The very complex mixture of aerosol types and their spatial distributions provide diverse warming and cooling influences on climate, and impact the formation of both water droplets and ice crystals in clouds. Our poor understanding of aerosol properties and distributions results in large uncertainties about the net impact of aerosols on climate and impairs our ability to project climate changes.

In this light, we have embarked on Phase I of a synthesis product entitled, "Aerosol properties and their impacts on climate," which addresses Goal 2, "Improve quantification of the forces bringing about changes in the Earth's climate and related systems," under the Strategic Plan of the U.S. Climate Change Science Program (CCSP). We present here the status of this first phase of work, which is focused on new assessment and synthesis information stimulated by the CCSP. Three topics have been selected as foci and will be reported on here: (1) dependence of radiative forcing by tropospheric aerosols on aerosol composition in the North Atlantic, North Pacific, and North Indian Ocean based on in-situ observations, (2) a review of measurement-based understanding of aerosol radiative forcing and aerosol sources derived from the analysis of remote-sensing observations, and (3) a model intercomparison study to quantify the uncertainties associated with indirect aerosol forcing.