Greenland's massive ice sheet is more than 3 kilometres thick in some places.Credit: Martin Zwick/REDA&CO/UIG via Getty Images

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.

Previous studies have shown record melting on parts of Greenland's ice, but the latest analysis includes the first estimate of historical runoff across the entire ice sheet. The results, published on 5 December in Nature1, show that the runoff rate over the past two decades was 33% higher than the twentieth-century average, and 50% higher than in the pre-industrial era.

“The melting is not just increasing — it’s accelerating,” says lead author Luke Trusel, a glaciologist at Rowan University in Glassboro, New Jersey. “And that’s a key concern for the future.”

Centuries of ice

A team led by Trusel drilled a series of ice cores, the biggest 140 metres long, in central West Greenland in 2014 and 2015. There, snow that melts in the summer later refreezes, rather than running off into the ocean — creating an annual record of ice melt. The researchers compared data from these ice cores, and an older core from the same area, with satellite observations of melting across Greenland, and estimates of melt and runoff from a regional climate model.

The team’s analysis suggested that the rate of melting at its drilling sites is representative of trends across Greenland. Armed with this knowledge, the researchers used the ice-core data as a proxy to estimate runoff rates going back centuries — before satellites and climate models existed.

The findings bolster a study published in March that found that West Greenland is melting faster than it has in at least 450 years2. “What this paper does nicely is expand that record to the whole ice sheet,” says Erich Osterberg, a climatologist at Dartmouth College in Hanover, New Hampshire, and a co-author of the March study.

Vicious cycle

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.

His team’s study shows that Greenland’s runoff hit a 350-year high in 2012, when the ice sheet released about 600 gigatonnes of water into the ocean — enough to fill 240 million Olympic swimming pools. Globally, average sea levels have increased by around 3.5 millimetres per year since 2005. Greenland is now the second-largest contributor to that rise, accounting for nearly 22% of the total, according to the latest analysis from the World Climate Research Programme3. And that number is poised to increase.

“I think the acceleration is the bell-ringer here,” says Mary Albert, a glaciologist at Dartmouth who co-authored an earlier study4 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.

“Climate change is impacting the cryosphere much sooner than we thought, and the impact is much larger than we thought,” she says. “And that is not comforting.”