Global Warming Effects on Sea Level Underestimated

By Cat Lazaroff

BOSTON, Massachusetts, February 19, 2002 (ENS) - Global sea levels could rise eight inches by the end of this century, more than the rise predicted last year by the Intergovernmental Panel on Climate Change. Melting glaciers and collapsing Antarctic ice sheets, such as the 58 square mile iceberg that calved from the Matusevich Glacier Tongue earlier this month, foreshadow the problems to come.

The projected sea level rise is due to a revised estimate of the ice melt from glaciers, said geological sciences emeritus professor Mark Meier of the University of Colorado. Meier presented the findings last weekend at the annual meeting of the American Association for the Advancement of Science (AAAS) in Boston.

In this 1958 photo, the South Cascade Glacier in Washington state fills the valley it has carved (Two photos courtesy U.S. Geological Survey) Meier and CU-Boulder colleague Mark Dyurgerov have collected new data showing the world's glaciers and ice caps have lost massive amounts of ice in the 20th century, with the process accelerating since 1988. That loss contributes at least 20 percent of the observed rise in sea level, said Meier. "Some glaciers around the world now are smaller than they have been in the last several thousand years," said Meier, a researcher and former director of CU-Boulder's Institute for Arctic and Alpine Research. "The rate of ice loss since 1988 has more than doubled."

Meier said last year's report by the Intergovernmental Panel on Climate Change (IPCC) might have underestimated the wastage of glaciers and ice caps around the word - excluding Greenland and Antarctica - for several reasons. The IPPC did not include increases in ice melt since the late 1980s, an apparent increase in the sensitivity of ice melt to both temperature and precipitation, and a probable increase in melting from small, cold glaciers surrounding the Greenland and Antarctic ice sheets, Meier contends.

New data from colleagues at the University of Alaska show that huge glaciers on the West Coast of Alaska and northern Canada are melting rapidly, said Meier. The melting of these large glaciers has contributed about 0.14 millimeters (0.0055 inches) per year in sea rise over the long term, jumping to more than 0.32 millimeters (0.0126 inches) per year during the last decade.

The IPCC, which estimated global ice wastage of only 0.3 millimeters (0.012 inches) per year, probably underestimated the contribution of glacier disintegration to sea level rise because little data on the large, maritime glaciers in Alaska was available, said Meier. But this region is the largest contributor to sea level rise, he said.

By 1995, the South Cascade Glacier had retreated far up the valley, leaving a new lake at the valley's base "The sensitivity of glacier melt to temperature rise depends largely on precipitation, which in some glaciered areas like southern coastal Alaska has been greatly under measured," said Meier. "The large glaciers of Alaska and adjacent Canada currently are contributing about half of the rate of global ice loss, exclusive of Greenland and Antarctic ice sheets," said Meier. "But they contain only 17 percent of the glacier ice area."

The new data suggests the IPCC calculation for the 21st century - a total of 0.16 to 0.36 feet (4.9 to 11 centimeters) - was an underestimate, said Meier. He calculated that glacier melting could contribute 0.65 feet (20 centimeters) or more to sea level this century.

The IPCC estimated that other processes such as ocean warming would cause an additional 0.36 feet to 1.4 feet (11 to 43 centimeters) of sea level rise by the year 2100, Meier said.

"These estimates in sea level rise may seem small, but a one foot rise in sea level typically will cause a retreat of shoreline of 100 feet or more, which would have substantial social and economic impacts," Meier said.

A one meter rise in sea level could cause coastlines, like this section of California coast, to retreat by 100 feet or more. Meier said that in the United States, some large coastal cities like Houston "are not much above sea level now." He noted that island nations such as Seychelles off the West Coast of Africa and Kiribati southwest of Hawaii are already within a meter (39 inches) of being inundated by sea rise. Sea rise of just one meter in Bangladesh would put one half of the nation underwater, displacing more than 100 million people, Meier warned.

