Unprecedented extremes or the new normal?

When Hurricane Helene struck the Southeast in late September, it quickly became one of the deadliest U.S. hurricanes of the last 50 years, second only to Hurricane Katrina. At least 228 people died as a result of the storm. Helene was also one of the largest storms in recent decades, similar in size to Katrina at 400 miles wide. Nearly 2 million people lost power as unprecedented winds and rain hit Florida, Georgia, South Carolina, North Carolina, Virginia, and Tennessee.

But in a changing climate, what does it mean for a storm to be unprecedented? How are hurricanes changing, what are the impacts, and what does the future have in store?

Two Tufts representatives joined a group of 21 researchers from around the world who were given 48 hours by the World Weather Attribution (WWA) to conduct a rapid study in the wake of Helene. Tufts climate risk expert, Erin Coughlan de Perez, helps explain how climate change is affecting the severity of hurricanes like Helene.

The cone of uncertainty

“As hurricanes become more intense, one of the most important tools in our toolbox are weather forecasts,” says Coughlan de Perez, associate professor in the Friedman School of Nutrition Science and Policy at Tufts University.

Coughlan de Perez works with groups like the American Red Cross and the United Nations to incorporate climate models into contingency and financial planning for storms and other weather events like droughts, floods, and heat waves. In hurricane forecasting, the most likely path a specific storm might take is marked by a “cone of uncertainty,” which gets updated as the storm progresses.

“Because of good forecasting and early warning systems, people deployed well in advance of Helene. So many people evacuated, which takes courage. Forecasting can be a matter of life and death,” Coughlan de Perez says.

But Coughlan de Perez also says that hurricane assessments are far too focused on wind speed, which dictates the category of a hurricane. Categories, therefore, have nothing to do with the expected amount of rainfall or the storm surge from the ocean.

“One of the major reasons Helene caused so much damage is because of the water that it dumped in North Carolina,” Coughlan de Perez says.

Extreme rainfall from tropical storms is increasing due to climate change—warmer air holds more moisture at a rate of 6 to 7% per degree Celsius. In today’s warmed climate, rainfall of the level brought by Helene occurs every seven years in coastal areas and every 70 years in inland regions.

According to the WWA report, climate change made the rainfall about 10% heavier and 70% more likely in the Appalachian region and 10% heavier and 40% more likely in the southern region.

The WWA report also notes that tropical storms in the North Atlantic that make landfall also tend to meander at slower speeds across the landscape and stall more frequently, which means they can sit over one area for a long period of time, dumping more rain than any one place can handle.

During Hurricane Sandy in 2012, sea level rise due to climate change created a higher storm surge, directly causing an additional $8.1 billion in damages. Storm surge forecasting is based on many factors, but the key to forecasting storm surges is locating the eye.

“Storms move counterclockwise in the northern hemisphere, so one side of the storm (the ‘dirty’ side) pushes the water into the land and the other side of the storm pulls it away from the land,” Coughlan de Perez says. “But getting the location of the eye forecasted perfectly is really difficult.”

The 15-foot-high storm surge that hit the Florida coast during Helene broke records.

Unprecedented extremes

“Unprecedented” can mean a lot of things: a hurricane can hit at an unprecedented time of year; wind speeds and rainfall can be at unprecedented levels; and hurricanes can hit in unprecedented locations.

“To study unprecedented extremes, we run weather models thousands of times to determine all the possible weather scenarios, only one of which we will end up experiencing. We look at what happened in the other ‘worlds’ and factor that into scenario planning,” Coughlan de Perez says.

The WWA report found that hurricanes as intense as Helene are now 2.5 times more likely and expected to occur once every 53 years because of climate change. Their intensity is determined by several complex factors, like sea surface temperature, air temperature, and humidity.

Helene’s wind speeds on the coast of Florida were about 13 miles per hour faster (or 11% more intense) because of climate change. Helene was fueled by high sea temperatures made 200 to 500 times more likely by climate change. Climate change is also contributing to the rapid intensification of hurricanes.

The Gulf of Mexico was 3.6 degrees Fahrenheit above average when Helene formed. The storm intensified rapidly, transforming from a Category 2 to a Category 4 in just a few hours. Hurricane Milton, which hit the Florida coast a couple weeks after Helene, was the third-fastest intensifying storm on record in the Atlantic, moving from a Category 1 to a Category 5 in less than a day. Coughlan de Perez says that rapid intensification makes forecasting much more difficult.

Compounding extremes, like heat waves, also complicate hurricane assessments. The highest density of power outages during Helene were observed in areas with the highest temperatures. Many areas of Florida declared heat advisories after Helene made landfall and, without power, people lacked air conditioning. Heat waves are the leading cause of death from climate-related disasters in the U.S.

Vulnerability & risk perception

The WWA report found that, in the 88 counties that declared disasters after Helene, 30% of people are below 150% of the federal poverty level, 29% are minority populations, 16% have a disability and 6% don’t have a vehicle. 22% of the housing units were mobile homes.

“Identifying who the most vulnerable people are allows us to create systems that prevent them from being disproportionately impacted by something like Helene or Milton,” says Carolyn Van Sant, project manager for the Feinstein International Center and one of the WWA report contributors.

Even evacuating is a privilege. Van Sant notes that it might require paying for a hotel and owning a car, and that it is made easier by having a remote job or some form of disaster insurance. But whether someone has disaster insurance largely depends on their long-term risk perception.

Leah Poole, a Ph.D. student in the Agriculture, Food, and Environment Program at the Friedman School and one of the contributors to the WWA report, says that even though tropical storm Fred brought flash flooding to the North Carolina mountains in 2021, killing six people, she found that those communities had very low risk perception.

According to the WWA report, enrollment in flood insurance was two times higher in Florida than in North Carolina.

The future of storms

The WWA report found that if warming reaches 2 degrees Celsius, storms similar to Helene will become an additional 3% more intense and 25% more likely in the Appalachian region, as well as 3% more intense and 16% more likely in the coastal region.

“Most studies on climate change adaptation find that what we’re doing is not fast enough to keep up with the current climate, so looking at the past is not helpful in understanding what is possible today and in the future. It’s like driving while looking in the rearview mirror,” Coughlan de Perez says.

Yet despite the work that still needs doing, Coughlan de Perez remains hopeful.

“I find it extremely encouraging to see how people came together, not only to respond to these hurricanes, but to prepare for them,” she says.