Warming culprit CO₂ has a cool side — and it’s in Antarctica

In a cold corner of the world, carbon dioxide is doing something surprising. Instead of causing warming, rising CO₂ levels over central Antarctica produce a net cooling effect, new research suggests.

That discovery does not undermine the fact that accumulating greenhouse gases raise temperatures elsewhere around the world (SN: 4/4/15, p. 14), the researchers say. The effect is instead a testament to the extreme and unique conditions in Antarctica’s interior.

“We’re not saying the greenhouse effect is rubbish,” says study coauthor Justus Notholt, an atmospheric physicist at the University of Bremen in Germany. “But in Antarctica, the situation is different.”

Central Antarctica is the only place on Earth where surface temperatures are regularly colder than those in the overlying stratosphere. So instead of primarily trapping heat radiating from the ground, accruing CO₂ above central Antarctica boosts the amount of heat escaping into space from the atmosphere, the researchers report in a paper to be published in Geophysical Research Letters.

Unlike the rest of the planet, the Antarctic interior has not warmed over the last few decades; it has even shown signs of slightly cooling (SN: 7/27/13, p. 18). A chilling effect from CO₂ in the atmosphere could explain some of that lack of warming, though further research is needed, Notholt says.

CO₂ absorbs and emits heat in the form of infrared radiation. When infrared radiation emanating from Earth’s relatively warm surface hits a CO₂ molecule in the atmosphere, the molecule can absorb the energy and later reemit it as infrared radiation. Like a pinball machine, the CO₂ molecule fires the infrared energy in a random direction. Sometimes, the emitted energy continues out into space, but other times it returns to the surface, creating warming called the greenhouse effect.

Satellites monitor the amount of radiation escaping into space. Where CO₂ blocks radiation from the surface, scientists see a dip in the amount of escaping infrared radiation. Over the Antarctic Plateau in the center of the icy continent, satellites instead see an increase in the infrared radiation escaping into space within the range of frequencies associated with CO₂.

This negative greenhouse effect, Notholt and colleagues propose, results from the region’s frigid temperatures, the coldest on Earth (SN: 1/25/14, p. 15). The Antarctic Plateau sits at a high elevation at the southernmost part of the globe and is covered by sunlight-reflecting ice and snow. Temperatures on the ground can drop as low as −93.2° Celsius and are typically colder than those in the stratosphere 20 kilometers or more up.

The ground is so cold that little infrared radiation comes from the planet’s surface. But, like in other places around the world, CO₂ in the stratosphere over Antarctica soaks in heat in the atmosphere and sends infrared radiation pinballing in different directions. That siphons some heat into space that would otherwise stay near Earth. Elsewhere, this effect is normally overshadowed by the trapping of heat from the ground, but in Antarctica, so little heat comes from the ground that the loss becomes significant, causing an overall cooling effect.

The researchers don’t know where in the atmosphere that cooling is taking place. Finding that out will be important, Notholt says, because the cooling could alter wind patterns or cause other climate shifts.

In addition, just because part of the atmosphere cools doesn’t mean the mechanism cools the ground below as well, says environmental scientist Scott Rutherford of Roger Williams University in Bristol, R.I. The new work predicts that other places around the world with frigid surface temperatures, such as Greenland, should see a reduced, but still positive, greenhouse effect. Temperatures in Greenland, however, are rising much faster than the global average, Rutherford says, suggesting that the effect doesn’t significantly affect surface temperatures there.