If world leaders need one more sign that they must reach a decisive climate agreement this week in Paris, it might be a new study showing that Greenland’s glaciers are retreating at least twice as fast as any other time in the past 9,500 years.
Melting ice from the Greenland Ice Sheet threatens to flood millions of people in low-lying coastal areas in the next few decades, and the study shows just how sensitive the glaciers are to warming temperatures. The results are based on analyses of ice sediment samples in a glacier-fed lake in southeastern Greenland, Iceland and Baffin Island.
“Two things are happening,” said study co-author William D’Andrea, a paleoclimatologist at Columbia University’s Lamont-Doherty Earth Observatory. “One is you have a very gradual decrease in the amount of sunlight hitting high latitudes in the summer,” D’Andrea said.
If that were the only thing happening, we would expect these glaciers to very slowly be creeping forward, forward, forward. But then we come along and start burning fossil fuels and adding carbon dioxide to the atmosphere, and glaciers that would still be growing start to melt back because summer temperatures are warmer,” he said.
Sediment cores from the glacier-fed Kulusuk Lake allowed the scientists to track changes in two nearby glaciers going back 9,500 years. Before the 20th century, the fastest rate of glacier retreat reflected in the core was about 8,500 years ago, at a time when the Earth’s position relative to the sun resulted in more summer sunlight in the Arctic.
“If we compare the rate that these glaciers have retreated in the last hundred years to the rate that they retreated when they disappeared between 8,000 and 7,000 years ago, we see the rate of retreat in the last 100 years was about twice what it was under this naturally forced disappearance,” D’Andrea said.
The history captured in the Kulusuk Lake cores shows that a warming period started about 8,500 years ago, when the glacier’s erosion rates fell rapidly, suggesting the glacier was getting smaller. Then, about 8,200 years ago, temperatures began to cool rapidly and erosion rates increased again. That cooling period has been well documented by other studies and has been connected with large changes in ocean circulation.
The glaciers started growing again about 4,000 years ago, experiencing a series of growth pulses that reveal their sensitivity to change. The glaciers expanded in bursts, advancing quickly, retreating briefly, and then expanding farther – until about 100 years ago, according to the study.
“This shows that there are internal responses within the climate system that can make glaciers grow and shrink on very short time scales. They’re really dynamic systems, which we have not had much evidence for prior to this,” D’Andrea said.
The pulses of growth match cooling periods documented in ocean sediment cores and in the continuous cores from Iceland and Baffin Island, suggesting that glaciers have responded in sync across the North Atlantic for at least the past 4,000 years, the authors write.
Understanding how glaciers melt and how ice melted in the past is a critical component to understanding past and future sea level rise and improving risk assessment in the future, said D’Andrea.