Across much of the Carolinas, people who haven’t already fled are bracing for massive rainfall from Hurricane Florence, more than 40 inches in some places. They can blame climate change for much of that misery, according to a novel new study that estimates how much climate change has “supercharged” the hurricane.
Florence, now a Category 2 storm, is slated to make landfall early Friday morning, but the Carolinas could start feeling the impacts on Thursday.
The study, led by researchers at Stony Brook University in New York and the Lawrence Berkeley National Laboratory in California, is a bold new step for the science of climate attribution, which assesses global warming’s fingerprints on extreme weather events.
Every time a weather disaster strikes, people want to know if and how climate change is involved. Scientists have raced to get answers out faster and faster. Now they’ve provided one before a storm has even struck land.
The researchers ran computer simulations of the hurricane under two sets of circumstances. First they used conditions predicted by current weather forecasts, then they subtracted the effect of climate change on temperatures and the amount of moisture in the atmosphere.
There was a big difference between the simulations. “We find that rainfall will be significantly increased by over 50% in the heaviest precipitating parts of the storm,” the researchers said, in a summary of their research released on Wednesday.
They also estimate that Florence, currently measuring about 500 miles across, will be about 50 miles wider when it makes landfall than it would in a world in which human activities had not warmed the planet.
Climatologists contacted by BuzzFeed News warned that there’s some big uncertainty around how much global warming has increased the menace from Florence. But they’re impressed that the researchers came up with an initial estimate so fast.
“This is indeed lightning speed,” Katharine Hayhoe, a climate scientist at Texas Tech University in Lubbock, Texas, told BuzzFeed News by email. “Of course, in order to be conclusive, we will need to see multiple studies using different methods and approaches, but this initial estimate is very impressive.”
“You should be cautious about the quantitative conclusions,” Gavin Schmidt, a climate scientist at NASA’s Goddard Institute for Space Studies in New York City, told BuzzFeed News by email. “But, yes, that people are doing this credibly is new and very interesting.”
“It gives you an idea of what the attribution could look like, but I wouldn’t put so much on the actual numbers,” Friederike Otto, acting director of the Environmental Change Institute at the University of Oxford told BuzzFeed News. Since this analysis is only based on one model, Otto said, “you cannot assess the confidence in the result very well.”
Late last year, Mark Risser and Michael Wehner of the Lawrence Berkeley National Laboratory used similar methods to estimate how much climate change increased rain from Hurricane Harvey, which brought historic levels of flooding in and around Houston. Over the worst hit areas, they estimated that climate change likely boosted rainfall by some 37%.
Other researchers have come up with similar findings. “We’re all within each other’s uncertainty bounds,” Wehner said.
This time around, Wehner and his colleagues decided to work from forecast data rather than waiting for actual weather measurements after the hurricane comes ashore. The simulations were run by Alyssa Stansfield, a PhD student at Stony Brook.
It’s no surprise that global warming should drive bigger hurricanes capable of dumping more rain. Storms form and gather strength in warm waters, and a warmer atmosphere is capable of holding more moisture, leading to heavier rain. (Sea level rise tied to climate change is also driving more damaging storm surge.)
But these factors alone can’t account for all of the increased rainfall estimated by Wehner and his colleagues. “The structure of the storm has to be changing,” Wehner said. “This will motivate even more analysis into the fine-scaled structure of tropical cyclones and how that changes in a warmer world.”
Wehner admitted that he and his colleagues are sticking their necks out in making an estimate of the effect of climate change before the storm makes landfall. But he said that it’s important to provide answers when a hurricane is in the news, not months later when most people are thinking about other issues.
“This kind of study really brings home the point that dangerous climate change is here now,” Wehner said.
Inland flooding risks from Hurricane Florence
Blue shading shows zones forecast to be at risk from flash floods from excessive rain on Sept. 14. Colored circles show flooding forecast for rivers at locations of stream gauges measuring water height; this is only partial guide to likely river flooding.