**5. Summary**

Located in central Asia, one of the most extreme continental regions on Earth, the Tian Shan, is particularly important because it possesses a high concentration of mountain glaciers which contribute a significant amount of freshwater to populated areas in the lowlands across the bordered countries. These glaciers are also sensitive to climate change and respond to temporal variations in the dominance of major climate systems, such as the mid-latitude westerlies and the Siberian high pressure. The Tian Shan has received little attention compared to other key mountains in the world, partly because of the accessing challenges.

As a WSW-ENE trending ~2500 km arc of mountain system, the Tian Shan includes many ranges across the Kyrgyz Tian Shan in the west and the Chinese Tian Shan in the east. The climatic settings show a high contrast of local differences. On the whole mountain scale, the temperature distribution is quite uniform regardless the change along the elevation, while the precipitation pattern shows strong variations from west to east and from northern slopes to southern slopes. Through some international and national efforts of glacier monitoring, glacier data set/inventories have been collected in the Kyrgyz and Chinese parts using recent remote-sensing data. Overall, there are more than 15,000 glaciers in the whole range, and large glaciers are mostly found in the west, whereas a high number of smaller ones with much less area and volume of ice are found in the east.

Our focus of the Little Ice Age (LIA) allows us to place the contemporary glacier changes under the warming climate into a broader context of glacier history. Although most evidence of the LIA climate and glaciers were assembled in Europe and North America, more and more studies have been conducted in an area like the Tian Shan to reconstruct the past climate conditions and decipher the timing and magnitude of the late Pleistocene glaciations. To date, a few studies have successfully constrained the chronology of the maximum or the successional LIA glacial advances in the Tian Shan, and the dating methods include lichenometry, radiocarbon dating and most recently, cosmogenic nuclide exposure dating. LIA glacial advances occurred around 200 years ago, 450 years ago and 650 years ago based on the moraine chronologies at some sites in the Chinese Tian Shan, but the sequence of the moraine/sub-moraines are not necessarily well-preserved at the front of every glacier. Climate reconstructions from the climate-proxy data, such as tree rings, lake sediments and ice core, revealed a cold and wet climate during the LIA in arid central Asia, and the timing generally agreed with the advances of glaciers. It is well recognized that glaciers respond to changes in climatic conditions, i.e. temperature and precipitation, and during the LIA, the cold, wet climate served as the major driving force to glacier expansions. Admittedly, the mechanism of glacier-climate relationship is complex and difficult to untangle, however, even if assuming similar climate changes at local scales, the variability of glacier changes still exists and implies the influence of other non-climate driving factors, for example, the topographic and geometric settings.

In a case study, we found that the selected local factors (elevation, area, slope, aspect, solar radiation and location) can explain more than 50% of glacier changes since the LIA in the eastern Chinese Tian Shan. This high level of explained variance indicates that local geomorphometric setting is important in determining glacier behaviour and its response to climate change. Among considered factors, glacier size and elevation are the two dominant factors, and this result is in agreement with findings in previous studies. The importance of the other factors is much less than that of these two. At a sub-regional scale, a decreasing trend is observed along a west-east gradient in the correlation between longitude and area change. Such a spatial variation of glacier changes reflected by the three sub-regions might not only be attributed to the difference in their local settings, but also to the gradient in climate systems (i.e. mid-latitude westerlies and the Siberian high pressure). Random forest model is helpful for us to better understand the mechanism between glacier retreat and local factors, and based on the split conditions in random forest model, scenarios resulting in greater or lower glacier retreat are identified.

**5. Summary**

52 Glacier Evolution in a Changing World

Located in central Asia, one of the most extreme continental regions on Earth, the Tian Shan, is particularly important because it possesses a high concentration of mountain glaciers which contribute a significant amount of freshwater to populated areas in the lowlands across the bordered countries. These glaciers are also sensitive to climate change and respond to temporal variations in the dominance of major climate systems, such as the mid-latitude westerlies and the Siberian high pressure. The Tian Shan has received little attention compared to other

**Figure 5.** The representative tree of the random forest model. All split conditions are significant at 0.05 level. Grey boxes

As a WSW-ENE trending ~2500 km arc of mountain system, the Tian Shan includes many ranges across the Kyrgyz Tian Shan in the west and the Chinese Tian Shan in the east. The climatic settings show a high contrast of local differences. On the whole mountain scale, the temperature distribution is quite uniform regardless the change along the elevation, while the precipitation pattern shows strong variations from west to east and from northern slopes to southern slopes. Through some international and national efforts of glacier monitoring,

key mountains in the world, partly because of the accessing challenges.

represent terminal nodes, and the second value is the predicted glacier change at the node.

Future studies in the Tian Shan should consider focusing on several aspects: (1) utilizing high-resolution satellite images to document glacier status and recent glacier changes; (2) developing new sites and improvements in dating techniques to create more well-constrained glacial chronologies; (3) filling the spatial gaps in past climate information by adding more site-specific climate reconstructions across the range and (4) modelling the response of glaciers to climate changes with considerations of local geomorphometric factors.
