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<em>Science</em>: Sea Level Rise Could Be Bad, But Not as Bad as Some Models Suggest

A decade-long record of nearly all of Greenland’s major glaciers suggests that Earth’s sea levels may not rise as dramatically over the next century as some studies have predicted as ice from those glaciers melts or calves off into the sea.

The new findings, reported in the 4 May issue of the journal Science, show that Greenland’s glaciers are not currently on track to raise the planet’s sea level 2 meters (6.6 feet) by 2100—the worst-case scenario of some predictive models. But if ice loss in that region continues to accelerate, researchers say that the world’s sea level could still rise up to 0.8 meters (2.6 feet) by the end of the century.

“With our observations of Greenland glaciers, we don’t see any evidence at this point that we will reach the worst-case scenario,” explained Science author Twila Moon of the University of Washington. “But, unfortunately, there is no ‘good-case’ scenario when it comes to sea level rise.”

Moon and colleagues found that different types of glaciers across Greenland display very different patterns of ice loss. Depending on where they are located, the glaciers grow and shrink at various rates.
[PHOTOGRAPH] Many fast-moving outlet glaciers around the Greenland coast are constantly calving ice into the ocean, where the melting ice affects sea level. Photograph by Ian Joughin. [Image © Science/AAAS]


Many fast-moving outlet glaciers around the Greenland coast are constantly calving ice into the ocean, where the melting ice affects sea level. | Photograph by Ian Joughin. Image © Science/AAAS

The researchers combined data from several satellites to produce velocity maps that tracked the movement and speed of more than 200 glaciers in Greenland between the years 2000 and 2010. Their record illustrates the complexity of Greenland’s ice flow and reveals how those glaciers have evolved so far in the 21st century.


After a comprehensive report by the Intergovernmental Panel on Climate Change (IPCC) in 2007 made it clear that scientists did not fully understand the dynamics of ice sheet behavior as they relate to climate change, many researchers turned their attention to the issue. And a few years later, after considering the range of ice sheet behavior that might be possible, scientists had come up with some predictions for the next 100 years.

According to those early predictions, melting of the world’s ice sheets was set to raise the global sea level by at least 0.2 meters and possibly as much as 2 full meters by the year 2100. However, this new study of Greenland’s glaciers indicates that a 2-meter rise is rather unlikely.

Moon and her colleagues do not discuss the impacts of sea level rise in their report. However, a 2007 study from the United National Environment Programme suggested that a 1-meter (a little more than 3 feet) rise in global sea level could threaten 145 million people worldwide, especially in densely populated, low-lying deltas in southeast Asia, Africa, and the Indian subcontinent.
[PHOTOG RAPH] Small outlet glacier, Greenland. Photograph by Ian Joughin. [Image © Science/AAAS]

Small outlet glacier, Greenland.

[Photograph by Ian Joughin. Image © Science/AAAS]

The authors of the new study in Science point out that various glaciers, whether they are land-, marine-, or ice shelf-terminating glaciers, contribute different amounts to sea level rise. For example, glaciers that terminate at the sea generally affect sea level more than those that dump ice off onto land.

“These glaciers act as conveyor belts that move ice from the interior of the ice sheet out to the glaciers’ far end, or terminus,” said Moon. “Some glaciers… end on the land instead of connecting to the ocean, and they might be analogous to a stream that leaves the lake but dries up before getting to the ocean. Such land-terminating glaciers are relatively slow-moving and don’t calve icebergs into the ocean.”

“In contrast, most Greenland glaciers end at the ocean, actually bringing ice all the way to the water,” she continued. “These marine-terminating glaciers calve off large icebergs into the ocean, and once that ice has been added to the ocean it contributes to sea level rise. Such marine-terminating glaciers tend to move quickly and the regions with more of them tend to be where most ice is lost from the ice sheet.”

The researchers also discuss another category of glaciers, known as ice shelf-terminating glaciers. They are similar to marine-terminating glaciers in that they haul ice all the way to the ocean, but instead of calving off right away, these glaciers extend out and create a long, floating ice shelf.

Read the abstract, “21st-Century Evolution of Greenland Outlet Glacier Velocities,” by Twila Moon and colleagues.

Read a related press release from the University of Washington.