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

The effect of radiative forcing by anthropogenic aerosols is one of the lar gest sources of uncertainty in climate predictions(1-6). Direct observation s of the forcing are therefore needed, particularly for the poorly understo od tropical aerosols. Here we present an observational method for quantifyi ng aerosol forcing to within +/-5 per cent. We use calibrated satellite rad iation measurements and five independent surface radiometers to quantify th e 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 a nd fly ash from the south Asian continent(7,8). 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 o f the atmosphere. This threefold difference (due largely to solar absorptio n by soot) and the large magnitude of the observed surface forcing both imp ly that tropical aerosols might slow down the hydrological cycle.