Changes in the global cryosphere and their impacts:A review and new perspective

ShiYin Liu,TongHua Wu,Xin Wang,XiaoDong Wu,XiaoJun Yao,Qiao Liu,Yong Zhang,JunFeng Wei,XiaoFan Zhu

1. Institute of International Rivers and Eco-security and Yunnan Key Lab of International Rivers and Transboundary Eco-security,Yunnan University,Kunming,Yunnan 650500,China

2. Cryosphere Research Station on the Qinghai-Tibetan Planteau State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources,Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

3. School of Resource, Environment and Safety Engineering, Hunan University of Science and Technology, Xiangtan,Hunan 411201,China

4.College of Geography and Environmental Sciences,Northwest Normal University,Lanzhou,Gansu 730070,China

5.Institute of Mountain Hazards and Environment,Chinese Academy of Sciences,Chengdu,Sichuan 610041,China

ABSTRACT As one of the five components of Earth's climatic system,the cryosphere has been undergoing rapid shrinking due to glob‐al warming. Studies on the formation, evolution, distribution and dynamics of cryospheric components and their interac‐tions with the human system are of increasing importance to society.In recent decades,the mass loss of glaciers,including the Greenland and Antarctic ice sheets,has accelerated.The extent of sea ice and snow cover has been shrinking,and per‐mafrost has been degrading. The main sustainable development goals in cryospheric regions have been impacted. The shrinking of the cryosphere results in sea-level rise, which is currently affecting, or is soon expected to affect, 17 coastal megacities and some small island countries.In East Asia,South Asia and North America,climate anomalies are closely re‐lated to the extent of Arctic sea ice and snow cover in the Northern Hemisphere. Increasing freshwater melting from the ice sheets and sea ice may be one reason for the slowdown in Atlantic meridional overturning circulation in the Arctic and Southern Oceans.The foundations of ports and infrastructure in the circum-Arctic permafrost regions suffer from the con‐sequences of permafrost degradation.In high plateaus and mountainous regions,the cryosphere's shrinking has led to fluc‐tuations in river runoff,caused water shortages and increased flooding risks in certain areas.These changes in cryospheric components have shown significant heterogeneity at different temporal and spatial scales. Our results suggest that the quantitative evaluation of future changes in the cryosphere still needs to be improved by enhancing existing observations and model simulations.Theoretical and methodological innovations are required to strengthen social economies'resilience to the impact of cryospheric change.

Keywords:cryospheric change;sea-level rise;water resources;climate change

1 Introduction

The term "cryosphere" describes those portions of the Earth's surface where water occurs in solid form.It includes sea, lake and river ice, snow cover, gla‐ciers, ice caps and ice sheets, and frozen ground (in‐cluding permafrost and seasonally frozen ground),where the Earth's terrestrial realm experiences nega‐tive temperatures. According to the existing forms of cryospheric elements, the cryosphere is generally di‐vided into the continental, marine and atmospheric cryospheres (Qinet al., 2017). There is a trend to‐wards conducting integrated studies on the formation,development and geographical distribution of, and dy‐namic changes in,cryospheric elements and their inter‐action with the biosphere (Qinet al., 2017).The conti‐nental cryosphere denotes ice that lies on the ground,including glaciers and ice sheets, frozen ground (per‐mafrost and seasonally-frozen ground), snow cover,river ice and lake ice. The marine cryosphere includes ice shelves, icebergs, sea ice and sub-sea permafrost,while the atmospheric cryosphere includes frozen wa‐ter in the troposphere and stratosphere. The cryo‐sphere has attracted widespread attention over the past century and is now acknowledged as one of the five major climate system components. The Climate and Cryosphere Project was founded in 1998, under the auspices of the World Climate Research Program,to highlight studies on the interactions among the cli‐mate's subsystems. In 2007, the International Union of Geodesy and Geophysics changed its standing ice and snow committee of the International Hydrological Association (founded in 1922) into a parallel panel of the International Association of Cryosphere Science.In addition, the Intergovernmental Panel on Climate Change (IPCC) has provided a thorough assessment of the cryosphere in its reports since the early 1990s,dedicating a whole chapter or a special report to it(IPCC,2007,2013,2019).As a country that has a wide distribution of cryospheric elements and is broadly im‐pacted by the cryosphere, China has been encouraging its scientists to investigate the cryosphere for a long time and established the discipline of cryospheric sci‐ence to integrate the study of all elements of the cryo‐sphere (Xie, 1988; Shi and Cheng,1991;Allisonet al.,2001;Qin and Ding,2009;Barry and Gan,2011).

