Melting Ice, Rising Seas

The rising temperatures of global climate change are melting the world’s ice. Most notable are the shrinking ice sheets of Greenland and west Antarctica, which have shown dramatic loss in recent years. Travel to the glaciers of Greenland and to fossilized coral reefs of the Florida Keys, where earth scientists are studying geologic records of past warming to predict future ice loss and associated sea level rise.

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The Fate of a Frozen Land

Bob Hawley faces the steep terminus of the Russell glacier, which is exiting the southwestern edge of the Greenland Ice Sheet. It's not big, as glaciers go, so it's not the most dramatic yardstick of what's occurring in Greenland right now. But the face of this small, slow-flowing river of ice is changing, visibly, constantly. "This is the sort of place you don't want to stand very close to," says Hawley, a glaciologist with the University of Washington and the University of Cambridge's Scott Polar Research Institute. "A piece might fall off."

As the glacier emerges from the constraints of the continental ice sheet, it is free to disintegrate. In the warm month of August, chunks of glacier fall, or calve, from Russell once or twice an hour. They collapse like mini-avalanches and splash into the ice-strewn water that flows readily from the glacier's base. These chunks will join the ocean, adding to its volume.

Greenland and the western and eastern sides of Antarctica are Earth's three ice sheets, where most of our world's freshwater resides. Both Greenland and western Antarctica are rapidly losing volume. In the last 50 years, humans have warmed the Arctic by over 2 degrees Celsius. This is hastening a number of melt mechanisms that affect the world's frozen water, including the speed at which Greenland's glaciers flow and crumble. A sobering example is the flow rate at the massive Jakobshaven glacier, which increased 95 percent from 1996 to 2005.

Were the Greenland Ice Sheet to disappear completely, global sea level would rise by about 7 meters (23 feet). "This doesn't seem like much," says Hawley, "until you look at places like the Mississippi Delta, New Orleans, the Netherlands, or the Philippines." Such a sea-level rise would drive out the great many people who live in these low-lying coastal areas. Could Greenland really unfreeze? And what might happen if it does? Observations of current conditions, paleoclimatic research, and computer models are providing some answers.

More and More Melt

Greenland's climate is particularly sensitive to global warming. As temperatures increase and the surface of snow and ice gives way to land and ocean, Greenland's reflectivity, or albedo, decreases. Lower albedo means more sunlight is absorbed rather than reflected back into space. This melts the remaining ice even faster, decreasing the elevation of the ice sheet. Because lower elevations are warmer than higher elevations, this thinning exposes the ice sheet's surface to even higher temperatures.

Most computer models that gauge Greenland's melt (and make predictions of when all its ice could disappear) account for albedo and ice-sheet thinning. But they don't account for the recent rapid ice loss from glacial calving, despite the fact that glaciers are moving faster than ever before. As ice sheets melt, the water seeps through cracks and crevasses to the bedrock, then eventually flows to the ocean. This meltwater percolating downward might be contributing to faster glaciers. "It acts as a lubricant to their flow," says Hawley.

The new projection of the 2007 Intergovernmental Panel on Climate Change (IPCC) is one of those that fails to account for increased glacial flow. The IPCC predicts that the Greenland Ice Sheet could disappear in "millennia" given the current rate of warming. But to Greenland-focused climatologists such as the University of Arizona's Jonathan Overpeck, the outlook is much bleaker. "There are a couple of answers to the question of what might happen in the future," he says. "One is the conservative answer--that we'll continue to get sea-level-rise acceleration, primarily due to expansion of the ocean as it continues to warm, and sometime after the turn of the century, we'll start to get a larger and larger contribution from the big ice sheets. Our work indicates that the future change could be a little more quick and large-scale. And that's because we actually looked at the last time this may have happened."

Submerged Before

The last time sea level was higher than the present day--by up to 6 meters (20 feet)was 115,000 to 129,000 years ago. This period is called the Last Interglacial, and the legacy of its sea levels is visible in the geological record. One of the best places to see it is an abandoned limestone quarry in southern Florida's Windley Key State Park. Much of the limestone here was formed from a coral reef community that lived during the Last Interglacial. The corals' calcium carbonate structures are now fossilized as rock, the traces of boulder-star and brain corals frozen in time in their original growth positions.

Boulder-star and brain coral are still among the most common reef species in the Caribbean and western Atlantic. Scientists observe that brain corals grow only in water about two or three meters deeper than they are high. The top of the quarry – where the coral heads stop – is about 5 meters high. Therefore, sea level at Windley Key was perhaps 6 to 8 meters higher than its present level some 130,000 years ago.

Models for the Future

The Windley Key corals show a sea-level rise of about 7 meters in the Last Interglacial. That's how much oceans would rise if the Greenland Ice Sheet melted completely. Coincidence? Not according to Overpeck. His research and that of a number of his colleagues indicate that Greenland was a major contributor to high sea levels at that time, and that the Antarctic may have also been an important contributor.

Higher concentrations of carbon dioxide in the atmosphere are causing much of Earth's current global warming. Yet during the Last Interglacial, carbon dioxide levels in the atmosphere were not elevated much above pre-industrial levels. Instead, the Earth's northern hemisphere tilted more toward the Sun in summer than it does now. Although some of the Western Antarctic Ice Sheet melted, most of the warming was over the Northern Hemisphere. So more – in fact much more – of Greenland's ice sheet melted. To find out how much this tilt drove up temperatures over the Arctic, Overpeck and his colleagues ran a state-of-the-art computer climate system model from the National Center for Atmospheric Research. "We found that you don't need a lot of warming to get large-scale Greenland and Antarctic melting," says Overpeck. The Last Interglacial, it seems, was only 3 to 5 degrees Celsius above present-day temperatures in the Arctic, and much less on a global scale.

"With our current 2 degrees of Arctic warming, we're already about halfway to 5 degrees Celsius," says Overpeck. He predicts that sometime later in this century – not in millennia – we'll cross the threshold beyond which the bulk of the Greenland Ice Sheet could melt. Although climate scientists understand that such predictions are extremely uncertain, a 7-meter rise in sea level would change the face of our planet, flooding vast stretches of what are now shallow coastal areas. The entire southern third of Florida including Windley Key's ancient coral reefswould again disappear into the blue. Just a 1-meter rise could put whole nations with unique cultures, such as Tuvalu and the Maldives, under water.

"The good news is that humans have a lot of control in how much more warming there will be," says Overpeck hopefully. "Our whole purpose of digging into what warmth is necessary to melt ice sheets is to give policy makers and citizens the chance to make a choice."