Global modeling of soot-cloud interactions

Recently, the climate influence of soot from fossil fuel and biomass burning has received a lot of attention in the climate science and policy communities because of its supposed contribution to global warming. It is generally accepted that the direct radiative effect of soot warms the Earth-Atmosphere system, as does the albedo effect of soot deposition onto ice and snow. However, soot-cloud interactions are not well understood, and both the magnitude and the sign of the radiative forcings associated with these processes are uncertain. Here, we present a global model sensitivity study of soot particles effects on clouds, investigating i) their semi-direct effect, ii) ther indirect effects, as well as iii) their influence on mixed-phase clouds. As the uncertainties associated with each of the above effects (i-iii) are significant, we test the sensitivity of the modeled radiative forcings to the most uncertain aspects of their parameterizations. In particular, we will focus on the uncertainties associated with soot contribution to heterogeneous freezing processes in mixed-phase clouds. Estimates of the net effect of anthropogenic soot on Earth's radiative balance and hydrological cycle will be presented, along with the separate contributions from each of the processes discussed above.