FORT LAUDERDALE, Fla. -- Ask anyone the most dangerous element of a hurricane, and they may mention wind. They may mention storm surge. But they're unlikely to name the biggest current killer: rain.
In the past three years, as the impact of climate change on hurricanes became more apparent, rain has pushed aside storm surge to emerge as the top source of deaths.
About 75% of the 162 fatalities in hurricanes and other tropical cyclones striking the United States from 2016 to 2018 were caused by rain-induced flooding, with most victims drowning in or near their vehicles, according to the National Hurricane Center. The number excludes the toll from Hurricane Maria, which dumped enormous quantities of rain on Puerto Rico, because of uncertainties in the death count.
The fatalities occurred in a series of particularly wet hurricanes that set state and national rainfall records. Topping these was Hurricane Harvey, a 2017 storm that brought more than 60 inches of rain to parts of southeastern Texas, setting a U.S. record for tropical weather systems. Torrents of water swept away cars, carried off drivers attempting to escape on foot, and threw vehicles off low-lying bridges.
Such scenes may become more common as heavier rain pours from hurricanes forming in a warmer world.
"I think we're at the beginnings of the new normal," said Ben Kirtman, director of the Center for Computational Science Climate and Environmental Hazards Program at the University of Miami's Rosenstiel School for Marine and Atmospheric Science.
"It's pretty simple. A warmer atmosphere holds more water vapor. And so when it comes time to condense all that water vapor and produce rainfall, there's more water vapor available. The second element is the engine for tropical storms; the energy source for that engine is warm ocean surface temperatures, and those have risen. As the climate system warms, the ocean warms. That means there's more fuel for these hurricanes, which can lead to enhanced rainfall."
Several studies attribute Harvey's torrential rain at least partly to climate change. A 2017 study by scientists at the Lawrence Berkeley National Laboratory found that climate change probably increased the hurricane's rains in the Houston area by up to 38 percent.
A study by scientists at the National Center for Atmospheric Research and other institutions found a direct link between Harvey's rain and the unusual amount of heat in the Gulf of Mexico at the time, when the water temperate reached 86 degrees. The study, funded by the U.S. Department of Energy and National Science Foundation, linked the amount of rainfall directly to the quantity of water that evaporated from the Gulf of Mexico, showing that the heat lost from the ocean manifested itself in the amount of water that fell as rain.
"While hurricanes occur naturally, human-caused climate change is supercharging them and exacerbating the risk of major damage," the study said. " ... Harvey could not have produced so much rain without human-induced climate change. Results have implications for the role of hurricanes in climate. Proactive planning for the consequences of human-caused climate change is not happening in many vulnerable areas, making the disasters much worse."
Suzana Camargo, executive director of the Initiative on Extreme Weather and Climate at Columbia University, said all the studies that examined Harvey's rainfall and climate change found a link, differing only in the percentage of the rain they attributed to climate change.
"You'd expect with a warmer atmosphere you can hold more water vapor so you'd have more rainfall, so it's not surprising that we can start seeing that in relation to hurricanes as well," she said. "That was one of the projections that we'd see by the end of the 21st century, but now in some specific hurricanes, we're starting to see it. The projections are for an increase on the order of 20% by the end of the century. But you see already that you can see the signal starting to appear in these storms."
Like a giant sponge, a hurricane absorbs water evaporating off the warm surface of the ocean. As the hurricane proceeds on its path, the sponge is constantly absorbing water and wringing itself out, producing rain. In a warmer climate, the sponge is bigger because warm air can hold more water vapor. And since the ocean itself would be warmer, more water would evaporate into the sponge to return to earth as rain.
With this warmer climate, other recent storms also set rainfall records. Last year Hurricane Florence set records in the Carolinas, with nearly 36 inches of rain, and Hurricane Lane set the Hawaii record with 52 inches.
Hurricane Maria struck Puerto Rico in late September 2017, and deluged the island with up to 38 inches of rain, triggering flash floods and landslides. A study in the journal Geophysical Research Letters found that because of climate change, the severe rainfall of Maria was nearly five times more likely to have taken place than it would have been in the 1950s.
"Our study concludes that extreme precipitation, like that of Hurricane Maria, has become much more likely in recent years and long-term trends in atmospheric and sea surface temperature are both linked to increased precipitation in Puerto Rico," the study said.
These heavy rains led inland flooding to exceed storm surge as a cause of hurricane deaths. From 1963 to 2012, storm surge caused 49% of hurricane deaths, with rain accounting for 27 percent, according to the National Hurricane Center. But in the past three years, storm surge accounted for just 4% of deaths.
Unlike storm surge, which affects only areas near the ocean, torrential rains can kill deep inland. The heaviest rain in Hurricane Florence fell over Elizabethtown, N.C., more than 50 miles from the ocean. The heaviest rain in Harvey fell on the town of Nederland, Texas, about 20 miles inland.
In Beaumont, just north of Nederland, a woman drowned after escaping from her car and being swept away. A minister and his wife tried to drive their pickup through a flooded intersection, got stuck and called 911. When rescuers arrived, they found their bodies in the submerged vehicle.
A family of six died after attempting to cross a flooded bridge in a van. The water picked up the van and carried it over to the adjacent bayou, where it started to sink. The driver escaped and yelled for the children in back to exit through the rear door, but they couldn't make it and were heard screaming as the van went under.
A study by scientists at the National Center for Atmospheric Research looked at how 20 Atlantic hurricanes would be different if they took place at the end of this century, if the average projection for global warming came true. The study found they would generate an average of 24% more rain.
Daniel Brown, senior hurricane specialist and warning coordination meteorologist, at the National Hurricane Center, said that while the future may well bring rainier hurricanes, it would be premature to conclude from a few recent storms that such a future has arrived.
"We've seen slow-moving storms in the past that have dumped copious amounts of rain, and we'll see that in the future," he said. "It takes more than a few storms to see a long-term trend."
Whether or not climate change is responsible for the heavier rains, scientists say there's evidence hurricanes have slowed their forward motion and will continue to do so. That would mean other parts of the country could experience the misery of Texas under Hurricane Harvey, which stalled over the state, pounding it with rain for four days.
A study in the journal Nature found that tropical cyclones have decreased their forward speed by 10% since 1949, and many scientists expect this trend to continue.
"Studies have suggested hurricanes could be slower," said Brown, of the National Hurricane Center. "That is a recipe for very heavy rain-producing hurricanes."
The familiar system for classifying hurricanes, where Category 1 represents a relatively weak storm and Category 5 represents a monster, relies solely on wind speed. Experts say this emphasis on wind carries over to the public, where there continues to be a lack of appreciation for the risks posed by rain.
"People are very wind-centric," said Bill Johnson, emergency management director for Palm Beach County. "We see the damage the wind does, and we pay a lot of attention to that, but unfortunately we don't pay attention to the real killer, and that is water."
Drivers underestimate the power and depth of floodwater. Just 6 inches of fast-moving water can knock down an adult, according to the National Weather Service. A foot of water can carry away a small car. And it's usually impossible to gauge the depth of floodwater.
"After a storm, the water doesn't look deep," Johnson said. "When the road signs are down, it's even harder to determine where the road ends and where the canal begins."
(This story was produced by the Florida Climate Reporting Network, a multi-newsroom initiative that includes the South Florida Sun Sentinel, the Orlando Sentinel, The Palm Beach Post, the Miami Herald, WLRN Public Media and the Tampa Bay Times.)
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