Editorial for the special issue in memoriam to Prof.Xie Zichu

Prof. Xie Zichu passed away on January 25, 2020 in Changsha, Hunan Province, China at the age of 83 years old. Being one of the internationally re‐nowned and highly respected glaciologists of China,this issue is dedicated with all respect in memory of him for his life-long effort and contribution to ad‐vance studies on glaciology in China. He has under‐taken field investigations on glacier mass balance, ice formation, snow avalanches, and glacial debris flows in the Arctic, Tianshan, Tibetan Plateau, Andes, Ant‐arctica, and laboratory measurements of ice samples taken from Antarctic ice sheet. He has been decorated with the first-rank national prize for natural science studies, the special award from the Chinese Academy of Science (CAS), and many provincial prizes for his outstanding contribution in the field mentioned above.Prof. Xie has published over 200 journal and confer‐ence papers, authored and co-authored 15 books/book chapters.We present a list of some representative arti‐cles and books by him in the references. Those inter‐ested in his research may trace back all his works.

He has pioneered in China the studies of mass bal‐ance and ice formation of glaciers in western China.Prof. Xie introduced and applied the living standard termed as the fix-date time system for mass-balance observations on Chinese glaciers. He later identified glaciers in western China characterized by their sum‐mer accumulation with simultaneous ablation due to the seasonality of precipitation dominated by mon‐soons and westerlies. Prof. Xie proposed three catego‐ries of glaciers in China based on mass change patterns observed in the second half of the 20thcentury, includ‐ing high negative mass-balance glaciers in the south‐east Tibetan Plateau and eastern Himalayas,mild nega‐tive mass balance glaciers in the eastern Tianshan, the north slope of the Karakoram and the eastern Kunlun Mountains, and weak positive mass-balance glaciers in the Qianlian Mountains and Tangula Mountains.This observation could be traced back to the classifi‐cation, with Prof. Shi Yafeng, of extreme continental,continental and maritime type glaciers in China.

He also proposed a standard to partition a glacier,from high to low elevations, into recrystallization zone, refreezing-recrystallization zone, cold percola‐tion-recrystallization zone, warm percolation-recrys‐tallization zone, percolation zone, percolation-super‐imposed zone, and ablation zone based on in-situ ob‐servations of moisture migration and meltwater infil‐tration as well as the refreezing processes in snow lay‐ers at different elevations.This knowledge is a refine‐ment of ice formation and glacier zones proposed by Shumskiy in his Theoretical Glaciology.With this theo‐retical fundament,he extended his interest in snow cov‐er and snow avalanche hazards in the central Tianshan Mountains. With his colleagues, Prof. Xie summarized knowledge from in-situ and laboratory experiments and proposed measures to mitigate the hazards from snow avalanches along the inner Tianshan Highway.

In addition,Prof.Xie has engaged in studies on gla‐cier changes and their impact on stream runoff and wa‐ter resources. He applied modern techniques of remote sensing, geographical information system and computer modeling to derive information of glacier geometrical changes and coupled the glacier module into the runoff model to project scenarios of glacial water resources in river basins originated from Asian high mountain re‐gions under climate change in the 21stcentury.

He has supervised over tens of students for their master's and PhD degree studies at Lanzhou Institute of Glaciology and Geocryology (LIGG) of the Chi‐nese Academy of Sciences and Hunan Normal Univer‐sity. These students and those who have been benefi‐cial from his help during his term as the director of LIGG are now active in the field of mass balance,gla‐cier and glacial lake inventories, glacial hydrology,physics of glaciers, permafrost, glacier geomorpholo‐gy, environmental records in glacier ice, glacial mete‐orology, remote sensing of snow and ice as well as glacier-related hazards. The special issue of SCAR covers the most recent progress in glaciology,as men‐tioned above. A brief introduction of each paper may be as follows. Readers can explore detailed knowl‐edge while reading each paper.

