Toronto: Greenland's ice sheet has traditionally been pictured as a bit of a sponge for glacier meltwater, but new research has found that climate change is changing its structure, thereby reducing its ability to buffer its contribution to rising sea levels.
For the study, the researchers analysd data from three expeditions to the Greenland ice sheet in 2012, 2013 and 2015.
The project involved drilling firn cores in the interior of the Greenland ice sheet. Firn is multi-year compacted snow that is not as dense as glacier ice. Instead, it forms a porous near-surface layer over the ice sheet.
"The study looked at very recent climate change on the ice sheet, how the last couple of years of melt have really altered the structure of the ice sheet firn and made it behave differently to future melt," said study co-author William Colgan, professor at York University in Toronto, Canada.
The team set up several camps and drilled a series of shallow firn cores about 20 metres deep during their time on the ice sheet.
They were surprised by what they found. An extreme melt that occurred in 2012 caused a layer of solid ice, several metres thick, to form on top of the porous firn in the low elevation areas of the ice sheet.
"In subsequent years, meltwater could not penetrate vertically through the solid ice layer, and instead drained along the ice sheet surface toward the ocean," Colgan said.
"It overturned the idea that firn can behave as a nearly bottomless sponge to absorb meltwater. Instead, we found that the meltwater storage capacity of the firn could be capped off relatively quickly," Colgan noted.
Because the models scientists use to project Greenland's sea level rise contribution do not presently take firn cap-off into consideration, it means that Greenland's projected sea level rise due to meltwater runoff is likely higher than previously predicted.
The study was published in the journal Nature Climate Change.