The greenhouse gases that are warming the globe actually cool Antarctica much of the year, a new study confirms. The odd trend doesn’t break the laws of physics, but it does highlight what a strange place Earth’s southernmost continent truly is.
Antarctica is home to many extremes. It’s the world’s highest continent, with an average elevation just a shade under 2300 meters. And despite its ice, it’s technically a desert thanks to a paucity of precipitation. This lack of moisture is one of the key factors behind the region’s “negative greenhouse effect,” says Sergio Sejas, an atmospheric scientist at NASA’s Langley Research Center in Hampton, Virginia, who led a newly published investigation of this atmospheric quirk.
Cool greenhouse gases high in Earth’s atmosphere typically trap heat by absorbing infrared radiation emitted by our planet’s warm surface before it reaches space. Thanks to its release by many human activities, carbon dioxide (CO2) is one of the most notorious of these planet-warming gases, but water vapor is a strong greenhouse gas, too. It is abundant in the atmosphere, giving it a much stronger overall warming effect. And when water vapor is scarce, as it is above central Antarctica, the continent’s greenhouse effect goes topsy-turvy, Sejas says. Add to it another weather phenomenon called a temperature inversion, where the atmosphere warms as altitude increases, rather than growing colder, and things truly start to go awry.
“Antarctica is the only place in the world where the surface is colder than the stratosphere,” says Justus Notholt, an atmospheric physicist at the University of Bremen in Germany. The continent’s surface temperatures are typically 20°C colder than the temperature a few hundred meters up in the atmosphere, he explains.
The persistent temperature inversion causes high-altitude greenhouse gases to actually emit more heat to space than they trap, Sejas says. Recent studies identified this negative greenhouse gas effect over Antarctica, but those analyses typically looked at the effect only in terms of CO2, Sejas notes. So he and his colleagues analyzed how water vapor might contribute to the cooling effect.
In general, CO2 is thoroughly mixed throughout the atmosphere. Because of this, Sejas and his colleagues found, some of the heat radiated toward space by CO2 at low altitude in Antarctica still ends up getting trapped by the gas at higher altitude. But that’s not true for water vapor, the researchers report this month in npj Climate and Atmospheric Science. Although there’s very little water vapor over Antarctica at low altitude, there’s even less in the overlying stratosphere, Sejas says. Any heat radiated toward space by low-altitude water vapor keeps on going, as if the continent’s heat-trapping comforter had been ripped away in the middle of the night. This negative greenhouse effect is in effect about 9 months out of the year, the team reports.
“I like how the team’s analysis breaks down the effect” between CO2 and water vapor, says Karen Smith, an atmospheric scientist at the University of Toronto in Canada who is unaffiliated with the study. That approach clearly highlights what makes Antarctica unique, she adds.
Unfortunately, adding more greenhouse gases to the atmosphere won’t strengthen Antarctica’s negative greenhouse effect, Sejas says. A warming world will, in fact, boost the amount of water vapor in the stratosphere globally, eventually rendering the continent as susceptible to greenhouse warming as the rest of the planet.