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Science That Connects Hurricane Dorian’s Features to Climate Change

The science relating climate change to hurricanes like Dorian is strong as warmer oceans will fuel more extreme storms and rising sea levels boost storm surges leading to worse floods.

This summer, after scrutinizing more than 70 years of Atlantic hurricane data, NASA scientist Tim Hall stated that storms are now much more likely to “stall” overland, extending the time when a community is exposed to devastating winds and drenching rain.

But his sheets of analysis could not prepare Hall for the devastating image of the Dorian swirling as a Category 5 storm, monstrous and nearly motionless, above the islands of Great Abaco and Grand Bahama, on his computer screen this week.

Hall described, “Seeing it just spinning there, spinning there, spinning there, over the same spot, you can’t help but be awestruck to the point of speechlessness”.

After annihilating the Bahamas for over 40 hours, Dorian finally swerved north on Tuesday as a Category 2 storm. It is now expected to skirt the coasts of Florida and Georgia before making landfall again in the Carolinas, where it could again deliver more life-threatening wind, storm surge, and rain.

Marshall Shepherd, an atmospheric scientist at the University of Georgia and former president of the American Meteorological Society, tweeted, “Simply unbelievable. I feel nausea over this, and I only get that feeling with a few storms.”

The overwhelming hurricane has equaled or broken records for its intensity and for its creeping pace over the Bahamas. But it also conformed to a trend: Dorian’s advent made 2019 the fourth straight year in which a Category 5 hurricane formed in the Atlantic making this the longest such streak on record.

In spite of its shocking devastation, meteorologists and climate scientists claim that it bears trademarks of what hurricanes will increasingly look like as the climate warms.

Dorian’s rapid intensification over this weekend was unprecedented for a hurricane that was already very strong. In the space of just nine hours on Sunday, its peak winds amplified from 150 mph to 180 mph (240 km/h to 290 km/h) and by the time the storm hit land, its sustained winds of 185 mph (298 km/h) were recorded as tied for strongest ever observed in the Atlantic.

Jennifer Francis, an atmospheric scientist at Woods Hole Research Center, said that the link between rapid intensification and climate change is robust. The primary source of fuel for a hurricane is the heat in the ocean, and the world’s oceans have absorbed over 90% of the warming of the last 50 years, according to the National Oceanic and Atmospheric Administration.

The water that Dorian developed over was about 1 degree Celsius warmer than normal, Francis commented, “That translates to a whole bunch of energy.”

As warm air can hold more moisture, climate change has raised the amount of water vapor in the atmosphere, causing wetter hurricanes that unleash more extreme rainfall. The warm, wet air similarly gives further fuel to a growing storm.

Francis explained, “When that water vapor condenses into cloud droplets, it releases a lot of heat into the atmosphere and that’s what a hurricane feeds off of. These factors are very clearly contributing to the storms we’ve been seeing lately.”

Models forecast that Category 4 and 5 hurricanes in the North Atlantic could become almost twice as common over the next century due to climate change, even as the total number of storms drops.

After a hurricane makes landfall, the rise in sea level created by global warming can worsen its effects by amplifying storm surge. The strong winds of a hurricane will push water towards the shore, causing extreme flooding in a fairly short time. The higher the water level on a clear day, the worse floods will be after a storm arrives and global sea levels are projected to rise by approximately a meter by the end of the century.

Hurricane Dorian was chiefly striking and devastating because of the manner in which it lingered over the Bahamas. Such “stalling” events have turned out to be far more common in the past three-quarters of a century, according to Hall, a senior scientist at NASA’s Goddard Institute for Space Studies.

In a study published in the journal Climate and Atmospheric Science in June this year, Hall revealed that North Atlantic hurricanes have slowed about 17 %  since 1944 while the annual coastal rainfall averages from hurricanes have increased by nearly 40 % over the same period.

A 2018 paper suggested that tropical cyclones worldwide have slowed significantly.

Hall said that in such stalling events, “You have a longer time for the wind to build up that wall of water for the surge and you just get more and more accumulated rain on the same region”.

“That was the catastrophe of Harvey,” he added, mentioning the hurricane Harvey that dumped over five feet of rain over Texas in 2017.

Both hurricanes Dorian and Florence, the latter of which swamped the Carolinas last year, also fit this pattern.

Hall and his colleagues are certain that there is a “climate change signal” in this phenomenon, though they are still working out the link between human-caused warming and slow-moving storms.

As hurricanes have no engines of their own, they are actually steered across the Earth’s surface by large-scale atmospheric winds almost similar to corks bobbing in a turbulent stream.

If these guiding winds collapse, or even just shift around, a hurricane can get caught in an eddy and “stagnate,” Hall described.

Climate simulations have demonstrated that atmospheric winds in the subtropics, where Dorian is located, are slowing down, thus making these types of eddies more likely.

“But there are a lot of points in the chain of cause and effect that remain to be elaborated,” Hall added.

These stalling events make hurricanes tougher to track. Without an identified large-scale wind to propel them, the storms are rocked about by small-scale fluctuations in their environments that are fairly harder to forecast.

Both Hall and Francis warned that scientists can’t connect any single weather disaster to climate change, especially not during the disaster’s occurrence.

But researchers can evaluate how much worse the disaster was made due to human-caused warming, and how likely it is that similar kind of disaster will occur again.

According to Hall the answers to both those questions are grim with respect to the Dorian.

“This is what we expect more of,” he commented but he admitted that doesn’t think he’ll ever get used to seeing it.

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