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


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 ±5 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.

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Figure 1: Temporal variation of aerosol optical depth.
Figure 2: Aerosol forcing in the atmosphere.
Figure 3: Clouds in the presence of aerosol.

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We thank the Government of Maldives for providing the land for KCO and for assistance in its operation. We also thank B. N. Holben for providing the AERONET data, and K. Priestley and N. Loeb for the CERES data. The US component of INDOEX is funded by the NSF and the US Department of Energy, and KCO was funded by the NSF and the Vetlesen Foundation. We also thank J. Fein for providing support to both INDOEX and KCO.

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Correspondence to V. Ramanathan.

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Satheesh, S., Ramanathan, V. Large differences in tropical aerosol forcing at the top of the atmosphere and Earth's surface. Nature 405, 60–63 (2000).

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