Climate change increased chances of record rains in Louisiana by at least 40 percent

Sept. 8, 2016

Human­-caused climate warming increased the chances of the torrential rains that unleashed devastating floods in south Louisiana in mid-August by at least 40 percent, according to a team of scientists from Princeton University and partner institutions with the international research network World Weather Attribution (WWA). The research team, which conducted a rapid assessment of the role of climate on the historic heavy-rain event, also found that climate change boosted the chance of rain volume by 10 percent.

"With this attribution analysis we looked specifically at the recent deluge in south Louisiana," said lead author Karin van der Wiel, a postdoctoral research associate in Princeton's Program in Atmospheric and Oceanic Sciences. "We found global warming can play a measurable role in events such as the August rains that resulted in such devastating floods, affecting so many people."

The scientists ​found this type of event is at least 40 percent more probable now than a century ago due to climate change. Given the range in the results, it is likely that climate change approximately doubled the odds of this event. The volume of rain, as measured by three­-day total rain volume in this type of event, is now about 10 percent greater due to climate change.

"This was by far the hardest fast-attribution study we have done, given all the different small­-scale weather types that cause precipitation in the region," said Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute, part of the WWA, which was formed to provide near-real­ time analyses of extreme-weather events. "It was encouraging to find that our multi­model methods worked even for such a complicated case."

For the assessment, scientists conducted a statistical analysis of rainfall observations and used high-resolution climate models to understand how the odds have changed for such three­-day events between the early 20th century and the early 21st century. T​he results were consistent using observational data and climate models.

The research focused on the central U.S. Gulf Coast, and investigated events as strong as that observed at the height of the storm (August 12­-14) to provide a regional context and a broader assessment of risk. ​The climate-model experiments involved altering the climate based on levels of greenhouse gases in the atmosphere, aerosols such as soot and dust, ozone and natural changes in the sun's radiation, and volcanic eruptions to assess how extreme rainfall events respond to climate change.

The storm began when a low­-pressure system carried massive levels of moisture from an unusually warm Gulf of Mexico over south Louisiana where the system stalled, leading to record-breaking precipitation in the region around Baton Rouge, followed by inland flash flooding and river flooding that was slow to recede due to flooding downstream. Louisiana officials reported that as of Aug. 17 the flood had claimed 13 lives, more than 30,000 people had been rescued, more than 8,100 slept in shelters, and more than 60,000 homes had been damaged.

"Extreme-event attribution analyses are meant to characterize the changing nature of weather risk," said Heidi Cullen, chief scientist at Climate Central, an independent climate science and news organization that leads WWA. "The information is critical to insurers, policymakers, engineers and emergency managers as they work to make communities more resilient."

The rapid assessment is published open-access in the journal H​ydrology and Earth System Sciences where it will be reviewed by peers online. While the research has not undergone peer review, the analysis was conducted using regularly used methods.