Think of the Earth’s climate system as a pair of dice. You never know exactly how a roll will end. But some outcomes, like rolling a seven, are much more likely than others, like snake eyes. But when we warm the globe, we essentially load the dice to favor extreme outcomes, including some of the most unpleasant weather possible in the United States.
A new study, rapidly conducted in September and published Monday in the Proceedings of the National Academy of Sciences, finds that the dice are increasingly likely to roll some very unpleasant weather indeed. Global warming may not have caused Hurricane Harvey, which ravaged Houston over the course of a week earlier this year, but it made it much more likely.
The study argues that storms that dump more than 20 inches of rain on Texas are about six times more likely now than they were at the end of the 20th century. Hurricane Harvey dropped 20 inches of rain across a swath of the state, and it deluged some parts of Houston with a record-smashing 60 inches of rain.
Harvey-like hurricanes will only get more likely as the century wears on. Between 1981 and 2000, there was about a one-in-100 chance that a hurricane would dump 20 inches of rain on Texas. By the end of this century—from 2081 to 2100, as children today enter old age—there will be a roughly one-in-5.5 chance that such a flooding hurricane would form if global warming continues apace.
“When you’re planning for the future of cities like Houston, it would be unwise to assume that the climate of the future is pretty much similar to what it’s been for the last 100 years or so,” said Kerry Emanuel, the author of the paper and a professor of atmospheric science at the Massachusetts Institute of Technology. “Even if the climate wasn’t changing, records of rainfall are too short, and the quality of them is too low, to really get a handle on flooding risks.”
“The main reason our technique shows increasing rainfall is that there’s more water in the air.”
Global warming will worsen hurricanes for two reasons. First, otherwise identical storms of all types—not just hurricanes—will retain more moisture in a warmer climate. “It’s one of the risks that we’re most confident of,” he told me. “The physics are so elegantly simple: Warm the air and it can hold more water.”
Second, global warming will make storms move slower. What made Hurricane Harvey so damaging was not its intensity but its longevity: The cyclone moved ashore, parked over Houston, and then rotated in place for three days. It became an atmospheric conveyor belt, picking up water from the Gulf of Mexico and raining it out over the Houston metropolitan area. These days of rain caused the city’s devastating inland floods.
In Emanuel’s study, global warming helped slow similar hurricanes down. Though tropical cyclones unleash some of the fiercest winds on Earth at their center, their movement across the surface of the planet—and thus their storm track—is determined by the slower high-altitude winds summoned by distant high- and low-pressure systems. This planetary engine is slowing down as global warming pushes land and ocean temperatures closer together.
“If [general circulation] slows down, then places near the coast will get more rain,” Emanuel said. “But the main reason our technique shows increasing rainfall is that there’s more water in the air.”
Emanuel conducted the study through the use of a computational tool he pioneered: embedding a hurricane and ocean model in a coarse, planetary-scale climate model. To determine the current climate’s propensity for a Harvey-like storm, he used three meteorological models (with the embedded hurricane tool) to simulate thousands of years of late-20th-century weather. Then, he used six different global climate models (each with the same hurricane tool) to simulate thousands more years of late-21st-century weather. By examining the models’ output of rainfall data and storm tracks, he could arrive at more accurate probability risks.
“When you are looking at extremes, it’s good to look at all rain-producing events.”
“This analysis provides strong evidence for why climate-change information should be incorporated into any plans for building Houston back stronger,” said Heidi Cullen, the chief scientist and director of the World Weather Attribution program at Climate Central, a nonprofit based in Princeton, New Jersey, in an email.
She said the study was a “very nicely constructed analysis” that “makes it clear that we must act quickly to both adapt to the changes that are already in the pipeline while also reducing our dependence on fossil fuels.”
But it did not capture the future risk of flooding rains in the Houston area or Texas more generally, she added. “About one-third of extreme precipitation events on the Gulf Coast are not associated with a hurricane,” she told me, citing a forthcoming Climate Central analysis of Hurricane Harvey. “When you are looking at extremes, it’s good to look at all rain-producing events.”
While this paper was prepared for publication quickly—Emanuel said he rushed it toward completion so it could inform Houston’s rebuilding efforts—it uses some of the most cutting-edge techniques in climate science. Embedding a cyclone model inside a climate model helped a team of researchers conclude last month that New York City will soon see major, hurricane-induced flooding every five years.
And Emanuel’s scholarship helped inform the Climate Science Special Report, released this month by 13 U.S. federal agencies, which predicted that global warming will increase the strength and destructiveness of hurricanes by the end of 21st century.
The paper also examined how likely it is that a Harvey-like storm would hit the Houston metro area specifically. While major flooding cyclones will be once-in-100-year events by the end of this century, they were as rare as once-in-2,000-year events at the end of the last. “By the standards of the average climate during 1981–2000,” the paper says, “Harvey’s rainfall in Houston was ‘biblical’ in the sense that it likely occurred around once since the Old Testament was written.”