At the current level of human-caused global warming, extreme rainfall from atmospheric rivers, as well as an extensive network of previously undetected micro-cracks, could degrade large parts of the Greenland Ice Sheet faster than expected and accelerate melting toward worst-case projections for ice loss and sea level rise, recent research shows.
One new study, published July 21 in Science, provides a precedent, describing a massive Greenland meltdown that happened relatively recently on the geological time scale, between about 424,000 and 374,000 years ago during a warm, interglacial period known as Marine Isotope Stage 11. That era is used as an analogue for today’s climate because temperatures in the Greenland region were about the same as now, and atmospheric carbon dioxide was about 280 parts per million, similar to the pre-fossil fuel era, but average global sea level was 20 to 40 feet higher than today.
The scientists showed that the area was ice-free at that time by measuring the luminescence signal of particles of rock and sand to determine when they were last exposed to sunlight. The deposits were at the bottom of a deep ice core drilled at Camp Century, in northwestern Greenland, more than 100 miles from the coast, just 800 miles from the North Pole and where the ice is almost a mile thick. Ice sheet modeling shows the location where the ice core was taken could have only been free of ice if other large parts of Greenland were also ice-free at the same time.
The results are “bulletproof evidence” that the region was thawed during that warm interglacial phase, suggesting that the ice sheet is vulnerable to a moderate global temperature increase if the warming is sustained long enough, said co-author Paul Bierman, a geoscientist at the University of Vermont’s Rubenstein School of the Environment and Natural Resources and a fellow in the Gund Institute for Environment.
“What struck me in the last week, as I thought about the study and talked with people about it, is that we’re recreating at least the carbon dynamics of MIS 11 by keeping the atmosphere rich in carbon and warm for a long time,” he said. “Except we’re doing it not at 280 ppm of carbon dioxide. We’re doing it 420, soon to be 450 or 500 parts per million. So that’s a really strong forcing on the climate.”
He said that’s important when you go to the “top level idea” of the many recent ice sheet studies, summarized in a 2016 Nature Climate Change paper led by Peter Clark, which is “that policy decisions in the next 100 years will determine the next 10,000 years of climate on Earth.”
The new research “suggests a warm, wet, and largely ice-free future for planet Earth,” Bierman said. Pinpointing the timing of ice-free conditions more accurately bolsters the evidence “that Greenland is more sensitive to climate change than previously understood, and at grave risk of irreversibly melting off.”
The study is a reminder that global warming is quickly driving the planet close to some “potentially critical climate tipping points,” said climate scientist Michael Mann, director of the Center for Science, Sustainability & the Media at the University of Pennsylvania.
“These findings raise additional concern that we could be coming perilously close to the threshold for collapse of the Greenland ice sheet and massive additional sea level rise of a meter or more,” said Mann, who was not involved in the research. The precise implications “remain a bit fuzzy,” he said, because the larger climate context isn’t exactly the same. The Greenland Ice Sheet may not respond to today’s human-caused warming the same way it responded to the warming during MIS 11, which was caused by changes in the seasonal patterns of solar radiation, he said.
Bierman said he approaches the question as a “field and lab geologist who measures things. I look at the past as an analogy for today and for the future,” he said. “And to me, the simple logic of this is, if the ice melted where it’s now nearly a mile thick and 130 miles from the ice margins, the ice sheet is in trouble.”
Networks of Previously Undetected Micro-Cracks
Paleoclimate studies show that Earth’s ice age cycles are slow build-ups to peak glaciation over millennia, with much more sudden phases of disintegration and collapse, when the rate of sea level rise spiked to as high as 16.4 feet per century in the most extreme cases in the paleoclimate record. Current research suggests that the warming caused by greenhouse gas emissions from a handful of highly developed countries during the last century could once again trigger similar phases of extreme sea level rise in coming centuries.
So what could melt a slab of ice that’s bigger than Alaska and up to 2 miles thick, holding enough water to raise average global sea level by 24 feet? Or perhaps more importantly, what could melt it fast?
The second recent study, published in June in Nature Geoscience, shows a possible new mechanism that could accelerate ice sheet disintegration. There are vast networks of previously undetected micro-cracks on the Greenland Ice Sheet that may run hundreds of meters deep, carrying warm surface water to the interior of the ice sheet and melting it from within, said David Chandler, a postdoctoral researcher at the Bjerknes Centre for Climate Research in Bergen, Norway, and a co-author.
That could help partly explain why, when researchers measure the interior temperatures of Greenland’s ice, it’s often warmer than models suggest it should be, and the “associated reduction in ice viscosity and increased damage accumulation act to enhance the vulnerability of ice sheets and shelves to dynamic instability” as the cracks and other drainage networks spread inland and to higher elevations, the authors wrote in the paper.
The narrow fractures described in the paper are different from the large drainage structures often featured in dramatic climate videos from Greenland, when entire meltwater lakes pour down off the surface through holes called moulins into a deep system of tunnels that can lead all the way to the base of the ice sheet, where the water, under the weight of the ice builds up so much pressure that it hydraulically lifts the ice, enabling it to slide faster.
Chandler said he first started wondering about the significance of the smaller cracks more than 10 years ago, when he was doing field research on the ice sheet, studying how water flowed through the larger drainage structures.
“I spent a long time camping on the ice sheet and in the spring, when they open, you can hear them banging and popping and banging all the time,” he said. “It’s quite noisy, actually. I was working at four different sites where this happened in West Greenland at high elevation, where the ice is about 900 meters thick. Even there, there were cracks opening and capturing streams and developing moulins, he added.
His field work was focused on the subglacial hydrology: how and where the drainage systems develop, because that can affect the inside and the base of the ice sheet. But then he started to realize the extent of the smaller cracks, which don’t end up draining out in a big rush, and that made him wonder how the micro-cracks affect the ice from a temperature, rather than a hydrological, perspective.
