Here’s How Global Warming Is Already Worsening Extreme Deluges In The U.S.

http://thinkprogress.org/climate/2014/09/09/3564989/global-warming-extreme-deluges/

by Joe Romm Posted on September 9, 2014

One of the most robust scientific findings is the direct connection between global warming and more extreme deluges. Scientists have observed a sharp jump in monster one- and two-day rainstorms in this country.

The 2014 National Climate Assessment (NCA), which is the definitive statement of current and future U.S. climate impacts, notes, “The mechanism driving these changes is well understood.” The congressionally-mandated report by 300 leading climate scientists and experts, which was reviewed by the National Academy of Sciences, explains:

Warmer air can contain more water vapor than cooler air. Global analyses show that the amount of water vapor in the atmosphere has in fact increased due to human-caused warming…. This extra moisture is available to storm systems, resulting in heavier rainfalls. Climate change also alters characteristics of the atmosphere that affect weather patterns and storms.

That final point from our leading scientists is very important. The worst deluges have jumped not merely because warmer air holds more moisture that in turn gets sucked into major storm systems. Increasingly, scientists have explained that climate change is altering the jet stream and weather patterns in ways that can cause storm systems to slow down or get stuck, thereby giving them more time to dump heavy precipitation.

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Dr. Kevin Trenberth, former head of the Climate Analysis Section at the National Center for Atmospheric Research and one of the world’s leading experts on extreme weather, wrote a must-read article in 2012 on how to “relate climate extremes to climate change” (PDF here, HTML here). As Trenberth explains:

The answer to the oft-asked question of whether an event is caused by climate change is that it is the wrong question. All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be….

The air is on average warmer and moister than it was prior to about 1970 and in turn has likely led to a 5–10% effect on precipitation and storms that is greatly amplified in extremes. The warm moist air is readily advected onto land and caught up in weather systems as part of the hydrological cycle, where it contributes to more intense precipitation events that are widely observed to be occurring.

Because global warming tends to make wet areas wetter and dry areas drier, this effect does not manifest itself the same way in every part of the country.

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Those of you in the Northeast who thought you’d noticed deluges becoming more intense were right. Thanks to climate change, when it rains, it pours, literally. As the NCA explained, “The heaviest rainfall events have become heavier and more frequent, and the amount of rain falling on the heaviest rain days has also increased.” Some 70 percent more precipitation falls in the heaviest rain events now than it did in 1958.

Ironically, what this means is that even for the regions that are expected to see a drop in total annual precipitation — such as the Southwest — more of the precipitation they do get will be in the form of deluges so intense they can create terrible flash floods. Sound familiar?

The bottom line is that scientists predicted that climate change would increase the intensity and frequency of the worst deluges — and we’ve observed that happening. Now scientists are telling us that things are going to get a lot worse. After all, the Earth has only warmed one degree Fahrenheit in the past half-century, while we are on track to warm as much as ten times that this century if we continue ignoring the warnings. Perhaps it’s time to listen and act…

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