The Greenland ice sheet contains enough water to raise sea levels by 17 to 23 feet. While this would take at least one thousand years, a new study has found that meltwater from the vulnerable ice sheet is already increasing flood risk around the world. 
The new research, published in Nature Communications, is the first to measure the water that melts off the sheet during the summer months from space.
“Here we reported that the runoff of surface meltwater from Greenland raised the global sea level by one centimetre [approximately 0.4 inches] during the past decade,” study lead author Dr. Thomas Slater, a Research Fellow in the Centre for Polar Observation and Modelling at the University of Leeds, tells Treehugger in an email. “While that sounds like a small amount[,] each centimetre of sea level rise will increase in the frequency of storm-related flooding in many of the world’s largest coastal cities and will displace around a million people around the planet.”
The Greenland ice sheet has begun to lose mass as global temperatures warm. This happens when the ice sheet loses more ice to summer meltwater and iceberg calving than it gains through snowfall in the winter. A 2018 study found the ice sheet began to lose mass in the 1980s and that this loss increased six-fold since then.
The new study adds to the understanding of this loss by being the first to use satellite data to measure the meltwater that flows off of Greenland in the summer. 
“Previously, we have had to rely on regional climate models because it’s not possible to get a complete picture of the entire ice sheet from the sparse network of ground-based measurements,” Slater explains. “While these models are very reliable, these new measurements should help improve them even further moving forwards.”
The researchers used data from the European Space Agency’s (ESA) CryoSat-2 satellite mission. What they found was that meltwater runoff had increased by 21% in the last four decades. In the last decade alone, the ice sheet sweated 3.5 trillion tonnes (approximately 3.9 trillion U.S. tons) of meltwater into the ocean, enough to swamp New York City beneath 4,500 meters (approximately 15 feet) of water. 
Further, they found the melting didn’t steadily increase year to year. Instead, it has become 60% more erratic between each summer in the last four decades. Significantly, one-third of the centimeter of sea level rise added this decade was attributed to two record-breaking melting events during heatwaves in 2012 and 2019.
This revelation is one example of how study can help researchers better model how the ice sheet will respond to climate change in the future. 
“[A]s the climate continues to warm[,] it’s reasonable to expect surface melt events similar to the summers of 2012 and 2019 will happen more often and become a major component of Greenland ice loss,” Slater says. “If we want to better predict Greenland’s sea level contribution by the end of the century, it’s vital that we understand these events and are able to capture them in our climate models.”
The reason this is all so important to understand is that what happens in Greenland does not stay in Greenland. 
“Sea level rise caused by the loss of ice on land raises the global sea level and increases the frequency of coastal flooding across the world’s largest coastal communities,” Slater says. “Coastal flooding occurs when events such as storm surges coincide with high tides; as the sea level rises the weather needed to create these conditions is less extreme, and flooding happens more often as a result.”
Protecting these cities means understanding how high water levels are expected to rise, but this isn’t simple to do
“Model estimates suggest that the Greenland ice sheet will contribute between about 3 and 23 cm to global sea level rise by 2100,” study co-author Dr. Amber Leeson, Senior Lecturer in Environmental Data Science at Lancaster University, says in a University of Leeds press release. “This prediction has a wide range, in part because of uncertainties associated with simulating complex ice melt processes, including those associated with extreme weather. These new spaceborne estimates of runoff will help us to understand these complex ice melt processes better, improve our ability to model them, and thus enable us to refine our estimates of future sea level rise.”
However, decisions made in the next decade can also influence how much of Greenland’s ice melts, and how much the world’s coastlines flood. 

“Reducing emissions can significantly limit the amount of ice lost from Greenland this century,” Slater says. “Hitting the Paris Agreement target of 1.5 degrees could reduce Greenland’s sea level contribution by up to a factor of three compared to our current trajectory.”
This will mean reducing emissions by nearly half by 2030, and will require that the world leaders who pledged to keep 1.5 alive in Glasgow earlier this month follow through with strong policies. 
“It’s still possible to achieve this but time is running out,” Slater says.

"Study Predicts More Long-Term Sea Level Rise from Greenland Ice." NASA, 2019.
Slater, Thomas, et al. "Increased Variability in Greenland Ice Sheet Runoff From Satellite Observations." Nature Communications, vol. 12, no. 1, 2021, doi:10.1038/s41467-021-26229-4
Mouginot, Jérémie, et al. "Forty-Six Years of Greenland Ice Sheet Mass Balance from 1972 to 2018." Proceedings of the National Academy of Sciences, vol. 116, no. 19, 2019, pp. 9239-9244., doi:10.1073/pnas.1904242116
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