Wind Theory May Clear Up Warming Mysteries

It is one of the biggest climate mysteries today: If the Earth is getting warmer, why are vast parts of Antarctica getting colder?

For the last decade, the behavior of Antarctica has become increasingly inexplicable. And the inability of scientists to figure out what they were seeing has been one of the most troubling gaps in explanations for why the Earth seems to be warming--one repeatedly pointed to by global warming skeptics.

Now a team of researchers thinks it has an answer to the Antarctic puzzle. The explanation involves the effect of wind patterns far above the Earth’s surface as well as the hole in the atmosphere’s ozone layer that opens over Antarctica for several months each year.

If correct, the explanation would close a big gap in theories about global warming. But the explanation also suggests that up to half the warming may be caused by changes in the wind patterns of the stratosphere, implying that human activity, while a major factor, is not the direct cause of as much of the warming as many experts and activists have thought.

The mystery of Antarctica is that one part of the continent has warmed dramatically even as the rest of the continent has gotten colder.

Along the slender Antarctic peninsula, which juts toward South America, ice shelves the size of Delaware and Connecticut have thinned, collapsed and broken off, disintegrating into bergs.

But in Antarctica’s frozen interior, the temperature has dropped and glaciers have grown. Instead of retreating, sea ice on the eastern side of the continent has thickened.

“There’s been lots of thinking on what is causing the warming, but the cooling has remained a mystery,” said David W.J. Thompson, an atmospheric scientist at Colorado State University.

Thompson studies the swirling patterns of wind in the stratosphere, the high, thin part of the planet’s atmosphere six miles overhead.

For decades, scientists thought nothing much happened in this wispy realm to directly affect the Earth’s weather, that the winds there merely responded to atmospheric turbulence closer to the Earth’s surface.

Work by a number of atmospheric scientists in recent years, though, suggests that the tail may wag the dog: The lightweight stratosphere doesn’t merely respond to the thicker lower atmosphere; instead, it shapes the weather far below.

Last year, Thompson and John M. Wallace of the University of Washington suggested in a paper in the journal Science that a stratospheric pattern that swirls an enormous doughnut of wind above southern Alaska and north-central Europe plays a central role in determining the climate throughout the Northern Hemisphere.

A weather pattern that is associated with those high-altitude winds is called the Northern Annular Mode, and it seesaws between two phases.

In one phase, air pressures are high over the polar cap and high-altitude winds are weak.

When this pattern is in effect, cold Arctic air masses flow downward and cool more temperate parts of North America and Europe.

In the other phase, high-altitude winds stay strong, holding cold northern air in and steering ocean storms north.

This means that North America and Europe stay much warmer than normal.

For the last few decades, the Northern Annular Mode has largely stayed locked in the second phase. This has allowed warmer weather to sweep out over the planet’s northern reaches.

That helps explain why parts of the Arctic have seen such dramatic warming in this time period and why winters throughout North America and Eurasia have generally been less bitter.

The atmospheric pattern could be responsible for about a third of the warming throughout the Northern Hemisphere in the last three decades.

That warming is so dramatic that many scientists think that year-round sea ice in the Arctic could disappear within 50 years.

“Any perception that winters have been less wintry is as much due to changes in atmospheric circulation than to increases in global mean temperature,” Thompson said.

“The big question is, was this also happening in the South?” said Thompson. He and Susan Solomon, an expert on Antarctic ozone depletion with the National Oceanic and Atmospheric Administration, decided to take a look. They paid particular attention to a major disruption in the southern stratosphere--the ozone hole.

The analysis was more difficult for the Southern Hemisphere: Temperature records in the uninhabited region are rare and sparse. The team, Wallace said, “really made use of just about all the data there is.”

As in the Northern Hemisphere, the winds high in the stratosphere over Antarctica also are organized in predictable patterns and have an associated weather pattern called the Southern Annular Mode. It, like its northern counterpart, has two variations.

For a great deal of the last few decades, the Southern Annular Mode has followed a pattern in which winds are strong and cold air remains trapped in polar regions. That pattern helps explain the Antarctic puzzle: Temperatures at the South Pole and in Antarctica’s interior remain colder than they otherwise would be while temperatures at the edge of the continent rise.

The ozone hole, which forms high over Antarctica in December, plays an additional role. Without the insulating layer of ozone, the stratosphere gets very cold and polar stratospheric winds tend to speed up.

Thompson and Solomon found that in the months after the ozone hole appeared, the high-altitude winds were strongest and the weather in those parts of Antarctica known to be cooling were coldest.

Not all scientists are convinced that the stratosphere--an atmospheric lightweight--plays such a powerful role or that the new study proves that the changes in the stratosphere cause changes at the surface and not vice versa.

“The evidence is there that there is indeed an effect. There’s just some question over what the magnitude is,” said James Hurrell, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colo., whose studies focus on the role that the oceans play in controlling global climate.

While the new findings explain most of the cooling seen in parts of Antarctica, the atmospheric circulation can account for only about half of the warming seen elsewhere on the continent, suggesting that other factors, including human-induced global warming, may be involved.

Humans’ use of chemicals has ripped apart the protective ozone layer. In addition, greenhouse gases may help create the stratospheric conditions that Thompson and Solomon described.

The same gases that warm the surface of the planet can make the stratosphere cooler by reflecting sunlight away from the Earth. Cooling the stratosphere may lead to stronger wind patterns and change the way that the Northern and Southern Annular Modes work. If that is the case, greenhouse gases may hijack these natural patterns and intensify them. The new findings on the planet’s southern edge add some credence to that idea.

“We’re seeing similar trends in both hemispheres, which adds weight to the argument we’re seeing changes that are truly global, fundamental changes in the structure of the atmosphere that could be human-induced,” Wallace of the University of Washington said.

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