The rapid development of cryospheric science, es‐pecially the cryosphere geography, has adapted to the broadening of global change research.The cryosphere has received a lot of attention because humans have encountered many practical problems in expanding their living spaces and adapting to climate change.Sea ice is one of the cryospheric elements to have been observed and studied first. There have been re‐cords of the ice margin along the coast of Northern Europe since 1550, relating to ocean exploration and harbour construction. Besides, the frozen soil in Baf‐fin Island,Canada and Siberia,Russia was first report‐ed in 1577. TheGuide to Frozen Soil, published in 1895 and updated in 1912, was used for the construc‐tion of the Trans-Siberian Railway (Barry and Gan,2011). Though the earliest studies were reported in 1920s (Zhouet al., 2000; Zhang, 2005), China began to study the cryosphere in earnest in the late 1950s.In‐tense investigations have since foused on serving the demand for water resources in China, improving the traffic conditions on the Qinghai-Tibet Plateau (QTP)and the northern cold regions, and protecting the eco‐logical environment in the fragile western regions.

Significant changes have occurred in the cryo‐sphere since the advent of industrialization, which has had various impacts on the natural environment and human activities in regions with a cryosphere and their associated areas. Studying spatiotemporal differ‐ences in the cryosphere from the perspective of geog‐raphy, examining the interacting mechanisms between the cryosphere and the terrestrial surface system, hy‐drosphere and biosphere under global change, and proposing countermeasures for adapting to cryospher‐ic changes, can enable a harmonious coexistence be‐tween humans and nature, which is one of the impor‐tant objectives for achieving the goals of the 2030 sus‐tainable development agenda(United Nations,2015).

2 Spatial distribution of the cryosphere

The global cryosphere is mainly located at high latitudes and in high mountainous areas at low and mid-latitudes (Figure 1) (Barry and Gan, 2011). The Greenland and Antarctic ice sheets and other gla‐ciers account for 10% of the global land area, cover‐ing about 14.78×106km2(IPCC, 2013), with the vol‐ume of these ice bodies being 27×106km3(Fretwellet al., 2013), 2.96×106km3(Bamberet al.,2013) and 0.158×106km3(Farinottiet al., 2019), respectively.The potential contribution of all land ice to sea-level rise has been calculated to be 66 m (IPCC, 2013). It has been estimated that the global surface runoff into the ocean is 46,268 km3/a (Schellekenset al., 2017).Compared with the solid cryosphere reservoirs, the surface water resources available to humans are a small fraction of the entirety of freshwater resources.The global land area containing permafrost is 16×106to 21×106km2and has been estimated as being up to 22×106km2considering the sub-sea permafrost. Most permafrost is distributed in the Northern Hemisphere,including Russia, the United States, Canada, and the Tibetan Plateau and high mountain areas of Asia, ac‐counting for about 24% of the land area (Zhanget al.,1999).Permafrost also spreads across Antarctica.Snow covers up to 30.6% of the global land area, with sea‐sonally-frozen ground making up 33% of the same. In addition, Antarctic and Arctic sea ice extends across 5.2%and 3.9%of the global ocean surface,respectively(IPCC,2013).

