SPATIAL PATTERN OF EQUILIBRIUM-LINE ALTITUDE IN CENTRAL ASIA
We estimated ELA’s for modern and paleo glaciers in climate transects across Central Asia. Glacier chronology was established in key areas with cosmic-ray exposure dating, augmented by dates reported in literature. We used ASTER satellite images to map glaciers and glacial deposits. ELAs for modern glaciers were estimated using ASTER digital elevation models to capture ice surface topography, and regional threshold values for established techniques used to recover paleo ELA from the topography of once-glaciated valleys. The accumulation-area ratio, toe-headwall altitude ratio, and elevation of highest lateral moraine techniques were used to estimate paleo-ELAs.
ELA depression during the “global” last glacial maximum (LGM; ~20 ka) relative to modern ELAs ranged from 500 to 1000 m in regions with >200 mm/yr of modern precipitation. In contrast, ELA depression was only 30–70 m in cold, hyper-arid parts of the Tibetan Plateau and the Gobi ranges, although it was much greater for some pre-LGM advances. This is the pattern of asynchronous glacial advances noted by Gillespie and Molnar (1995).
Ablation of precipitation-starved glaciers in cold, hyperarid Central Asia is dominated by sublimation and is less sensitive to temperature (Rupper and Roe, 2008). We hypothesize that LGM temperature depression (~5°C) could not produce big glacial advances in these regions. Mapping of the glaciers with minimal ELA depression thus reveals regions with paleo-precipitation of <150 mm/yr.