Rain is becoming more common across Greenland’s ice sheet and it may be playing an important role in rising sea levels.

Greenland’s 660,000-square mile ice sheet contains enough fresh water to flood coastal cities around the world. Warm air over the sheet is causing it to melt, but new work reveals that rainfall is also causing more melting than previously thought.

An analysis of satellite and weather station records suggests that around 300 melt events in Greenland between 1979 and 2012 were linked to rainfall. Over this time, rain-associated melting became twice as frequent in summer, and three times as frequent in winter. Rain now appears to account for 28 per cent of the ice sheet’s melt.

The analysis highlights an under-monitored area, says Robin Smith, at the University of Reading, UK, who was not involved in the study. “It tells us that we need to pay more attention to all the processes, and all the weather, all-year round, not just what’s obvious,” he said.

Nicholas Barrand, at the University of Birmingham, UK, says rainfall could have “profound effects” on the density of Greenland’s snowpack, where meltwater goes, and the total amount of meltwater that runs off the sheet into the sea. “Each of these make up the Greenland ice sheet’s contribution to global sea-level rise, and will require close monitoring in the coming years,” he says.

Winter melting

Rain contributes to melting because it contains heat. It is becoming more common in Greenland due to higher temperatures, and is increasingly falling further north, even during the winter in some areas.

When precipitation falls as rain, it causes some of Greenland’s ice sheet to be covered in ice rather than snow. Come the summer, this ice reflects less of the sun’s energy, exacerbating summer melting.

Historically, Greenland’s melt season has run between May and August, but rainfall means melting is now happening in winter too. “The rain events are extremely important because they are one of the only triggers for melting in winter,” says Marco Tedesco, of Columbia University in New York, who was involved in the analysis.

Warmer period

However, the team acknowledge that the period they studied was particularly warm. Natural variability means the decades between 1979 and 2012 were hotter than average, on top of the long-term trend of global warming, so are not necessarily a good guide to future melting

Jason Box at the Geological Survey of Denmark and Greenland says that if the period had extended to 2017 the trend would not have been so strong. That’s because 2012, which saw intense melting in Greenland , was the end of a string of years with increasing temperatures.

Rainfall’s role in Greenland’s melting ice sheet has ramifications not just for sea level rises, which still need to be quantified, but for undertaking vital climate science too.

Liz Thomas of the British Antarctic Survey in Cambridge, UK, is heading to Greenland this summer to drill ice cores to study signs of previous climate change. “From my prospective, new evidence of rain in the winter and increased melting is alarming. Surface melt water will percolate through the ice and potentially wash away the valuable climate proxies contained in ice cores,” she says.

Journal reference: The Cryosphere