BY ALEX SCHWARTZ republished from Popular Science
Release date: April 23, 2019
More grim news from the Arctic: Greenland’s ice sheet has shrunk six times faster than normal since the 1980s, and it could keep melting for decades even if humans significantly reduce carbon emissions.
Researchers from the University of California, Irvine and Utrecht University in the Netherlands used decades of data on the melting of Greenland’s snowcap and glaciers to construct a 46-year history of change on the world’s largest island—and it wasn’t reassuring. They found that Greenland alone caused sea levels to rise by 13.7 milimeters since 1972, half of which occurred during the last 8 years.
Scientists studying rising sea levels have been keeping an eye on Greenland for decades, as its 656,000 square miles of ice—three times the size of Texas—is land-based. Sea ice, while sensitive to warming temperatures, doesn’t affect the volume of the oceans. Think of a glass of water with ice cubes in it: If those cubes melt, the water level in the glass won’t change. But when land-based ice sheets melt, it’s like adding more water to the glass that wasn’t there before.
Antarctica is the only other land mass in the world covered in a year-round ice sheet, and it’s a lot bigger than Greenland’s. The gigantic southern landmass is also just starting to lose ice, while its northern counterpart has been shrinking for decades.
While previous studies have tracked ice loss in Greenland since 2000, scientists doing this latest research wanted to see how the island’s ice mass balance (a budget for how the ice sheet gains and loses ice over time) had changed since 1972, when satellites first began collecting data on it.
Two factors determine this balance: snowmelt and glacier movement. In the summer, when warmer temperatures abound, some of the snow on the surface of Greenland’s ice sheet melts into the sea. If less snowfall occurs in the winter to replace it, a net loss results. This process varies yearly, as it’s affected by seasons.
Hundreds of glaciers dot Greenland’s coast. These massive, slow-moving rivers of ice slice through bedrock and deposit icebergs directly into the sea. Scientists measure how fast these glaciers move, which affects how much ice they drop into the ocean during a given period of time. That, in turn, also impacts the mass balance of the ice sheet, as well.
Eric Rignot, a professor of earth system science at the University of California, Irvine and one of the study’s authors, says he wanted to put the rapid change in Greenland’s mass balance observed in recent decades in the context of a broader time scale.
“It’s [about] understanding how climate warming is affecting the ice,” Rignot says.
The team found that Greenland actually gained ice mass in the 1970s but began to lose it at an increasing rate starting in the 1980s. Rignot says this decade-by-decade approach, rather than year-by-year, helped him identify a larger trend: Glaciers have played a more significant role in the loss of Greenland’s ice than previously thought.
Because glaciers are so massive and slow-moving, they react to long-term changes in climate rather than short-term changes in the seasons. Rignot says despite the occasional colder year when the loss from snowmelt might decrease, a glacier that has been dislodged from its bedrock will keep flowing quickly.
“Once you push these glaciers out of equilibrium because of a large warming wave, it’s difficult to bring them back to where they were before,” Rignot says. “It’s almost like the damage has already been done.”
Jeremie Mouginot, another author of the study, says this research shows that even in recent years when Greenland gained snow, glaciers were still rapidly discharging ice.
“The ice sheet has lost mass every year since 1998,” Mouginot says.
While Greenland’s dangerous downsizing is nothing new to scientists, this research helps reveal where the ice loss is coming from—and just how rapidly it’s happened over the past four decades.
Rignot says thinking about Greenland beyond seasonal time is important, especially when considering that climate change happens on a long-term scale, too. Regardless of how global warming plays out, it’s already set off a slow-motion domino effect that will go on for decades.
“The glaciers are the floodgates, and clearly breaking the floodgates is a big part of what’s happening in Greenland, and also something that we need to consider for the future,” Rignot says.
Kristin Poinar, a geophysics professor at the University at Buffalo who has done extensive research on glaciology and ice sheet modeling and wasn’t involved with the study, says these results prove what scientists had already suspected about Greenland—that its melting is occurring on a decadal time scale rather than a seasonal one.
“I’ve been waiting for this study,” she says.
Poinar says while snowmelt levels may have a bad year, a glacier will have a bad decade, making it difficult to reverse any damage done.
“The day that we reduce or even halt carbon dioxide emissions is not the day that we return to normal,” Poinar says. It will take years for temperatures to return to normal no matter how swiftly and broadly humans are able to slash emissions, and even longer for the glaciers to restabilize in response to that.
Poinar says understanding how Greenland has behaved over the past few decades can give us insight into its next few decades, helping us plan for what will happen when—not if—sea levels rise.
“If Greenland has been losing mass for 30 years, then that’s just one more big piece of evidence that we need to plan for the sea level rise that the Greenland ice sheet contains,” she says.
Were all of Greenland’s ice to melt off into the ocean, scientists estimate that global sea levels would rise by around 24 feet—just a quarter of that would be enough to engulf most of the world’s coastal cities and displace over 650 million people. Poinar says knowing how that will play out over time will be vital to urban planners, who will have to come up with ways to deal with the flooding.
Rignot says humans have launched a machinery that operates on a much longer time scale than our activities do, and that it’s crucial to keep that in mind as we develop solutions:
“The forces of nature at play here are very, very big, and we should not underestimate them.”
Sustainable Development Goals:
13. Climate action: Taking steps to combat climate change and its impacts
14. Life below water: Conserving and managing the marine resources and oceans to promote sustainable development of our world