While rivers continued to crest in the Carolinas as the last of Florence’s rainfall finally made its way back to the sea, scientists and others already were asking what role the warming climate played in the deadly hurricane’s torrential rains and flooding.
They're also studying the impact of rising sea levels on the storm surge that poured into communities along Pamlico Sound.
Frank Tursi, mayor pro tem of Swansboro, is not a scientist. But the long-time resident and long-time environmental journalist believes he’s seeing more intense hurricanes than in the past.
“My sense is that minor hurricanes have a much greater opportunity to build into major storms because of the warmer water temperatures,” he said. Tursi, who started writing about climate change in the 1980s, said it follows predictions he’s heard for years that warming would bring stronger storms and more rainfall.
But can Florence’s impacts — especially the up to three feet of rain over three days — be directly linked to a warming climate and rising seas? That depends who you ask.
“There’s an active debate,” said Jason Evans, an associate professor of environmental science at Stetson University in DeLand, Florida, who’s studied the impacts of sea level rise across the Southeast, including Hyde County and Nags Head in North Carolina. “Of course everyone wants to know the answer. There are scientists who say you can do the attribution and scientists who say you can’t.”
The impact throughout the eastern Carolinas is consistent with broad patterns scientists warned would emerge, with larger and wetter hurricanes, Evans said. But whether Florence can be specifically attributed to a warming climate is “hard to say.”
Florence's devastating flooding draws obvious comparisons to Hurricane Harvey, which dumped more than 50 inches of rain as it lingered over Houston for a week last year, killing 89 people and damaging or destroying 200,000 homes and businesses.
At least 47 deaths have been attributed to Florence, including 36 in North Carolina.
Experts say a combination of natural and man-made factors could be working together to make storms more devastating and more deadly.
“You’ve got more people in harm’s way, you’ve got more property in harm’s way,” said David Easterling, a climate scientist with the National Center for Environmental Information in Asheville. “You combine that with the fact that we’re seeing more of these heavy rain events occur and it’s just a recipe for disaster.”
“I can’t say absolutely positively 100 percent” that Florence is directly linked to climate change, Easterling said. "We could have had a hurricane either way, but Florence was very likely made worse by human-introduced climate change."
Heavy rainfall events are increasing, and not just with tropical cyclones like hurricanes, according to data collected by the National Oceanic and Atmospheric Administration.
Meanwhile, the tropics seem to be trending to lighter winds, particularly the upper level winds that steer a hurricane, he said. That could be contributing to storms like Florence and Harvey slowing down as they reach the coast, where they remain close enough to the ocean to pull in massive amounts of moisture off the ocean.
“And, the slower a storm like that moves, the more rain it’s going to drop," Easterling said. "We have a warming ocean, a warming atmosphere and we have more moisture in it, and all of that is setting up the possibility for more of these kinds of storms to occur.”
Chris Landsea, chief of the tropical analysis and forecast branch at the National Hurricane Center, is among those waiting for more data and research.
There is a link between global warming and increased rainfall in tropical cyclones, said Landsea. But it’s important to assess how much of that change is measurable.
Hurricanes of the past also had heavy rainfall, such as the 48 inches from Amelia in Texas in 1978 or the 45.2 inches from Hurricane Easy on Florida's Gulf Coast in 1950, which remains the state's highest recorded rainfall.
The expected increase in rainfall in a tropical cyclone from man-made influences — the extra greenhouse gases altering the climate — is expected to be “pretty tiny, maybe 3 to 4 percent,” Landsea said. That would equate to about an inch of the rain that fell in the areas that received Florence’s heaviest rainfall, so that alone would not explain Florence’s huge totals.
“We know what causes extreme rainfall in hurricanes and that is it stops moving,” said Landsea. “If the storm changes speed, that’s going to dramatically change how much rainfall you get.”
“When a hurricane meanders ashore and stays around,” he said, like a house guest visiting and not leaving when they should, “you get a huge amount of rain.”
Both Landsea and Easterling said the link between warming and the speed at which a hurricane moves merits further research. They cite a study published in the scientific journal Nature in June by one of their NOAA colleagues, Jim Kossin with the Cooperative Institute for Meteorological Satellite Studies in Madison, Wisconsin.
Are hurricanes slowing down?
Kossin’s research suggested warming in the atmosphere slowed the speed of tropical cyclones by an average of 10 percent between 1949 and 2016, and by as much as 20 percent in land areas impacted by storms in the North Atlantic.
The changes might be compounding or dominating increases in rainfall totals, according to the study. Kossin cited several studies that indicated warming is contributing to “a general weakening of summertime tropical circulation” in the atmosphere, which led him to examine how that might impact hurricanes.
No one knows precisely why the atmospheric circulation is weakening and that needs more research, said Easterling, Landsea and others.
Once forward direction is better understood, especially over a long time scale, said Landsea, “that will give us a better indication of whether we’re going to have more rain from hurricanes or not.”