Being one of the three components, terrestrial cryosphere including glaciers, frozen soil, snow cov‐er,lake and river iceetc.has intimate relation with hu‐man beings and is the most vulnerable to climate change. Studies based on benchmark measurements or representative basins are key to understand the forc‐ing mechanism of cryopsheric changes.The special is‐sue includes some papers on the theme of glacier changes.An integrated result emphasized the acceler‐ated recession of the benchmark Glacier No.1 at the head of Urumqi River in Tianshan Mountains from over 60 years' observations of changes in supraglacial zones, mass balance, frontal position and its stream flow (Liet al., 2020). The test and validation of ice thickness estimation were conducted on Longbasaba Glacier based empirical equations and the field thick‐ness measurements from ground penetrating radar(GPR) (Heet al., 2020). They proved that the glacier has experienced thinning since 1980 by a rate of 0.23 m/a based on modeled ice thickness.While gla‐ciers in Qaidam Basin in-between Qilian and Kunlun Mountains have undergone widespread shrinkage, by 8.12 km2annually during 1977?2019 as indicated by multi-temporal remote sensing images (Zhou, 2020).Another attempt was made to estimate the thickness of debris cover on glaciers in area along the China-Pakistan Economic Corridor based on the thermal-in‐frared channel of satellite images and an energy bal‐ance model(Zhenget al.,2020).Debris cover is char‐acterized by its spatial heterogeneity either of thick‐ness or impact on glacier ablation in CPEC. The data is valuable for the impact assessment of debris on gla‐cier meltwater runoff.

Permafrost is also undergoing degradation in the past decades in the Tibetan Plateau.How this degrada‐tion influences the engineering in permafrost region is challenging. Shenget al. (2020) developed a hierar‐chical model to evaluate this effect and applied their model to the Chaider-Muli Railway for the effective‐ness of the roadbed treatment with rubber material.About 86% of which is proved to be in the preferable condition. In addiation, a team has developed a distrib‐uted model of water-heat exchange coupling with re‐motely sensed data for frozen soil and applied in Naqu of the east-central Tibetan Plateau (Gao and Zhang,2020). With the assimilation of soil data derived from remote sensing datasets, the model performance was highly improved especially during the transitional peri‐od between freezing and thawing of the frozen soil.

Either lakes with contact to modern glaciers or nourished by glacier meltwater have been experienc‐ing remarkable variability under the warming climate in the Qinghai-Tibetan Plateau. These changes are of implications to the ecological settings or hazard poten‐tial in these lake areas. Zhou (2020) presented the insitu measurements of the lake level of Nam Co during 2005?2019.They showed a general rising trend of the lake level since 2005 but with sharp descending by 59 cm during 2014?2017 and increase by 20 cm dur‐ing 2017?2018.They identified the discrepancy of lake level derived from ICESat altimetry and proposed fur‐ther validation of the altimetry with available data from other lakes.Qiet al.(2020) displayed a general expan‐sion of glacial lakes in the two bordering counties of Zhangmu and Gyirong with Nepal, of which 11 glacial lakes demonstrated the high risk of outburst floods.Be‐sides there will be over 100 new lakes developed fol‐lowing continuous retreat of glaciers in future while warming sustains. With respect to widespread distribu‐tion of glacier-related hazards in High Mountain Asia,Wang Xet al.(2020)summarized the available knowl‐edge of triggering mechanism of glacier-related haz‐ards and proposed perspectives for future researches by integrating in-situ-aerial-space borne techniques and the development of coupled models for predicting these cascading disasters from glaciers.

The special issue has collected 4 review papers on issues of cryospheric changes and their impacts.Wang Ket al. (2020) digested the progress on cryo‐sphere evapotranspiration in the Tibetan Plateau.They provided a thorough summarization on the spa‐tiotemporal heterogeneity of evapotranspiration on the Plateau and emphasized the interesting topic of the emerging vegetation greening and its influence on evapotranspiration since this is of importance to un‐derstand carbon cycles in cryospheric region. Liuet al. (2020) present the geography of cryosphere as a discipline that the rapid recession and degradation of global cryosphere has exerted profound impacts on so‐cietal-economic sectors in regions suffering from cryospheric changes and proposed integrated research efforts in competing the adverse and use the favorable effects of the changes.Dinget al.(2020) provided the new knowledge on the interaction between cryo‐sphere and the atmosphere. They identified some key indicators and hotspot regions like the Arctic amplifi‐cation associated with the shrinking sea ice and its consequence to mid-latitude weather and climate.They proposed more attention should be paid in un‐covering and quantifying these complex interactions between cryospheric and atmospheric systems.

Raoet al.(2020)discussed that the positive trends in δ18O records from ice core and peat in Holocene in Xinjiang confirms the long-term warming trend dur‐ing the Holocene.

Finally, Lüet al. (2020) gave us a portray of cli‐mate change in countries along "The Belt and Road Initiative" based on the Sixth Phase of Coupled Mod‐el Intercomparison Project (CMIP6). He indicated that our region may experience a warmer and wetter climate at the end of this century with pronounced in‐crease of precipitation in the Polar and arid regions.How such pattern of climate change exerts the im‐pacts on cryosphere and the societal-economic sectors are receiving attention for in-depth researches.