As shown in Table 1, thermal conditions deter‐mine that cryospheric elements can only be found at high latitudes or high altitudes where cold conditions can support the development of the cryosphere. Gla‐ciers between 35°N and 62°S account for 18.9% of the total area and 8.5% of the total water reserves of global glaciers, excluding ice sheets. Permafrost,snow cover and its water equivalent are mainly con‐centrated in the 50°N ?78°N region of the Northern Hemisphere, accounting for 81.7%, 76.4% and 85.5%of the global totals, respectively (Zhonget al., 2018).The permafrost at high latitudes features large thick‐nesses and high ice content, whereas the high-altitude permafrost at low and mid-latitudes has thin thick‐nesses and low ice content(Zhanget al.,2008).

Figure 1 Distribution of global cryospheric elements,based on RGI(Randolph Glacier Inventory Version 6)glacier inventories,sea-ice(1978?2019)and maximum snow extent(2000?2019)at NSIDC(national snow and ice data center),and Gruber's(2012)permafrost distribution data(based on permafrost zonation index(PZI))

Table 1 Global cryosphere distribution statistics(%of the total,with latitudinal bands,taking geographic unit completeness into account)

3 Cryospheric change

3.1 Glaciers

The most recent assessment shows that global gla‐cier mass loss was 322±144 Gt/a between 1961 and 2016 (Zempet al., 2016; IPCC, 2019). The volume loss of the Greenland ice sheet has accelerated, and the mass loss likely increased from 34 Gt/a between 1992 and 2001 to 215 Gt/a between 2002 and 2011.During the same periods (1992 and 2001, 2002 and 2011), the rate of ice loss from the Antarctic ice sheet has also increased significantly, from around 30 Gt/a to 147 Gt/a; this has mainly occurred in the northern Antarctic Peninsula and Amundsen Sea area in the western Antarctic. Since 2000, the Greenland ice sheet has been continuously melting, with a relatively higher melting rate in 2012. In 2019, the area of the ice sheet that was melting reached around 28.3×106km2,and the ice loss exceeded 300 Gt (IPCC, 2019). In recent de‐cades, the length, area and volume reserves of gla‐ciers outside of the ice sheets have been continuously decreasing. This accelerated retreating trend has be‐come more pronounced in just the past 20 years(IPCC, 2019).There are significant regional heteroge‐neities in the changes in glaciers. For example, small glaciers in low-latitude regions have been disappear‐ing, with the areal reduction reaching 5.2%/10a in western China from the 1960s to 2010. The glaciers in the Karakorum and West Kunlun Mountains, the eastern Pamirs and surrounding regions have exhibit‐ed slight change,however,and have even advanced or surged over that time (Yaoet al., 2012; Shangguanet al.,2016;Zhanget al.,2016;Liuet al.,2017;Kaabet al.,2018;Mouet al.,2018).

3.2 Permafrost

Since the 1980s, permafrost degradation has been characterized by rising of ground temperatures, thick‐ening of the active layer, thawing of ground ice, and thinning and shrinking of the seasonally-frozen lay‐er. From 2007 to 2016, the average annual tempera‐ture of permafrost increased by 0.29±0.12°C global‐ly, 0.39±0.15 °C in continuous permafrost regions, and 0.20±0.10°C in discontinuous permafrost regions.More‐over, temperatures in the mountainous and Antarctic permafrost regions have increased by 0.19±0.05 °C and 0.37±0.10 °C, respectively (AMAP, 2017; Biskabornet al., 2019). Over the last 10 years, the thickness of the active layer on the QTP increased by 21.7 cm/10a,and the average warming rate of this layer was 0.45 °C/10a (Xuet al., 2017; Wuet al., 2018; Chenget al., 2019; Zhaoet al., 2019). In the 1960s to the 1990s, the lower limit of permafrost was raised by 50 ?80 m, and the southern boundary moved north‐ward by about 1?2 km(Wanget al.,2019).The south‐ern edge of the permafrost in the Daxinganling and Xiaoxinganling Mountains in northeastern China has moved northwards by 40 ?120 km, with an areal re‐duction of about 7×104km2from the 1970s to 2005,and some permafrost islands have disappeared (Zhanget al., 2019). In Russia and Northern Europe, the per‐mafrost thickness and area decreased significantly from 1975 to 2005.

