Researchers from Columbia University have quantified the mechanisms by which carbon dioxide (CO2) causes cooling in the Earth's stratosphere, a phenomenon observed since the mid-1980s.
How Stratospheric Cooling Works
In the lower atmosphere, CO₂ traps heat. However, in the stratosphere—the layer of the atmosphere located 11 to 50 kilometers above Earth—the dynamic is different. CO₂ absorbs infrared energy rising from below and radiates some of that energy into space. As CO₂ levels increase, this radiative cooling is enhanced, directly lowering stratospheric temperatures.
Research Method
The team employed an iterative approach to pinpoint the cause. They identified key processes, assigned mathematical values to them, and compared their results against both computer simulations and real-world atmospheric data. They then adjusted their equations until they matched reality.
Their critical discovery is that CO₂'s interaction with a specific range of infrared wavelengths—termed the "Goldilocks zone"—is the primary driver of the cooling. As CO₂ accumulates in the atmosphere, this wavelength range expands, which increases the efficiency of the cooling process.
Quantitative Findings
The study reveals that the cooling effect is not uniform:
- Stratospheric cooling varies by altitude, being weakest at the stratosphere's lowest level and strongest at its highest point.
- Each doubling of CO₂ leads to approximately 8°C of cooling at the stratopause, the boundary region in the upper stratosphere.
- A cooler stratosphere reduces the amount of infrared energy that escapes to space from that layer, which paradoxically amplifies CO₂'s heat-trapping effect in the lower atmosphere.
Role of Other Factors
While ozone and water vapor also contribute to stratospheric cooling through similar radiative processes, their influence is small in comparison to the dominant effect of carbon dioxide.
Implications
The research provides a quantitative theory for stratospheric cooling, confirming and extending the Nobel Prize-winning climate models developed by Syukuro Manabe in the 1960s. The findings may also prove useful for studying the stratospheres of other planets.
The study was published in Nature Geoscience. Co-authors include Robert Pincus, Sean Cohen, and Lorenzo Polvani from Columbia University.