A "negative greenhouse effect" means rising concentrations of CO2 and methane have slightly cooled parts of Antarctica’s upper atmosphere, but that could change as the air becomes more humid
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| Above some parts of Antarctica, their is curious negative greenhouse effect (Credit: imageBROKER.com GmbH & amp / Alamy) |
Rising concentrations of methane and carbon dioxide have led to a “negative greenhouse effect” above parts of Antarctica, but this slight cooling effect could reverse as the air becomes more humid alongside rising temperatures.
Greenhouse gases like CO2 and methane heat the planet by trapping radiation from its surface. However, in certain rare circumstances they can have a cooling effect, where the atmosphere radiates more heat out into space from its warm upper layers than it traps nearer to Earth’s surface.
Justus Notholt at the University of Bremen in Germany and his colleagues first identified this “negative greenhouse effect” about a decade ago above high-altitude parts of Antarctica, where the air is particularly frigid and dry. Antarctic surface temperatures were still warming. But the researchers argued a cooling effect higher in the atmosphere could partly explain why temperatures were not rising as fast as in the more humid Arctic, where a negative greenhouse effect appears to be uncommon.
They have now run simulations to explore how water vapour in the atmosphere over Antarctica influences the warming – or cooling – effects associated with increasing atmospheric CO2 and methane at every layer of the atmosphere.
The researchers ran simulations in which they made the amount of water vapour in Antarctic air match levels seen over the Arctic. They found CO2 and methane would have a similar effect on temperatures in Antarctica as they do on Arctic temperatures.
Notholt says this isolates humidity as the key factor controlling the negative greenhouse effect. As humidity increases with rising temperatures, he says the cooling effect could switch to a warming one, although it is unclear if upper layers of the atmosphere would mix enough with lower layers to affect surface temperatures.
Tim Cronin at the Massachusetts Institute of Technology also thinks water vapour levels help explain the different ways greenhouse gases affect each polar region. But he says factors like changes in sea ice extent are probably much more important for temperatures there than the negative greenhouse effect. “It’s more likely to be sort of a piece of trivia than a key piece of understanding climate change,” he says.
Journal reference:
Geophysical Research Letters DOI: 10.1029/2023GL105600
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