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Harvey’s ‘Biblical’ rainfall is becoming more likely

WASHINGTON – The chance that a hurricane, flood parts of Texas, as Harvey did, it soared six-fold in only 25 years, as a result of global warming, and will probably triple again before the end of the century, a new study says.

Study author Kerry Emanuel, a professor of meteorology and hurricane expert at the Massachusetts Institute of Technology, found that what was once an extremely rare event — 20 inches of rain over a large part of Texas — may soon be a nearly general.

From 1981 to 2000, the probability of 20 inches of rain happening somewhere over a large part of Texas was 1 in 100, or even less, Emanuel said. It is now 6 on the 100, and by 2081, these opportunities will be 18 on the 100, he said.

“The changes in the odds are, because of global warming,” Emanuel said.

The study was released Monday in the Proceedings of the National Academy of Sciences.

Emanuel said he hurried to the study to help Houston officials to think about what conditions they should consider when they rebuild.

Texas state climatologist John Nielsen-Gammon said he was struck by the possibilities for much more precipitation that Emanuel, the simulations predict for the future and how important it is for the design of critical structures such as dams and nuclear power plants.

“If the worst-case rainfall scenario is getting worse, if Kerry’s study and other evidence, implies that the safety margin is shrinking,” Nielsen-Gammon said in an e-mail, with the emphasis Emanuel of the results also show the worst-case storms wetter and more common.

Gabriel Vecchi, a climate scientist at Princeton university, who is not formally a part of the research, said the study confirms what scientists have already thought: “that the most extreme precipitation events will become more likely as the planet heats up.”

“These results underline the importance of finding ways to incorporate our understanding of climate change in the long term of urban planning, storm water management and flood mapping,” Vecchi said in an e-mail.

For the study, Emanuel had to make use of a number of innovative modeling techniques. Global climate models used for future global warming studies are not detailed enough to simulate hurricanes. Hurricane models that say nothing about the larger climate. So Emanuel combination of the models and then created thousands and thousands of fictional storm “seedlings” to see what would happen.

Emanuel’s calculations of the 20-inch (half a meter) rainfall total for that was the first figure discussed when the storm was finished.

Subsequent measurements revealed that Harvey, the rain was much heavier and much rarer than initially reported. After Emanuel had his work began, data showed Harvey Houston-wide rainfall ended up closer to 33 inches (84 centimeters). And in the individual areas, pit peaked at 60 inches (1.5 meters).

Emanuel called those numbers “bibles.”

“By the standards of the average climate during 1981-2000, Harvey’s rainfall in Houston was the ‘biblical’ in the sense that it probably occurred about once because the Old Testament was written,” Emanuel, the study said.

While some scientists praised the study of the technique, Chris Landsea, science and operations director of the National Hurricane Center, had a number of reservations. He said Emanuel, the results do not fit with the other climate change model projections that are more rainfall totals, but also show a decrease in the number of storms.

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