Third Pole climate warming and cryosphere system changes

Figure 1. Glaciers distribution over theTP and its surrounding areas.

Figure 1. Glaciers distribution over theThird Pole and its surrounding areas.

Mountains are sources of water, energy, minerals, forest and agricultural products as well as popular recreational areas. High mountain regions are the largest reservoir of ice and snow after the Arctic and Antarctic. Asia’s high mountain region hosts the world’s 14 highest peaks and some 100 000 km² of glaciers. This so-called Third Pole (TP) encompasses the Tibetan Plateau, the Himalayas, the Hindu Kush, the Pamirs and the Tien Shan Mountains. Melt-water from ice and snow in the Third Pole feeds many of Asia’s large lakes and rivers, including the Indus, Brahmaputra, Ganges, Yellow and Yangtze. Known as the Asian Water Towers (AWT), this mountain region is critical for the water security and socio-economic sustainability of many nations. It supports a population of 1.7 billion and gross domestic product (GDP) of US$ 12.7 trillion (Figure 1).

The Third Pole has experienced significant environmental changes over the last five decades. The conditions and stability of the Asian Water Towers are impacted by warming-induced glacier retreat, ice collapse, glacial lake expansion and frequent Glacier Lake Outburst Floods (GLOFs) with repercussions on the socio-economic progress of countries in the region. The rapid changes in Third Pole glaciers, permafrost, snow cover, lakes, rivers and their downstream effects have been studied since 2010 under the Third Pole Environment (TPE) programme with funds from the Chinese Academy of Science. This key international research initiative addresses the multi-sphere interaction of the Earth system across the Third Pole. Mountain cryosphere changes and their impacts on regional hydrology and water resources are important research aspects of the TPE programme.

Ten years into the study, it is useful to synthesizes the research results and to discuss water availability and the social-economic implications over the broader mountain regions. This report, the 1st part of synthesis, focuses on climate warming and changes in the Third Pole cryosphere system, that is to say changes in snowcover, glacier and permafrost conditions. The 2nd part (to be published in the next issue of the Bulletin) will cover hydrologic response to the Third Pole cryosphere changes, water security and social-economic sustainability.

Climate warming over the Third Pole

The Third Pole is one of the most sensitive areas to climate change. It has been considered as the place to observe for early warning signals of global warming (Yao et al., 2019; You et al., 2019). The region has warmed by about 1.8 °C over the past half century (Figure 2), significantly higher than the warming rates for the Northern Hemisphere and the globe mean (Kang et al., 2010; Liu and Chen, 2000; Yang et al., 2014). Annual and seasonal temperatures increased more at higher elevation zones across the Third Pole (Figure 3; Gao et al., 2018; Liu and Chen, 2000; Liu et al., 2009; Yao et al., 2019). This elevation dependent warming is especially pronounced during the winter and fall seasons (Yao et al., 2019) in areas below the 5 000 metres above sea level (m asl) mark.

Figure 2. Regionally mean surface temperature for China and Tibetan Plateau in the period from 1850 to 2100 from the multi-model ensemble means of 21 Coupled Model Intercomparison Project (CMIP) 5 under the IPCC’s Representative Concentration Pathway (RCP) 8.5 and 4.5. The vertical brown line marks the boundary between the historical and the RCP CMIP5 simulations (You et al., 2019)

Third Pole Figure 3 Figure 3. The elevation dependence warming of winter mean air temperature overTibetan Plateau in the period from 1970 to 2014 (Yao et al. 2019).

Climate warming over the Third Pole

The Third Pole climate is characterized by a wet and humid summer and a cool and dry winter. Approximately 60% to 90% of annual precipitation falls between June and September. Since 1960, annual precipitation, with significantly high inter-annual variability, slightly increased in most parts of the Third Pole, except the southern and southeastern regions (Gao et al., 2015) (Figure 4a,b). Because of inaccessibility and the complex terrain, precipitation in most parts of the northwestern Third Pole remains largely unknown due to lack of observations (See Figure 4b). Similar to the elevation dependent warming, there is a significant increasing trend in summer precipitation with elevation over the Third Pole (Figure 4c), that is by 0.83% decade⁻¹ km⁻¹ during 1970 to 2014, and 2.23% decade⁻¹ km⁻¹ for 1991–2014 (Li et al., 2017). Third Pole precipitation is projected to increase in the 21st century particularly over the north and west regions.

Figure 4 (a) Variations of annual mean precipitation averaged over the Third Pole from 1971–2011 (Gao et al., 2014) (b) Spatial pattern of the trends in annual precipitation on the Tibetan Plateau from 1979–2011. The filled symbols indicate increasing trend, while the hollow symbols indicate decreasing trend. Larger symbols represent significant trends (Gao et al., 2015). (c) The elevation dependence of trends (% decade⁻¹) in summer precipitation for three time periods (1970–1990, 1991–2014, and 1970–2014) over Tibetan Plateau.

