Hurricane Dorian left at least 20 people dead in the Bahamas before making its way up the east coast of Florida and the U.S.
Scientists have said the catastrophic storm was made worse by rising global temperatures.
“While the science has yet to come in on the specifics of just how much worse climate change made Dorian, we already know enough to say that warming worsened the damage,” Penn State University Distinguished Professor of Atmospheric Science Michael Mann and Texas A&M University Professor of Atmospheric Sciences Andrew Dessler recently wrote in an op-ed for The Guardian.
The eye of #Dorian has made a second landfall at 2 pm EDT (1800 UTC) on Great Abaco Island near Marsh Harbour. Maximum sustained winds were 185 mph at the time. This is tied for the strongest Atlantic hurricane landfall on record with the 1935 Labor Day hurricane. pic.twitter.com/O9hrotTTbS
— National Hurricane Center (@NHC_Atlantic) September 1, 2019
“I think that we are at the point where we can now officially start saying hurricane climate is changing. We're watching it happen,” said Jill Trepanier, a hurricane climatologist and associate professor in the Department of Geography and Anthropology at Louisiana State University. “Every season the storms are bigger, they're stronger, they're more devastating. There's a lot of reasons for that, but one of them, and a very significant one, is that the climate is changing.”
There are three main factors making hurricanes like Dorian more extreme: warmer ocean waters, warmer air temperatures, and rising sea levels.
For every degree Celsius, or 1.8 degrees Fahrenheit, that the oceans warm, hurricane wind speeds increase by about 7%. But according to Mann, a hurricane’s destructive potential is proportional to the third power of the wind speed. “That means that we see a roughly 23% increase in destructive potential,” he said. “That's a large enough signal that we can expect to see it with our own two eyes. We can see it playing out with these more destructive storms.”
Here are the current sea surface temperature anomalies (departure from 'normal') in the tropical Atlantic. #Dorian's track took it over waters that are 0.5C-1.0C warmer than "normal" (thread) pic.twitter.com/Vcf50RAstJ
— Michael E. Mann (@MichaelEMann) September 1, 2019
Generally speaking, wind isn’t the main concern when it comes to hurricanes. It’s the water, and hurricanes bring it from both above and below.
Warmer ocean surface temperatures result in more water vapor in the atmosphere and warmer air holds more moisture. Those factors lead to more rainfall during storms.
Meanwhile, sea levels are rising due to melting ice sheets and thermal expansion (like most substances, water expands as it warms). On top of that, more than 17,000 square miles of the U.S. are being affected by subsidence, or the gradual settling or sudden sinking of the surface, according to the U.S. Geological Survey.
That combination of more rain and higher water levels exacerbates flooding and storm surge.
“For example, if you take Superstorm Sandy, that devastating 13 foot storm surge that impacted New York City, Battery Park, that devastated the New Jersey coastline… had around a foot, we’ll say, of sea level rise baked into it,” said Mann. “In the absence of sea level rise, it would have been a 12 foot storm surge rather than a 13 foot storm surge. That might sound like a small difference, but it actually meant the difference between literally billions of dollars of flood damage and I think something like 25 square miles of increased coastal flooding.”
“Imagine now if we get six or seven feet of sea level rise, which is not out of the picture if we continue on the path that we're on when it comes to the burning of fossil fuels and the increase in greenhouse gas concentrations in the atmosphere,” he said.
Many of these intense hurricanes are also exhibiting what climate scientists refer to as rapid intensification - a phenomenon that occurs when a storm passes over very warm water.
When storms ramp up at a rapid pace, as Hurricane Dorian did, it leaves less time to plan evacuations and set up coastal defenses.
“The Bahamas really got the brunt of that rapid intensification,” Mann said. “They really didn't have much time to prepare for what was an historic landfalling hurricane for them.”
Climate scientists increasingly think the slowing down of weather systems like Hurricane Dorian, which was moving at 1 mph as it battered the Bahamas, can be tied to climate change as well.
“It turns out that in the work that we've done, it actually ties into the accelerated warming in the Arctic. The melting of the sea ice is actually changing global temperature patterns in a way that slows down the jet stream,” Mann explained. “And if they become stalled, if they just don't get moved along because of a slow jet stream, then you get the sort of thing that we saw [with Dorian]. It was so tragic in the Bahamas, where it was literally just sitting under this record strength storm for days at a time, which led to devastating flooding and wind damage.”
Hurricane Harvey took a similar track in 2017, stalling over Texas and dumping several feet of water on Houston. “It was the worst flooding event on record and that was in part true because the system slowed down upon landfall,” said Mann.
The combination of all these factors are making devastating storms like Hurricane Dorian more common, the scientists said.
There have only been 35 recorded Category 5 strength hurricanes in the Atlantic, going back to 1924. Dorian is the fifth since 2016.
“Since 2000, since I began studying hurricanes, we see more storms reach closer to that maximum potential intensity. We see more storms that are rapidly intensifying, more storms that are dumping more rainfall and have higher gusty winds at landfall, all of these things in the last 19 years, compared to the 100 years before it,” Trepanier said.
And these storms could get even more powerful as temperatures continue to rise, which is why there has been recent talk of adding a hypothetical sixth category to the scale.
— Alex Harris (@harrisalexc) September 4, 2019
“There isn't an upper limit to how intense they might become, at least not in the near future,” said Mann. “Things only get worse if we continue down this path of our reliance on fossil fuels.”