This gigantic "conveyor belt" redistributes heat between the tropics and the poles. It's one way an Antarctic melt-down could alter global climate.
  The big meltdown
SEPT 1997. What's up with Antarctic sea ice? Is it growing, shrinking, or just biding its frozen time? In winter, the combined area of floating and continental ice on the frozen continent is considerably larger than the United States. Still, the question seems pretty academic, given the remoteness of Earth's largest ice mass.

oceanconveyor


But the size of the floating ice sheet is not academic to climatologists who fear that a major melting could signal a rise in temperatures caused by global warming. That's a hot topic as governments prepare for the December conference on restricting greenhouse gases in Kyoto.

Nor was the location of sea ice academic to whalers, who seem to have hunted their mammalian prey near the edge of the sea ice.

But until satellites started spying on the ice in 1973, nobody knew how much ice actually ringed the frozen continent, nor how it changed from year to year.

minke whale
  Now it turns out that 1.5 million records of whale kills stored on a computer in Norway may substitute for direct information on the ice pack.

Minke whale, one of the last whales to be hunted at the edge of the Antarctic ice pack.   That's the word from Australian scientist William de la Mare, who studied more than 40,000 records of whaling kills in an attempt to fill in the blanks about the ice. His theory was that while ships never left the water, they hunted close to the ice, where whales found the most food. Therefore, ship positions indicated the position of the ice edge. And since the whaling records began in 1931, they would reveal the decade to decade shifts in ice.

Writing in Nature (Sept. 4, 1997, pp. 57-60, see also pp. 20-21), de la Mare, who's with the Australian Antarctica Division of the Department of Environment, Sport and Territories, found that the edge of the summer ice moved almost 3 degrees south between the mid-1950s and early 1970s.

That's a big shift -- almost 200 miles -- enough to reduce the area of the sea ice by 25 percent.

Does de la Mare's finding signals an increase in planetary temperature? Perhaps, but even though scientists have recently reported disturbing shrinkage of glacial ice in many other locations, there's no proof. Still, many observers expect that polar ice would shrink if greenhouse gases do warm the planet.

In the big cooler
Such a precipitous decline in ice area "poses a challenge to model simulations of recent climate change," de la Mare wrote, since climatologists have assumed that such changes would be much more gradual.

Want some satellite maps of changing Antarctic ice?

The shrinking Antarctic ice sheet is a hot topic; it could signal a rise in global temperaturesAny change in ice distribution could affect ocean circulation. Ocean currents redistribute heat from the tropics to the poles and are a key part of the global climate system.

By altering plant and animal growth in the southern ocean, the melting could affect atmospheric concentrations of carbon dioxide, the major greenhouse gas.

And since ice reflects more sunlight back to space than sea water, less ice could translate into a warming planet.

But some skeptics question whether the data reflects hard facts -- or convenience. Did the first whalers actually travel all the way to the ice edge? Or did they find prey in a more handy location, further north, and only move to the ice when they'd exhausted the northerly whales? If the latter is true, then the de la Mare data could mean little about the actual location of the ice.

An ominous message
In a commentary in Nature, two British scientists observed that de la Mare's work shows that climate can change more quickly than once thought possible: "There is now increasing evidence globally supporting the view that such rapid changes in the Earth's climate system can occur naturally and indeed probably have taken place in the past in the Southern Ocean."

And while there's no proof that human-caused greenhouse gases produced the Antarctic meltdown, it's further proof that climate is like mercury -- the thing looks solid, but moves quickly. That means greenhouse warming could cause quick -- and unwanted -- changes in our climate, more like lurches than the gradual shifts we've come to expect.

--David Tenenbaum

The Why Files
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