and the dissociated NGHs will be the equivalent of approximately 1.34×1010‒8.88×1011 m3 and 1.57×1010‒1.04×1012 m3 of CH4, respectively by 2030 and 2050 (Fig. 16).

    As shown by preliminary calculation results, in the past 50 years, the mean ground temperature of the shallow layers on the Qinghai-Tibet Plateau increased by 1.155‒1.575°C (average: approximately 1.38°C), and the permafrost area on the plateau decreased by approximately 0.34×106 km2, and the NGH thickness decreased by approximately 29‒39 m (average: approximately 34 m). Moreover, the preliminary calculation results also revealed that the released or reduced NGHs due to NGHs dissociation were the equivalent of approximately (1.69‒2.27)×1010 m3 of CH4 at least and the equivalent of approximately (1.12‒1.51)×1012 m3 of CH4 at maximum in the past 50 years under the influence of global warming. It is predicted that by 2030 and 2050, the mean ground temperature of the shallow layers on the Qinghai-Tibet Plateau will increase by 0.90°C and 1.095°C, respectively, and the NGHs thickness will continue to decrease by approximately 23 m and 27 m, respectively, and the NGHs will continue to decompose and the released or decreased NGHs will be the equivalent of approximately 1.34×1010‒8.88×1011 m3 and 1.57×1010‒1.04×1012 m3 of CH4, respectively.

    Theoretically, the thermal properties, the geothermal flow, the warming magnitude and timing, and the dynamic behavior of permafrost due to global warming can also be predicted based on its original thermal state. The theoretic calculations show that the maximum thickness of the permafrost melted is 18 m in the case that the temperature between the surface and the solidification level increases by 2.5°C within 55 years and 33 m in the case that the temperature increases by 5°C within 55 years (Lunardini VJ, 1996). This finding on the permafrost is sketchy at best at present, while global warming relies on plausible scenarios. Overall, the predicted thickness of permafrost melted is comparable to the results obtained in this study.

    Recently, Cheng F et al. (2022) carried out a similar study of the impact of climate change on the Alpine permafrost on the Qinghai-Tibet Plateau. The plateau was warm during the mid-Pliocene (3.3–3.0 Ma), and the MAAT of the plateau decreased from 1.7°C at 4.3–2.7 Ma to −7.7°C at 2.7–2.6 Ma or −6.4°C at 2.7–0.8 Ma (Cheng F et al., 2022; Fig. 5). The Qinghai-Tibet Plateau has a local modern MAAT of −6°C to −7°C and is facing with global warming. Since the two circumstances are similar and comparable to each other, the Middle Pliocene Warm Period (MPWP; 3.3–3.0 Ma) is often used as a geological analogue for the near future to study the impacts of climate change on sea level and the extent of glaciers and ice sheets and to estimate the permafrost-climate feedback on the present global warming. Cheng F et al. (2022) assumed conditions similar to the MPWP and the modeled results show that the northern and southern edges of the Qinghai-Tibet Plateau will become unstable under an MPWP-like climate. It is estimated that approximately 20% of the circumarctic permafrost will be destabilized (approximately 3.9×1012 m2) but approximately 60% of alpine permafrost (approximately 1.9×1012 m2) will be destabilized (Cheng F et al., 2022). It is estimated that approximately 254 petagrams (1015 g) of organic carbon in the circumarctic permafrost zone and 85 petagrams of organic carbon in the alpine permafrost zone will be affected in the scenario of MPWP-like climate (Cheng F et al., 2022). These estimates highlight that the Alpine permafrost will account for approximately 25% of thawed carbon due to the permafrost-climate feedback. The Qinghai-Tibetan Plateau is the largest Alpine permafrost region on the Earth and there is an urgent need to take measures as soon as possible.

    It is certainly true that the temperature increase is not completely consistent in different regions of the Qinghai-Tibet Plateau and that the permafrost or NGHs are not evenly distributed on the plateau. Nevertheless, the massive dissociation of permafrost-associated NGHs on the Qinghai-Tibet Plateau under the background of global warming will further increase regional and even global air temperatures. Therefore, it is necessary to take relevant artificial measures to reduce the negative climatic effects of NGHs dissociation.

