A new assessment that gauges the views of leading ice sheet experts finds that scientists think global sea level rise during this century may be far more significant than previously thought, possibly eclipsing 1 meter, or 3.3 feet, by 2100. Such a sharp rise in sea level would inundate heavily populated coastal areas around the world, potentially forcing the relocation of 187 million people, the study said.
The new report adds to a growing body of evidence showing that the globe’s ice sheets in Greenland and Antarctica, which were once thought to have a slow and delayed response to climate change, are melting more rapidly, and are becoming more vulnerable to reaching tipping points from which they may not recover.
Recent summer air temperatures for Greenland ice are the highest in at least 172 years, according to data from polar researcher Jason Box. This graphic shows surface temperature departures from the 1951-1980 average during the period from 1840-2011.
Click to enlarge the image. Credit: Meltfactor.org
The Greenland and Antarctic ice sheets have enough water locked away in their frozen depths to raise global sea level by 65 meters, or 213 feet, if they were to melt completely, which is not likely to happen anytime soon. Greenland alone holds 680,000 cubic miles of ice.
Warmer air temperatures and ocean waters are causing increased melting of both the Greenland ice sheet and the West Antarctic ice sheet. A recent study found that temperatures in West Antarctica have increased by 4.3°F over the past 50 years or so, which is far more than scientists have thought, and nearly as much as the 5°F rise on the nearby Antarctic Peninsula, the fastest-warming region on Earth.
This year a rare melt event occurred in Greenland, when 97 percent of the Greenland ice sheet — including the normally frigid high-elevation areas — experienced some degree of melting. That was the first time that melting was documented at the highest elevations of the ice sheet.
Unlike other studies of how the giant ice sheets of Greenland and Antarctica are responding to global warming, this one does not directly rely on computer models or satellite data. Instead, authors Jonathan Bamber and Willy Aspinall from the University of Bristol in the U.K., used a method known as “expert elicitation,” in which they sought the views of about two dozen ice sheet researchers via a detailed survey, and received responses from a smaller subset of this group. The survey questions focused on the respondents’ understanding of ice-sheet processes, the factors that are driving climate change in Greenland and Antarctica, and how the ice sheets are likely to behave over the short to long-term.
The researchers then used an approach that mathematically pools and weighs experts’ opinions, and used this to arrive at a “virtual expert” opinion, Bamber said in an interview.
Bamber said that the study’s methods provide “a kind of snapshot of the state of the art at this time,” rather than a specific forecast of sea level rise. The study found that the ice sheets are likely to contribute a median estimate of 29 centimeters (11.4 inches) of sea level rise by 2100, with a 5 percent chance that it could exceed 84 centimeters (33 inches). When combined with other contributors to sea level rise, such as melting mountain glaciers and thermal expansion of seawater, this implies a “conceivable risk” that sea levels could rise more than 1 meter by 2100.
To put that in perspective, the 8 inches or so of sea level rise the world has experienced since 1900 is already enough to have boosted the power of storm surges and exposed millions of Americans to the danger of coastal flooding. The 3 feet of additional rise expected by 2100 will make all of these problems vastly worse.
Sea level rise is already aggravating coastal flooding from extreme weather events, such as Hurricane Sandy, which flooded the Hoboken PATH transit station.
Credit: Port Authority of New York and New Jersey.
The experts who contributed to this study agreed that the sea level rise estimates contained in the 2007 Fourth Assessment Report from the U.N. Intergovernmental Panel on Climate Change (IPCC) were too low, and Bamber said he expects the next IPCC report — to be released beginning later this year — will contain a higher estimate of sea level rise during the rest of this century.
The study found there is very little agreement among ice sheet experts on what is causing the ice sheets to melt at an accelerating rate since reliable continental-scale observations of ice sheet mass began in the early 1990s. This uncertainty is complicating scientists’ efforts to produce reliable sea level rise projections. “This is a key question and knowing the answer to this question very much colors what we think the ice sheets are going to do in the future,” Bamber said.
The study frames the uncertainty as being similar to the difference between long-term climate, which exhibits overall trends, and short-term weather, which is far more variable. “In broad terms, are we observing ice-sheet ‘weather’ or ice-sheet ‘climate’ variations, or both?” the study asked.
Eric Rignot, a professor of earth system science at the University of California at Irvine who has been observing ice sheet and glacier dynamics for more than two decades, said it’s clear that what is going on in Greenland is the result of manmade climate change, not natural variability. He said the expert consensus as expressed in the study is far too conservative compared to the rate and scale of the ice sheet changes that are taking place.
“It is certainly VERY disappointing to hear that the majority of experts are not sure whether the changes are human-driven or the result of internal variability,” Rignot said in an email conversation.
“My work in the last 25 years has focused on observations, and what these are telling me is that the changes taking place in the polar region are very significant, rapid and large; and of course these changes are attributable to human-induced climate change. Most experts are afraid to think that the system is able to evolve so quickly, and feel more comfortable to sit on the cautious side. While in general this may be a wise behavior, the impact of this resolve to prudence is enormously negative and potentially devastating,” Rignot said.
Rignot was one of the experts consulted in the study, but he did not participate in the post-survey analysis.
Despite the lack of consensus revealed in the study, Bamber said a key message of the research is that ice sheets are likely to melt more, and thereby raise sea levels by a greater amount than previous studies had estimated.
“One thing we are certain about for Greenland and, I think, West Antarctica as well is that in general if you warm the planet, ice melts,” Bamber said. “And we have seen accelerated mass loss in Greenland over the past two decades, which is undoubtedly due to warming that has taken place there at the same time.”
Climate scientists know that the ice sheets can respond in nonlinear, unstable ways, and that once they are pushed past a particular threshold, there may no stopping them. “If we push them into a state where they’re really losing [a lot more] mass,” Bamber said, “it’s going to be very difficult to turn them around again.”
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