The role played by black carbon (BC) in the Earth’s climate is highly uncertain: Black carbon not only absorbs sunlight, but it also has a semi-direct effect on clouds, and when deposited on snow and ice it changes the reflectivity of the ground surface. These mechanisms generally have a warming effect on the climate, but emissions of BC are also associated with other emissions, including some that have a cooling effect. This project will study the net effect of current BC levels and future possible reductions of BC aerosols, taking into account scientific, economic, and political perspectives.