    Since NGHs have a positive feedback mechanism on climate warming and the CH4 released from NGHs dissociation has a higher greenhouse effect than its oxidation product—CO2, it is preferable to enhance the exploration and exploitation of permafrost-associated NGHs on the Qinghai-Tibet Plateau rather than leaving the NGHs to be dissociated and wasted in vain under the influence of global warming, which will continuously accelerate climate warming. Although CO2—the oxidation product of CH4—is still an important greenhouse gas, CH4 has a larger greenhouse effect than CO2. Therefore, burning CH4 has a smaller greenhouse effect than the direct CH4 emission, thus reducing the global warming effect. Accordingly, in terms of mitigating global climate change, the exploitation and utilization of NGHs energy resources in the permafrost on the Qinghai-Tibet Plateau is an important option at present. It is preferable to fully utilize NGHs energy resources rather than leaving them to be naturally dissociated and released due to global warming, which, in turn, will further exacerbate global warming. In other words, oxidizing CH4 in NGHs into CO2 by utilizing NGHs resources serves as a method of reducing global warming. Specifically, oxidizing large amounts of CH4 and converting them into CO2 and water using surface facilities will mitigate the global greenhouse effect.

    In addition, the greenhouse effect of the CH4 or natural gas contained in NGHs is lower than that of other fossil energy resources. It is well known that natural gas is superior to coal because the CO2 per unit of energy produced by burning CH4 (50 g/MJ) is less than that produced by burning coal (92 g/MJ) and coal combustion produces particulates. It is estimated that approximately 98% of carbon emissions come from the combustion of fossil fuels (coal, oil, and natural gas), with coal combustion accounting for 30%‒40% of CO2 emissions from fossil fuel combustion. Among the fossil fuels, natural gas has the least impact on CO2 emissions during combustion, followed by liquefied petroleum gas (LPG) and then coal. For example, the combustion of 1 g of CH4, butane, and charcoal produces 2.75 g, 3.03 g, and 3.67 g of CO2, respectively (Demirbas A, 2010).

    In fact, there are vast amounts of NGHs resources in the energy structure and NGHs are known as clean alternative energy in the 21st century. For example, according to the study of the utilization strategies of renewable energy, fossil energy, and NGHs carried out by Letcher TM (2021), it is predicted that renewables will increase their share of electricity production from 26.7% in 2020 to 29.2% in 2040. Approximately 40% of all energy sources used to produce electricity are either renewable (wind, solar, hydropower, biomass, tide, and geothermal) or nuclear (10%). The major electricity-producing energy source is coal, which still produces 37% of the world’s electricity. The transition from fossil fuel to renewables is slow. It is predicted that renewables would produce 45% of the world’s electricity by 2040, with coal still being a significant supplier of energy. Therefore, replacing fossil fuels will be a mammoth task. Statistics show that globally, the equivalent of more than 11 billion tons of oil from fossil fuels is currently consumed every year and crude oil reserves are vanishing at a rate of more than four billion tons a year. At this rate, the known oil deposits could run out in just over 50 years. If gas production is increased to fill the energy gap left by oil, the known gas reserves would meet the needs for only 50 years. If coal production is increased to make up for depleted oil and gas reserves, the known coal deposits could be gone in 150 years (Letcher TM, 2021). Another set of estimates have been given by BP in 2018. Their estimation of the time left for oil, gas, and coal as a result of current usage was predicted to be 30 years, 40 years, and 70 years, respectively (Letcher TM, 2021). Therefore, NGHs is an important strategic option in addition to accelerating the development and utilization of renewable and nuclear energies.

    Given the environmental risks of permafrost-associated NGHs on the Qinghai-Tibet Plateau, it is recommended to take the following measures:

    (i) Putting more efforts into the exploration, determining detailed information on the resources, and conducting a comprehensive resource-technology-economics-environment evaluation for the resource potential of permafrost-associated NGHs on the Qinghai-Tibet Plateau.