Snow cover characteristics and changes

Climate warming directly affects the Third Pole’s cryosphere system, leading to significant glacial retreat, snow cover changes, and permafrost degradation (Yao et al., 2019). Remote sensing data reveal changes in snow cover conditions from 1980 to 2018. The average, maximum and minimum snow extent in the accumulation period (November to March) were large in 1980s and 1990s, but have consistently decreased since 2000 (Che et al., 2008). Maximum snow extent was approximately 2.5 × 106 km² in the winter of 1994/1995. Most of the Tibetan plateau experienced less snow cover days from 1980–2016, with an average decrease of less than 2 days/year over almost half of the region, and more than 4 days/year in some areas. The decrease in snow duration is also evident in the Tibetan plateau since 1980s. Similarly, snow depth decreased from 1980 to 2018, with large inter-annual fluctuation before 2000 and less variation after 2000 (Che et al., 2019). Spatial inconsistencies exist in snow depth change across the Tibetan plateau, with a clear decrease by 0.1-0.2 cm/a in the Nyenchen Tanglha Mountains, and a slight increase (less than 0.1cm/a) in the Qilian mountain, the HohXil Mountain and the North slope of the Himalayas (Figure 5).

Figure 5. Snow depth distribution and change during 1980-2018 over the Third Pole.

Snowmelt processes vary over space and time across the Third Pole. Although spring snowmelt is not the dominant contributor to river flow, it comes at the end of spring, the critical period for irrigation and plant growth. Therefore, snowmelt is an important water supply to soil moisture and river runoff in Third Pole. Climate change significantly affects hydrological processes across the Tibetan Plateau. In recent years, an increase in runoff and an earlier peak of snowmelt runoff were found in several studies (Immerzeel et al. 2010; Wang and Li 2006). Model results suggest regional differences in snowmelt process and runoff in response to climate warming in the Himalayas (Rees and Collins 2006). For example, increase of spring snowfall in the eastern part of Himalayas would reduce the increase in snowmelt runoff, and push back the timing of peak flows.

Permafrost condition and change

Figure 6: Distribution of permafrost and ground ice on the Third Pole

Permafrost is ground – soil or rock, including ice or organic material – that remains at or below 0 °C for at least two consecutive years (Harris et al. 1988). Permafrost covers approximately 40% of the Third Pole surface area, around 1.06 × 106 km² (Zou et al. 2017). Permafrost exists at the source of many major river basins, its coverage in the watersheds vary from less than 10% to more than 60% in Qilian Mountains and in between Kunlun and Tangula Mountains. During the period of permafrost formation and repeated ice segregation, a large amount of water is reserved underground and stored in the solid state as ground ice buried near the permafrost table. The ground ice reserve near the permafrost table on the Qinghai-Tibet Plateau is about 1.27 × 1013 m³ (Zhao et al. 2019) (Figure 6).

Global warming due to climate change has widely degraded permafrost across the Third Pole. In situ monitoring shows significant increases in active layer depth and ground temperature. Borehole observations along Qinghai-Tibet Highway from 2004 to 2018 show warming by 0.48 °C/decade, on average, at the bottom of the active layer, and by 0.02~0.31 °C/decade at depth of 10 metres (Cheng et al. 2019) (Figure 7). Modeled results also indicate a thickening trend in the active layer depth by 19.5 cm/decade (Hu et al. 2019). The change in the active layer displays spatial heterogeneity; it is more prominent in the cold permafrost regions, high elevation, high-alpine meadows and regions with fine-grained soils.

Permafrost degradation can lead to changes in hydrological processes, including alteration of water storage in surface reservoirs (e.g., lakes, wetlands), hydrological connectivity and surface water-groundwater interaction (Connon et al. 2014). In permafrost terrain, the interaction between groundwater and surface water is restricted because permafrost acts as an impermeable layer. With permafrost degradation, groundwater storage and recharge are expected to increase (Niu et al. 2011, Bense et al. 2012). Permafrost degradation has been identified as a potential associated cause for winter streamflow increase in the upper Heihe basin (Gao et al. 2018) and Lhasa River (Gong et al. 2006).

Figure 7: Soil temperature at the bottom of active layer during 2004-2018.

The thickening of the active layer affects production, convergence and ecological processes in permafrost regions; ground ice melt leads to additional water release and participation in the water cycle. In permafrost regions, during the thawing season, soil water content increased in the soil profile (Zhao et al. 2000). In response to the thickening of the active layer and ground ice melt, water content at the bottom of the active layer increased overall between 11%–32% from 2004 to 2018, while the surface soil moisture decreased or remain constant (Wu et al. 2017; Zhao et al. 2019).

