UC San Diego

Black carbon (BC) aerosols are significant contributors to anthropogenic climate change and are considered as the second largest warming agent only after CO₂. To better quantify the present-day Asian BC aerosol forcing, in Chapter 2 we utilize both a top-down approach using ground -based and satellite observations, as well as a bottom-up approach using a latest global climate model. By comparing the observations with the model simulations, we show that the emission inventory over Asia used in the model needs to be adjusted by a factor of two to four. The global mean BC forcing since pre-industrial in the model is 0.5 to 0.8 W/m², larger than the unconstrained model forcing of 0.2 W /m². In understanding the climate system response to external forcing such as BC, we first show in Chapter 3 that the north-south contrast of land and ocean distribution determines the latitudinal asymmetry of climate change, which is an intrinsic characteristic of the climate system. The main thrust of this dissertation is that in Chapter 4, we demonstrate with the improved CESM that BC aerosols play a vital role in causing the observed surface warming, snow cover and snow depth reduction over Tibet and Himalaya region. Due to BC aerosols, the simulated snow cover fraction over Tibet is reduced by more than 2% and snow depth is reduced by 5%. The model simulated warming enhancement at high altitude surface (more than 1 K at 5000 m) due to the direct atmospheric heating of BC and the BC deposition on the snow surface are similar to observations. The relative contributions from greenhouse gases and cooling aerosols are also assessed with the model. In Chapter 5, we evaluate the climate mitigation benefits associated with a rapid reduction of BC aerosols. We show that aggressive BC reduction leads to warming mitigation and snow recovery over Tibet. We further discussed the major sources of BC emissions from China and the potential practical measures to cut BC emissions. The costs and benefits on public health due to cook-stove improvement are also discussed