SCOTT SIMON, HOST:
One of the deadliest and most costly hurricane seasons ever seen in the Atlantic ends today. More than a hundred and fifty people died from direct causes in Hurricane Helene, which tore through Florida and Georgia and brought severe flooding and destruction to North Carolina. But that was just one storm in a very busy season. NPR's Greg Allen reports.
GREG ALLEN, BYLINE: Before the season began, scientists warned there were likely to be a lot of hurricanes. Record-high ocean temperatures in the Atlantic 2 to 3 degrees warmer than normal and other atmospheric conditions set the stage for above-normal activity. As it turned out, because of a lull in August, the 18 named storms and 11 hurricanes were fewer than had been predicted. But after that break, in September, things picked up again in a big way. Hurricane Helene left a path of destruction from Florida to North Carolina.
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ROY COOPER: We did an aerial overview to see the damage here in western North Carolina, and it is extensive and devastating.
ALLEN: That's North Carolina Governor Roy Cooper. The director of the National Hurricane Center, Michael Brennan, says Helene's impact and the number of fatalities were the greatest seen in the U.S. since Hurricane Katrina almost two decades ago. Days before Helene hit, the Hurricane Center's forecasts were remarkably accurate. Meteorologists warned there would be catastrophic flooding in western North Carolina days in advance. Despite that, Brennan says as many as 90 people died in Helene's flooding.
MICHAEL BRENNAN: It's difficult when you have an event that's never been seen before in a community to convey what that impact is going to necessarily look like on the ground. And it's also challenging because that level of flooding happened over such a large area.
ALLEN: Helene made landfall as a powerful Category 4 hurricane with 140 mph winds. It was 1 of 5 major hurricanes this season, several of which reached Category 5 but weakened before coming ashore. Brennan says it's clear climate change is making hurricanes more powerful. Michael Mann, a climate scientist at the University of Pennsylvania, says the warm sea temperatures are helping storms strengthen dramatically over the course of several hours - what's known as rapid intensification.
MICHAEL MANN: It could be a tropical depression one day, and, all of a sudden, within 24 or 48 hours, it's a major hurricane, and it becomes extremely difficult to plan for.
ALLEN: In October, Hurricane Oscar went from a small tropical depression to hurricane strength in just five hours. The National Hurricane Center had to scramble to issue warnings to Cuba and other Caribbean islands. Climate scientist Daniel Gilford says warming ocean temperatures are pushing hurricane intensities up by an average of 18 mph. In a recent study, he analyzed the last several seasons.
DANIEL GILFORD: Five out of every six hurricanes had this really strong, statistically robust signal where human-caused climate change was really clearly increasing the intensity of these storms.
ALLEN: Gilford says because of climate change, hurricanes are now a full category higher than they would have been in earlier decades. Penn climatologist Michael Mann says, with the laws of physics, higher wind speeds make hurricanes much more destructive.
MANN: So if you have a 10% increase in wind speeds from human-caused warming, that'll lead to, like, a 33% increase in the destructive potential of these storms.
ALLEN: Fueled by the extra heat from the ocean, storms are also picking up more moisture and then dropping it in heavy rainfall events, such as that seen when Helene hit North Carolina. Gilford, a researcher with Climate Central, says as hurricanes grow larger, stronger and wetter, they're now posing an increased threat to inland areas far from the coast.
GILFORD: Hurricane Helene is especially a lesson that certain places that maybe wouldn't have been experiencing these intense effects before really are today because of climate change.
ALLEN: Along with hundreds of deaths, damage from all storms this season is estimated at more than $190 billion - second only to 2017, the year of hurricanes Harvey, Irma and Maria.
Greg Allen, NPR News, Miami.
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