But to some, Kossin’s study seems prescient as scientists begin to dissect Florence.
The hurricane not only set rainfall records in North Carolina, it became one of the rainiest storms ever to strike the United States over the past 70 years, concluded Ken Kunkel, a climate researcher and meteorologist with NOAA and North Carolina State University, in preliminary research released this week. Based on four-day rainfall totals, Florence dropped an average of 17.5 inches of rain over a 14,600 square mile area. That ranks Florence second only to Harvey for an area of that same size.
Harvey dropped an average of 25.6 inches of rain over a 14,600 square mile area. Harvey also ranks first for an area 20,000 square miles in size, but Kunkel's research showed Florence dropped to seventh in that category.
Similar to Harvey, Florence was blocked to the north and west, with no steering currents pushing it around. At one point, the storm was moving at only 2 mph, said Jessica Whitehead, coastal communities hazards adaptation specialist with North Carolina Sea Grant. “That circulation coming in over the warm water just funnels the water and in this case, it was a fire hose aimed at the Cape Fear River basin."
But Whitehead isn't ready to definitively link that rainfall to climate change.
“I prefer to see those rigorous attribution studies after the event before I say 'x' amount of this rainfall was caused by climate change,” she said. “It certainly fits the pattern of what we expect to see from these storms as a result of climate change.”
A very wet year
One factor that contributed to Florence’s flooding — rain that fell earlier in the year — also may be linked to the warming climate, NOAA scientists said this week. Data collected in locations around the globe show an increase in moisture content as the atmosphere warms.
Rainfall across parts of the eastern United States this year rank among the top 30 percent on record, including many sites that rank among the top 10, including Wilmington, the NOAA data shows. And that reinforces a trend seen in recent decades.
By July 30 this year, rainfall records had been set six times in Wilmington, including a monthly record for May with 14.36 inches. Records were also set 11 times in North Myrtle Beach, South Carolina, National Weather Service records show. That rain likely diminished the area's capacity to absorb rainfall during Florence, exacerbating the flooding as rivers in the region rose with the hurricane's torrential rainfall.
The gauge at Wilmington International Airport recorded 23.05 inches of rain during Florence, pushing the station to a new annual record of 86.25 inches. That’s nearly double the normal — 45.63 inches — through Sept. 26 and topped an 83.65-inch annual record set in 1877, with three months still to go.
The above-normal rainfall this year continues a long-term trend meteorologists are seeing in the Wilmington area, said Tim Armstrong, a climate specialist with the Weather Service office in Wilmington. “The 30-year average rainfall totals have sloped upward by 15 inches over the past 100 years.”
The storm surge from Florence and whether it was affected by the six inch rise in sea levels along the North Carolina coast since the 1970s will be another area that gets intense study in the weeks ahead, said Whitehead.
The wind- and pressure-driven water surged through Pamlico Sound in exactly the right direction to flow into New Bern, said Whitehead. “Belhaven and Little Washington also took some of the bigger surge totals.”
Just how rising sea levels might have had an impact is hard to sort out, she said. So many ingredients go into storm surge, such as the angle of a storm's approach, wind direction and geography where it hits the coast. Complicating factors even further was Florence’s arrival in the days just after a seasonal new moon high tide.
It’s also hard for scientists to compare floods from rainfall or storm surge to determine how they’ve changed over time because landscapes changed so dramatically over the past 50 or 75 years. Landsea and others pointed out the impacts from hurricanes are getting worse because of the urbanization of the coastline, which reduces the capacity for the rain to infiltrate into the ground.
And, while the rise in sea levels may have made coastal flooding worse, so has coastal development, said Evans. “We put a lot of structures in harm’s way.”
The New York-based First Street Foundation is among the groups that believe sea level rise already is making the impact of storms like Florence worse.
In a quick-turn study in the days after Florence, Foundation scientists took readings from 75 stations collecting data when Florence pushed ashore. They compared that with previous studies and concluded some 51,000 homes on the coast were threatened by Florence’s storm surge, said Steven McAlpine, chief of data science for the non-profit foundation's Flood IQ project. Of those homes, some 28,000 were constructed after 1970.
“We also looked at what that could look like in the future,” McAlpine said. “Twice as many homes would be affected by a Florence like storm in 2050, even if there was no additional development.”
Preparing for the worst
Just before Florence hit Swansboro with 34 inches of rain, the second highest total in the state, Whitehead and local officials had completed initial meetings on preparing for climate change impacts. Town officials had mentioned their drainage improvements and thought things would function as needed, she said. And, they’d mapped out all the places they thought might flood as the result of rising sea levels.
But nobody could have imagined the impact of the rain that fell during Florence, she said. “It’s very hard to visualize 34 inches of rain. Every place on that asset map that we said we were worried about flooded."
Tursi said his coastal community could have survived Florence "pretty well, had it just moved on."
“I fully expect that Florence is sort of our wake-up call that we here are on the right path,” he said. “And, that other communities are going to have to start realizing that these kinds of storms are our future."