4. Warming Southern Ice

4. Warming Southern Ice

The break-up of the Larsen B Ice Shelf in Antarctica over 34 days in 2002. Courtesy NSIDC

The history of climate is seldom smooth.

Ice shelf at Larsen-B collapses, 13 March 2002. Courtesy Pedro Skvarca, Instituto Antartico Argentino

Tabular icebergs remove a great deal of ice from the Antarctic Ice Sheet. Icebergs are made during rapid ice-shelf collapse, and during the normal transfer of mass from the ice sheet to the ocean. This tabular berg, measuring about 20 meters above the water, has run aground on the sea floor near the Antarctic Peninsula. Photo: Julian Dowdeswell, courtesy Andrew Shepherd

Four views of the collapsing ice shelf, taken from space.

Given the role of ice in this scientific detective story, it's only appropriate to look at how abrupt warming might affect frozen water. Could it jack up sea level, squeezing an already crowded globe?

If a big portion of the ice squashing Greenland and Antarctica enters the ocean, sea level could rise: the West Antarctic Ice Sheet alone has enough water to raise the ocean by 5 meters.

A one-meter rise would be expected to cost Holland alone $10-billion, says Andrew Shepherd of the University of Cambridge (U.K.). The cost in Bangladesh could be measured in the lives of the millions who live on the fertile, coastal delta.

The Day After Tomorrow would seem prescient fiction, not science fiction.

In a study published last October (see "Larsen Ice Shelf..." in the bibliography), Shepherd looked at the breakup of the Larsen Ice Shelf, off the Antarctic Peninsula. (The sea-level implications of an ice shelf are tricky: Ice shelves float, and water level does not change when floating ice melts. But the breakup of an ice shelf may "uncork" the glaciers behind it. If that ice enters the ocean, sea level would rise.)

Breaking up is not hard to do
Previously, scientists had blamed the Larsen break-up on a 5-degree C rise in air temperature around the peninsula. Meltwater, they explained, had formed on the surface, seeped through the ice, and cracked the shelf.

Since air temps are not rising fast elsewhere in Antarctica, a bigger breakup seemed unlikely. But Shepherd and colleagues offered another explanation for the disintegration -- a 0.2 degree C warming of water under the ice shelf. "We tried to show that it's thinning at the base, where it rests on water, and that's more likely to have caused the disintegration," Shepherd says.

If such a slight warming of the ocean can destroy ice shelves, more could disintegrate. By itself, as we said, that would not affect sea level -- unless it uncorks land ice. Glaciologists are not sure it does, but some glaciers on the Antarctic Peninsula have recently accelerated after an ice-shelf breakup (see "Glacier Surge..." in the bibliography.)

Giant ice shelf moves away from the ice sheet, with open water at the break.

So the warming will continue forever?
No, says geologist Richard Alley. "It certainly won't warm indefinitely, the warmer it gets, the harder it gets to warm more. It's not 'end-of-life-on-Earth' stuff. We have reasonably good evidence that the dinosaurs stomped around with a lot of carbon dioxide in the air. It was really warm but they were fine -- until a meteorite hit."

Pleasant thought!

A second reason why warming will end has to do with rocks, which will eventually take up excess carbon from the air. But to time-starved humanity, a few million years of roasting might be too long.

As a geologist, Alley is used to thinking long-term. He observes that the carbon in fossil fuels came from plants that took up airborne carbon dioxide during photosynthesis. As industrialized societies continue burning fossil fuels, Alley says, "We will, in 500 years, undo what nature did in 500 million years."

Wide white iceberg sits against gloomy Antarctic sky

The Intergovernmental Panel on Climate Change (IPCC), a United Nations organization charged with anticipating global warming, has predicted several degrees C of warming over the next century. But the graph lines trend steadily upward, without the kind of blips and bounces that have marked some climatic changes in the past.

To many scientists, abrupt change means unpredictable change. "If it comes fast, unexpectedly, you're not ready for it," says Alley, "so those smooth curves are optimistic in some sense. If the IPCC is wrong, it's more likely that they are being optimistic, not pessimistic. We can be confident in climatic history that it's never smooth, it always has jumps, bumps and wiggles. The question, is how big are they?"

Bump on over to our bibliography.

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