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May 062010
 

From Green Right Now Reports

Seeing the pictures of the flooding in Nashville this past week may have reminded you of other recent U.S. floods — in Fargo, Iowa City and the Mississippi River Valley.

Nashville Flooding, May 2010 (Photo: ABC News)

Nashville Flooding, May 2010 (Photo: ABC News)

And if you keep up with global warming, you may be wondering if this trend isn’t proving what scientists have been telling us about extreme rain events growing more severe and more frequent under climate change.

That question certainly came up in Nashville, according Rich Hayes, deputy communications director at the Union of Concerned Scientists and a Nashville resident.

“A lot of my friends here have asked me if this disaster is related to global warming. The fact is that climate change increases the probability of some types of weather, including heavy rains and flooding. As average temperatures rise, more rain falls during the heaviest downpours. Unfortunately, that is exactly what we experienced in Nashville over the weekend,” Hayes said in a prepared statement on Thursday.

“Warmer air holds more moisture. We’ve all seen it. Next time you take a shower, notice how the water vapor hangs in the warm air after you turn off the hot water. When warm air holding moisture meets cooler air in the atmosphere, the moisture can condense onto tiny particles to form floating droplets. If those drops get bigger and become heavy enough, they fall as precipitation.

Hayes went on to cite a report by the United States Global Change Research Program, a collaborative effort involving 13 federal agencies, which found that “one of the most pronounced precipitation trends in the United States is the increasing frequency and intensity of heavy downpours.”

As for those climate skeptics who might think the Nashville event is just an extreme example of an otherwise good thing, i.e., more rain. Listen in to the rest of what that report predicts:

“More precipitation is falling during very heavy events, often with longer dry periods in between.  Climate models project more heavy downpours and fewer light precipitation events.”

In other words, we get rain that doesn’t really work that well for us anymore. It comes in sudden, heavy downpours that produce a lot of runoff and erosion, and when the water can’t escape, flooding. In between, we get dry spells. Ask any farmer if this is a good thing.

It would be akin to having water to drink one week, but no water the next. Most organisms can’t live that way.

Hayes goes on to note that the rain that fell on Nashville was undoubtedly outside the norm. A record of 13 inches fell on Saturday and Sunday, nearly double the previous record set in 1979, and that followed a hurricane.

All these flood events are different. Fargo and the Mississippi Valley had their own distinct issues. Fargo faced rapid heavy snow melt. In Iowa, the flooding was exacerbated by monoculture farming that has left the flat lands susceptible to run off. But they seem to be happening more frequently.

Increases in Very Heavy Precipitation 1958-2007 (Image: U.S. Global Change Research Program)

Increases in Very Heavy Precipitation 1958-2007 (Image: U.S. Global Change Research Program)

In Nashville, the sheer volume of rain in a short time overwhelmed the city. Unfortunately, under climate change models, what’s been outside the norm is becoming the norm.

The report cited by Hayes notes that “very heavy rain and snow events, defined as the heaviest 1 percent of all precipitation events, now drop 67 percent more water on the Northeast; 31 percent more on the Midwest and 20 percent more on the Southeast than they did 50 years ago.” (See image above from the USGCRP.)

“If the fossil fuel emissions that cause global warming continue unabated, scientists expect the amount of rainfall during the heaviest precipitation events across the country to increase more than 40 percent by the end of the century. Even if we dramatically curbed emissions, these downpours would still increase, but by only a little more than 20 percent,” according to the UCS statement.

“It’s going to take Nashville a long time to recover from the flooding,” says Hayes. “But when the flood waters do recede, and local officials turn to the question of how we plan for the future, they need to take climate change into account.”