Why does Carbon Dioxide matter?

Found on the InterTubes:

What property does CO2 have that causes it to retain heat, reflect heat, or transmit heat, that does not exist in other invisible gases in the atmosphere, nitrogen (78%), oxygen (21%), argon (1%), humidity (0.4% to 1%)? Where are the photo-thermal studies demonstrating the overwhelming properties of carbon dioxide (0.03% to 0.04%) …

That’s a good question, and we even know the answer.

The simplest version of the answer is all about the balance between heat coming down to the Earth from the Sun, and heat leaving the Earth and heading off to deep space.

Carbon Dioxide (CO2) is transparent to the sunlight that warms the Earth, but it absorbs the infra-red energy (i.e. the heat radiation) that the Earth emits. So, light from the sun streams down through the atmosphere and heats the Earth, but the Earth has trouble emitting the heat back to outer space. The more CO2 we have, the harder it is for excess heat to leave.  So, CO2 causes warming: this is the “greenhouse effect.”

But there’s another part to the question: “why is CO2 important compared to much more abundant gases like N2, O2, and water?”

The answer uses two concepts: “saturation”, and different wavelengths. [Infra-red isn’t uniform.  There are many wavelengths within the infra-red range, and each wavelength is treated differently by different gases.  Details here, including a beautiful plot showing how many different absorption lines there are in the infra-red.]  You should think of the infra-red as a combination of hundreds of different wavelengths. (Metaphorically, think of a room with hundreds of different windows, one for each different color of infra-red; each window has its own shade.)

For instance, water has absorption lines in the infra-red, so it contributes to the greenhouse effect. But, there’s a lot of water around, so that — at the wavelengths where water absorbs — the atmosphere is opaque.  There is far more absorption at those wavelengths than necessary to stop a photon from escaping to outer space; that’s what “saturation” means.

Imagine a infra-red (heat) photon leaving the ground. If the photon’s wavelength is in the middle of a water absorption line, it will get stopped within a few meters; it won’t even make it past the treetops. That means all the water above that height is “wasted”; it doesn’t trap any photons because the photons from the ground won’t even make it that high.  As a result, water has less effect than you might think, because all of its blocking is concentrated at a few wavelengths, and the absorption at those wavelengths is saturated.  (In our window analogy, the windows that correspond to water vapor lines are already painted black.  Adding more water is like adding a second coat of black paint to those windows: it won’t make the room much darker.)

However, at other wavelengths, water doesn’t absorb and the heat can freely escape to outer space.

“Can escape”, well actually “Could escape if it weren’t for the CO2.” CO2, being a different molecule, absorbs at different wavelengths from water. So, it blocks some wavelengths/windows that water leaves open.

With CO2, since it’s not as common as water, this “saturation” effect isn’t as important. Thus CO2 contributes more to the greenhouse effect than you’d expect from it’s concentration.

And Oxygen and Nitrogen? They simply don’t have absorption lines at the infra-red wavelengths. Even though there are a lot of N2 molecules, they don’t stop the outgoing heat.

Where are the Studies?

There are hundreds; go to Google Scholar and search for “infra-red absorption water vapor“.

  • Here’s a modern one from University College, London.
  • Some 1970 work.
  • Here’s a 1946 paper on details of the old methods that they needed to use before modern laser spectroscopy and computers.
  • Here’s a 1918 one from the University of Michigan.
  • An 1896 paper that refers to Paschen’s 1893 work.

As you can see, people have been measuring the infra-red absorption of water vapor for a long time, and (other than improvements in measurement techniques) the answers haven’t been changing.

The greenhouse effect has also been known for a long time.  We’ve understood the basic mechanism since Arrhenius published his model in the 1890s.

And, for a long time, we’ve understood the limits of the simple model: it doesn’t include dust and clouds. Thirty years ago, it was possible to believe that changes in clouds might save us from global warming. But, over the last couple of decades, we’ve gradually improved our understanding of clouds, and the hope that clouds will cancel out effects from is mostly gone.

One of the more important “experiments” happened when Mt. Pinatubo exploded in 1991. It let us find out whether changes in water vapor emphasize or counteract climate change.  [Paper here: Global Cooling After the Eruption of Mount Pinatubo: A Test of Climate Feedback by Water Vapor; Brian J. Soden, Richard T. Wetherald, Georgiy L. Stenchikov, Alan Robock]  Unfortunately, the evidence suggests that as the climate warms, water vapor will increase and make the warming worse.

A Philosophical Note

Don’t be comforted by the uncertainties in climate models.  There is no law of nature that says our mistakes will always make things look worse than they really are.

Uncertainties go both ways…