05/09/2019

How Has Climate Change Affected Hurricane Dorian?

New York Times - John Schwartz

Credit NASA/EPA, via Shutterstock
The links between hurricanes and climate change are complex, but some aspects are getting clearer.
Tropical storms draw their energy from ocean heat — and more than 90 percent of the heat trapped by greenhouse gas emissions is being stored in the ocean. Storms that survive the cradle of formation can intensify quickly and become immensely powerful.
While it’s common to hear the question, “Was it caused by climate change?” scientists argue that this is an unhelpful way to look at the issue. As Katharine Hayhoe, a climate scientist at Texas Tech University, put it recently on Twitter, “that’s the wrong question. The right one is, ‘how much worse did climate change make it?’”
Like so many hurricanes, Dorian’s origins were unassuming. At 11 a.m. on August 24, the National Hurricane Center in Miami announced a new tropical depression east-southeast of the Lesser Antilles. At the time, it was just Tropical Depression Five.
Now, as that same storm slowly moves away from the Bahamas — which experienced a nightmare scenario of a Category 5 storm stalling over it for 24 hours — it begins its slow roll toward the East Coast of the United States.
A number of recent storms have stopped in one place for extended periods of time, including Harvey, which sat over Houston for days in 2017 and caused unprecedented flooding.
Recent research suggests that climate change has made stalled Atlantic storms more common since the mid-20th century, and that they are more dangerous because they stay in one place for a longer period of time, potentially concentrating their destruction.
Jennifer Francis, a scientist with the Woods Hole Research Center, said, “This is yet another example of the kind of slow-moving tropical systems that we expect to see more often as a response to climate change. Upper-level steering winds are slowing over the continents during summer, so stalling weather systems are more likely.”
Credit Johnny Milano for The New York Times
Hurricanes are steered in part by high-atmosphere winds not directly related to the storm. Dorian slowed to a crawl — about one mile an hour — because the tropical winds that were pushing it westward over the Bahamas weakened, said Joel Cline, a meteorologist with the National Oceanic and Atmospheric Administration in Silver Spring, Md.
Climate change is making hurricanes more destructive in many ways.
Andrew Dessler, a climate scientist at Texas A&M University, said that some of the links between hurricanes and climate change are still being worked out. But, he said, some attributes of storms, particularly the increasing amount of rainfall associated with many of them, has reached a very strong consensus.
A similarly solid consensus has developed about storms getting stronger. There is somewhat less consensus, he said, around the idea that storms are likelier to stall.
The problem with looking for answers in individual storms, he said, is that “because these are rare events, it’s really hard to get good statistics. Picking out trends is difficult.” And so the field draws conclusions from physics and models, and “physics rules the system.”One of the other characteristics of Dorian: Its course has, at times, been difficult to forecast. Gabriel Vecchi, a professor of geosciences at Princeton University, said that forecasts of the storm’s path have actually been “in relative agreement,” but that the people find the remaining degree of uncertainty unsatisfying. “The problem is our standards have gotten so high, because the forecasts have gotten so good,” he said.Mr. Cline said that over the previous two days, Dorian’s track had more closely matched the forecasts. As it travels up the East Coast the storm’s behavior may become even more predictable, he said, as it encounters the typical west-to-east wind flow that defines most of the weather in the Mid-Atlantic region. That should drive Dorian increasingly to the northeast, he said.
Dr. Vecchi warned against trying to attribute too many elements of an individual storm to climate change right away, however, since attribution science has so far been most successful in terms of rainfall. He cautioned against saying that every intense storm was made more powerful because of global warming, since “there have been intense storms in the past.”
Instead, he put it this way: Dorian “looks like what we’re going to have more of in the future.”

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