Carbon Dioxide is more Important than Water Vapor as a Greenhouse Gas

Carbon Dioxide is more Important than Water Vapor as a Greenhouse Gas

This may seem strange. If you track what happens to the radiation leaving the Earth's surface, some is absorbed on the way, and some goes straight out to space. Water vapor, carbon dioxide, and clouds dominate the absorption, with all the others (methane, ozone, chlorofluorocarbons, nitrous oxide, etc.) also enough to be important if taken together. (Clouds also have a slightly more important role in blocking the sun, with the net effect of clouds being slight cooling under modern conditions.) Because some radiation is blocked almost entirely by only one gas type, but other wavelengths may interact with both water vapor and carbon dioxide, there is a bit of uncertainty in the bookkeeping of the exact importance of a single type of greenhouse gas. Overall, though, it is fairly accurate to say that water vapor supplies close to half of the total greenhouse effect, clouds and carbon dioxide each a little under a quarter, and all others just under a tenth.

But, the amount of water vapor in the air is equal to the amount of rain that falls on the Earth in just over a week. As water vapor rains out very rapidly, it is replaced by evaporation of more water. Any extra water vapor we put in the air from burning of fossil fuels or irrigating crops just doesn't stay up there very long. And, because the natural source of water vapor is so huge (evaporation from a giant ocean and a lot of plants that together cover almost the entire Earth), the human source is actually tiny in comparison. The only practical way we know of to greatly change water vapor in the air is to change the temperature. A hair dryer has a heater for good reasons, and warming the air will allow it to pick up and carry along more water vapor, whether the warming is caused by carbon dioxide, or a brighter sun, or some sort of heat ray from space aliens, or anything else.

Some research has looked at what would happen if carbon dioxide were removed from the atmosphere. Loss of the carbon dioxide cools the planet, but that condenses some of the water vapor, which cools the planet more, and the Earth turns into an ice-covered snowball. If water vapor is removed, a lot more evaporates quickly before the Earth can freeze.

So, yes, water vapor is blocking more energy than carbon dioxide today. But, carbon dioxide is much more important for changing the climate than is water vapor. Carbon dioxide can be a forcing—add it to the air, and you force the climate to change. Carbon dioxide also can be a feedback—change something else (such as reducing oxygen in the ocean to allow more fossil-fuel formation), and that changes carbon dioxide in the air, which in turn changes the temperature. But, water vapor is almost entirely a feedback, because there aren't any natural or human processes other than changing the temperature that can put water vapor up fast enough to make a big difference to climate.