1. Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093-0221, USA
2. Brookhaven National Laboratory, Upton, New York 11973, USA
3. *These authors contributed equally to this work

Correspondence to: Dan Lubin^1,3 Correspondence and requests for materials should be addressed to D.L. (Email: dlubin@ucsd.edu).

Abstract

The warming of Arctic climate and decreases in sea ice thickness and extent^{1, 2} observed over recent decades are believed to result from increased direct greenhouse gas forcing, changes in atmospheric dynamics having anthropogenic origin^{3, 4, 5}, and important positive reinforcements including ice–albedo and cloud–radiation feedbacks⁶. The importance of cloud–radiation interactions is being investigated through advanced instrumentation deployed in the high Arctic since 1997 (refs 7, 8). These studies have established that clouds, via the dominance of longwave radiation, exert a net warming on the Arctic climate system throughout most of the year, except briefly during the summer⁹. The Arctic region also experiences significant periodic influxes of anthropogenic aerosols, which originate from the industrial regions in lower latitudes¹⁰. Here we use multisensor radiometric data^{7, 8} to show that enhanced aerosol concentrations alter the microphysical properties of Arctic clouds, in a process known as the 'first indirect' effect^{11, 12}. Under frequently occurring cloud types we find that this leads to an increase of an average 3.4 watts per square metre in the surface longwave fluxes. This is comparable to a warming effect from established greenhouse gases and implies that the observed longwave enhancement is climatologically significant.

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