Conflicting signals of climatic change in the upper Indus Basin

Abstract

Temperature data for seven instrumental records in the Karakoram and Hindu Kush Mountains of the Upper Indus Basin (UIB) have been analyzed for seasonal and annual trends over the period 1961-2000 and compared with neighboring mountain regions and the Indian subcontinent. Strong contrasts are found between the behavior of winter and summer temperatures and between maximum and minimum temperatures. Winter mean and maximum temperature show significant increases while mean and minimum summer temperatures show consistent decline. Increase in diurnal temperature range (DTR) is consistently observed in all seasons and the annual dataset, a pattern shared by much of the Indian subcontinent but in direct contrast to both GCM projections and the narrowing of DTR seen worldwide. This divergence commenced around the middle of the twentieth century and is thought to result from changes in large-scale circulation patterns and feedback processes associated with the Indian monsoon. The impact of observed seasonal temperature trend on runoff is explored using derived regression relationships. Decreases of ∼20% in summer runoff in the rivers Hunza and Shyok are estimated to have resulted from the observed 1°C fall in mean summer temperature since 1961, with even greater reductions in spring months. The observed downward trend in summer temperature and runoff is consistent with the observed thickening and expansion of Karakoram glaciers, in contrast to widespread decay and retreat in the eastern Himalayas. This suggests that the western Himalayas are showing a different response to global warming than other parts of the globe. © 2006 American Meteorological Society.