Letters to Nature

Nature 405, 60-63 (4 May 2000) | doi:10.1038/35011039; Received 16 October 1999; Accepted 31 March 2000

Large differences in tropical aerosol forcing at the top of the atmosphere and Earth's surface

S. K. Satheesh & V. Ramanathan

  1. Center for Clouds, Chemistry, and Climate (C4), Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093 USA

Correspondence to: V. Ramanathan Correspondence and requests for materials should be addressed to V.R. (e-mail: Email: ram@fiji.ucsd.edu).

The effect of radiative forcing by anthropogenic aerosols is one of the largest sources of uncertainty in climate predictions1, 2, 3, 4, 5, 6. Direct observations of the forcing are therefore needed, particularly for the poorly understood tropical aerosols. Here we present an observational method for quantifying aerosol forcing to within plusminus5 per cent. We use calibrated satellite radiation measurements and five independent surface radiometers to quantify the aerosol forcing simultaneously at the Earth's surface and the top of the atmosphere over the tropical northern Indian Ocean. In winter, this region is covered by anthropogenic aerosols of sulphate, nitrate, organics, soot and fly ash from the south Asian continent7, 8. Accordingly, mean clear-sky solar radiative heating for the winters of 1998 and 1999 decreased at the ocean surface by 12 to 30 W m-2, but only by 4 to 10 W m-2 at the top of the atmosphere. This threefold difference (due largely to solar absorption by soot) and the large magnitude of the observed surface forcing both imply that tropical aerosols might slow down the hydrological cycle.