POSTED 6 APRIL 2006
Some of the first studies of global warming warned that a rising sea could drown low-lying coastal communities. These pioneering scientists cited two factors in the sea rise: Water expands as it warms, and melting ice increases the mass of water in the oceans.
Certainly, the ocean is warming (which is apparently making hurricanes more intense). And it's rising. Last year, Richard Alley of Penn State reported that sea level has risen by 1 to 2 millimeters per year over the last century, with about 0.5 millimeters due to expanding water, and the rest due to added meltwater (see "Ice-Sheet and Sea-Level..." in the bibliography).
Alley also showed how CO2 and sea level have moved in tandem over the past 21,000 years. Present predictions call for a further rise of roughly 1 meter by 2100. But that number is uncertain, and Alley, for one, expects no significant change due to glacial melting through 2100.
Others are not so sanguine, and after years of hearing that ice would take centuries to melt, we see alarming signs of melting in the gargantuan glaciers in Greenland and Antarctica. Combined, these ice caps store enough water to raise sea level about 80 meters.
Sea level matters because of where we have chosen to live. Much of Bangladesh, a nation of 145 million, occupies a river delta located barely above sea level, and according to Chris Rapley, director of the British Antarctica Survey, "half of the world population lives fairly close to sea level." The gradually rising sea is adding up. In 2001, the island nation of Tuvalu began planning to evacuate in the face of rising seas.
Developed countries have more money to cope with the rising ocean, but they are also having problems. The Thames Barrage, a giant, movable dike built to protect London from stormy seas, was originally expected to be closed less than once a year. Now, Rapley said, the average year sees six closures. A flood over the dike would cost the equivalent of 2 percent of the United Kingdom's gross domestic product, he added.
Coastal floods are increasing everywhere, Rapley told the AAAS in St. Louis, a city 700 miles upriver of flood-wracked New Orleans. The birthplace of jazz is already below sea level. Due to subsiding land and rising seas, New Orleans may sink another 1 meter below sea level over the next century. A similar threat is facing Calcutta, India, and the little Chinese burg of Shanghai, which also rests on a river delta, and is subsiding by about a centimeter a year.
Ice: Breaking up is easy to do
In the renewed concern about rising seas, 2002 was crucial. Glaciologists ogled satellite photos of the sudden breakup of the Larsen B ice shelf in Antarctica, and saw the 3,250 square-kilometer hunk of ice shelf shatter into icebergs and drift away. In the following weeks, the glaciers behind Larsen B accelerated their lurching march to the ocean. With the ice shelf gone, some started moving six times faster, while ice stuck behind intact sections of Larsen B did not speed up.
This big breakup had a dramatic impact on glaciologists, and on sea level. "That increase in the delivery of ice has contributed to rise in sea level," said Rapley. "We have realized in the last five years that there is much greater sensitivity to ice shelf loss than we thought. It is much more dynamic than we assumed."
While the huge East Antarctic Ice Shelf is high, dry, and still quite cold, the smaller West Antarctic Ice Sheet is a major cause of concern because much of it rests on bedrock that is below sea level. Parts of the sheet, Rapley said, "have begun to discharge ice fast enough to make a significant contribution to sea level rise. Understanding the reason for this change is urgent in order to be able to predict how much ice may ultimately be discharged and over what timescale."
Which is a polite way of saying, for predicting how wet you're going to get on the South Shore of Long Island, say. Check out these maps of flooding in the United States due to rising seas...
Watching and waiting
One key to predicting glacier movement is understanding how water flows down to the rock beneath the glacier. Warmer temperatures cause ponds of meltwater to gather on the surface of a glacier, and when this water seeps through to the bottom, it greases the bedrock, which allows the ice to accelerate. But present estimates of glacier behavior don't account for this, Rapley said. "The models do not include the effect of liquid water on ice sheet sliding and flow, and so provide only conservative estimates of future behavior. Only five years ago, Antarctica was characterized as a slumbering giant in terms of climate change. I would argue that this is now an awakened giant and we should take notice. ... Sliding is the joker in the deck. We can't model it, we can only sit and wait."
So the cliché, "Moving at a glacial pace" deserves a whole new meaning....
One glaciologist who used to view the Antarctica glaciers as slow-moving beasts is Charles Bentley, a professor emeritus of geophysics at the University of Wisconsin-Madison. Bentley, who has studied Antarctica ice for almost 40 years, now agrees that the mammoth West Antarctic Ice Sheet has melting on its mind. In particular, he cites a new satellite measurement of the total ice mass of Antarctica, which revealed that about 150 cubic kilometers is melting each year. That's about one-third the volume of Lake Erie and enough to raise sea level 0.4 millimeters per year all by itself.
"Glacial pace" indeed...
Still, Bentley suggests that the meltdown is probably a holdover from the steady warming after the last ice age, which ended in Antarctica about 10,000 years ago. "It could be largely independent of today's global warming, because these are large dynamic systems, and global warming is a recent phenomenon," Bentley says.
So should we quit worrying about global warming and melting ice? No, since greenhouse-gas warming could start adding to the ongoing melting of the giant glaciers, Bentley says. If glaciers that are now melting due to historic warming also begin responding to the recent spike in temperatures due to global warming, then sea level could rise faster than expected.
Greenland near the tipping point?
Rapid melting is equally obvious in the planet's other large ice house -- Greenland. "There has been a measurement of rapid change all around Greenland, a rapid acceleration of ice into the ocean," Bentley says. "It seems clear that Greenland is losing mass all around the periphery." A total melt-down of the Greenland ice cap would raise sea level by six or seven meters.
You might think that glacial melting could just as easily reverse course, until you consider the problem of "tipping points." Tipping points are "a serious concern, a totally open question," says Bentley. He explains that an ice sheet has an accumulation zone where snow gathers and solidifies into ice, and an ablation zone where the ice melts. "The snow accumulates in the high, cold interior, and ice escapes through the borders of the island where it is lower and warmer." When the ice sheet melts, the center falls to lower, warmer altitudes, shrinking the accumulation zone. "In Greenland the ablation zone is around the coasts and at some considerable distance inland, and dry snow accumulates in the deep interior," Bentley says. If the accumulation zone shrinks, less ice will gather. "You can reach a point where there is not enough high elevation left to have substantial accumulation, and it will melt away."
The melting glaciers could contribute to another tipping point around Greenland. Many earth scientists think a huge influx of fresh water could change the flow of the Gulf Stream, causing a rapid freeze-off in Northern Europe.
This is more evidence that climate is a skittish creature, not the lumbering beast we've long imagined.
Tipping points are also a concern in the Amazon rainforest.
Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive