Atmospheric aerosols cause scattering and absorption of incoming solar radiation. Additional anthropogenic aerosols released into the atmosphere thus exert a direct radiative forcing on the climate system¹. The degree of present-day aerosol forcing is estimated from global models that incorporate a representation of the aerosol cycles^{1, 2, 3}. Although the models are compared and validated against observations, these estimates remain uncertain. Previous satellite measurements of the direct effect of aerosols contained limited information about aerosol type, and were confined to oceans only^{4, 5}. Here we use state-of-the-art satellite-based measurements of aerosols^{6, 7, 8} and surface wind speed⁹ to estimate the clear-sky direct radiative forcing for 2002, incorporating measurements over land and ocean. We use a Monte Carlo approach to account for uncertainties in aerosol measurements and in the algorithm used. Probability density functions obtained for the direct radiative forcing at the top of the atmosphere give a clear-sky, global, annual average of -1.9 W m^-2 with standard deviation, 0.3 W m^-2. These results suggest that present-day direct radiative forcing is stronger than present model estimates, implying future atmospheric warming greater than is presently predicted, as aerosol emissions continue to decline¹⁰.

1. Met Office, Exeter EX1 3PB, UK
2. NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey 08540, USA

Correspondence to: Nicolas Bellouin¹ Correspondence and requests for materials should be addressed to N.B. (Email: nicolas.bellouin@metoffice.gov.uk).

Received 1 August 2005; Accepted 17 October 2005

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