The Impact of Water Vapor Feedback

The Web site Skeptical Science argues that the water vapor feedback effect multiplies by two the impact of global warming. In other words, without water vapor feedback, doubling the amount of CO2 in the atmosphere would raise global temperature by about 1.8°F (about 1°C). Counting the water vapor feedback effect, the temperature would rise by about 3.6°F (about 2°C).

Some debate exists about whether water vapor has such a strong feedback effect, however, and about how or whether this effect might increase or decrease as the earth warms further. David Archer notes that the earth currently has an average relative humidity of about 80 percent—relative humidity being the amount of water vapor that exists in the atmosphere. Archer suggests that an increase in global temperature and global CO2 might increase the global average relative humidity, possibly resulting in a larger feedback effect. On the other hand, if increases in temperature cause the relative humidity to drop, the feedback effect might be smaller than expected.

One of the most controversial arguments about water vapor feedback has to do with clouds. When water evaporates, it causes clouds to form. Clouds have a high albedo—which is to say, they reflect sunlight rather than absorbing it. A lot of clouds would therefore increase the albedo of the earth. With a higher albedo, the earth would reflect more sunlight, and global temperatures would cool. Science educator Randy Russell has noted therefore that "an increase in water vapor, and hence cloudiness, might actually serve as a 'self-correcting' mechanism (or 'negative feedback loop') that would 'put the brakes on' global warming; or possibly induce a period of 'global cooling.'"8

Summer sea ice in the Arctic has fallen by about 386,102 square miles (about one million square kilometers) in the past 30 years.

It is not at all clear how the water vapor feedback loop and the cloud albedo feedback loop interact. As just one complicating factor, consider the idea that global warming may change regional climates and precipitation patterns in unexpected ways, which in turn could have a powerful effect on humidity and cloud formation. Because of the difficulty of the problem, and because of the importance of both of these loops to overall climate, Russell argues that "Predicting the net influences these feedback loops produce is possibly the greatest challenge facing modern climate scientists who are trying to determine our future climate."9

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