“How deep are they?” he said. “If they can get quite deep in the ice, then it’s interesting because of the englacial warming.”
“Turbocharged Melt Seasons”
The study documents the opening of “abundant hairline fractures associated with ice flow acceleration following the onset of seasonal melt,” he and co-author Alun Hubbard wrote. “Typically, we observed sets of multiple parallel thin fractures extending hundreds of meters perpendicular to prevailing ice flow, even across zones distant from crevasse fields and supraglacial lakes.”
The cracks are generally 1 to 2 centimeters wide and “hence remain undetectable by satellites or drone remote observations.” But because they are so ubiquitous, the study suggests they can not only heat the ice from the inside-out, but can also contribute to accumulating damage that could promote disintegration of the glaciers farther downstream, thousands of tiny stress fractures that can eventually lead to catastrophic failure.
“When we think of water, it’s kind of a mundane, innocent feature of ice,” said James Kirkham, chief science advisor and coordinator to the International Cryosphere Climate Initiative, set up to draw attention to Earth’s vanishing ice. “But it’s extremely powerful, one of the most powerful driving forces of shifting these ice sheets today, both in Greenland and Antarctica.”
The changes wrought by the effects of water on the ice sheets are most visible in Greenland right now, he said, which is “seeing melt seasons turbocharged by recent warming.”
Multiple recent studies suggest the beginning of some chain reactions that could rapidly accelerate the ice loss, he said. “That is really concerning for everyone around the world, including the U.S., given that 10 percent of the U.S. population lives quite close to the coast where they could be affected by the sea level rise.”
He said the worst-case scenario for Greenland is approaching faster than previously thought. More studies a are showing “fairly extreme” melt projections, pointing toward some of the high-end outcomes for ice mass loss and sea level rise, including potentially a foot of sea level by 2100 from Greenland, at the current level of warming.
Atmospheric River “Rapids”
Melting surface ice isn’t the only source of water on the Greenland Ice Sheet. Rainfall is playing an increasing role, and the amounts of rain now falling are “insane,” said Jason Box, a snow and ice climatologist at the Geological Survey of Denmark and Greenland and lead author of the third recent study, published in July by the Royal Meteorological Society documenting an increase of extreme rainstorms in Greenland.
Box said his study, the study by Chandler on the ice cracks and Bierman’s new research on Greenland’s meltdown in Marine Isotope Stage 11 “add insight to an all-too-long list of factors not yet encoded in ice sheet models used to project future sea level rise.” The increased meltwater delivery from rainfall is an important aspect of the hydrofracturing processes that are cracking up the ice sheet, he added.
“Greenland Ice Sheet rainfall exemplifies how climatology has been undergoing a paradigm shift,” Box said in a video outlining the study results. “Our attention is now a lot less focused on the gradually upward creeping averages. Extremes in weather are increasingly disrupting our world and dominating the climate conversation.”
His new study measured a 33 percent increase in rainfall on the Greenland Ice Sheet since 1991, and includes new readings of extreme rainfalls since gauges were installed at several climate stations in Greenland.
Box said there is already strong theoretical evidence that the frequency and intensity of extreme rainfall increases because the warmer the atmosphere is, the more moisture it can hold. The study found that, since 1991, there have been 16 times when daily rainfall somewhere on the ice sheet exceeded 300 millimeters (11.8 inches).
“The single event I detail here was ranked second for total ice sheet rainfall,” he said. “The amount of rainfall that day would run the Thames River for two years.”
The study also focused on the role atmospheric rivers, wide streams of very moist air from the subtropics, play in bringing extreme rains to Greenland. The main impact of extreme rainfall on the ice sheet is the warm air surrounding the rain, which wipes out snow cover and reveals dark ice, and leads to the darkening of the remaining snow at the higher elevations.
“The dark snow effect is from the rounding of ice crystal edges,” he said. “Elevated melting becomes sustained for days or even another week, when otherwise snow would have brightened the surface, protecting the underlying ice from melting.”
Embed video: https://youtu.be/Tua4p9ns2JY
After analyzing wind speeds and directions at different elevations, Box said he detected “atmospheric river rapids” over southern Greenland, where the moisture is squeezed out of the clouds as the south-to-north flowing winds push up over the mountains and the ice sheet.
High rainfall rates are eroding the so-called cold content of the snow, making the ice sheet more prone to melting because less heat is later required to bring the temperature up to the melting point, he said. The flooding quickly drains into the ice sheet, heating the ice internally.
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All are signs that the “warming Greenland Ice Sheet continues its approach to its tipping point threshold and viability,” he added. “The more we can reduce carbon emissions, the more we can get into carbon dioxide removal at scale, the slower this disaster will be forced upon us.”
Kirkham, the chief science advisor and coordinator to the International Cryosphere Climate Initiative, said the increase in atmospheric rivers over the Arctic is one of the factors driving increasing melting of Arctic sea ice at the end of the summer, which prevents winter sea ice from reforming.
“If you have less sea ice, you have more open water, you have less sunlight and heat being reflected back into space,” he said. “It’s all an interlinked system … and we know that there are several tipping points built into the ice sheet system. Some people even are talking now about potentially having already crossed these thresholds, which commit us to sea level rise for decades to centuries.”
He said policymakers tend to think on the timescale of the next election, but “you have to really factor in these long-term changes when planning for suitable defense systems or extreme weather impacts on your countries.”
“Some of this is already locked in now for the coming decades and centuries, and it’s only getting worse,” he said. “This is why we really are pushing to say, this is not a problem for the future. It’s a problem for today, and you need to act now to prevent the very worst from happening. But right now, it’s only accelerating and that’s quite a concerning picture for us.”