Processes leading to emissions of black carbon (BC) aerosols from incomplete combustion affect the Earth’s climate through several forcing mechanisms: Direct absorption of sunlight, a semi-direct effect on clouds, by deposition on snow and ice thus changi ng the surface albedo, and through associated emissions of other climate agents. The proposed project will study the net effect of reducing emissions of BC aerosols, taking into account scientific, economic and political perspectives. This will be done fi rst by measuring the spectral reflectance of snow and sea ice surfaces, and by measuring and analyzing BC content in the snow and sea ice in Svalbard and the Greenland Sea. This information will then be used along with other observations and chemical tran sport model (CTM) calculations of BC deposition in other regions to drive a general circulation model (GCM) to quantify the global climate impact. In collaboration with two leading European research groups, GCM calculations of the direct and semi-direct e ffect will be carried out with the aerosol optical properties and distribution from the CTM included. To investigate the climate effects of BC emission reductions, three emission scenarios (including associated emission changes of other climate relevant s pecies) will be constructed: 1) A pure natural science perspective, 2) Scenario 1 with an added economy perspective by choosing the most cost-effective emission reduction, and 3) A focus on political feasibility, but building on scenarios 1 and 2. The cli mate effects (including long-term effects of changes in long-lived greenhouse gases) will be estimated by combining the CTM and a simple climate model (SCM) and analyzed with the perspectives of economics and political feasibility.