As Climate Changes, Hurricanes Get Wetter

When a tropical storm is approaching, its intensity or wind speed often gets the bulk of the attention. But as Tropical Storm Barry bears down on the Gulf Coast this weekend, it’s the water that the storm will bring with it that has weather watchers worried.

The National Weather Service is calling for roughly 10 to 15 inches of rain to fall from late Thursday night through Saturday. The average rainfall for July in New Orleans, which is in the path of the storm, is just under six inches.

And Tropical Storm Barry, which may become a Category 1 hurricane before making landfall, will drop rain on already saturated land. This week’s rainfall came after the regions extremely wet spring, causing rivers to swell and raising concerns that the looming storm may overtop levees in New Orleans.

Increased rainfall isn’t just a problem for the Gulf Coast this weekend, it’s also a sign of climate change, said Christina Patricola, a research scientist at Lawrence Berkeley National Laboratory and a co-author of a study that found that climate change is making tropical cyclones wetter.

Researchers have been studying the effects of climate change on tropical cyclones because those sorts of storms are driven by warm water. Tropical cyclones include both hurricanes and tropical storms, which are hurricanes’ less speedier kin.

Though storms can form at any time, Atlantic hurricane season stretches from June 1 through Nov. 30 because that’s typically when the Atlantic Ocean’s waters are warm enough to sustain storms. Climate change is altering that system because the oceans are now warmer than ever: They have absorbed more than 90 percent of the heat caused by human released greenhouse gas emissions.

“We wanted to understand how climate change so far could have influenced tropical cyclone events,” she said. “And then the second part is to understand how future warming could influence these events.”

The researchers used climate models to simulate how tropical cyclone intensity, or wind speed, and rainfall would change if hurricanes like Katrina, Irma and Maria had occurred absent climate change and under future climate scenarios. They found that for all three storms climate change increased rainfall by up to 9 percent.

This study is not the first to find that climate change is causing tropical cyclones to drop more rain. Studies on hurricane Harvey found that climate change contributed as much as 38 percent, or 19 inches, of the more than 50 inches of rain that fell in some places. Dr. Patricola’s study, by using global climate models and analyzing a number of storms, broadens the research.

“What’s really interesting is that, regardless of the methodology that you use, we’re starting to see more and more evidence that climate change so far has been enhancing the rainfall on some of these recent hurricane events,” she said.

When the researchers looked at the effect on storms under some possible future conditions, they found that under scenarios with higher greenhouse gas emissions, there would be more rainfall associated with storms. The largest increases would occur over regions, like the Gulf Coast, that have the heaviest historical rainfalls.

In other words, the wetter places are just going to get wetter.

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