3.3 Snow cover

Since the middle of the 20th century, the extent of snow cover in the Northern Hemisphere has been shrinking.From 1981 to 2018,the snow cover has de‐creased by 5×104km2in November, December, March and May, and the average annual snow water equiva‐lent has reduced by 5 Gt from November to June. In the Arctic, the reduction rate in average snow extent in May and June in 1967?2018 relative to the same months in 1981?2010 was 13.4%/10a (Brownet al.,2017; Mudryket al., 2017), with a reduction of 0.7 to 3.9 in snow-cover days in the Spring (Anttilaet al.,2015; Chenet al., 2018). The snowlines in the Euro‐pean Alps, Carpathians and Pyrenees generally climbed in 1982?2017 (Huet al., 2019). The maxi‐mum area of snow cover on the QTP occurred in 1994?1995,but then decreased significantly after 2000,accompanied by a decline in snow-cover days. Snow depth also fell between 1980 and 2018, especially af‐ter 2000, although with prominent spatial heterogene‐ity(Cheet al.,2019).

3.4 Sea ice

Since 1979, the extent of sea ice in the Arctic Ocean has shown a decreasing trend (Barberet al.,2017; Comisoet al., 2017; Stroeveet al., 2018),with greater variability in summer than winter. From 1979 to 2018, its extent in September decreased by 8.3×104km2compared with the mean area in 1981 ?2010,or at an interdecadal rate of ?12.8%/10a,whereas its extent in March was ?4.1×104km2, or ?2.7%/10a.In summer, the most obvious decrease in sea ice ex‐tent has occurred in eastern Siberia, and the Beau‐fort, Chukotka, Laptev and Kara Seas (Onarheimet al., 2018). In winter, the extent of sea ice in the Bar‐ents Sea has also shrunk significantly (Onarheimet al., 2017), with the lost ice extent accounting for 27% of the total in the pan-Arctic Ocean. In addi‐tion,the thickness of sea ice has also reduced signifi‐cantly, as evidenced by 65% decrease in the central Arctic Ocean from 1975 to 2012 (Walshet al.,2017). Based on reconstructed time-series data, sea ice has experienced unprecedented retreats in the sum‐mers of the past 30 years in the Arctic Ocean due to unusually high sea-surface temperatures for the past 1,450 years (Polyaket al., 2010; Kinnardet al., 2011;Halfaret al., 2013; Walshet al., 2017). However,there has been no apparent trend in sea ice around Antarctica since the satellite era (1979), (Ludescheret al., 2019).A weakly increasing trend in sea ice extent between 1979 and 2015 was reversed after 2016(Comisoet al., 2017). This might represent an offset‐ting effect in the sea-ice dynamics around the Antarc‐tic continent, such as significant shrinkage in the ex‐tent of sea ice near the Antarctic Peninsula versus a noticeable expansion in the Amundsen Sea (Hollandet al.,2014).

3.5 River ice and lake ice

Global river ice and lake ice have also shrunk in the past few decades, as evidenced by delayed dates of freezing or earlier thawing, as well as shorter fro‐zen periods, although the change has been spatially heterogeneous (Sharmaet al., 2019). Compared with the mean during 1984 ?1994, global river ice de‐creased from 2008 to 2018, with a variability between 47% and 75%, with the most significant reduction oc‐curring in the QTP, eastern Europe and Alaska (Yanget al., 2020). A study on the ice phenology during 2000?2015 based on 103 lakes in the QTP and the re‐gion north of 30°N revealed that the frozen period has generally shortened, with an earlier date of complete thaw and a later date of complete freeze (Duet al.,2017;Guoet al.,2018).

4 Impacts of cryosphere change

Cryospheric change affects about 10% of the world's population living within 100 km of a glacial area, in terms of, for example, water resources and glacial hazards. By raising sea-level, it has an impact on about 28% of the world's population in low-eleva‐tion coastal areas (<100 m above sea level) within 100 km of the sea, including 17 megacities with a to‐tal population exceeding 5 million (Kummuet al.,2016).

