The study, published Tuesday in the journal Climate of the Past, offers new proof that glaciers can grow and shrink rapidly - over decades and centuries rather than hundreds or thousands of years - in response to temperature and snowfall changes.
The findings could help scientists better forecast how fast and how much sea levels will rise due to global warming in coming decades. The rapid melting of glaciers worldwide in the past century is a major contributor to increased sea levels.
In November, scientists reported that the melt of northern Greenland’s Zachariæ Isstrøm glacier, which alone holds enough water to hike sea levels 19 inches, has accelerated since 2012 to about 5 billion metric tons of meltwater a year.
“There is no reason to expect that glaciers should be retreating this fast based on natural forcing mechanisms alone,” said D’Andrea, a paleoclimatologist with the Lamont-Doherty Earth Observatory at Columbia University. “But what we’re seeing all over the Arctic is glaciers melting, at rate that can’t be explained by natural long-term changes in the energy budget of the Arctic.”
To reconstruct a multimillennial record of change in Arctic glaciers, D’Andrea and colleagues took sediment core samples from Kulusuk Lake in Greenland, which is fed solely by two nearby glaciers. They tested the core’s layers, which went back roughly 10,000 years, to determine what sat on the lake bed from century to century.
While glaciers look inert, they are actually flowing downhill, grinding up the bedrock at their bases, said D’Andrea.
So it followed that when the glaciers near Kulusuk Lake were larger and heavier, they ground more silt, rocks, and boulders out of the bedrock, and meltwater then carried that material into the lake, said D’Andrea. When the glaciers were smaller, the deposits of dirt and rocks decreased.
“It’s a pretty simple story: You either have a lot of eroded material from these glaciers, or you don’t,” he said.
The mineral record in the core showed that the glaciers dwindled, possibly disappearing completely, around 8,000 years ago, and began growing again about 4,000 years ago. From that point their size was on a general upward trend until roughly 100 years ago, when the mineral record shows a sharp decline.
“There is definitely always natural variability in the climate system, especially in the North Atlantic. We see that the glaciers have had natural intervals when they’ve grown and shrunk,” said D’Andrea, and “that the whole climate system can undergo fairly rapid changes.”
But given the firm scientific evidence that human activities are increasing global temperatures, “it’s a logical extension that the retreat we’ve seen in the past 100 years is due to warming, and that that warming is due to greenhouse gas emissions,” he said.
Asked if the great Arctic thaw could still be stopped, D’Andrea was cautious. “The extent of summer sea ice is getting smaller and smaller each year, which has huge implications for the amount of heat that can enter the Arctic each year,” he said. “Stopping emissions alone at this point probably isn’t enough.”
But cutting greenhouse gas emissions remains crucial, said D’Andrea. “Every CO2 molecule that doesn’t go into the atmosphere is not causing warming,” or increasing the rate of ocean acidification, he said. “There are things that are not just related to something beautiful like a glacier, or charismatic like a polar bear, that are threatened by CO2 - not to mention sea level rise.”