Sub-alpine trees testify late 20th century rapid retreat of Gangotri glacier, Central Himalaya

Our understanding on glacier-climate link in the Himalayan region is constrained due to lack of long-term observational and high-resolution proxy records. Hostile weather conditions and difficult approaches to access glaciers due to rough terrains largely limit observational studies on glaciers in the Himalaya. Sub-alpine trees and shrubs close to glaciers, growth of which is very sensitive to fluctuations in temperature, provide precise continuous record on climate and glacier behaviour to supplement the observational records back to several centuries. To unfold the Gangotri glacier dynamics in the past, we developed tree-ring chronologies and studied the colonization pattern of the trees in the region. Tree-ring chronologies of Himalayan birch and Himalayan pine developed from their respective upper tree line ecotone in the Gangotri glacier forefields showed impact of temperature on the growth variations. A comparison of Himalayan pine chronology with temperature proxies revealed regional and hemispheric scale temperature signal. Using tree-ring chronologies we show expansion (retreat) of the Gangotri glacier during cool (warm) phases that are also in agreement with the glacier fluctuation records from Central Asia and Southern Tibet. Moreover, using tree colonization pattern in the glacier forefields we established for the first time that the Gangotri glacier terminus receded similar to 1.853 km since the late 16th century (1571 C.E.), major part of which (1.79 km) receded since 1935 C.E. The glacier retreat, associated with the onset of 20th century warming got accelerated since 1957 C.E. (1.567 km). In view of our findings, the Gangotri glacier might further face accelerated recession in the 21st century under the projected warming.