What goes up, must come down. This basic rule of gravity on Earth's surface also applies to water vapor in the atmosphere. And as the air, earth and sea warms with climate change the atmospheric water vapor load increases by as much as 6.5 percent per degree Celsius, according to satellite data from the past 20 years. As the water vapor increases, so, too, will rainfall, argues physicist Frank Wentz, director of Remote Sensing Systems (RSS) in Santa Rosa, Calif., a provider of climate data records contracted by NASA.

Reviewing data from Special Sensor Microwave Imagers on Earth-observation satellites, the RSS team evaluated how much water evaporated into the atmosphere and how much fell as rain. "When averaged globally over monthly time scales, [precipitation] and [evaporation] must balance except for a negligibly small storage term," the researchers note in Science.

Yet, current climate models predict that evaporation will increase slowly and rainfall will hardly rise at all—in part due to a potential decrease in global winds. This is not borne out by the satellite data. "We found that when averaged over the world oceans,'' Wentz says, "the winds have slightly increased over the last 20 years by about 0.2 meter per second (or 0.4 mile per hour)."

In addition, the satellites reveal that evaporation and precipitation are increasing much faster than the models predict. "The observations suggest that maybe global rainfall will increase at a higher rate—three times higher according to these results—than climate models predict," Wentz says. "The additional rain may be beneficial for some of the drier areas and pose a significant climate risk for other areas of the world."

If the climate warms just two degrees Celsius (3.6 degrees Fahrenheit)—as predicted by the recent Intergovernmental Panel on Climate Change (IPCC) report—13 percent more water vapor will be in the atmosphere at the end of this century, the scientists note. Rainfall will likely increase by a similar amount. "Where that additional rain falls is the sixty-four-million-dollar question,'' Wentz says, "and I don't think anyone can say that with any confidence."

Ultimately, climate models will have to improve their accuracy in predicting how global warming will affect precipitation. (They currently struggle to match observations of seasonal events like El Niño). Modelers may be able to do that by incorporating better data on the total amount of water vapor observed in the atmosphere.

But even if the forecast calls for more potentially beneficial rain, that is no reason to welcome climate change. "Man is going to impact the climate, there's no doubt about that. If we can slow that rate down, that gives everyone more time to adapt," Wentz stresses. "We know what we have right now in terms of climate and we know how to deal with it. Thirteen percent more rainfall is going to be more unknown."