4.1 Impacts on regional and global climate

Cryospheric change influences the global tempera‐ture by altering the planet's albedo and the latent heat of phase change when transforming solid and liquid water (Qinet al., 2014). The increase in snow cover in the eastern QTP in winter and spring has reduced precipitation in eastern China and the Indo?China Pen‐insula but increased rainfall in India and the north‐western Bay of Bengal. When an abnormal increase in snow cover occurs in eastern North American, the frequency of air-mass intrusion also surges (Zhuet al.,2007).In addition,a decrease in the extent of Arc‐tic sea ice always correlates with a weaker westerly jet and intensified storms at mid-latitudes, as well as stronger subtropical Westerlies,andvice versa(Barnes,2013). Atlantic meridional overturning circulation(AMOC) transports warm ocean currents from low to high latitudes in the Northern Hemisphere, through oceanic conveyor belts, and brings cold seawater from the North Atlantic to the Equator in the form of deep-sea currents (Lozieret al., 2019). The decrease in ice volume in the Antarctic and Greenland ice sheets has released a large amount of low-tempera‐ture, low-density freshwater into the ocean. There may be a critical tipping-point in the freshwater re‐lease from ice sheets that may trigger a suddenly change of the AMOC (Lentonet al., 2008). A weak‐ened AMOC is expected under continuous global warming. Thus, there is significant potential for an abrupt change in the hemispheric or global climate.Permafrost degradation and glacier retreat can aggra‐vate climate warming by releasing tremendous amounts of greenhouse gases and decreasing surface albedo, respectively (Schuuret al., 2011; Schaeferet al., 2014; Hoodet al., 2015). The storage of soil or‐ganic carbon in the subsurface layer (0?2 m) of per‐mafrost regions over the QTP has been estimated to be about 20 Pg (Zhaoet al., 2018), while that in the subsurface soil layer(0?3 m)in the circum-Arctic per‐mafrost region is 1,330 ?1,580 Pg (Hugeliuset al.,2014). The total organic carbon stored in permafrost is far greater than that in the atmosphere (770 Pg).The Arctic permafrost region has been projected to re‐lease 37?174 Pg of carbon under various climatic sce‐narios by 2100, whereas the soil carbon in the QTP could decrease by 2%?3%by 2050(IPCC,2013).

4.2 Impacts on water resources

The cryosphere stores 75%of the world's freshwa‐ter,and therefore the mountains hosting cryosphere ar‐eas are termed 'water towers' for the world's 1.9 bil‐lion people (Immerzeelet al., 2020).The QTP and its surrounding area are the headwater region for more than 10 major rivers in Asia, among which are the In‐dus, Tarim and Amu Darya basins with the most in‐tense irrigation networks in the world. Agriculture in these basins is highly reliant on water availability from the mountainous cryosphere. Glacier meltwater accounts for about 25%?40%of the river runoff in ar‐id inland areas of northwestern China (Yang, 1991),and its contribution to the annual runoff of the Yellow,Lancang, Yangtze, Nujiang, Yarlung Zangbo and In‐dus Rivers is 0.5%,2.8%,7.8%,8.3%,15% and 45%,respectively(Lutzet al.,2014;Huss and Hock,2018).Cryosphere change affects not only the amount of riv‐er runoff,but also its seasonality,and hence the gover‐nance of water resources in downstream regions. Gla‐ciers in the QTP and its surrounding area were project‐ed to decrease by 49% and 64% under representative concentration pathways(RCP)4.5 and 8.5,respective‐ly, directly affecting the vulnerability of water re‐sources in these areas (Luoet al., 2018; Biemanset al.,2019;Immerzeelet al.,2020).Nearly one-sixth of the world's population relies on meltwater resources for irrigation in spring,especially in the western Unit‐ed States and Central Asia. 75%?85% of agricultural water depends on snowmelt resources (Diazet al.,2018), with the oases in the Tarim Basin entirely rely‐ing on the meltwater from Tianshan and the northern QTP (Ding and Zhang, 2015). The thickening of ac‐tive layer has increased the base flow in autumn and winter (Zhaoet al., 2019; Kanget al., 2020). It has been estimated that recent permafrost degradation in the QTP has increased water release by 50×108to 110×108m3annually.The water equivalence of ground ice in the top 25 m of permafrost equals the total run‐off of all main rivers in northern Eurasia (Zhao and Sheng, 2019).The ground ice stored in permafrost re‐gions in the Northern Hemisphere has been approxi‐mated at 11.37×103to 36.55×103km3, and permafrost degradation will surely alter the hydrological cycle in the region to some extent(Zhanget al.,2008;Qin and Ding,2009).

