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Why Is Tropical Storm Barry Such a Threat? The Science Behind the Brewing Hurricane

3 minute read

Tropical Storm Barry, which is forecast to become Hurricane Barry as it hurls forward on a collision course with the Louisiana coast, has experts worried about potentially catastrophic effects, from storm surge to a potential breach of levees meant to prevent flooding.

“The more information we get, the more concerned we are,” said Louisiana Gov. John Bel Edwards at a news conference Thursday as Barry gathered strength in the Gulf of Mexico. “It’s going to be an extreme rain event.” Officials implemented mandatory evacuation orders in some parts of Louisiana, while other residents are being told to prepare to shelter in place and have an emergency plan ready.

Several factors tied to man-made climate change are contributing to the urgent alarm over soon-to-be Hurricane Barry: unusually warm ocean temperatures in the Gulf of Mexico and water levels elevated by a year of record rainfall.

Higher temperatures in the Gulf of Mexico are a key factor as Barry heads toward land. As the planet has warmed, ocean water has absorbed much of the heat trapped in the atmosphere by greenhouse gases — some 93% of excess heat, according to a U.S. government study. That’s helped slow warming in the air. But the phenomenon has contributed to a slew of other problems, including the formation of new storms. The science is relatively simple: warmer water temperatures generate increased water evaporation, providing more rain for a storm to dump when it makes landfall.

Sea-surface temperatures in the Gulf of Mexico specifically were among the warmest in recent decades, according to data from the National Oceanic and Atmospheric Administration (NOAA).

Higher air temperatures also contribute to the formation of stronger storms. Simply put, the air holds 7% more water for every degree Celsius of higher temperatures. Researchers have concluded that global warming made the massive downpours in 2017 Hurricanes Maria and Harvey, which struck Puerto Rico and Houston respectively, much more likely.

“The whole background of the atmosphere has changed since the 1800s,” says meteorologist Jeff Berardelli. “The atmosphere is like a sponge: because we have a bigger sponge it can hold more moisture.”

The slow-moving Hurricane Barry is also likely to produce enormous downpours as it crawls inland. Forecasters said Thursday that Barry is likely to lead to 20 inches of rainfall in parts of Louisiana, which will in turn cause flash flooding. “Flash flooding and river flooding will become increasingly likely,” NOAA’s National Hurricane Center said Thursday, “some of which may be significant.”

The flooding threat is compounded by Louisiana’s vulnerability even before Barry strikes. The previous 12-month period in the U.S. has been the wettest since record keeping began 125 years ago, with average precipitation topping 19 inches, according to NOAA data released this week. That record downfall has left rivers, including the Mississippi River, which runs through New Orleans, more vulnerable to storms than they would be otherwise. Forecasters said the Mississippi River could crest at nearly 20 feet over the weekend, bringing it close to the height of some of the levees built to protect the city after Hurricane Katrina. The U.S. Army Corps of Engineers, which maintains the levees, said Wednesday that a breach is unlikely, according to a report in The Times-Picayune | New Orleans Advocate.

Tropical Storm Barry is already a natural disaster that’s threatening lives and property. But July is still early in the Atlantic hurricane season, and, with water levels near record highs, the worst could still be yet to come.

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Write to Justin Worland at justin.worland@time.com