    (ii) Enhancing research on techniques and equipment for the development, storage, and transportation of the NGHs resources. Tibet and Qinghai suffer a severe shortage of hydrocarbon resources. The successful exploitation of permafrost-associated NGHs resources on the Qinghai-Tibet Plateau will contribute greatly to the economy and people’s livelihood in the frontier regions and the global reduction of carbon emissions.

    (iii) Permafrost is widely distributed in the world, and global warming is irreversible in a short time. As a result, permafrost-associated NGHs will be inevitably dissociated and released into the atmosphere. Therefore, governments and scientists from various countries and funds must cooperate to provide strong support for research on the negative effects of permafrost-associated NGHs on global warming. It is expected to find suitable ways to turn the harm into a benefit to mankind.

    (i) The Qinghai-Tibet Plateau holds huge amounts of permafrost-associated NGHs resources. It is estimated that the plateau has natural gas resources contained in the permafrost-associated NGHs of approximately 1.2×1011 ‒ 2.4×1014 m3 (Chen DF et al., 2005), which account for 0.007%‒14% of the current atmospheric CO2 volume and are equivalent to 0.017‒34 times the current atmospheric CH4 volume. These NGHs serve as a large natural pool of carbon release.

    (ii) Under the influence of global warming, the mean annual air temperature and mean annual shallow ground temperature on the Qinghai-Tibet Plateau increased by approximately 1.54‒2.1°C and 1.155‒1.575°C, respectively in the past decades. Moreover, the permafrost region on the plateau decreased from approximately 1.4×106 km2 to the present 1.06×106 km2. As indicated by simulated calculation results, the thickness of the NGH-bearing permafrost on the Qinghai-Tibet Plateau decreased by 29‒39 m in the past 50 years, with the equivalent of (1.69‒2.27)×1010 ‒ (1.12‒1.51)×1012 m3 of CH4 being released due to NGH dissociation. It is predicted that the thickness of the NGH-bearing permafrost will decrease by 23 m and 27 m, respectively and that the dissociated and released NGHs will the equivalent of (1.34‒88.8)×1010 m3 and (1.57‒104)×1010 m3 of CH4, respectively by 2030 and 2050. The permafrost-associated NGHs on the Qinghai-Tibet Plateau have become a major uncertainty factor that hinders the future achievement of the carbon neutrality goal.

    (iii) Permafrost is widely distributed in the world, covering an area of approximately 6.968×108 km2 in total (Zhao SM et al., 2022). Moreover, the simulated calculation based on the line of zero-degree annual mean ground temperature shows that the terrestrial permafrost in the Northern Hemisphere has an area of approximately 20.8×106 km2 (Obu J et al., 2019) and bears NGHs mostly. It is estimated that the global permafrost-associated NGHs resources are the equivalent of approximately (1.4‒3400)×1013 m3 of CH4 (Kvenvolden KA, 1993). Regarding the permafrost-associated NGHs resources in individual regions in the Northern Hemisphere, Mallik, Mackenzie Delta, Canada has resources of the equivalent of approximately (2.8‒28)×1010 m3 of CH4 (Majorowicz J and Osadetz K, 2001), Eileen, Northern Slope of Alaska, USA has resources of the equivalent of approximately (1.0‒1.2)×1012 m3 of CH4 (Collett TS, 2007), and Messoyakha, West Siberia, Russia has resources of the equivalent of approximately 2.4×1010 m3 of CH4 (Makogon YF and Omelchenko RY, 2013). Moreover, global warming is irreversible in a short time. As a result, permafrost-associated NGHs will be inevitably dissociated and released into the atmosphere. Therefore, governments and scientists from various countries must cooperate to make more efforts to study the negative effects of permafrost-associated NGHs on global warming and to find suitable ways to turn the harm into a benefit to mankind.

    The authors would like to extend their gratitude to the reviewers for their constructive comments and suggestions. This work was supported by the projects of the China Geological Survey (DD20190102, DD20221857).

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