Dear Umbra,

Coming from a scientific background, I was under the assumption that water vapor was the worst — or you could say the best — at causing global warming. Do you believe this to be false, and if not, why is no one talking about it?

Erik Nash

Dearest Erik,

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I’ve decided to use your letter as a continuation of B’s from earlier this week.

Spouting off on global warming.

Photo: iStockphoto.

Water vapor is a powerful greenhouse gas. It is the dominant greenhouse gas in the atmosphere by mass and volume, but scientists don’t seem to agree on a quantification. It provides maybe 35 to 70 percent of the natural greenhouse effect, 80 percent of the greenhouse gases by mass. But these numbers can be misleading, because the way gases combine is an important component of the greenhouse effect; if water vapor accounts for, say, 55 percent of the effect, that doesn’t mean the rest of the gases make up 45 percent. If you are interested in more exactitude, check out this post on the RealClimate blog.

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From my understanding of others’ understanding, the reason “no one talks about it” — i.e., the reason we focus on carbon dioxide — is that we are not directly creating water vapor. We are loading the atmosphere with carbon, resulting in the many metaphors — adding an extra blanket to the atmosphere, overheating the well-managed greenhouse — and the techno-speak, “direct forcing.” We release the carbon by burning it, the carbon goes into the atmosphere, the atmosphere retains more of the sun’s radiation. That is a direct impact we have and should stop having.

Water vapor does offer us “climate feedback.” On the simplest level: as the atmosphere warms, it is able to, and will, hold more moisture. Moisture in the form of water vapor, which will be retained and itself make the atmosphere warmer, leading to higher water retention in the atmosphere, and on and on. Comprehending the properties of water vapor in the atmosphere begins to tire my little climate-obsessed head, because it is quite technical. If you (coming from a scientific background) wish to understand theories of how long this moisture feedback will continue, the scary factors by which it will increase projected global temperatures, and more, try that RealClimate link and the Intergovernmental Panel on Climate Change’s water vapor feedback page as starting points.

Basically, our “take-home learning” (hee hee, I love these horrible corporate gerunds) is that increased water vapor will amplify anthropogenic carbon emissions. Water vapor is not “the best” at causing global warming, because it has a short life in the atmosphere. Also, we are causing global warming and we have found the easiest, best way to do so is to burn fossil fuels and deforest. No, we don’t need to worry about lids on pasta pots. We should, however, feel very sober about the avalanche of climate changes we are bringing upon ourselves.