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Global Warming Is Rapidly Raising Sea Levels, Studies Warn

John Roach
for National Geographic News
March 23, 2006
 
Water from melting ice sheets and glaciers is gushing into the world's
oceans much faster than previously thought possible, sending scientists
scrambling to explain why.

The unexpected deluge is raising global sea levels, which scientists say could eventually submerge island nations, flood cities, and expose millions of coastal residents to destructive storm surges.

By the end of this century the seas may be three feet (one meter) higher than they are today, according to a pair of studies that appear in tomorrow's issue of the journal Science.

"After that we'll be committed to multiple more meters of sea level rise that will occur at rates of up to a meter—or three feet—per one hundred years," said Jonathan Overpeck, an earth scientist at the University of Arizona in Tucson, who co-authored the studies.

"And it could go faster," he added.

But scientists don't know if it will. They believe global warming triggered the ice's seaward gallop, but they say the dynamics at play are poorly understood.

"We did not expect that the ice sheets can react to warming on such a short time scale," said Konrad Steffen, a geographer at the University of Colorado at Boulder who has spent the past 15 years monitoring ice sheets in Greenland (map).

Scientists thought ice sheets and glaciers would respond to warming slowly over hundreds of years. The current acceleration could be a short-term adjustment to the warmer temperatures, Steffen said.

"Something dramatic is happening," said Göran Ekström, a seismologist at Harvard University in Cambridge, Massachusetts.

Ekström and colleagues report tomorrow in Science that glacial earthquakes—seaward lurches of glaciers—in Greenland have more than doubled in number since 2002.

Most of the glacial earthquakes occur in July and August, at the height of the Northern Hemisphere's summer melt.

The finding complements a study published in Science last month that found some of Greenland's glaciers have doubled in speed over the past five years, said Jay Zwally, a glaciologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

Zwally added that both findings are "alarming" given that Earth has only experienced the full effects of greenhouse gases for about a decade.

"As these changes take place, we're still in the process of learning what happens to the ice. We are discovering new things," he said.

Ancient Rise

About 130,000 years ago, global sea levels were 13 to 20 feet (4 to 6 meters) higher than they are today. Scientists have determined this by studying ancient coral reefs that now sit high and dry, and other so-called paleo-climate clues.

The University of Arizona's Overpeck and his colleagues wanted to understand what sort of climate conditions were necessary to create such high sea levels.

Scientists believe that Earth's orbit had shifted slightly at the time, giving the Northern Hemisphere greater exposure to the sun.

When Overpeck's team plugged those orbital conditions into a computer model, they found the Arctic warmed 5º to 8ºF (3º to 5ºC), sufficient to melt enough Arctic ice to explain the sea level rise.

But the researchers also know how much the Greenland ice sheet, which holds most of the Arctic water, melted at the time. When they plug that data into the model, the melt only accounts for 7.2 to 11.2 feet (2.2 to 3.4 meters) of the water rise.

"That means we got a substantial amount [of water] from Antarctica," Overpeck said. "And that is a big discovery."

The finding suggests that, even though the Antarctic itself did not warm, the ocean warming and sea level rise in the Arctic were sufficient to drive melting in Antarctica.

When the team used the same climate model to predict what will happen over the next 140 years from increasing greenhouse gases, they found that by 2100 the Arctic will be at least as warm as it was 130,000 years ago.

But unlike 130,000 years ago, today's atmospheric warming is global.

"So it will be even more conducive to melting parts of the Antarctic ice sheet in the future than it was 130,000 years ago," Overpeck said.

"And indeed that jibes nicely with a lot of observations coming in that suggest parts of the Antarctic ice sheet are already melting."

Irreversible Decline?

According to Zwally, the NASA glaciologist, humans can limit the effects of global warming by acting now to reduce emissions of greenhouse gases.

But if we continue to pollute at the current pace, he says, by the end of the century the Greenland ice sheet and part of Antarctica could be undergoing irreversible decline.

"Man is doing an experiment with the ice sheets, which is a scientifically interesting experiment, except it is going to have some serious consequences," he said.

"And the longer we wait to do something about climate warming, the more serious it's going to be."

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