The Greenland Ice Sheet managed to withstand the warming brought by the first 150 years of the industrial age, with enough snow piling up each winter to balance the ice lost to spring and summer melting. But, according to a new study, that all changed 20 years ago.
Starting in 2000, Greenland's glaciers suddenly began moving faster, their snouts rapidly retreating and thinning where they flow into the sea. Between 2000 and 2005, that acceleration led to an all-but irreversible "step-increase" of ice loss, scientists concluded in the new research, published this week in the journal Nature Communications Earth & Environment.
If the climate were to stop warming today, or even cool a little, Greenland's ice will continue to melt, said Ohio State University Earth scientist Ian Howat, co-author of the research paper. "Glacier retreat has knocked the dynamics of the whole ice sheet into a constant state of loss," he said. "Even if we were to stabilize at current temperatures, the ice will continue to disintegrate more quickly than if we hadn't messed with the climate to begin with."
Lead author Michalea King, a glaciologist at the Ohio State Byrd Polar Research Center, said the team looked at nearly 40 years of monthly satellite data for 200 Greenland glaciers to measure the meltdown, finding that "the ice that's discharging into the ocean is far surpassing the snow that's accumulating on the surface of the ice sheet."
"The kind of discouraging thing is, even if we were to go back to the old amounts of snow we were adding, at this new elevated rate of melting, we would expect it to stay out of balance for some time," she said.
Before 2000, the ice sheet would have about the same chance to gain or lose mass each year, she added, but in the current climate, the ice sheet will gain mass only once every 100 years.
Other research suggests that, in previous geologic eras, nearly the entire Greenland Ice Sheet melted when global temperatures were near today's levels, showing that even the thickest, coldest parts of Greenland are vulnerable to just a few degrees of warming.
How Long Will Greenland's Ice Last?
When all Greenland's ice melts, it will raise sea level by 20 feet. That could take 10,000 years, but jolts to the climate system like the glacier acceleration in the early 2000s, combined with the effects of the growing spread of ice-darkening algae and black carbon, the amount of meltwater saturating of the snow atop the ice or changes in ocean currents flowing near the glaciers could speed that up by thousands of years, Howat said. When a tongue of ice breaks free from its seafloor anchor, the flow of the glacier above it accelerates.
Melting ice is the main cause of sea level rise, and Greenland contributes the most meltwater to the ocean. With millions of people threatened by the rising ocean, the rate of sea level rise in the coming decades is critical for planning seawalls and other defenses, or deciding to abandon some coastal areas for higher ground. The Fourth National Climate Assessment estimates sea levels will rise 1 to 4 feet by 2100. The new study warns that accelerating melting could push the ocean toward the higher end of that range.
"If climate warming continues, technically the ice sheet is doomed," said Greenland ice researcher Jason Box, with the Geological Survey of Denmark and Greenland, who was not involved in the new study.
Ice loss would accelerate even more once the top of the ice sheet, which rises to 10,000 feet above sea level, melts down to a lower and warmer level of the atmosphere. Only a long series of very cold years could stabilize the ice, Box said. "What matters more is the climate trajectory, Is there any chance of cooling in the foreseeable future?"
Tipping points are complicated, but it's clear the Greenland Ice Sheet is not going to miraculously recover, said co-author Brice Noël, a climate scientist at Utrecht University.
"It's going to be difficult to reverse the trend," he said. "The ice discharge is high now, and it might increase more. In northwest Greenland, glaciers are still retreating fast."
Only a huge increase in winter snowfall could build up the ice sheet. Current models suggest that won't happen, but rather that more snow and ice will melt and evaporate, he said. "All the indicators right now are that snowfall will not change much, and ablation will increase."
Alfred Wegener Institute climate researcher Ingo Sasgen, who was not involved in the study, doesn't necessarily identify a tipping point, because those are hard to specify in the dynamic and complex climate system, and oversimplification can be misleading.
But the new research shows that the ice loss from Greenland is so great that "you would need a lot of snowfall to compensate for this," he said. "But we're seeing exactly the opposite; more extreme melt years. Rain will probably increase."
That rain is another factor accelerating the meltdown.
"The concerning message of this paper is the likelihood that we'll have a recovering ice sheet in the near future is close to zero," Sasgen said. It's also concerning that, during the 40-year satellite record, the five years with most ice loss have all been within the last 10 years, tracking with other indicators of the overheating planet, he added.
Even if the Greenland ice sheet is headed for a complete meltdown, curbing greenhouse gas emissions today could delay the process by 5,000 years, he said, giving people much more time to adapt as the sea encroaches across thousands of miles of densely populated coastlines.
Howat, the Ohio State co-author, said the findings identify the threshold at which global warming shifted the slow geologic pace of ice sheet melting into overdrive. Glaciers that flow into the ocean grow very slowly, but they can melt very fast, erasing centuries of advance in just a few decades, he added.
The sudden glacier acceleration early this century increased the amount of ice flowing to the ocean so much that it's going to be nearly impossible for the ice sheet to grow, he said.
"What it's done is put us on a different trajectory," he said. "Tipping points are where we go from one trajectory to another. It's hard, if not impossible to go back."