As Louisiana struggles to recover from last month’s historic flooding—which displaced thousands and is expected to total at least $15 billion in damages—the National Oceanic and Atmospheric Administration has issued grim predictions for the future. Due to climate change, the odds have doubled that the region will experience another extreme downpour like the type that caused the flooding.
Such heavy rainfall—Louisiana received more than 2 feet of rain during the flood—has typically been considered rare. Historically, that kind of rain at any given location is classified as a once-every-500-years event. The historical frequency of an event of this level happening somewhere along the Central Gulf Coast, according to a NOAA, is usually around once every 50 years.
According to a recently published study by NOAA, however, man-made global warming has led this kind of devastating rainfall to occur at least 40 percent more often. The report’s lead author, Karin van der Wiel, also said in a statement that, “while we concluded that 40 percent is the minimum increase in the chances of such rains, we found that the mostly likely impact of climate change is a near doubling of the odds of such a storm.”
As NOAA explained in a press release, Louisiana had the ideal conditions for a destructive storm:
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. The rains were followed by inland flash flooding and river flooding that was slow to recede due to flooding downstream.
To reach their conclusion, the researchers—which also include climate scientists at Princeton University, the Dutch think tank Climate Centre, and the news site Climate Central—applied a widely used statistical method on historical data to study how the frequency of a three-day rainfall similar to last month’s has changed along the Gulf Coast from the pre-Industrial era to now. They also studied the frequency under different climate scenarios, using two high-resolution models from NOAA that altered the levels of man-made greenhouse gases. In some scenarios, greenhouse gases increased at the current rates; in others, the levels stayed closer to those of the pre-Industrial past.
What once occurred only once every 50 years now happens more like every 30 years on the Gulf coast. And a 30-year event today results in 10 percent more rain than a similar event in 1900.
“We found human-caused, heat-trapping greenhouse gases can play a measurable role in events such as the August rains that resulted in such devastating floods, affecting so many people,” van der Wiel said.
The study, which was conducted shortly after last month’s flooding, is published in the journal Hydrology and Earth System Sciences. It has not been peer-reviewed, but researchers said they will release an updated version in coming months with comments and criticism from other experts. In the meantime, when the Associated Press reached out to outside scientists regarding the data, most of them called the methods appropriate and thorough.
This isn’t NOAA’s first paper linking climate change to extreme downpour, nor are they the first ones to make the connection. In 2014, the agency found that the amount of rainfall during heavy storms has increased by 20 percent over the past century. And as CityLab reported after Hurricane Sandy, scientists have long been making dire predictions:
Rainfall totals that were considered 1-in-100-year events in the 1950s through 1970s had become 1-in-60-year events in the 1980s through 2007. Some scientists say that rain events in the Northeast that are now 1-in-20-year events will take place every four to six years by the end of this century.