Why do methane, and other secondary greenhouse gases such as CFCs have such a potent effect on trapping infrared radiation?

Carbon dioxide is the leading driver of climate change because so much of it is emitted into the atmosphere by humans, primarily from burning fossil fuels. But other gases, climate “super-pollutants,” like methane, nitrous oxide and fluorinated gases including CFCs, are much more potent pound-for-pound than carbon dioxide. 

So what makes super-pollutants so effective at warming the planet?  

“The short answer is they absorb outgoing terrestrial radiation more efficiently molecule for molecule than CO2,” Drew Shindell, an Earth science professor at Duke University, said.

Energy from the sun travels as sunlight, most of which passes through the atmosphere and is absorbed by the Earth’s surface, warming it. Some of that heat then radiates back out toward space in the form of lower energy, thermal waves that can be absorbed by the atmosphere. This absorption or trapping of thermal energy as it is radiating back out toward space is the greenhouse effect that warms the planet.

Most of the thermal energy that the atmosphere absorbs is absorbed by water molecules that occur naturally. Carbon dioxide absorbs thermal energy at many of the same wavelengths as water molecules. The net effect is that carbon dioxide is not a very potent greenhouse gas on a per molecule basis, because much of the thermal energy that can be absorbed by carbon dioxide is already being absorbed by water vapor.

Other greenhouse gases like methane, nitrous oxide and CFCs absorb thermal energy at wavelengths that differ from the wavelengths absorbed by water and CO2 making them highly potent greenhouse gases.

“The importance of any individual gas is a function of what wavelength it absorbs at, and what else might be absorbing at those wavelengths,” Shindell said. 

Methane and nitrous oxide are tens to hundreds of times more efficient at absorbing thermal radiation in the atmosphere, “but some of these fluorinated gases, they are just super absorbent at some of those wavelengths, more than 10,000 times as efficient as CO2,” Shindell said.  

Another factor that has an impact on warming is how long each of the different gases remains in the atmosphere. Carbon dioxide can stay in the air for centuries, while other greenhouse gases like methane and some fluorinated gases are much shorter lived, sticking around for a matter of years. Reducing emissions of these so-called “short-lived climate pollutants” can be an effective way to combat climate change in the near term, while working to address longer-lived carbon dioxide emissions. 

Phil McKenna

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