4.3 Impacts on the ecosystem

The ecosystems of the terrestrial and aquatic envi‐ronments in cold regions have suffered from the im‐pacts of cryosphere change through modifications of thermal and hydrological conditions. The cryosphere plays a vital role in improving the climate adaptation capacity of species in high mountainous areas. Due to glacier retreat, permafrost degradation, and snow reduction in the Alps and Himalayas, some animals and plants have migrated and expanded their ranges into higher altitudes. The cryosphere region is also the habitat of various microorganisms. So changes may affect the ecological processes of microbial ni‐trogen azofication and transformation, organic mat‐ter decomposition, and, consequently, the local and global biogeochemical cycles (Hotalinget al., 2017;Margesinet al., 2019). The decrease in ice and snow meltwater has directly altered the community structure and species richness of many freshwater or‐ganisms. Permafrost degradation adjusts hydrologi‐cal systems and soil conditions and, accordingly, af‐fects ecosystem health (Chenget al., 2019). Perma‐frost degradation on the QTP has led to vegetation degradation through thickening active layer, rising soil temperatures and drying surface soils.On the oth‐er hand, the thawing of permafrost in the Arctic re‐gion has triggered water release, increasing surface runoff and soil moisture, which benefits vegetation growth(Sistlaet al.,2013).

4.4 Impacts on hazard occurrences

Various types of cryosphere-related hazards occur in high-altitude mountains and high-latitude regions(Figure 2). Frequent global occurrences of such haz‐ards have indicated a spatial connection with regions experiencing rapid responses to climate change and recurrent extreme weather(Wang and Xiao,2019).

Figure 2 Spatial distribution of various global cryospheric hazards(redrawn from Wang and Xiao,2019)

The retreat of the cryosphere has changed the fre‐quency,intensity and locations of related hazards.Per‐mafrost degradation has had a significant impact on frozen ground engineering and buildings (Dinget al.,2019). Permafrost degradation and debris flows can severely damage infrastructure. Furthermore, perma‐frost degradation increases the frequency and extent of slope failure in mountainous areas. Glacier retreat often trigger rock/ice avalanches or glacier collapses.Although snow cover has been continuously shrink‐ing (Bormannet al., 2018), the frequency and magni‐tude of extreme snow hazards have been gradually in‐creasing (Moet al.,2017;Vavruset al.,2017;Fontro‐donaet al.,2018).The Hindu Kush,Himalaya,Nyain‐qentanglha, Hengduan, Alps and Rockies mountains are avalanche-prone areas, with recent increases in frequency and losses of assets and casualties (IPCC,2019). Glacier surge, ice collapse and avalanche in some regions of the QTP have been frequently re‐ported, such as the collapse events associated with the No. 53 and No. 50 glaciers in Arucuo Lake in Ngari,Tibetan Autonomous Region (Zhanget al., 2018;Yaoet al., 2019), and the blocking event in 2018 at the bend of the Yarlung Zangbo River in southeastern Ti‐bet(Liuet al.,2014).

Intense melting of glaciers and seasonal snow can trigger outbursts from glacier lakes or cause floods by meltwater, often resulting in debris flows. Most of these disasters form cascading chains, enlarging the risk to people and the environment. Floods happen predominantly in the high-altitude mountains, espe‐cially in the transition period from autumn to winter and winter to spring (IPCC, 2019). Since the 1930s,more than 30 floods due to glacial lake outburst (also called J?kulhlaups for glacier dammed lake outburst)have been recorded in the Tibetan Autonomous Re‐gion (Wanget al., 2011; Yaoet al., 2014; Kelvinet al.,2019).The moving sea ice and icebergs can gener‐ate massive thrust and impact forces, which can dam‐age ships, offshore facilities and coastal works. In ad‐dition,the forming of sea ice may also block sea chan‐nels,trap ships,close ports and damage aquaculture.

