Greenland, home to the Earth’s second largest ice sheet, has lost ice at an accelerating pace in the past several decades - a nearly sixfold increase that could contribute to future sea level rise, according to a new study based on nearly a half century of data.
The findings, published Monday in the Proceedings of the National Academy of Sciences, estimate that Greenland's glaciers went from dumping only about 51 billion tons of ice into the ocean between 1980 to 1990, to losing 286 billion tons between 2010 and 2018.
The result is that out of nearly 14 millimeters of sea level rise in total caused by Greenland since 1972, half of it has occurred in just the last 8 years, researchers found.
And the losses are likely to get worse. The regions with the biggest potential ice loss - the frigid far northwest and northeast of the island, which sit up against the Arctic ocean - have not changed as quickly as other parts of Greenland. Should they begin to melt and lose chunks of ice more rapidly, then Greenland's overall ice loss - and contribution to sea level rise - could grow even more.
Eric Rignot, an Earth-systems scientist for the University of California at Irvine and NASA who was one of the study's authors, said in an email Monday that the study places the recent mass losses in Greenland in a longer-term context.
"The 1980s marked the transition time when the Earth's climate started to drift significantly from its natural variability as a result of man-made emissions of greenhouse gases," Rignot said.
He said that shift is worrisome in part because of what it portends for the future, especially when combined with potential ice loss in Antarctica.
"The entire periphery of Greenland is affected. I am particularly concerned about the northern regions, which host the largest amount of potential sea level rise and are already changing fast," said Rignot, who led research last year that found that the Antarctic region also is losing six times as much ice as it was four decades ago - an unprecedented pace in the era of modern measurements.
"In Antarctica, some big sleeping giants in East Antarctica are waking up, in addition to a large part of West Antarctica being significantly affected. None of this is good news," Rignot added. "We ought to prepare ourselves for what is coming up and take action as soon as possible to avoid the most drastic scenarios."
Greenland is the world's biggest island. It's the home to more than 200 major glaciers, many of which extend all the way from the mile-thick central ice sheet out into deep waters of the ocean. They flow outward in fjords, narrow canyons that are partly submerged. At the leading edges of glaciers, large pieces of ice frequently break off in spectacular fashion, causing "icequake" events that can be detected across the globe.
But much of the ice loss is far less dramatic, consisting of a steady melt that pours out in streams on the ice sheet's surface, but also in the form of undersea flows, a process that may be partly fed by the sudden disappearance of meltwater lakes on Greenland's surface.
Researchers have known for some time that the ice losses are getting worse. Greenland lies in a zone of the Arctic that has warmed by more than 2 or even in some regions 4 degrees Celsius (3.6 or 7.2 degrees Fahrenheit) above preindustrial levels. That warming has led to some dramatic changes, such as when the enormous Petermann glacier lost several "ice islands" larger than Manhattan in 2010 and 2012.
But precisely how much ice has Greenland lost? That's been a difficult question to answer. But it matters a great deal, since every 360 billion tons equates to a millimeter of sea level rise.
Stitching together records from satellites, gravity measurements, and other tools, Rignot and his colleagues at the University of California, Irvine, and NASA's Jet Propulsion Laboratory, along with colleagues at institutions in France, Denmark, and the Netherlands, say that they can now answer that question going back 46 years.
The researchers found that from 1972 through 1990, Greenland was more or less in balance. It lost mass as glaciers flowed out into the sea and broke off large icebergs, but it also gained it back as snow fell on top of the ice sheet.
But that changed rapidly in the last 30 years. Ice losses in the 1990s were about 41 billion tons per year, but by the 2000s they were 187 billion tons -- and by the 2010s, 286 billion tons.
Based on the results of models that simulate Greenland's past climate, "the current melting and runoff is unprecedented over the past 150 years," Marco Tedesco, a Greenland expert at Columbia University's Lamont Doherty Earth Observatory, said in an email. He said the paper "also highlights the importance of understanding and quantifying how the glaciers respond to warming and enhanced surface melting."
The melting in Greenland came from different spots. The famed Jakobshavn glacier, for instance, lost 323 billion tons during the time period, contributing nearly 1 millimeter of sea level rise on its own. The long but shallow Humboldt glacier, the fifth biggest loser, lost an estimated 152 billion tons.
And there are glaciers not losing much yet, but changing fast, including Petermann, Nioghalfjerfjorden, and Zachariae Isstrøm. These are enormous glaciers far to the northeast and northwest of Greenland that have begun to change. If they speed up further, the losses could be enormous: Nioghalfjerfjorden and Zachariae Isstrøm, for instance, each contain more than half a meter (or over 1.6 feet) of potential sea level rise.
The causes of the current changes, according to Rignot, are multiple. They include warmer temperatures melting more ice at the surface of Greenland, but also warmer Atlantic-originating ocean waters reaching the glaciers.
"This is consistent with our understanding of how climate change affects ice, only that it is happening sooner and faster than anticipated by models," Rignot said.
Rignot said that a certain amount of ice loss - and corresponding sea level rise - is likely already unavoidable, given that carbon dioxide emissions remain in the atmosphere for decades. But he said what humans do going forward certainly can impact the nature of ice in Greenland and Antarctica.
“If we do something now, it will take 30 years to affect the climate and another few decades to turn the melt down of glaciers, so probably half of that signal is already written in stone,” he said. “But the impact sea level will have on humanity increases with every 10 [centimeters] of sea level rise, and right now we are about to commit to multi-meter sea level rise in the coming century if we don’t do something drastic.”