Insideclimatenews.org, Dec. 6, 2018
Melting on Greenland's ice sheet has gone into "overdrive," with meltwater runoff increasing 50 percent since the start of the industrial era and continuing to accelerate, new research shows. As more water runs off the ice sheet, it drives sea level rise, putting new pressure on coastal communities around the world.
"Once the ice sheets get kicked into motion, they just keep going. This is a wake-up call that shows how fast Greenland is changing," said Rowan University climate researcher Luke Trusel, co-author of the new study published Wednesday in the scientific journal Nature.
The scientists found that the trend of increasing surface melting across the ice sheet began in the mid-1800s as greenhouse gas emissions from the growth of industries was ramping up, and that it shot up over the past three decades. More meltwater is running off Greenland's ice sheet now than at any time in the last 350 years, and probably since long before that, going back 6,000 to 7,000 years, Trusel said.
As a result, Greenland is also adding more to sea level than at any time over those centuries, he said.
The findings support previous estimates that melting ice from all sources will raise sea level between 8 and 12 inches more by 2050, but what happens after that will still partly depend on future greenhouse gas emissions and other factors, said co-author Sarah Das, who studies ice and ancient climates at the Woods Hole Oceanographic Institution.
The researchers used ice cores—long tubes of ice drilled out of the ice sheet that capture a history of ice thickness and melting year to year—from several locations across the ice sheet. Combined with satellite measurements and climate model output, the ice cores help establish one of the most accurate estimates of how the ice sheet responds to the warming caused by greenhouse gases.
Satellites "are good at telling you when it's melting and when it's frozen," as said. The intensity of that melting can be calculated with ice cores by measuring the thickness of the seasonal ice layers. By combining the data, the scientists can accurately measure melting and runoff across the entire ice sheet.
"Right now, that runoff component is the greatest contributor to sea level rise," Das said.
"We think there's a tipping point around 1.5 Celsius of [global] warming to 2 degrees Celsius of warming that would commit the ice sheet to melting for centuries or millennia, based on paleoclimate records," Trusel said. The average global temperature is expected to start hitting that benchmark between 2030 and 2052, according to a recent IPCC report, and the Arctic is warming at twice the average global rate.
Is Greenland headed toward a tipping point?
There are several factors that affect how fast the region reaches that point, said co-author Xavier Fettweiss, a climate scientist at the University of Liege Climate Lab in Belgium.
Since about 2000, weather patterns that bring warm air to Greenland have become more common, said Fettweiss, who has done related research on the patterns, known as blocking highs.
The increased melting is a "combination of a global warming signal and a change in the atmospheric circulation over Greenland," he said. If the warm weather pattern continues to become more common, it could amplify the surface melting and runoff documented in the current study even more, potentially even doubling the reported estimates, he said.
University of Lincoln (UK) climate scientist Edward Hanna said the research shows the ice sheet is so sensitive to warming temperatures that sea level rise estimates may need to be revised upward again.
"We can't rule out that the IPCC (Intergovernmental Panel on Climate Change) sea level rise scenarios are too conservative," said Hanna, who was not directly involved in the study. "Greenland is a bit like a sleeping giant that is awakening. Who knows how it will respond to a couple of more degrees of warming? It could lose a lot of mass very quickly," he said.
The new research helps place satellite measurements, which only started in 1979, into a longer-term context. It shows how the meltdown started accelerating about the same time greenhouse gases from fossil fuel burning started building up in the atmosphere, said Nerilie Abram, a paleoclimate researcher at Australian National University who was not involved in the study.
"From satellites, we see a lot of years when there is a lot of melt and other years with less melt, and an overall upward trend, but it's important to be able to test that against what happened in the past, before satellites were watching," she said. "It shows the increase in the melting of the ice sheet during summer is really unusual."
The precise data will also potentially help identify climate tipping points more closely, she said. "We think about climate change as gradual melting over time, which gives us time to get ready, but not all parts of the climate system change that way," she said.
At some as-yet unknown temperature threshold, there's a chance that ice sheets in Greenland and Antarctica could start collapsing much faster than expected and swamp highly populated coastal areas with sea level rise before they have a chance to get ready. Climate feedbacks like increasing soot from wildfires or increased algal growth on the ice that drive more melting could hasten the meltdown.
Some Melting Is Already Locked In
If all of Greenland's ice were to melt, it would raise sea level by about 20 feet. The latest IPCC report estimates that global sea level will rise somewhere between 10 and 30 inches by 2100 if global temperatures warm by 1.5 degrees Celsius above pre-industrial times. With 2 degrees Celsius warming, sea level would rise an additional 4 inches, putting 10 million more people at risk of flooding, the report says.
Beyond 2100, recent research suggests that at least 30 feet of sea level rise have already been locked into the system, said Greenland ice researcher Jason Box, who was not involved in the new study.
"The point and benefit of mitigation (carbon drawdown) is it will buy us time to prepare for that sea level rise and save lives," Box said.
In a related study published online Nov. 27 in the journal Polar Science, Box and other researchers highlighted the widespread impacts of Arctic global warming, showing that, even with big cuts to emissions, the Arctic is expected to warm another 4 degrees Celsius from today by 2050.
That warming will also affect lower latitudes by thawing permafrost, which releases more greenhouse gas emissions, causing further warming, and by shifting ocean currents and storm tracks, all with "large ecological and social impacts," Box and his co-authors wrote.