4.5 Impacts on engineering and shipping

The impacts of cryosphere change on engineering and buildings are mostly observed in permafrost re‐gions, with frost heave and thawing subsidence being the primary factors. Degrading permafrost can lead to thermal collapse and thermokarst terrains,which threat‐en the foundation of roads, oil pipelines and residen‐tial buildings (Zhaoet al., 2020). The infrastructure along the coast of the Arctic Ocean has already been af‐fected by permafrost degradation and intensification of wave erosion (Guet al., 2020). In the permafrost re‐gion of the Northern Hemisphere,nearly 4 million peo‐ple and 70% of the current infrastructure are located in the potentially thawing zone (Figure 3). And roughly one-third of the pan-Arctic infrastructure and 45% of the oil and gas exploitation areas in the Russian Arctic regions have already been affected(Hjortet al.,2018).

Figure 3 Risk map of the infrastructure in northern Hemisphere(after Hjort et al.,2018)

The asphalt pavement used for the Qinghai-Tibet Highway has a strong heat absorption effect, which may cause thawing and settlement-related hazards,es‐pecially in ice-rich, warm permafrost regions. Al‐though temperature increase and ecological environ‐mental change were considered in the design and con‐struction of the Qinghai-Tibet Railway, traditional methods such as raising the subgrade and increasing the thermal resistance are no longer guarantees for the stability of the embankment. Therefore, engineering stability is still a potential problem (Niuet al.,2015). In recent years, engineering measures, such as heat pipes, ventilation tubes and crushed rock,have been widely used in the design and mainte‐nance of the Qinghai-Tibet Highway and Railway.These measures have been effective in improving the thermal stability of the highway and railway and pro‐tecting the structures from damages caused by per‐mafrost degradation (Wu and Niu, 2013). Oil pipe‐lines in permafrost regions, such as the trans-Alaska crude-oil pipeline in the USA, the Roman pipeline in Canada, and the Golmud-Lhasa pipeline in China,have also encountered thaw-settling and frost-heave problems. Although the Alaska pipeline was raised above ground in some areas, this does not guarantee that it is not impacted by permafrost degradation. The cost of the construction,operation and maintenance of such engineering infrastructures is prohibitive (Jinet al., 2006). The permafrost along the Sino ?Russian crude-oil pipeline has also shown rapid degradation.Large-scale frost heave and thermokarst occurrence pose a significant threat to the pipeline's stable opera‐tion(Gaoet al.,2019).

The influence of cryosphere change on shipping mainly occurs in the Arctic and Antarctic regions.Changes in the distribution of sea ice affect the selec‐tion of shipping routes and vessel safety.In the Arctic,it mainly affects waterways. It has been predicted that there will be an ice-free summer in the Arctic Ocean around 2035. In the next five years, the start and end times for navigation along Arctic routes are expected to expand from June?September to April?November each year. The opening of the Arctic waterways will prompt a transformation in the world's transportation trade patterns(Guo,2019).

5 Perspective

Due to recent rapid changes in the cryosphere,studies on the spatiotemporal heterogeneity of cryo‐spheric dynamics have raised significant concern.Limited by accessibility, observation and simulation methods,there is still a large gap in the study of cryo‐sphere-change processes, their driving mechanisms and future trends, and the quantitative assessment of,and adaptation to,the influence of cryosphere change.

