Combo of chilly surface temps and accumulating gas creates cooling in the atmosphere.
At the bottom of the world, carbon dioxide is doing something surprising. Rising levels of this gas normally cause warming. But over central Antarctica, they produce a cooling. That’s the finding of a new study.
This discovery does not undermine climate science, the researchers note. Rising levels of greenhouse gases, such as carbon dioxide, do raise temperatures elsewhere. The effect is instead a sign of how extreme and unique the conditions are in Antarctica.
“We’re not saying the greenhouse effect is rubbish,” says Justus Notholt. He is an atmospheric physicist at the University of Bremen in Germany. It’s just that “in Antarctica, the situation is different.”
Earth is warm enough to support life because of the greenhouse effect. Greenhouse gases prevent some of the sun’s energy from escaping back into space. But human activities, such as burning fossil fuels, have increased the amount of these gases in the atmosphere. As a result, temperatures have risen most everywhere. Except central Antarctica.
This is the only place on Earth where surface temperatures are regularly colder than those some 8 to 50 kilometers (5 to 31 miles) above the surface. This second layer of the planet’s atmosphere is known as the stratosphere. And over central Antarctica, excess carbon dioxide actually boosts the amount of heat escaping into space. This is the opposite of what happens everywhere else.
The surprising finding will appear in Geophysical Research Letters.
Antarctica is unique
Unlike the rest of Earth, the Antarctic interior has not warmed over the last few decades. It has even shown signs of cooling slightly. A chilling effect from carbon dioxide in the atmosphere might partly explain this. More research would be needed to prove that, though, Notholt says.
Carbon dioxide absorbs and emits heat energy as infrared radiation. A form of light, it’s outside human vision. This light emerges from Earth’s relatively warm surface. When it hits a molecule of carbon dioxide in the air, the molecule can absorb the radiation’s energy. The molecule later reemits that energy as infrared light.
Like a pinball machine, the carbon dioxide molecule fires off that energy in random directions. Sometimes the energy continues out to space. Other times it returns to the surface. And when it does that, it creates what’s known as a greenhouse warming.
Where carbon dioxide blocks some of this radiation from the surface, satellites see it as a dip in the amount of energy — heat — escaping into space. So the ground level warms. But over central Antarctica, satellites see an increase in heat escaping into space.
Notholt and colleagues now call this the “negative” greenhouse effect.
And they propose that it’s due to the region’s super-frigid temps. This is the coldest place on Earth. Surface temperatures can fall to −93.2° Celsius (−136° Fahrenheit). That’s typically colder than the stratosphere.
The surface is so cold that little infrared radiation leaves. That is true even though the Antarctic Plateau is covered by ice and snow. Those surfaces reflect sunlight. But, as in other places around the world, carbon dioxide in the stratosphere over Antarctica soaks in heat. And that sends infrared radiation pinballing in different directions. That siphons some heat into space that might otherwise remain 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. What results is an overall cooling.
Just because part of the atmosphere cools doesn’t mean the mechanism cools the ground below too, says Scott Rutherford. He is an environmental scientist at Roger Williams University in Bristol, R.I.
The new work does predict that other places with frigid surfaces, 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 notes. That suggests that the effect doesn’t significantly affect surface temperatures there.
Source | student.societyforscience.org