New Delhi: Despite the temperature rise on the surface and lower atmosphere of the Earth, the upper atmosphere has paradoxically cooled dramatically. The
pattern was well known, but not the underlying physics. Now, researchers from Columbia University have shed light on the mechanism. The research shows that rising carbon dioxide cools the atmosphere, with the cooling largely controlled by how the greenhouse gas interacts with infrared light. The range of infrared wavelengths involved in stratospheric cooling expands with the rise in carbon dioxide levels.
In the lower atmosphere, carbon dioxide traps heat that would otherwise escape into space. Higher up in the atmosphere, the dynamics change. In the stratosphere, between 11 and 50 km above the Earth, the carbon dioxide molecules act almost like a radiator, absorbing infrared energy from below, and transporting some of that energy into space. When more carbon dioxide is injected to the atmosphere, the heat is radiated away more efficiently, cooling the atmosphere. Since the 1980s, the stratosphere has cooled by 2℃, over 10 times the expected amount without anthropogenic carbon dioxide emissions. The researchers iterated on identifying the key processes by comparing pen-and-paper models to sophisticated simulations and real world data, deducing the best fit equations over months.
Process driven by carbon dioxide interacting with light
The central factor was how carbon dioxide molecules react with light. Not every infrared wavelength passes through them the same way, with some wavelengths contributing more to cooling than others. Ozone and water vapour are both implicated in similar processes, trapping heat in the lower atmosphere and radiating away heat in the upper atmosphere. A cooling stratosphere lets less infrared energy escape into space, increasing the heat-trapping effect in the lower atmosphere. The Earth ends up losing less heat to space overall, which strengthens the warming in the lower atmospheric layers and the surface. A paper describing the research has been published in Nature Geoscience.
