Black Carbon Aerosols and the Third Polar Ice Cap

Menon, S., Lawrence Berkeley National Laboratory

General Circulation and Single Column Models/Parameterizations

Cloud Modeling

Menon S, D Koch, G Beig, S Sahu, J Fasullo, and D Orlikowski. 2010. "Black carbon aerosols and the third polar ice cap." Atmospheric Chemistry and Physics, 10(10), 10.5194/acp-10-4559-2010.

Recent thinning of glaciers over the Himalayas (sometimes referred to as the third polar region) has raised concern about future water supplies, since these glaciers supply water to large river systems that support millions of people inhabiting the surrounding areas. Black carbon (BC) aerosols, released from incomplete combustion, have been increasingly implicated as a cause of large changes in the hydrology and radiative forcing over Asia and its deposition on snow is thought to increase snow melt. Here, we examine aerosol distribution and their climate impacts over India for the last decade using a global climate model.

New estimates indicate that Indian BC from coal and biofuel is large, and transport is expected to expand rapidly in coming years. Using two different BC emission inventories, we quantify the impact of BC aerosols on the climate from 1990 to 2010 over the Indian subcontinental region. Simulations include the aerosol direct and indirect effects, BC effect on snow/ice cover, and changes to sea-surface temperatures and greenhouse gases (GHGs). In general, for changes driven by changing aerosol emissions only, with increased atmospheric forcing, snow/ice cover decrease. The reduction in surface radiation and the positive forcing at the top of the atmosphere result in the high value of atmospheric forcing simulated for the higher BC emissions used that indicates stronger heating effects of fossil/bio-fuel BC over the column.

This decreases cloud and snow/ice cover, and precipitation is reduced compared to the changes obtained when the standard emissions are used. We also find an increase in summer precipitation for the eastern parts of India with a decline in most other areas from the increased fossil/bio-fuel BC emissions for 2000. For the future projection, with an increase in fossil/bio-fuel BC emissions the decrease is confined to the north-central (not statistically significant) and southern parts. With the new and higher Indian BC emissions, precipitation changes over most parts of India are similar to observed trends for similar time periods. However a notable exception is the simulated increase over eastern India/Bangladesh obtained for simulations without the climate influence that is opposite to observed trends, but consistent with summer rainfall trends from the Indian Meteorological Department data for the 1990–2000 period. These observations indicate a decrease over much of central India but an increase over eastern India.

Although we do not preclude the influence of large-scale circulation or GHGs on the spatial patterns of precipitation or snow cover changes, our results indicate that aerosols and the enhanced Indian fossil/bio-fuel BC aerosols in particular may be responsible for some of the observed patterns and trends in snow/ice cover and precipitation. Over the Himalayas, from 1990 to 2000, simulated snow/ice cover decreases by ~0.9% due to aerosols. The contribution of the enhanced Indian BC to this decline is ~30%, similar to that simulated for 2000 to 2010. Spatial patterns of modeled changes in snow cover and precipitation are similar to observations (from 1990 to 2000), and are mainly obtained with the newer BC estimates. The range in climate impacts from the two emission inventories examined provide an estimate of the expected uncertainty in climate change from aerosols for the last decade (1990 to 2000) and illustrate future expected challenges from aerosols and BC emissions in particular.