Hurricane Harvey, which inundated the Houston area with up to 60 inches of rain last August, was one of the most outlandish storms ever to hit the U.S. Ironically, it crossed a Gulf of Mexico that had been calm for days and quickly quieted again afterward. This rare situation allowed scientists to obtain unusually specific data about the ocean before and after the hurricane, and about the storm’s energy and moisture.
Last week researchers published that data in Earth’s Future. The numbers indicate the amount of energy Harvey pulled from the ocean, in the form of rising water vapor, equaled the amount of energy it dropped over land in the form of rain—the first time such an equivalence has been documented. Investigators say this revelation supports assertions climate change is likely to make Atlantic hurricanes bigger, more intense and longer-lasting than in the past. The researchers calculate climate change caused Harvey’s rainfall to be 15 to 38 percent greater than it would have been otherwise.
Kevin Trenberth, a senior climate scientist at the National Center for Atmospheric Research, led the team. Scientific American asked him why Harvey behaved so strangely, how it confirms predictions about changing hurricanes and what the U.S. and other nations prone to the storms should prepare for in the future.
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[An edited transcript of the interview follows:]
You say hurricanes will get bigger, and last longer. Why?
As climate change makes oceans hotter there is more heat—more energy—available, so there is likely to be an increase in hurricane activity. That can be the size of the storms, their duration and their intensity.
So hurricane dynamics are really driven by ocean energy?
Right. The way that energy moves around is transporting water vapor in the atmosphere—in this case, pulled up into the storm and dumped over the land in Texas. As the water vapor condenses it releases the latent ocean heat into the atmosphere. Our study was the first time anyone has been able to match up these two numbers.
Is moving energy a hurricane’s main role in the climate system? Why do we even have them?
A hurricane moves heat out of the ocean rapidly. It keeps the oceans cooler. A hurricane is actually a relief valve for the tropical ocean.
Why does it need a relief valve?
In general, the global weather system doesn’t like to have big temperature contrasts. If it’s hot in one place and cold in another, that produces wind that blows the warm air toward the cold and the cold air toward the warm. The atmosphere is always trying to remove those temperature gradients.
Similarly, thunderstorms move heat upward from a hot ocean to a cooler atmosphere but they don’t have the strong winds that a hurricane does, which produce the very large evaporating out of the ocean into the atmosphere. A hurricane is a collection of thunderstorms, but having the same number of thunderstorms without hurricane winds doesn’t cool off the tropical ocean to the same extent. Without hurricanes the tropical ocean would get really hot and the contrast between there and the middle latitudes would create different weather systems than we have now.
Warmer oceans mean more intense hurricanes. But you note that the number of large storms might actually decrease. Why?
By pulling up an ocean’s heat, a hurricane leaves a colder ocean in its wake. One big storm creates more cooling than, say, four smaller storms. It leaves a cooler ocean that is less favorable for a new storm.
That explains why Harvey got so big; your paper notes that the Gulf of Mexico water temperature was several degrees hotter than usual for late August. But why did Harvey stay big?
Over the ocean a hurricane’s circulation typically reaches about 1,000 miles in all directions, grabbing moisture and bringing it into the storm. Once over land the storm dries and weakens. But even when two sides of Harvey were over land, the spiral arm bands reached well out over the Gulf, which was still very warm. That kept the storm going.
After several days the storm moved back over the Gulf before it came inland again, and then moved north. We don’t attribute that movement to climate change. But the fact that when it did come back over the Gulf it reintensified is very much related to climate change. Despite the fact that Harvey had taken a lot of energy from the Gulf and cooled the waters in the upper 100 meters in particular, the deeper Gulf was still warm enough to well up and sustain hurricane-force winds.
The related question is why did Harvey remain over Houston so long? Some experts say it had to do with climate change altering the nature of the jet stream—giving it bigger bends, causing it to meander more slowly from west to east across the U.S.— which can help weather systems get stuck in one place.
Some people suggest that may have played a role. I’m inclined to think it didn’t. The storm track does depend on the weather; a high-pressure system blocked Harvey from moving north or northeast over land as it normally would have. But that high wasn’t part of a big jet steam wave structure. Also, the idea of a slow jet stream, because of changing conditions in the Arctic, is still a controversial topic. I and others think the bigger factor is the tropical Pacific—systems like El Niño and so on. We need to research this more, but I think the effect of the tropics is much greater than the effect of polar regions.
The end of the paper addresses what society needs to do to prepare for stronger hurricanes. It’s commentary, which is unusual. Why did you decide to include this?
Physical scientists often do not talk about impacts and consequences. They often leave that to social scientists or economists. But we need to connect the physical and social effects much more. The Earths’ Future journal is designed to help bridge that divide. Our study is relevant to a lot of policy. And it’s especially relevant to the current administration, whose operational rule seems to be to do away with regulations even when they make sense and are based in science.
You also recommended actions hurricane-prone regions should take. What is the most crucial?
The hurricane damage last season in Puerto Rico, Florida and Texas shows that infrastructure should have been hardened for such storms, which were certainly going to come sooner or later. There were many meetings—I was at some of them—in which politicians, heads of countries and states, were very aware of two main threats: sea level rise and intense hurricanes. But they haven’t done much to prepare.
In Texas, after Hurricane Ike in 2008, there were proposals to add flood control measures in Houston, and they were voted down. In contrast, places like Taiwan, in the typhoon belt, hardened their infrastructures: drainage systems, building codes to withstand category 4 and 5 storms, emergency systems. In 2015 Taiwan had four typhoons that caused flooding and damage—but each time, within about four days they were back up and fully operational because they had built-in resilience and were prepared. Places in the U.S. need to make that kind of investment.