Is water vapor more important than carbon dioxide for global warming?

Illuminating answers to environmental questions.
Jan. 22 2008 11:16 AM

Is Global Warming Caused by Water Vapor?

How to think about the No. 1 greenhouse gas.

Illustration by Robert Neubecker. Click image to expand.

Every week, you blather on and on about carbon dioxide and methane. Yet you never mention a single word about the most important greenhouse gas of all, water vapor, which accounts for 98 percent of the greenhouse effect. Doesn't this inconvenient truth wholly discredit your little global-warming charade?

Variations on this question appear in the Lantern's inbox every few days, occasionally accompanied by family-unfriendly slurs. For folks who doubt that human activity is causing global warming, citing water vapor's role in the greenhouse effect is a common retort. Though such enviroskeptics are technically correct to some extent, the water-vapor argument by no means proves that anthropogenic (i.e. man-made) global warming is a fiction.

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It's unassailably true that water vapor is the gas most responsible for the greenhouse effect. Greenhouse gases let shortwave solar radiation through the atmosphere, but impede the escape of long-wave radiation from the Earth's surface. This process keeps the planet at a livable temperature: Without a suitably balanced mixture of water vapor, CO2, methane, and other gases in the atmosphere, the planet's average surface temperature would be somewhere between -9 and -34 degrees Fahrenheit, rather than the balmy 59 degrees it is today.

By mass and volume, water vapor is the most prevalent greenhouse gas in the atmosphere. According to both the International Panel on Climate Change and many global climate models, water vapor accounts for somewhere between 60 percent and 70 percent of the greenhouse effect. (The 98-percent figure, much beloved by global-warming skeptics, seems to have been first used in a 1991 article by Richard Lindzen. He cites a 1990 IPCC report as his source, but the report doesn't appear to contain that number.)

The skeptical argument thus goes something like this: Since water vapor is the most potent greenhouse gas, and since this vapor is created through natural evaporation rather than human activity, the current warming trend is nothing to worry about—just the Earth going through a normal climatic cycle.

But this viewpoint ignores the reactive nature of water vapor—in other words, the gas doesn't cause warming all by its lonesome. The amount of water vapor the atmosphere can hold is almost purely a function of temperature—the warmer the air gets, the more vapor it's able to glean from the planet. We know, for example, that the atmospheric water content over the oceans has increased (PDF) by 0.41 kilograms per square meter every 10 years since 1988.

So, what's causing the temperature rise that's resulted in greater evaporation? Well, over that same time period, global emissions of carbon dioxide have soared. And unlike water vapor, which returns to Earth as precipitation within a week of entering the atmosphere, CO2 sticks around for between 50 and 200 years. Carbon dioxide accounts for approximately 25 percent of the greenhouse effect, so it's pretty clear that the dramatic increase in atmospheric CO2 is playing a significant role in recent warming. (This warming might have been even greater if not for the ability of the planet's oceans to absorb heat.)

Warmed by CO2, the atmosphere is thus able to absorb more water vapor. And that water vapor, in turn, causes further warming—it amplifies the effects of carbon dioxide. So anthropogenic CO2 serves as the chief engine of global warming, with water vapor playing a crucial secondary role. According to the IPCC, if CO2 emissions were to double, water vapor would amplify the resulting temperature change by another 60 percent. Furthermore, a 2005 article in the journal Science forecast that the amount of water vapor in the upper troposphere will double by the end of this century, as a result of higher temperatures caused in part by the vapor itself. (Scientists refer to this situation as positive feedback.)

A common skeptical rebuttal to these assertions cites the role of water vapor in forming clouds; those clouds, the argument goes, will help block solar radiation and therefore compensate for the greenhouse effect. But a 2005 report by Swiss researchers concluded that this wasn't the case in the Alps, where they monitored climactic conditions over a seven-year period. Even though the mountains' northern slopes experienced increasing cloud cover over this span, temperatures nevertheless rose steadily; the clouds' cooling effects couldn't compensate for the warming associated with elevated greenhouse-gas levels.

There are many skeptical assertions worth engaging, such as the questionable efficacy of carbon offsets and the potential for our species to endure moderate warming in exchange for greater economic growth. But the water-vapor argument is designed only to mislead, taking a kernel of scientific truth and blowing it up into a risible call to inaction. And so the Lantern vows to continue blathering on about CO2.

Is there an environmental quandary that's been keeping you up at night? Send it to ask.the.lantern@gmail.com, and check this space every Tuesday.

Brendan I. Koerner is a contributing editor at Wired and a columnist for Gizmodo. His first book, Now the Hell Will Start, is out now.

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