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How humans are transforming the hurricanes of the future

Global warming is already increasing hurricane rainfall, a new study says — and it could get worse

Hurricane Harvey from the International Space Station, August 28th, 2017.
Hurricane Harvey from the International Space Station, August 28th, 2017.
Photo by Randy Bresnik/NASA

Climate change is already making devastating hurricanes wetter — and similar storms are likely to unleash more rain and faster winds by the end of the century, new research says.

When Hurricanes Harvey, Irma, and Maria struck the US in rapid succession in the fall of 2017, the storms and their aftermaths killed thousands of people and caused an estimated $265 billion in damage. Harvey became the second costliest storm after Hurricane Katrina, which devastated New Orleans in 2005. Now, new research published today in the journal Nature suggests that climate change made Irma, Maria, and Katrina drop more rain than they would have in a pre-industrial world. The study also predicts that in a warmer future, these storms would be even wetter and windier. A second paper (also published today in Nature) took a closer look at Hurricane Harvey. It shows another way people may have made the storm’s rain and flooding more severe: by building cities.

Hurricanes should, theoretically, be more intense and wetter on a hotter planet. After all, warm ocean water fuels hurricanes, and warmer air holds more moisture — which means more rain. Recent studies back that up; analyses of Hurricane Harvey, for instance, reported that 15 percent or more of the rainfall could be attributed to global warming.

Today’s paper provides yet more evidence about the ways in which climate change shapes extreme weather events, says Dim Coumou, an atmospheric scientist at the Potsdam Institute for Climate Impact Research who was not involved in the study. “It’s a very important confirmation of our understanding of how hurricanes change with global warming.” Climate scientist Michael Mann says that some of the results make perfect sense. “We expect greater rainfall and flooding from hurricanes as ocean surface temperatures and atmospheric moisture content increases in a warming world.” But he cautions that the papers use a single climate model, so “it is difficult to draw other general conclusions from these studies.”

The study started because Christina Patricola, a climate scientist at Lawrence Berkeley National Laboratory, wanted to find out how exactly climate change fit into the recent spate of devastating storms. So she simulated more than a dozen hurricanes, including Hurricane Katrina. She and her co-author Michael Wehner plugged in ocean temperature, atmospheric conditions, and watched the simulated storms spin toward land.

The models weren’t perfect, the study acknowledges: the simulations didn’t quite capture Hurricane Maria’s track, or the rapid intensifications of Katrina, Maria, and Irma. But the models were close — and that’s where the experiments could start. Patricola tweaked the conditions to watch what would happen if Katrina, Irma, and Maria had unfolded in a pre-industrial world, and found that global warming boosted the deluge by between 4 and 9 percent. The team also analyzed wind speeds, but didn’t see much of an effect for the recent storms.

That could change in the future. Patricola found that if we continue to burn fossil fuels unchecked through the year 2100, some of those same storms would drop 25 to 30 percent more rain, and the winds would blow up to 33 miles per hour faster. It’s a thorough, convincing study, Coumou says — but there are still some big questions left to answer. The study doesn’t examine how climate change might alter the speed at which hurricanes travel along their paths, for example, or how future hurricane rainfall and storm surge might combine to drive heavy flooding.

The question of flooding is where today’s second study comes in. Hydrologist Gabriele Villarini at the University of Iowa and his colleagues at Princeton University took a close look at Hurricane Harvey, which dumped five feet of rain on Houston and killed at least 93 people. Previous studies analyzed how much of that rainfall could be attributed to global warming, and found it was around 15 precent or more.

We’d expect that when developers slap down impermeable materials like concrete and asphalt, the amount of water slicking off those surfaces would increase — leading to more flooding. But Villarini’s team took a slightly different tack and looked at how the city’s footprint and skyline might have changed both runoff and the rain. They modeled how much rain the hurricane would have dropped on the city had the buildings been replaced with farm fields. And they found that the rough texture of the city with its buildings and complicated skyline snagged and slowed down the humid air flowing over it, boosting conditions for precipitation.

To be clear, Hurricane Harvey would have dropped rain over Houston no matter what was there. But the presence of the city exacerbated the flooding across Houston by about 21 times, on average, the study says. The team doesn’t know precisely why. “The skyline, the tall buildings, the city footprint — we don’t know which one is more important,” Villarini says. They also don’t know if these findings are specific to Harvey, or whether they’d hold up when another storm hits a different city.

Still, the findings suggest that urbanization could make the flooding impacts of hurricanes worse, and in multiple different ways. “This is, as far as I know, a new result and, as the authors point out, has important implications for urban planning,” Kerry Emanuel, a professor of atmospheric science at MIT who was not involved in the study, says in a statement emailed to The Verge.

Together, the two studies show that we need to be ready for the complex ways a warmer world could change extreme weather events. And preparing for these events won’t be easy, according to Villarini. Politicians, urban developers, and engineers will all have to be involved, and will have to work together to figure out ways for their communities to survive future storms. “I don’t think there’s one single way of dealing with this problem,” he says.