Isotope studies reveal that the contributions of thawing permafrost to thermokarst lakes could reach 61.3% in the Beiluhe region (Yang et al. 2016). The contributions of meltwater from ground ice to runoff reaches 37.4% in the typical alpine river in Kunlun Mountains Pass (Yang et al. 2016) and 13.2% to 16.7% in the source region of Yellow River (Yang et al. 2019). The amount of ground ice melt and its effect to regional water cycle is difficult to quantify, since the response of permafrost to climate warming is relatively slow. Consequently, the impact of permafrost degradation on the hydrological process is also gradual. The specific processes of permafrost changes/variations in warmer, wetter climate and their effects on the hydrological conditions in the Third Pole need further research.

Glacier change and mass loss

Glacier changes and their impact on water resources and sea level rise have attracted attention around the world [Immerzeel et al., 2019; Zemp et al., 2019]. In the middle latitudes, the Third Pole is the region with the most concentrated distribution of glaciers (Figure 1). According to the new version of the world glacier inventory Randolph v.6.0 [RGI, 2017], there were 97 760 glaciers with an area of 98 739.7 km² in the Tibetan Plateau and its surrounding areas, including Hindu Kush, Pamir, Tien Shan and Altai. The ice volume of these glaciers was estimated to be about 7 481 km3 [Zemp et al., 2019]. It is important for downstream water resources management and sustainable socio-economic development to understand the glacier changes and their influences on river runoff.

Figure 8. Spatial and temporal changes of glacier mass balance in the Third Pole and its surrounding areas. Data used from [Bolch et al., 2017; Cao et al., 2014; Chen et al., 2017; Gardelle et al., 2013; Gardner et al., 2013; Kääb et al., 2012; Ke et al., 2015; Maurer et al., 2019; Neckel et al., 2014; Pieczonka et al., 2013; Scherler et al., 2011; Shangguan et al., 2010; Wang et al., 2008; Wang et al., 2013; Wei et al., 2015a; Wei et al., 2015b; WGMS, 2017; Wu et al., 2018; Xu et al., 2013; Zhang et al., 2016; Zhou et al., 2018; 2019].

Glacier changes is mostly reflected by changes in equilibrium line altitude (ELA), area and mass balance. The changes in glacier mass balance and area are directly controlled by the changes in ELA. In the Tibetan Plateau and its surrounding areas, there are only a few monitored glaciers. Over the past 50 years, all those glaciers displayed a general retreat trend, and their ELAs showed an increasing trend. For example, the ELAs of Glacier No.1 at the source of the Urumuqi River in Tien Shan, the Maliy Aktru glacier in the Altai Mountains and the Qiyi Glacier in Qilian Mountains have risen by about 110 m, 140 m and 250 m respectively since 1960s (Wang et al., 2010; Ye et al., 2016). Remote sensing data provide an important basis for the study of glacier changes and glacier inventory over the large spatial scale. Wang et al (2019) assessment of glacier area changes in the Tibetan Plateau and its surrounding areas via the synthesis of many studies results indicated a clear spatial pattern of glacier area changes over the past 40 years. That is glacier shrinkage of less than 0.2%/year in west Kunlun Mountains, Pamir and Karakoram – only 0.04%/year in central Karakoram – of 0.4%/year in east Altai, Tien Shan, Qilian, east Kunlun, Tanggula, Gangdis, southeast Tibet and Himalayas and greater than 0.7%/year in southeast Tibet. Wang et al. (2019) also summarized the results of studies on the changes of glacier mass balance obtained by geodetic and glaciological methods, and reported that the glacier specific mass balance over the past 50 years was close to zero and/or slightly larger or lower than zero in Karakoram, west Kunlun and Pamir, but was a significantly negative value in the other regions (Figure 8). On the other hand, glaciers were in a state of mass gain or less mass loss in Karakoram, west Kunlun and Pamir after 2000. Although other glaciers experienced accelerated mass loss in the other regions after 2000. This seems to imply that the “Karakoram anomaly” might partially extend to nearby west Kunlun and Pamir (Farinotti et al., 2020), and that water supply from the glaciers in the region of “Karakoram anomaly” to the downstream should be relatively stable.

The glaciers in the Third Pole are mostly concentrated in the Trim, Indus and Amu Darya basins (see Figure 1, about 60% of the total glacier area in these three basins). The large glaciers in these basins can lead to abundant glacier meltwater resources. For example, more than 40% of total runoff of Trim River comes from glacier meltwater. Even though the area of glaciers in the Trim Basin has shrunk, glacier meltwater runoff increased over the period of 1961–2006 (Gao et al., 2010). However, as glaciers recede further, the key question is when will annual glacier runoff reaches a maximum? This point is often referred to as “peak water,” and beyond it runoff decreases as the reduced glacier cannot supply rising meltwater anymore. A recent study estimated that peak water in the most large rivers basins in the Third Pole will be reached from 2030 to 2050, depending on the different greenhouse gas emission scenarios (Huss and Hock, 2018). This timing is critical for to current and future water resources management in the lower reaches.

