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How 'rapid intensification' fueled Hurricane Ida

Climate change is altering some of the atmospheric and ocean dynamics that power hurricanes. That doesn't mean more hurricanes — but it does mean more dangerous ones.
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Long before Hurricane Ida made landfall Sunday in southern Louisiana, climatologist David Keellings was already filled with dread.

As the storm passed over the western end of Cuba and moved over the Gulf of Mexico, Keellings, an assistant professor at the University of Florida, knew it would become fierce and unruly.

"I was saying, 'Uh oh, it’s going to head over some really warm water,'" he said. "And it did."

Once the storm began churning up the Gulf's unusually warm water — 86 degrees at places, even at depths of more than 100 feet — things escalated quickly. Over the next 24 hours, the hurricane underwent a process known as rapid intensification, growing from a Category 1 to a Category 4 storm only shortly before it roared over the coastline.

"I was thinking: 'Oh dear, this is happening too quickly and too close to land,'" Keellings said. "It's going to have major impacts."

Keellings and other scientists are paying close attention to Ida's rapid intensification. Although hurricanes are unique systems driven by a complex interplay of atmospheric and ocean dynamics, experts say how Ida formed and how it behaves portend the types of hurricanes that are more likely because of climate change.

"We expect global warming to make this process of rapid intensification much more likely going forward," said Gabriel Vecchi, a professor of geosciences at Princeton University. "That's a concern for the coming decades."

Scientists are still trying to understand precisely why some hurricanes strengthen so quickly; compared to other extreme events, such as heat waves, drought or wildfires, it's much more difficult to tease out the exact fingerprints of climate change on individual hurricanes. But what's becoming clear is that global warming is creating the ingredients for storms to become more intense, Vecchi said.

A study last year published in the journal Proceedings of the National Academies of Sciences analyzed satellite images from the past four decades and found that the chance that a hurricane will develop into a Category 3 storm or higher increased by around 8 percent per decade, as global warming has accelerated.

And along with strengthening them, climate change is making hurricanes wetter, allowing them to dump enormous amounts of rain.

"The physical mechanism that makes hurricanes wetter in a warming climate is really straightforward," Vecchi said. "A warmer atmosphere holds more moisture, which simply means a storm can rain more."

Scientists have estimated that for every 1 degree Celsius of temperature rise, the atmosphere can hold 7 percent more evaporated moisture.

After it made landfall in Port Fourchon, Hurricane Ida battered Louisiana with winds of more than 170 mph and 15 inches of rain in places. Storm surges up to 7 feet were recorded along parts of the Louisiana and Mississippi coasts.

Rising sea levels due to climate change increase the risk of flooding from storm surges, which are produced when wind pushes water from the ocean inland, causing abnormally high ocean levels.

"All other things being equal, if sea levels are higher, then storm surges will be more damaging," Vecchi said. "The total height of the water will be a combination of whatever tides are there, as well as the sea level rise."

It's not clear whether global warming is making hurricanes more frequent, but warmer sea surface temperatures are increasing the chances that storms will become major hurricanes when they form, Keellings said.

"Storms are not changing in terms of what ingredients they need," he said. "What we're doing is giving them more of all these things that they feed on."

That also means that when these types of storms make landfall, the results can be even more disastrous.

In August 2017, Hurricane Harvey stalled over southeast Texas, dumping up to 60 inches of rain over parts of Houston and the surrounding area. A study published later in the year in the journal Geophysical Review Letters found that rainfall from Harvey was up to 38 percent higher than would have been expected without global warming.

So far, Ida hasn't drenched Louisiana with as much rain as Harvey did in Texas in 2017, but as Ida swings over the Tennessee Valley up into the Northeast in the coming days, rain is one of the biggest concerns, said Texas' state climatologist, John Nielsen-Gammon, a professor of atmospheric sciences at Texas A&M University.

"Even if rainfall totals don't come close to Harvey, places farther inland typically get lower rainfall totals, so they're less prepared for anything like 15 inches of rain in a day," he said.

Large swaths of the storm's track, including parts of Tennessee, have also had wet summers, which means water levels are already higher than normal and soils are more saturated, which can increase the chances of flooding. Vecchi said the dangers of torrential rain should be top of mind for communities along Ida's path.

"It's easy to focus on the center, but the impacts of a hurricane can extend many miles away," he said. "As it moves to the whole middle of the country, through Mississippi and Tennessee and the northern parts of Alabama and Georgia, that's the thing to keep an eye on."