UW study says Antarctica now feeling the heat, too

While average temperatures have been rising on all other continents, Antarctica has remained a paradox.

The Antarctic Peninsula, an arm of land that reaches toward South America, is one of the planet's most rapidly warming spots and the site of spectacular ice-sheet collapses in recent years. But it has appeared that temperatures across the bulk of the ice-covered continent were either unchanged or dropping in places.

Now, a team led by a University of Washington scientist has combined satellite data and temperature measurements to find that, on average, Antarctica has gotten a little bit warmer over the past 50 years.

"That means all of the continents are warming, not just six out of seven," said geochemist Eric Steig, who leads the UW's Quaternary Research Center.

The effect is particularly strong in West Antarctica, which is lower in elevation and more moderate in climate than the high, frigid plateau of East Antarctica. "West Antarctica is more like Seattle and East Antarctica is more like central North Dakota," Steig said.

A recent cooling trend that scientists believe is caused by a thinning of the ozone layer continues during some parts of the year in East Antarctica, where sea ice also is expanding, the researchers say. But warming elsewhere on the continent was strong enough to yield a net temperature gain.

Global warming is a likely factor, though the study published today in the journal Nature says it's impossible yet to sort greenhouse effects from natural patterns and the role of human-caused ozone depletion.

"The greenhouse effect is almost certainly contributing to these warming trends, but we can't say how much," said co-author Drew Shindell, of NASA's Goddard Institute for Space Studies.

The continentwide warming trend is modest — slightly more than one degree Fahrenheit, which is roughly on a par with worldwide trends. The scientists caution that Antarctic records only go back five decades, a much shorter period than on other continents. But the finding does raise concerns about the future of Antarctica's glaciers and ice sheets, which hold 90 percent of Earth's frozen water.

"The reason most people worry about Antarctica is its potential contribution to long-term sea level rise, and I would say this study is moderately worrisome in that respect," Shindell said.

Melting of the ice in West Antarctica alone could raise global sea level by 20 feet, but that would take centuries or even millennia, he said.

Climate studies in Antarctica are hampered by remoteness, punishing weather and the lack of data. The land mass — the size of the continental United States and Mexico combined — has only 42 permanent weather stations, most of them along the coast.

"It's a really tough job, and that's why this picture keeps changing," said Antarctic researcher David Bromwich, of Ohio State University's Byrd Polar Research Center.

Steig and his colleagues compared all the data from the weather stations with 25 years' worth of satellite measurements of surface temperatures. Then they used the relationship between the two data sets to fill in gaps in temperature trends across the continent.

"It makes a lot of sense to me," said Andrew Carleton, professor of climatology at Penn State University. "One of the main advances in this [study] is their use of the satellite data ... to show trends."

By bringing Antarctica in from the cold, the new study could undermine the small cadre of global-warming skeptics who still argue that the planet is not getting hotter, or that humans are not to blame. Many have used the apparent cooling in Antarctica to attack global climate models and point out perceived weaknesses in the scientific consensus that emissions from automobiles and factories are beginning to change global climate.

"The skeptics apparently have one less leg to stand on," Bromwich said.

But he doesn't agree completely with the new study. In a new analysis of his own, as yet unpublished, Bromwich also finds evidence of significant warming in West Antarctica. But not enough to cancel out the cooling in East Antarctica for a continentwide temperature increase.

The cooling in East Antarctica started in the late 1970s, Steig said, coinciding with a seasonal hole in the ozone layer caused by aerosol chemicals.

Lower ozone concentrations cause the atmosphere to cool, and the effects are most pronounced over East Antarctica.

Temperature gradients due to the cooling in turn strengthen westerly winds, which at least partly explain the rapid warming on the Antarctic Peninsula — though the new study suggests that global warming or other factors may have overtaken the ozone hole in shaping Antarctic climate.

The culprit chemicals were phased out years ago, and the ozone hole is expected to heal by the middle of the century.

"If that happens, all of Antarctica could begin warming on a par with the rest of the world," Steig said.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com