Black carbon’s climate impact in CESM

Carbonaceous aerosols, including black carbon (BC) and organic carbon (OC), are significant contributors to the anthropogenic climate change. However, the direct radiative forcing of carbonaceous aerosols is still quite uncertain, in particular over Asia. To better constrain the present-day Asian carbonaceous aerosol forcing, we utilize both a top-down approach that is primarily based on ground-based and satellite observations over the first decade of the 21st century, as well as a bottom-up approach that is based on the latest global climate model coupled with an interactive chemistry and aerosol module (CESM1). (1) By making the comparisons of top-down observational estimates with bottom-up model simulations, we show that the model considerably underestimates atmospheric heating of BC. The major source of discrepancy between observations and models are speculated to be emission inventory, which is developed from emission of BC due to limited economic activity data reported by developing countries. (2) By applying a new partitioning scheme to the observed aerosol optical properties, we show that OC can contribute up to 20% of atmospheric heating, and thus the overall TOA cooling of OC is previously overestimated.

With the improved model representation of black carbon aerosols, we then study the Tibet snow and Himalayas glacier response to black carbon aerosols. Himalayas mountain glaciers are the headwaters of several major rivers in Asia region, which provide vital fresh water resources for billions of people. The observations have suggested that the Himalayas mountain glaciers are subject to gradual retreat during the last few decades. The attribution of the glacier retreat and associated high elevation warming is still unclear. Here we show that black carbon aerosols, emitted from both sides of Himalaya regions, play a major role in causing the glacier retreat. The high elevation warming associated with the direct heating effect of black carbon and BC deposition on the surface are both important. The surface albedo under all-sky conditions is shown to be reduced by more than 0.05 and the snow depth is reduced by more than 5%.


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