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."

7 Comments

1. 1. timemachinetony 12:55 AM 6/24/08

   The only way to save this planet is to reduce the human population. All countries must make an efford to reduce its population.

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2. 2. G. Karst 08:47 PM 12/28/08

   I can tell you, with good confidence. what 13% more rainfall means...
   
   Deserts receive more moisture, flowers bloom.
   Prairies receive more moisture, wheat grows.
   Forests receive more moisture, wood produced.
   
   Funny... seemed self-evident.
   

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3. 3. hankroberts 03:40 PM 12/31/08

   Clue: this happens.
   You don't get more tomatoes.
   You lose topsoil in vast amounts very rapidly.
   
   Abrupt increase in seasonal extreme precipitation at the Paleocene ...
   ic.ucsc.edu/~jzachos/eart120/readings/Schmitz_Puljate_07.pdf

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4. 4. hankroberts 04:27 PM 12/31/08

   PS, this old thread got revived because of a mention on Tamino's blog; I'm more or less duplicating a response there:
   
   This is not news, and there is a good bit of commentary available that needs to be read, e.g. just as a convenient source:
   
   http://www.newscientist.com/article/dn11977-warming-will-bring-more-rain-study-claims.html
   
   which begins: "
   
   Climate experts have cast doubt on the conclusions of a new study predicting that a warmer world would lead to more rainfall - a contradiction of the prediction of most climate change models - which was based on just 20 years of data....." and
   
   "... Gavin Schmidt, a climatologist at the NASA Goddard Institute points out that the 20 years studied were dominated by a couple of El Niño events, which increased precipitation during that time. "The trends are not really significant," he says. "I think some more work would be necessary to really pin their argument down."
   Rain or humidity
   
   "Two decades is a relatively short period of time for this type of analysis, but it is all we have," Wentz told New Scientist...."
   
   The author of the study agrees 20 years is too short; it's the total available to date.
   
   Point being -- don't rely on some guy on a blog; look the actual studies up, see when they were done, who's cited them, who's commented on them, then figure out if someone's brought it up on a blog to make some point of their own by leaving off the context.
   
   I understand these days it's called "retroactive research" -- write what you believe, then Google until you find something you can spin as supporting it, and cite that as though it were your source.
   
   

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5. 5. G. Karst in reply to hankroberts 12:47 PM 1/2/09

   Are you or are you not... some guy on a blog.
   
   Our planet is mostly water. Of course increasing temps will will result in increased absolute humidity and rainfall. Even a grade 6 science student knows this. If you do not understand the dynamic relationships of temperature, humidity, and rainfall, then perhaps you should move to a less scientific based forum.

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6. 6. Mat 08:22 AM 7/25/09

   I can tell the researchers exactly where the extra water is falling; in my backyard (near Montreal, CAN). It snowed almost every single day during last winter and it's been raining every day this summer. In July, we busted the rainfall normal in less then two weeks in...But it doesn't matter, weather changes and shifts. Sedentry people just have a hard time accepting that things change (especially nature).

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7. 7. hankroberts 12:52 PM 9/11/09

   This study is being referenced elsewhere, for example
   
   http://global-warming.accuweather.com/2009/07/plants_kept_the_earth_from_fre_1.html
   
   would the Scientific American folks consider following up by giving a reference to subsequent papers citing the one described here? It was not the last word, just a news story.
   

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