Climate change is already melting ice at the Earth's poles and high latitudes, according to research presented by Meier and others at an AAAS session called "Deciphering the Complex Changes in Snow and Ice."

Ted Scambos of the University of Colorado at Boulder argued that ice shelves - the steep ice cliffs at the edge of an ice sheet - in the Antarctic may be more vulnerable to warming induced break up than previously thought.

Sea ice in the Arctic Circle is melting at a rate of 37,000 square kilometers per year (Photo courtesy SHEBA Project Office, University of Washington) He and his colleagues developed a model for how cracks push their way through an ice shelf, and applied it to the so called Larson B Ice Shelf in the Antarctic. In the model, a relatively small amount of melted water on the surface seeps into fractures in the ice, breaking up the ice shelf when it freezes. The shelf then collapses surprisingly quickly, without first having to warm all the way through, as scientists had generally assumed.

"We found that ice shelves thought to be stable are probably susceptible to breakup," Scambos said.

Since the late 1970s, ice shelves in some of the northernmost areas of the Antarctic ice sheet have exhibited the dramatic breakup style that Scambos and his colleagues attribute to this inside out disintegration process. Before that, the shelves shed their ice in a more gradual fashion, he said.

The glacier in the bottom right corner of this satellite image (marked by a dashed blue line) calved off the Matusevich Glacier Tongue in February 2002. The larger glacier to the left was first spotted in the Eastern Ross Sea and has now drifted into the Western Ross Sea (Photo courtesy National Ice Center) Earlier this month, for example, a massive new iceberg calved from the Matusevich Glacier Tongue, an extension of the Matusevich Glacier from the Antarctic mainland into the northwestern Ross Sea. The new iceberg, designated C-17, is about 11 nautical miles long and four nautical miles wide, and covers an area of about 58.24 square statute miles (150.84 square kilometers). "This area is clearly experiencing a strong regional warming," Scambos said. It is too early, however, to make a connection between this trend and greenhouse warming caused by humans, Scambos cautioned.

Melting ice from somewhere else in the Antarctic ice sheet is known to be contributing to sea level rise, but the source of the meltwater has been a mystery. Scientists are now closing in, according to Robert Bindschadler of the National Aeronautics and Space Administration's Goddard Space Flight Center.

Bindschadler and his colleagues initially thought the fast moving ice streams within the West Antarctic Ice Sheet might be the culprit.

"We studied these ice streams for a long time, but they're not providing the signal for a large increase in sea level rise. One has stopped, and another major one is decelerating," Bindschadler said.

On the other hand, a certain region making up about 20 percent of the ice sheet does appear to be retreating and thinning quite rapidly, Bindschadler and his colleagues have found. This region includes the basin that feeds the Pine Island and Thwaites Glaciers.

In this computer generated image of Greenland, blues indicate areas where the loss of ice is greatest, and yellows indicate regions that are apparently thickening. Gray areas indicate no significant change in ice thickness (Image courtesy NASA, Goddard Space Flight Center, Scientific Visualization Studio) Bindschadler noted that these glaciers may behave quite differently from the well studied ice streams, so more research is needed before scientists can say exactly what the West Antarctic ice sheet is doing now and what it will do in the future. Evidence of recent warming is not limited to the Antarctic. The northern high latitudes are showing key signs of change, according to Mark Serreze of the University of Colorado at Boulder.

Serreze cited the thinning and breakup of sea ice, the warming of water masses deep in the oceans, the diminishing of snow cover, and the thawing of permafrost in Alaska and Russia. New research by Serreze also suggests that Siberian rivers have begun discharging more freshwater into the sea.

"Together, these data create a coherent picture of high latitude change," Serreze said.

Researchers have yet to determine whether this change is the result of human activity, or whether it simply corresponds to natural atmospheric variations that occur on the order of decades, according to Serreze.

"It isn't resolved yet. To be a responsible scientist you have to be a fence sitter on this issue," he said.