Climate scientists have known for years that fine particles known as aerosols cool Earth and partially offset the warming from carbon dioxide and other greenhouse gases. But scientists at the U.K.'s Meteorological (Met) Office and the U.S. National Oceanic & Atmospheric Administration now find that the cooling caused by aerosols may be much greater than previously estimated (Nature 2005, 438, 1138). Aerosols consist of natural windblown dust and airborne sea salt as well as particles from fossil fuel and biomass burning. The new study finds the magnitude of man-made aerosol cooling to be about 0.8 W/m2, which is at the upper end of the range of 0.2-1.0 W/m2 estimated in the latest United Nations Intergovernmental Panel on Climate Change report. For comparison, the buildup of long-lived greenhouse gases accounts for a warming of 2.4 W/m2. By combining satellite and surface-based data with results from chemical transport models, the researchers, led by Nicolas Bellouin at the Met Office, were able to separate the effects of anthropogenic and natural aerosols. Bellouin's work implies that efforts to reduce power-plant and vehicle emissions will lead to substantial warming that is now hidden by the cooling effects of haze.