The Why Files The Why Files --

The fact of global warming

POSTED 24 FEB 2005
1928: Large river of ice snakes down mountain side. 2000: The  mountainside is exposed due to melting.Major modern melting
So much for salt water. Most of the world's fresh water is not liquid; it's ice. And the ice is melting fast. The meltdown is unmistakable in the Arctic, where the melting of sea ice, glaciers and permafrost is causing havoc with ecosystems and people alike.

Glaciers respond to a changing climate. The South Cascade Glacier in Washington's Cascade Mountains shrank dramatically between 1928 and 2000. Photo: USGS

According to Richard Alley, a glaciologist at Penn State University, the melting of mountain glaciers started speeding up around 1985. Today, about 250 cubic kilometers of mountain glaciers is liquidated each year.

The melting is not seen on every glacier, Alley admits. "Glaciers do all kinds of fun stuff, they surge and stop, if you watch one glacier for one year, all sorts of things can happen. 'The world's glaciers are a thermometer. Any way you look, any place you look, the large samples show a shrinkage of glaciers.  It's likely this is really global warming.'But if you watch all the world's glaciers, they are a thermometer. Any way you look, any place you look, the large glacier samples show a shrinkage of glaciers, and it's likely this is really global warming."

Antarctica is the big kahuna of ice, and while the massive ice sheet is not indisputably melting, there are worries about its stability. When the Larsen B ice shelf collapsed in 2002, "It fell apart in five weeks," says Alley. "It was like a car wreck, you didn't want to watch, but you couldn't take your eyes off it, either."

Almost immediately, the glaciers behind the broken shelf accelerated their march toward the ocean. "The velocity was eight times faster," says Alley. "You pull the dam out, and the ice just goes."

Ice shelves don't raise sea level when they break up and melt, because they are already floating. But when land-based ice enters the water, it does raise sea level. And rising sea level is one problem associated with global warming.

Antarctic ice is not behaving according to the science reflected in the third IPCC's assessment, Alley adds, which suggested that faster melting would be counterbalanced by greater snowfall on the Antarctic ice sheet. "The snowfall does seem to be accelerating, but the ice flow is accelerating more, " Alley says. "Now, the best estimate is that ice sheets are contributing to sea level rise, and doing so by a process that's not in the model."

Massive blocks of ice and slush float amidst dark waters
Ice eventually melts. These icebergs were near the Antarctica Peninsula. Photo: Commander Richard Behn, NOAA

Greenland: A green future?
As questions remain about melting in Antarctica, they are largely settled regarding the second-hugest hunk of ice. "Ice is thinning all around Greenland; the ice shelf at Jakobshavn is falling apart, probably because of warm water underneath it," says Alley. And, as in Antarctica, land ice is accelerating toward the ocean. Jakobshavn glacier "was possibly the fastest glacier on the planet when it was measured in 1992," he says. When it went through a speed trap in 2000, its speed had doubled.

The worst-case is this: Greenland's ice cap disappears, and sea level rises seven meters. But even with rapid global warming, that could take centuries, even a thousand years. But there are plenty of reasons to worry about melting ice:

Massive melting around Greenland could halt the global ocean conveyor, freezing Europe in winter.

Glaciers provide water for cities in South America and Asia; some cities could run dry in a decade or two.

Melting of massive snowfields starts a feedback loop: Solar absorption can double or even quadruple as fresh snow starts to melt. In other words, melting can bring more melting.

Sea level has been rising for years, encroaching on coastlines and threatening to flood low-lying nations.

The catastrophic Dec. 26 tsunami was not caused by rising sea level, notes Alley, but "it catches people's attention, Opposite trends in sea ice coverage for northern and southern points out how many of us live near the coast, and how unhappy we are when the sea comes in. It says, we should take a look, maybe we should chat about sea level."

Annual cycle in sea-ice area for three climate models (Black = actual temperatures. Colors: climate models.) Results were similar in the Southern Hemisphere. Graph: PCMDI

How bad is the situation with ice around the world? "We don't know enough to start scaring real people, but we don't know enough to reassure them, either," says Alley. "Changes are happening, and the changes are consistent with warming. There's a whole lot of science to do, but the ice is getting smaller, and that getting smaller is consistent with the warming we know about."

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Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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