The scientists had to act fast.
On Aug. 11, 2016, the sky turned the color of a deep bruise over southern Louisiana. The rain fell hard, and the clouds refused to budge. Rain fell at a rate of two to three inches per hour.
The lazy rivers that usually meandered through thick forests and working class towns hulked into angry conquistadors. After just four days, 10 of them — Amite, Bogue Chitto, Calcasieu, Comite, Mermentau, Pearl, Tangipahoa, Tchefuncte, Tickfaw and Vermilion — had raged over their banks. Eight set all-time flood records. The Amite River swallowed entire neighborhoods in the capital of Baton Rouge, the city of Denham Springs, the town of French Settlement and the small village of Port Vincent.
In total, more than 7 trillion gallons of rain poured from the skies onto Creole country. That’s the equivalent of four Lake Pontchartrains. Or three Hurricane Sandys. The storm did more than $10 billion in damage and flooded 146,000 homes. Thirteen people died, including a woman trying to save her 4-year-old grandson, who had to be rescued as he clung for life to a tree.
Alongside the queries about how to help victims emerged a new question: Did climate change cause this storm?
“We received inquiries from the media and other scientists asking what the risk was of such an event and if that risk is changing or not,” says National Oceanic and Atmospheric Administration climate scientist Sarah B. Kapnick, one of nine World Weather Attribution scientists that studied the unnamed Louisiana storm.
The team, divided between Princeton, N.J., and the Netherlands, ran a climate attribution study — a burgeoning field of research that determines the probability that climate change is responsible for an extreme weather event. It also calculates the chance that climate change made a storm, a heat wave or extreme drought more severe.
In the past, these studies have taken years. But the Louisiana team didn’t have that long. Unless they used the best available science to answer the question quickly, they would risk losing the public’s attention to the next news cycle and having political rhetoricians answer the question about climate change’s influence, with no new sound study. As of Aug. 19, neither President Barack Obama or Democratic presidential nominee Hillary Clinton had visited Louisiana, both saying they didn’t want to draw law enforcement resources away from rescue efforts. But on that day, as part of his presidential campaign, during which he referred to climate change as a hoax, Republican candidate Donald J. Trump visited the flood zone.
Climate attribution studies have grown in scope and accuracy since 2004, when one of the first was completed after scorching temperatures in Europe killed 70,000 people. Scientists’ confidence in them is the greatest when analyzing a relatively staid weather event that’s spread out over a large area.
“Heatwaves are really obvious; I can make a statement that’s defensible about a heatwave that’s going to happen next week,” says Michael F. Wehner, climate scientist at the Lawrence Berkeley National Laboratory, who spent years studying the role of climate change in a deadly 2013 Colorado rainstorm. “Storms are a much more complicated problem.”
With attribution studies, scientists “hindcast” instead of forecast. They took all the recorded data from the Louisiana storm — rain amounts, temperatures, barometric fluctuations, wind speeds, chemical and carbon levels in the atmosphere — and ran it through complicated computer model simulators to understand why the storm unfurled the way it did.
They then ran a second set of simulations using data points from 1850 — before the Industrial Revolution when the air wasn’t saturated with mankind’s aerosol and greenhouse gas emissions.
The differences between the two revealed the probability that the Louisiana storm was made more probable and more terrible by climate change.
“Our study for Louisiana concluded that such rains are now at least 40 percent more likely than a hundred years ago,” climate scientists Karin van der Wiel and Sjoukje Philip, from the Royal Netherlands Meteorological Institute, wrote in an email.
The report states, “What used to be an event with a return time of 100 years should now be expected to occur, on average, once every 70 years or likely even more frequently. This trend is expected to continue over the 21st century.”
The attribution study had taken just three weeks — in time to retain the public’s attention, in time to interject sound science into the political debate.
While its influence on executive and legislative branches of governments remains to be gauged, attribution studies may one day find their most receptive audience with the judicial branch, activists say. If an act of God is legally accepted to have probably been an act of men and women instead, victims could sue for damages.
“We expect that claimants will wish to use such evidence when bringing claims against a range of potential defendants,” said Robyn Meadwell, spokeswoman for the environmental law firm Client Earth, “Including states and private actors.”
So in the future, the title “climate attribution study” might become shorter and punchier: “Exhibit A.”
Correction: A previous version of this article stated that Michael F. Wehner is a climate scientist at University of Berkeley, California. NationSwell apologizes for the error.
Homepage photo by Scott Olson/Getty Images.