Earth’s Steamy Blanket

But what does water have to do with global warming? When asked that question, Dessler turned to a white board in his office and began drawing pictures of clouds in a swath of space he had wiped clean of equations. Soon the figure was accompanied by arrows and a graph plotting water and temperature. Waving his dry-erase marker in the air, he explained why a little bit of extra water in the atmosphere is such a big deal.

At the root of global warming are greenhouse gases. The atmosphere acts as a global thermostat, letting sunlight in, but trapping outgoing heat. In this way, it keeps the Earth’s surface temperature in a range suitable for life. In theory, we could turn up the global thermostat by increasing the proportion of greenhouse gases in the atmosphere. “As humans add carbon dioxide, and carbon dioxide absorbs outgoing radiation,” Dessler says, “the Earth warms up.” As it does, more water evaporates from the oceans and into the atmosphere. “Since water vapor is also a greenhouse gas, the additional water in the atmosphere further heats the surface, leading to even more water evaporating,” he explains. And even though carbon dioxide is the greenhouse gas that gets all of the attention, it can’t compete with water vapor in heat-trapping power.

“Water vapor is the most important greenhouse gas in the atmosphere,” says Steven Sherwood, a professor in the Department of Geology and Geophysics at Yale University. As anyone who lives in a humid climate can attest, water traps heat being radiated from the Earth. In effect, water vapor envelops the Earth in a thick, steamy blanket. Warming due to carbon dioxide emissions from fossil fuel combustion evaporates even more water, increasing the thickness of the blanket, which leads to more heating, which leads to more water vapor… The loop is called the water vapor feedback, and it has the potential to be a serious problem. Sherwood explains. “If you have enough of this positive feedback, then of course the whole climate system would be unstable.” Today’s climate, he quickly adds, is not unstable. “But as you pile on more and more of this sort of thing, you get closer and closer to an unstable situation. So if the climate is unstable, small differences in how strong these feedbacks are can become relatively important, more important than you might think.”

Atmospheric gases, especially carbon dioxide and water vapor, prevent some heat (thermal infrared radiation) from escaping from the Earth into space. In this graph, the dotted line represents the energy that would be emitted from the Earth without an atmosphere, and the solid line shows the effects of water vapor, ozone, and carbon dioxide. (Graph adapted from Walter Roedel, Physik unserer Umwelt Die Atmosphäre, 2000)

Clearly, detailing how the water vapor feedback works is essential in predicting and mediating future climate change. “I was drawn to this problem by its importance,” says Dessler. “As a scientist, you want to work on the most important problem that you can find, and I think everyone would agree that water vapor feedback falls into this category.” To understand how much the water vapor feedback could boost the Earth’s temperature in the future, Dessler and Minschwaner decided to focus on finding out how much water will enter the atmosphere as the temperature climbs.

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Although carbon dioxide in the atmosphere traps some heat near the Earth’s surface, its effect is much less than that of water vapor. The small amount of warming caused by carbon dioxide may be greatly magnified by increased evaporation from the ocean surface as global temperatures rise. (Photograph copyright Corel)