Inevitable: Melting Greenland ice sheet will send seas nearly a foot higher, study finds

Even if the entire world stopped burning fossil fuels today, a new study finds the Greenland ice sheet would still lose enough ice to add nearly a foot to rising sea levels.

Melting over the past century has altered the ice sheet's equilibrium, according to the study led by two glaciologists at the National Geological Survey of Denmark and Greenland. For the ice sheet to correct that imbalance, it will lose an estimated 100 trillion tons of ice, adding at least 10.8 inches to global average sea levels.

That’s “a very conservative rock-bottom minimum,” said Jason Box, a glaciology professor with Denmark's geological survey.

Greenland's contribution to sea level rise could be more than 2 feet within the century if the pace of warming continues, the authors reported in the journal "Nature Climate Change," even though the study doesn't attach specific time frames.

Many nations committed during the 2015 Paris Agreement on climate change to hold the line on emissions to rein in rising global average temperatures, but it could be decades before the world reaches net zero emissions.

What's the takeaway?

William Colgan, a senior researcher at Denmark's geological survey, said the work is a "strong cautionary tale."

"If we can stop the warming to the point where we're not flickering around the 2012-type climate indefinitely, we can save ourselves a huge amount of potential sea level rise," he said. "We can reduce the harm and reduce the sea level rise."

Either way, the world is already on the hook for at least the 274 millimeters, roughly 10.8 inches, of sea level rise from Greenland, he said: “That’s going to come out over the next century, no matter what we do.” 

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That's their best-case scenario, said David Bahr, a study co-author and glaciologist at the Institute for Arctic and Alpine Research at the University of Colorado Boulder.

"None of us who wrote this paper are going to be at all surprised when we blast right past that," he said. "It’s entirely plausible that it’s going to be twice as bad.”

It is "entirely likely" the sea level could rise an estimated 30 inches – a projection not out of line with previous forecasts, Bahr said: "That’s a very bad case scenario. We're talking about large portions of places like New York, Miami and Bangladesh disappearing." 

What does melting ice have to do with sea level rise? 

Greenland's melting ice has been a key driver of the 11-inch average sea level rise observed along U.S. coastlines over the past 100 years, according to the National Oceanic and Atmospheric Administration. That rate has accelerated during the past 40 years.

Second only to the thermal expansion of warming ocean water, Greenland melting accounts for an estimated 20% of sea level rise, said Bahr. 

Any additional melting from Greenland's ice sheet would be on top of whatever global average sea rise occurs from warmer oceans, melting glaciers and ice sheet melting in Antarctica.

The 10.8 inch estimate is a bit higher than the International Governmental Panel on Climate Change projected last year as Greenland's contribution to sea level rise by 2100. But the study's findings won't mean an immediate change to U.S. projections for sea level rise issued earlier this year because the estimates are built to capture a range of plausible sea-level rise, said Ben Hamlington, a research scientist with NASA's sea level and ice group. 

“Studies like this could change how we interpret one scenario vs. another, but wouldn’t require a revision in the scenarios themselves,“ Hamlington said.

What did researchers study?

They looked at the "snow line," the boundary between ice sheet areas exposed to melting and those that were not between 2000 and 2019. They also studied meltwater runoff, tidewater ice flow and snow accumulation. 

The study is unique, the researchers said, because they used satellite measurements and personal observations rather than computer modelling typically used to make sea level rise projections.

They also looked at two extreme years, 2012 and 2018, both influenced by fluctuating pressures over the ocean known as the North Atlantic Oscillation.

During 2012, the negative phase of the oscillation pushed in excess heat and Greenland saw the greatest ice melt of this century so far. During a positive phase in 2018, colder air moved over the island's west side, suppressing surface melt.

If warmer conditions such as those of 2012 occur more often, the study's co-authors said Greenland's contributions to sea level rise could double or even triple. 

What does equilibrium mean for the ice sheet?  

The ice sheet is mapped in two zones: the upper area where snow and ice accumulates, and the lower section, which receives less snow and includes the area that is melting.

When one area gets larger than the other, the ice sheet becomes out of balance, Colgan said. At this point, the melt zone is growing larger, and warmer temperatures are moving the line between the two zones upward, shrinking the top accumulation area.

“We have caused the ice sheet to go out of equilibrium because we’ve melted it in all the wrong ways,” Bahr said. “We’re melting it faster than the ice can move downstream and replenish areas that are melting.” 

The changes aren’t just apparent to scientists, Colgan added. Even casual visitors, such as wealthy tourists who take helicopter sightseeing tours, notice the freshly exposed rocks and retreating ice.

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