A climatologically significant aerosol longwave indirect effect in the Arctic

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 feedbacks 6 . 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 summer 9 . The Arctic region also experiences significant periodic influxes of anthropogenic aerosols, which originate from the industrial regions in lower latitudes 10 . 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.