July 2012 Greenland melt extent enhanced by low-level liquid clouds

Abstract

In July 2012, a heat wave swept across Greenland, resulting in extensive melting of surface ice and flooding; this is shown to have been enhanced by liquid clouds forming in such a way that sufficient incoming shortwave radiation could penetrate to the surface while downwelling longwave radiation increased. The heatwave that struck Greenland in July 2012 melted surface ice across virtually the entire ice sheet, causing extensive flooding. Ice core data suggest that such events occur only about once every 150 years on average, with the last one in 1889. Ralf Bennartz and colleagues have examined the physical mechanisms behind the 2012 event. At high elevations thin liquid-bearing clouds had an important role in enhancing surface warming, as they were optically thick enough and low enough to significantly enhance the downwelling infrared flux at the surface. At the same time they were thin enough to allow sufficient solar radiation through to raise surface temperatures above the melting point. Melting of the world’s major ice sheets can affect human and environmental conditions by contributing to sea-level rise. In July 2012, an historically rare period of extended surface melting was observed across almost the entire Greenland ice sheet1,2, raising questions about the frequency and spatial extent of such events. Here we show that low-level clouds consisting of liquid water droplets (‘liquid clouds’), via their radiative effects, played a key part in this melt event by increasing near-surface temperatures. We used a suite of surface-based observations3, remote sensing data, and a surface energy-balance model. At the critical surface melt time, the clouds were optically thick enough and low enough to enhance the downwelling infrared flux at the surface. At the same time they were optically thin enough to allow sufficient solar radiation to penetrate through them and raise surface temperatures above the melting point. Outside this narrow range in cloud optical thickness, the radiative contribution to the surface energy budget would have been diminished, and the spatial extent of this melting event would have been smaller. We further show that these thin, low-level liquid clouds occur frequently, both over Greenland and across the Arctic, being present around 30–50 per cent of the time3,4,5,6. Our results may help to explain the difficulties that global climate models have in simulating the Arctic surface energy budget7,8,9, particularly as models tend to under-predict the formation of optically thin liquid clouds at supercooled temperatures6—a process potentially necessary to account fully for temperature feedbacks in a warming Arctic climate.