RALEIGH, N.C. — Global warming made Hurricane Florence wider and wetter, a study published this month in the journal Science Advances confirmed.
Days before Florence made landfall in September 2018, a team from Stony Brook University’s School of Marine and Atmospheric Sciences compared 10 forecast ensembles to a hypothetically cooler world to determine that the actual storm would produce as much as 50% more rainfall and be about 50 miles wider than it would have been without a warmer climate.
After the storm, the team refined its methodology and ran 96 different ensembles, comparing forecasts under the actual conditions in which Florence developed to those of a world where the sea surface temperature near the Carolina coast was about 1.4 degrees Fahrenheit cooler. Those ensembles showed that Florence’s average rainfall increased about 4.9% in the warmer world, while the storm was about 5.6 miles — or 1.6% — wider than it would have been without warming, the researchers reported.
In a prepared statement, Kevin Reed, the Stony Brook professor who led the study, said, “We found predictions about increases in storm size and increased storm rainfall in certain areas to be accurate, even if the numbers and proportions are not exact. More importantly, this post-storm modeling around climate change illustrates that the impact of climate change on storms is here now and is not something only projected for our future.”
Slow-moving Florence dropped record-setting rain across Eastern North Carolina in September 2018, with the National Weather Service reporting local highs of nearly 36 inches in Elizabethtown and 34 inches in Swansboro. Flooding from the storm damaged an estimated 75,000 structures in North Carolina, including in many areas that suffered flooding during 2016’s Hurricane Matthew.
Studies such as Stony Brook’s are no longer surprising, said Kathie Dello, North Carolina’s state climatologist. Dello noted that similar research after 2017’s Hurricane Harvey had indicated that storm’s rains ranged from 15% to 38% higher than they would have without the impacts of global warming.
Hours after discussing the Stony Brook study, Dello and her colleagues at the North Carolina State Climate Office announced that 2019 had been North Carolina’s warmest year on record. In a blog post discussing the new record, Dello and Corey Davis, a climatologist in the office, wrote, “We are moving rapidly into a different North Carolina than the one we used to know. It is one that is warmer, wetter, and generally more prone to extremes — intense daytime heat, hot nights, and heavy downpours.”
Generally, a temperature increase of 1.8 degrees Fahrenheit means the atmosphere will hold 6% to 7% more moisture. That has implications for hurricanes, which pull their energy from heat and moisture at sea, said Walter Robinson, a professor in North Carolina State University’s Department of Marine, Earth and Atmospheric Sciences.
With Florence, Robinson said, “It was warmer, so right away you expect more moisture. Even if a storm was dynamically exactly the same, you would get more rainfall.”
The Stony Brook study is consistent with that thinking, Robinson added.
And while a 4.9% increase in rainfall may not seem substantial, Robinson made clear that it has major implications in areas where a powerful storm has already pushed rivers and streams into flood stage.
“If the Lumber River is already in flood stage, you make that flood higher because you’ve got more rain. Each increment of flood adds a lot more damage. … Once you’re flooded it’s a very, very steep curve for someplace like downtown Lumberton because more and more buildings suffer flood damage,” Robinson said.
Showing that warmer temperatures are linked with heavier rainfall and larger storms also could be a consideration in how the state recovers and influence future building patterns.
The State Climate Office estimates that the state will see warming of 4 to 10 degrees by the end of the century, meaning the influence of warmer waters on storms is going to become more significant over time. If a Florence-type storm happens a decade from now, Robinson said, rainfall will be heightened by another increment and more places that had previously remained dry during storms will suffer flood damage.
“This is a case where history is not a good indicator of future performances,” Robinson said, “because the future is changing.”