Glacial hazard and disaster

Figure 9: Rescue for potential survivors of the Aru glacier collapse, July 2016 (photo by Xinhua Net)

Rapid glacier changes in the Third Pole may lead to disasters related to natural hazards such as glacier collapse, glacier surging, glacial debris flow, and glacial lake outburst flood (GLOF). These glacial events have their spatio-temporal distribution characteristics, dynamic processes and mechanisms. Glacier surging actively occurs in Karakorum, Himalayas and southeast Third Pole. The velocity of ice surface movement can reach hundreds of metres every year. There were 27 advancing glaciers in Third Pole in the period from 1978 to 2015 with significant increases in area and length. The speed of change for the western side of the Wood Stark glacier was 904 m/a from 1996 to 1998, 446 m/a for the eastern side of the K2 glacier from 2007 to 2009, and 238 m/a from 1978 to 1990 for the 5Y654D497 glacier (Xu et al., 2016). Surging glaciers can quickly move into glacial lakes and cause outburst floods. Glacier debris flow can be triggered by strong and quick glacier melt, GLOF, and glacier collapse or avalanche. High temperature and heavy precipitation are the two major meteorological factors that directly connect to the occurrence of glacier debris flow.

New types of glacier -related disasters are occurring the Third Pole. On 17 July and 21 September 2016, two massive ice collapses occurred in Aru Range, Ngari, west Third Pole (Kääb et al. 2017). The Aru glacier collapses caused nine human casualties – shepherds – and the lose of hundreds of livestock (Figure 9). On 17 and 29 October 2018, glacier collapse caused debris flow and blocked the Yarlung Zambo River at Sedongpu valley, southeast Third Pole. The fact that both the continental (Aru) and maritime (Sedongpu) type glaciers have experienced collapses seems to suggest that glaciers on the Tibetan Plateau might be in an unstable state.

Important implications

Over half of the world’s population lives in watersheds of major rivers with mountains sources – from glaciers and snow melt (Kaltenborn et al., 2010). Third Pole cryosphere changes affect regional hydrology, ecosystem and humans living in the entire watersheds. For instance, due to the decreased contribution of glacier runoff, streamflows will be more sensitive to precipitation fluctuations, leading to more stochastic hydrological processes. In the long-run, glacial meltwater in the streamflow would decrease if mountain glaciers continue to lose mass or disappear.

In the greater Himalayan region, up to 45% of the total river flow comes from seasonally snow and ice meltwater (World Resources Institute, 2003; Kehrwal et al., 2008). The downstream areas of the High Himalaya have escalating demands for water due to rapid population and economic growth. Changes in glacial runoff – decrease is likely decrease in the future – would reduce irrigation water availability, diminish agricultural productivity and threaten food security in the region. Due to shortages in water supply, food security for 4.5% of the population in the Brahmaputra, Indus, Yangtze, and Ganges basins will be threatened by reduced glacial runoff (Immerzeel et al., 2010).

It is clear that Third Pole cryosphere changes will have very broad implications. There is an urgent need to:

monitor Third Pole cryosphere changes and understand their impacts to water resources
develop adaption strategy, not just at the regional or national level, but at the basin scale involving all riparian countries, especially in the Third Pole region, in order to take account of and balance the demand for water from all the parties in the large watersheds.

Authors

Tandong Yao, Institute of Tibetan Plateau Research, Chinese Academy of Sciences

Lonnie Thompson, Byrd Polar and Climate Research Center, Ohio State University

Deliang Chen, University of Gothenburg

Yinsheng Zhang, Institute of Tibetan Plateau Research, Chinese Academy of Sciences

Ninglian Wang, Northwest University

Lin Zhao, Nanjing University of Information Science & Technology

Tao Che, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences

Baiqing Xu, Institute of Tibetan Plateau Research, Chinese Academy of Sciences

Guangjian Wu, Institute of Tibetan Plateau Research, Chinese Academy of Sciences

Fan Zhang, Institute of Tibetan Plateau Research, Chinese Academy of Sciences

Qiuhong Tang, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences

Walter Immerzeel, University of Utrecht

Tobias Bolch, University of St Andrews

Francesca Pellicciotti, Swiss Federal Institute for Forest, Snow and Landscape Research WSL

Xin Li, Institute of Tibetan Plateau Research, Chinese Academy of Sciences

Wei Yang, Institute of Tibetan Plateau Research, Chinese Academy of Sciences

Jing Gao, Institute of Tibetan Plateau Research, Chinese Academy of Sciences

Weicai Wang, Institute of Tibetan Plateau Research, Chinese Academy of Sciences

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