What do CO2, methane, and water vapor have in common? If your first thought was “greenhouse gases,” you’d be correct! Greenhouse gases trap heat in the atmosphere, in a process called the “greenhouse effect”(1). But how do these molecules actually warm our planet?

We’ll start our exploration of greenhouse gases with a single carbon dioxide (CO2) molecule. Let’s say this CO2 molecule came from the exhaust in your car. From your tailpipe, it drifts up into the atmosphere, diffusing among the other gases. There, particles of light—photons—hit our molecule.

So what happens to those photons? “Greenhouse gas molecules will absorb that light, causing the bonds between atoms to vibrate,” says Jesse Kroll, Professor of Civil and Environmental Engineering and Chemical Engineering at MIT. “This traps the energy, which would otherwise go back into space, and so has the effect of heating up the atmosphere.” Basically, the bonds between the carbon and oxygen atoms in our CO2 molecule bend and stretch to absorb photons. (With other greenhouse gases, the molecular bonds are different, but in all cases, they absorb photons, stopping them from leaving the atmosphere.)

Eventually, our CO2 molecule will release these photons. Sometimes, the photons continue out into space. But other times, they rebound back into the Earth’s atmosphere, where their heat remains trapped.

And importantly, greenhouse gases don’t absorb all photons that cross their paths. Instead, they mostly take in photons leaving the Earth for space. “CO2 molecules absorb infrared light at a few wavelengths, but the most important absorption is light of about 15 microns,” says Kroll. Incoming light from the sun tends to have much shorter wavelengths than this, so CO2 doesn’t stop this sunlight from warming the Earth in the first place. But when the Earth re-emits this light (2), it has a longer wavelength, in the infrared spectrum.

And the range of wavelengths around 15 microns is a particularly crucial window. The most common greenhouse gas, water vapor, doesn’t efficiently absorb photons in this range. So when CO2 grabs photons with wavelengths around 15 microns, it’s selecting for the same light that normally has the easiest time escaping Earth’s atmosphere.

There’s another reason why CO2 is such an important greenhouse gas: it has a long atmospheric lifetime. This has to do with the way CO2 reacts (or rather, doesn’t react) with the atmosphere. “The atmosphere is a very oxidative environment due to the presence of oxygen and ultraviolet radiation,” says Kroll. Oxidation occurs when oxygen steals electrons from another atom—it’s the same chemical reaction that causes iron to rust. Methane, another greenhouse gas, reacts easily with oxygen, which removes it from the atmosphere within around 12 years. That’s long enough to affect the climate, but nowhere near the lifetime of CO2, which does not react with oxygen and can last over a century.

CO2’s long lifespan is the key reason that human activities are leading to climate change. As we keep taking carbon-based compounds like coal and oil out of the ground, and put that carbon in the atmosphere in the form of CO2, the added CO2 piles up much faster than it can be naturally removed.

February 19, 2021

FOOTNOTES

1 This name is a little misleading. A real greenhouse traps heat because its glass stops the warm air inside from transferring heat to the colder surrounding air. Greenhouse gases don’t stop heat transfer in this way, but as this piece explains, in the end they have a similar effect on the Earth’s temperature.

2 Most of the sun’s radiation is absorbed by the Earth; only some is re-emitted as infrared light.