Monitoring and modelling cryosphere change:It is difficult to obtain background and change data for some cryospheric elements, such as the thickness of the terrestrial permafrost and sub-sea permafrost distribution, the volumes of ground ice and organic matter, and the depth of glaciers,etc..These compo‐nents cannot be directly obtained or reliably estimated by remote sensing. In addition, the accuracy of snow water equivalent data derived from satellites is low in rugged terrains. The challenge is how to understand the spatiotemporal heterogeneities of rugged moun‐tainous terrains, control the data quality and optimize the data products (e.g., snow cover/water equivalent,precipitation and albedo), based on the different tem‐poral and spatial scales involved in land surface pro‐cesses(i.e.,dynamic,physical,biogeochemical and hu‐man activities in the cryosphere) and the heterogene‐ity of the driving force.The coupling of these process‐es is also a challenge to conduct more detailed analy‐ses of the changes and impacts of the cryosphere on the earth system model. Therefore, launching micro‐satellites to monitor cryospheric elements and devel‐oping advanced model systems will help gain a better understanding of the interaction between the cryo‐sphere and the other components of the Earth system,and improve the ability to predict the changes and impacts associated with cryosphere dynamics.

Mechanism of the spatiotemporal pattern of cryosphere dynamics:As with other geographical en‐vironments, the cryosphere distribution shows signifi‐cant zonal and highly-differentiated characteristics. The southern boundary of snow cover and seasonally-frozen ground, the elevation of glacier fronts, and the south‐ern edge of permafrost distribution are controlled by zonal factors, as is the influence of altitude on the mountainous cryosphere. The cryosphere is highly sensitive to climate change, and this sensitivity is characterized by spatiotemporal heterogeneity. The impacts of cryosphere change on many aspects of the societal and economic sectors are unprecedented, as manifested through water resource scarcity, environ‐mental degradation, greenhouse gas emissions in per‐mafrost areas, and coastal zone inundation.The build‐ing of adaptation capacity to vulnerable environ‐ments will require a robust understanding of the changing cryosphere. Besides, abnormal responses of the cryosphere to climate change need to be better understood, such as glacier advance and surging in the Karakorum?Pamir?West Kunlun Mountains,gla‐cier collapse in the Aru Co in the central western Ti‐betan Plateau and Gyala Peri regions of the eastern Himalaya, and J?kulhlaups in the Yigongzangbu Riv‐er and Anyemaqen Range, which have caused, and likely will cause in the future,disasters,casualties and property loss. Therefore, changes in the temporal and spatial patterns and abnormal responses of the cryo‐sphere are topics worthy of future study.

Adaptation to cryosphere change:The cryo‐sphere is highly sensitive to climate change. With global warming, the negative impact caused by cryo‐sphere change has gradually increased.The regions di‐rectly inolving high-altitude cryosphere are being sig‐nificantly affected, bringing challenges to the ecologi‐cal environment and sustainable development in these areas (Wanget al., 2018). Most cryosphere regions are vulnerable environments with weak economies and lack of resilience in the face of cryosphere change. Therefore, it is of practical importance to study,and improve the capacity of,people in these re‐gions to enable them to cope with the approaching im‐pacts. In addition, it is necessary to develop a system‐atic model by combining cryospheric geography and human geography to identify early warning signs of potential disasters and risks to the economic and so‐cial systems in these regions, as well as ultimately providing guidelines for social, economic and cultural activities in such areas.

6 Conclusions

This paper summarizes our current understanding of the distribution and dynamics of the global cryo‐sphere. The overall shrinking cryosphere is the direct result of current climate warming, and is impacting human society at the regional, hemispheric and global scales. These effects include sea-level rise, a weaken‐ing thermohaline oceanic circulation, climate fluctua‐tions in Eurasia and North America, and variations in the annual and interannual runoff in cryospheric ba‐sins. The high mountain regions of Asia, the Andes and others will face the impacts of water shortages and glacier-related hazards. Nonetheles, there may be various temporal windows for extended navigation in future summers due to sea-ice extent reduction in the Arctic and Southern Oceans. Further studies on the cryospheric change will provide practical support for humans'sustainable development.

Acknowledgments:

This research was supported by Yunnan University(YJRC3201702), the National Natural Science Foun‐dation of China (Grant Nos. 41761144075, 41690142,41941015, 41771075, 41871096, 41671057, 41801052,41561016, 41701061, 41861013) and the Ministry of Science and Technology(2013FY111400).