Observed High-altitude Warming and Snow Cover Retreat over Tibet and the Himalayas Enhanced by Black Carbon

Abstract:

Himalayan mountain glaciers and the snowpack over the Tibetan Plateau form the headwater of several major rivers in Asia. In situ observations of snow cover extent, since the 1960s and satellite measurement of surface albedo during the last decade, both suggest that the glaciers and snow pack in the region have gradually retreated, accompanied by surface warming of about 2.5ºC observed over the peak altitudes of the Tibetan Plateau. To-date none of the climate models have simulated either the record surface warming or its anomalously strong altitude dependence.

Using a high-resolution (100 km) ocean-atmosphere global climate model and an observationally constrained black carbon (BC) forcing that is about a factor of 2.5 larger than the IPCC-model forcing values, we reproduce the observed altitude dependence of warming trends and the spatial pattern of reductions in snow depths and snow cover extent. We conclude that BC aerosols, by direct atmospheric heating and deposition on the snow surface, play a major role in causing the trends, especially the vertical gradient of temperature change and a decrease in the snow cover. Despite the fact that CO2 effects on global mean warming are five times larger, BC has a disproportionately stronger effect on climate change over the elevated Himalayan/Tibetan region and its impact needs to be properly accounted for in future regional climate projections. The findings also provide practical intervention options for slowing down the dramatic changes over the Third Pole.