Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 16, 1303-1315, 2016
https://doi.org/10.5194/acp-16-1303-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
05 Feb 2016
Observed high-altitude warming and snow cover retreat over Tibet and the Himalayas enhanced by black carbon aerosols
Y. Xu1, V. Ramanathan2, and W. M. Washington1 1National Center for Atmospheric Research, Boulder, CO, USA
2Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA
Abstract. Himalayan mountain glaciers and the snowpack over the Tibetan Plateau provide the headwater of several major rivers in Asia. In situ observations of snow cover extent since the 1960s suggest that the snowpack in the region have retreated significantly, accompanied by a surface warming of 2–2.5 °C observed over the peak altitudes (5000 m). Using a high-resolution ocean–atmosphere global climate model and an observationally constrained black carbon (BC) aerosol forcing, we attribute the observed altitude dependence of the warming trends as well as the spatial pattern of reductions in snow depths and snow cover extent to various anthropogenic factors. At the Tibetan Plateau altitudes, the increase in atmospheric CO2 concentration exerted a warming of 1.7 °C, BC 1.3 °C where as cooling aerosols cause about 0.7 °C cooling, bringing the net simulated warming consistent with the anomalously large observed warming. We therefore conclude that BC together with CO2 has contributed to the snow retreat trends. In particular, BC increase is the major factor in the strong elevation dependence of the observed surface warming. The atmospheric warming by BC as well as its surface darkening of snow is coupled with the positive snow albedo feedbacks to account for the disproportionately large role of BC in high-elevation regions. These findings reveal that BC impact needs to be properly accounted for in future regional climate projections, in particular on high-altitude cryosphere.

Citation: Xu, Y., Ramanathan, V., and Washington, W. M.: Observed high-altitude warming and snow cover retreat over Tibet and the Himalayas enhanced by black carbon aerosols, Atmos. Chem. Phys., 16, 1303-1315, https://doi.org/10.5194/acp-16-1303-2016, 2016.
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Short summary
We show that black carbon aerosol pollution is likely the dominant factor in causing the accelerated retreat of snowpack in Himalayas. The simulated snow fraction and surface albedo change at the surface, as well as the enhanced warming at higher elevations, are remarkably similar to observations in past decades. The reason for the model's ability to simulate the observed trends is that we replace the model-simulated black carbon forcing with one that is constrained by observations.
We show that black carbon aerosol pollution is likely the dominant factor in causing the...
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