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Small Melting Glaciers Will Speed Sea Level Rise, Study Says

Mason Inman
for National Geographic News
July 19, 2007
 
The rapid melt of small glaciers and mountain ice caps will be the main source of sea level rise over the next century, according to a new study.

The research, led by Mark Meier of the University of Colorado at Boulder, also suggests that sea levels could rise more during the 21st century than had previously been thought.

From 1996 to 2006, small melting glaciers dumped water into the oceans at growing rates year over year, the study found.

If this acceleration continues, sea levels could rise faster than some models had predicted.

Many people have heard that the two big ice sheets—on Greenland and Antarctica—are "the big players," Meier said.

"They will be, on time scales of centuries or millennia," he added. "But for the next few generations, it is the small glaciers that will be most important."

Big Impact of Small Ice

Most of Earth's ice is locked in the Greenland and Antarctic ice sheets, which are up to 2.5 miles (4 kilometers) thick.

The bulk of these ice sheets are in deep freeze.

"The ice sheets are well below the melting temperature throughout," Meier said. This means that it would take many decades of warming to put a big dent in the massive sheets.

But it's a different story for smaller patches of ice.

"Many of the glaciers are at or close to the melting point," Meier said.

(Read related story: "Mountain Glaciers Melting Faster Than Ever, Expert Says" [February 16, 2007].)

These glaciers, and also ice caps at high latitudes (closer to the Poles), are individually much smaller than the big ice sheets.

But together they currently contribute about two-thirds of the water that's raising sea levels, Meier said.

He and his colleagues surveyed studies of how these glaciers responded to warming over the past decade. They then estimated how much these glaciers might contribute to sea level rise by the end of the 21st century if these trends continue.

"We predict up to 24 centimeters [9.5 inches] of rise from small glaciers" by 2100, Meier said.

Factoring in the additional, gradual thaw of Greenland and Antarctic ice sheets, Meier and colleagues estimated that sea levels could rise as much as 22 inches (56 centimeters) by 2100 due to ice melt alone.

As the oceans warm, the water also expands, raising sea levels further, Meier added.

(See an interactive map of the effects of global warming.)

This forecast is worse than the worst-case forecast in the report issued this year by the International Panel on Climate Change (IPCC), which reflects the scientific consensus on global warming.

(Get the facts on global warming.)

That report estimated that by 2100, sea levels would rise between 8 and 19 inches (20 and 50 centimeters), of which about 3 to 7 inches (8 to 18 centimeters) would be from melting ice.

"We think that [the forecast from the] IPCC is low," Meier said.

Since ice-loss from most glaciers is accelerating, the glaciers could contribute twice as much to sea-level rise in the second half of this century as they did in the first half, he added.

The new study appears tomorrow in the journal Science.

Predictions

The researchers are "right on the mark with their discussion of tidewater glacier instability," said Richard Williams, a glacier expert with the United States Geological Survey.

"This is an important change in emphasis and in the way we traditionally understood changes in glaciers," he said.

Other researchers agree that it is important to look at these glaciers and how they respond to climate change, but they're skeptical about the estimates in the new study.

"The fact that most glacier systems on Earth are losing mass at an accelerating rate is of great interest," said Andrew Shepherd of the University of Edinburgh in the United Kingdom.

But in the new study, "the sea level projections are rudimentary at best," he added.

"There is no evidence to suggest that the recent accelerating trend will continue for a hundred years, so [it is] speculative," Shepherd said.

Kevin Trenberth, of the National Center for Atmospheric Research in Boulder, Colorado, says he's skeptical about the new study's predictions, in part because some glaciers will disappear completely over the coming century.

"They don't take that into account properly," Trenberth said. "But yes, I think sea levels will rise more than IPCC suggests."

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