Tuesday, November 14, 2017

Texas’ Odds of Harvey-Scale Rainfall to Increase by End of Century

Study finds state’s annual risk of extreme rainfall will rise from 1 to 18 percent.

“You’re rolling the dice every year,” says professor Kerry Emanuel. “And we believe the odds of a flood like Harvey are changing.” Pictured is an aerial view of Houston during the Hurricane Harvey flooding. (Credit: news.mit.edu) Click to Enlarge.
As the city of Houston continues to recover and rebuild following the historic flooding unleashed by Hurricane Harvey, the region will also have to prepare for a future in which storms of Harvey’s magnitude are more likely to occur.

A new MIT study, published online this week in the Proceedings of the National Academy of Sciences, reports that as climate change progresses, the city of Houston and Texas in general, will face an increasing risk of devastating, Harvey-scale rainfall.

According to the study, the state of Texas had a 1 percent chance of experiencing rainfall of Harvey’s magnitude for any given year between 1981 and 2000.  By the end of this century, the annual probability of Hurricane Harvey’s record rainfall returning to Texas will rise to 18 percent, if the growth of greenhouse gas emissions to the atmosphere continues unmitigated.

If the risk for such an event during this century increased in a steady, linear fashion, it would mean that there was a 6 percent chance of having Harvey’s magnitude of rainfall in Texas this year.

“You’re rolling the dice every year,” says study author Kerry Emanuel, the Cecil and Ida Green Professor of Atmospheric Science and co-director of the Lorenz Center at MIT.  “And we believe the odds of a flood like Harvey are changing.”

When the past isn’t a guide
In the wake of a large disaster, Emanuel says it is natural, and in some cases essential, to ask whether and how soon such an event will occur again.  

“Suppose you’re the mayor of Houston, and you’ve just had a terrible disaster that cost you an unbelievable fortune, and you’re going to try over the next few years to put things back in order in your city,” Emanuel says.  “Should you be putting in a more advanced storm-sewer system that may cost billions of dollars, or not?  The answer to that question depends upon whether you think Harvey was a one-off — very unlikely to happen any time in the next 100 years — or whether it may be more common than you thought.”

Looking at historical records of extreme rainfall will not provide much insight into the future, Emanuel says.  That’s because past measurements have been spotty and difficult to extrapolate across larger regions, and the period over which rainfall data have been recorded is relatively short.  What’s more, climate change is shifting the odds in terms of the frequency of high-intensity storms around the world.

“If the underlying statistics are changing, the past may not be a good guide to the future,” Emanuel notes in the paper.  

Instead, scientists are turning to climate models to try and forecast the future of storms like Harvey.  But there challenges also arise, as models that simulate changing climate at a global scale do so at relatively coarse resolution, of around hundreds of kilometers, while hurricanes require resolutions of a few kilometers.

“[Climate models] do simulate slushy hurricane-like storms, but they’re very poorly resolved,” Emanuel says.  “We don’t have the computational firepower to resolve storms like hurricanes in today’s climate models.”
Hurricanes, embedded
Emanuel and his colleagues had previously devised a technique to simulate hurricane development in a changing climate, using a specialized computational model they developed that simulates hurricanes at high spatial resolutions.  The model is designed so that they can embed it within coarser global climate models — a combination that results in precise simulations of hurricanes in the context of a globally changing climate.

Read more at Texas’ Odds of Harvey-Scale Rainfall to Increase by End of Century

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