New study explains why carbon dioxide cools the upper atmosphere while warming the lower atmosphere

A new study from Columbia University researchers explains the physics behind a well-known climate fingerprint: carbon dioxide warms the lower atmosphere but cools the stratosphere. Published in Nature Geoscience, the study shows that in the lower atmosphere CO2 traps heat, but in the stratosphere it acts like a radiator, absorbing infrared energy and emitting it into space more efficiently as CO2 concentrations rise. The researchers developed a quantitative theory for stratospheric cooling, identifying how CO2 interacts with specific infrared wavelengths. They found that a particular 'sweet spot' of wavelengths becomes more effective at radiating heat as CO2 accumulates. The stratosphere has cooled by about 2 degrees Celsius since the mid-1980s, more than ten times the cooling that would have occurred without human emissions. The team's equations successfully reproduce observed patterns of stratospheric cooling at different altitudes and show that each doubling of atmospheric CO2 produces around 8 degrees Celsius of cooling at the stratopause. Ozone and water vapor play a smaller role compared to CO2 in driving this effect.