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.

Flooding devastated area in Port Vincent, Louisiana along the Amite River southeast of Baton Rouge. Credit: NOAA Remote Sensing Division

This story was last updated on August 18, 2016, at 9:05 AM ET to reflect the rising number of fatalities cause by the Louisiana floods.

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 result 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 13 were killed. Thousands fled their homes.

Louisiana's capital, Baton Rouge, was among the worst-hit communities. Last Friday, Gov. John Bel Edwards had declared a state of emergency and even the governor's mansion was evacuated. President Barack Obama initially declared the event a disaster for four Louisiana parishes on Sunday night, and other parishes have since been added.

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.

As global warming worsens, 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.

Hayhoe walked through the Louisiana event's climate link in a Facebook post on Tuesday: "What's the connection? As the atmosphere warms, so does the ocean. Evaporation speeds up, making more water available for a storm to pick up and dump as it sweeps through."

Whether such slow-moving storms would become more common in a warming world, however, both Hayhoe and Kunkel said it was not as easy to quantify. "I'm not aware of anyone having done definitive work on that," said Kunkel.

Historical rainfall data shows most of the nation had already experienced an uptick in the intensity of the heaviest rainfall events in recent decades. For the southeastern U.S., for example, there's been a 27 percent increase in precipitation during the most intense events between 1991-2014 compared to 1901-1960, according to the National Climate Assessment.

Meanwhile, a Climate Central ranking of U.S. cities based on how the intensity of rainfall events has increased there found Baton Rouge ranked 13th, with a 120 percent increase in heavy rainfall between 2005-2014 compared to 1950-1959; New Orleans ranked 34th, with a 62 percent increase in intense precipitation over that time.

So far, more than 20,000 people have been rescued from flooded areas. Insurers are estimating the economic damages from this event could be $1.5 billion or more.

"This event is ongoing, it is not over," said Edwards in a statement on Monday. "Even if the sun has come out in your area, we do not know when the floodwaters will recede, and they will continue to rise in some areas."

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.

In reference to this year's extreme flooding in Texas and Louisiana, former Vice President Al Gore said on Tuesday: "These kinds of record downpours—that's one of the manifestations of the climate crisis."

Across the Atlantic, historic rainfall and related flooding beset Paris in May. A preliminary study looking at the climate fingerprints of the event, called an attribution study, determined such a disaster was made almost twice as likely because of man-made global warming. The analysis was conducted by a group of scientists from multiple research institutions and academia, including the French National Center for Scientific Research and the University of Oxford.

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