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Study: Climate change made the deadly Louisiana flood far more likely

Days of rain swallowed roads and tens of thousands of buildings in water throughout Louisiana. (Video: Jenny Starrs/The Washington Post)

On Aug. 10, it started raining in Louisiana and didn’t stop for seven days. In that time, as much as 30 inches of water fell from the sky — around 7.1 trillion gallons, which is three times what fell during Hurricane Katrina. Thirteen people died in the resulting flood. Rescuers saved and relocated 30,000 residents. Tens of thousands of homes were damaged. The American Red Cross called it “the worst natural disaster to strike the United States since Superstorm Sandy.”

After a rapid-response study, scientists at NOAA say that climate change made this rainfall event at least 40 percent more likely today than in say, 1900.

The disaster was caused by two weather-related features, extreme humidity and near-stationary low pressure that hovered over the Gulf Coast for days. The moisture component is where climate change comes in — warmer air can hold more water vapor. As the air warms due to greenhouse gas emissions, the air gets more humid, and there’s more moisture that generates more rain.

Precipitable water — a measure of how much moisture is in the air over a certain location — was off the charts in Louisiana that week. Day after day, data from weather balloons showed precipitable water that came close to or exceeded any other weather event on record in the region.

Given the magnitude of the disaster, scientists at NOAA’s Geophysical Fluid Dynamics Lab in Princeton, N.J., immediately teamed up with World Weather Attribution to determine what, if any, role climate change played in the event. They used historical weather observations along with a new, high-resolution climate model to reach a conclusion.

A rainfall event like the one that displaced 30,000 people in Louisiana in August “is now expected to occur at least 40 percent more often than it was in our preindustrial past,” the team wrote.

It’s more likely a doubling, said Karin Van der Wiel, the lead scientist on the study, in a conversation with The Washington Post. “That’s a very significant change in probability,” she noted.

Put another way, scientists expect this kind of event to happen, on average, every 1-in-30 years somewhere along the northern Gulf Coast from Houston to the panhandle of Florida. But in 1900, it used to be 1-in-50 years. In less than a century, climate change has cut our recovery time in half.

They also found that 1-in-30-year events have become 10 percent more intense since preindustrial times.

NOAA’s team of researchers appreciates that no data set or climate model is perfect, but the tools they took advantage of for this study were the best of what science can offer. In particular, the models the team used were triple the resolution of other models often used for studies like this.

“These models are at the forefront of climate change modeling,” said Van der Wiel, “and they really do a great job.”

Areas around Baton Rouge continue to clean up and rebuild — an expensive process for the majority of residents who did not have flood insurance. The Red Cross alone suspects it will spend $35 million to $40 million on flood relief in the coming months.

After the flood, the federal government declared 12 parishes major disaster areas, and the event will in all likelihood be named one of the country’s billion-dollar weather disasters. In light of this research, Louisiana may not have as much time to prepare for the next one.