Climate Change Loaded the Dice for Louisiana Flood, Experts Say

It would be a gross understatement to say that the mid-August deluge in central Louisiana was a thousand-year flood, according to an analysis by the National Oceanic and Atmospheric Administration. The amount of rain that drenched Baton Rouge and neighboring communities in three days would have constituted a 1-in-1000 year event even if it had fallen over a 20-day period, according to the agency’s “annual exceedance probability” analysis.

Yet a top official at NOAA acknowledged at the time that this analysis is based upon backward-looking statistics rather than forward-looking assessments that incorporate the increasing atmospheric volatility associated with a warming global climate. Such events are likely to occur much more frequently in the future, said David Easterling, director of NOAA’s National Centers for Environmental Information.

Now a new rapid assessment by an international team of experts has put a number to the new probability. The World Weather Attribution (WWA) project is an international effort designed to sharpen and accelerate the scientific community’s ability to analyze and communicate the possible influence of climate change on extreme-weather events such as storms, floods, heat waves and droughts. WWA’s analysis of the Louisiana flood presented its first quantitative estimate of the role climate change plays — both in terms of frequency and intensity — in an extreme precipitation event like the August deluge in Louisiana.

The bottom line: The Louisiana torrent is best characterized as a 1-in-550 year event locally based upon climatic conditions in 2016. And that’s a moving target as the new normal shifts toward an ever-changing likelihood of more frequent and extreme events. Given projections of accelerating global warming, the probability of such events will only increase into the future.

“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 WWA’s international research network. “It was encouraging to find that our multi-model methods worked even for such a complicated case.”

It’s worth noting that the study did not examine how infrastructure and urban planning in Louisiana affected flooding or its impacts. Land-use patterns, of course, determine to what extent a torrential downpour translates into damaging floods. The research focused more narrowly on the role of climate change and natural variation in causing such heavy rains.

The World Weather Attribution project also has analyzed the May 2016 floods that killed at least 18 people in Germany, France, Romania, and Belgium. Their analysis concluded that the probability of 3-day extreme rainfall in Spring has increased by at least 40 percent in France, with the best estimate of about 80 percent on the Seine and about 90 percent on the Loire.

As reported by FloodList in June, World Weather Attribution also is involved in efforts to understand and communicate the role of climate change on extreme weather events in East Africa and South Asia.