In a Warming World, Deluges Like Louisiana's Expected to Increase

Warmer air and sea temperatures conspire to add water vapor to the atmosphere, fueling intense rain events like the one that has devastated Louisiana.

The devastating rainstorm that unleashed terrifying flooding last weekend in Louisiana, with thousands of people escaping their homes and whole parishes being overtaken by water,  comes in recent succession to similarly extreme and deadly storms across the country—in Texas, Maryland, West Virginia and South Carolina. These intense storms have become seemingly commonplace, raising questions about climate change's role.

Of the two factors that made Louisiana's storm so devastating, one (increased moisture in the air) wears the fingerprints of man-made climate change from mostly fossil-fuel burning, while the other (how slowly the storm was moving) is not so easily explained.

"This storm is a good example of why we care about a changing climate," said Katharine Hayhoe, a climate scientist at Texas Tech University, "because Louisiana is a place that is already at risk of flooding and climate change is taking the risk that we already face, and it's exacerbating" the threat.

These storms have all happened as the planet is on track to have its warmest year on record. (In fact, July was the hottest month ever recorded).  "With such a warm year, you're going to see much higher-than-average sea surface temperatures," Hayhoe said.  Those water temperatures, in this case the Gulf of Mexico where the storm system formed on Aug. 7, mean the air above it has more than its usual share of water vapor, Hayhoe explained.  

Then this slow-moving, tropical low-pressure system came along and "there was virtually an unlimited supply of water vapor for this system," climate researcher Kenneth Kunkel told InsideClimate News.  When it reached Louisiana on Aug. 11, "it just didn't move and as a result, it rained over the same place for an extended period of time," said Kunkel, a senior scientist at the Cooperative Institute for Climate and Satellites, in North Carolina.

The results was a storm that lashed the state for days, dumping more than 20 inches of water in 12 towns and triggering widespread flooding. At least 11 were killed. Thousands fled their homes.
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For the worst-hit areas, federal scientists on Tuesday estimated that for a 48-hour period, the event had the intensity above what's expected for a so-called 1,000-year storm, which has a likelihood of 0.1 percent in any given year.  For an even larger swath of the state, the levels of rainfall observed for the same two days were above the rainfall thresholds expected with a 500-year event, which has a 0.2 percent likelihood of occurring.

In a warming world, climate scientists say, the heaviest rainfall events like these are expected to become more intense and more frequent in the Southeast—and across most of the United States.
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Louisiana's extreme flooding comes on the heels of several major flood disasters nationwide. Last month, Maryland's Ellicott City experienced intense record rainfall and damaging flooding. The city saw about 6 inches of rain in two hours,  a 1-in-200-year event, according to the National Weather Service and National Oceanic and Atmospheric Administration.  Earlier in the year, Texas also experienced strong rain-induced floods.

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