Ice melt across Greenland is accelerating, and the volume of meltwater running into the ocean has reached levels that are probably unprecedented in seven or eight millennia. The findings, drawn from ice cores stretching back almost 350 years, show a sharp spike in melting over the past two decades.
The latest analysis also suggests that warming is altering the structure of the ice sheet’s top layer. Thawing and refreezing sets up a vicious cycle: bright snow is replaced by darker patches of ice that absorb more heat from the Sun, further warming Greenland. The melting and freezing cycle also makes ice below the surface less permeable, so more runoff is shunted to the ocean rather than trickling down into the ice sheet.
The overall effect, Trusel says, is that melting begets even more melting and runoff. “I think the acceleration is the bell-ringer here,” says Mary Albert, a glaciologist at Dartmouth who co-authored an earlier study suggesting that the record 2012 melt was exacerbated by black-carbon air pollution from forest fires. When the dark particles landed on the ice, they absorbed heat from the Sun and warmed Greenland, her analysis found. And with both forest fires and temperatures projected to increase in the coming decades, the danger to Earth’s cryosphere — or icy regions — will only grow, she says.