Strengthening Storm Takes Aim at Vulnerable Florida Coastline

Scientists say that unusually warm Atlantic surface temperatures have helped to increase storm activity.

“It’s very likely that human-caused climate change contributed to that anomalously warm ocean,” said James P. Kossin, a climate scientist with the National Oceanic and Atmospheric Administration. “Climate change is making it more likely for hurricanes to behave in certain ways.”

Here are some of those ways.

1. Higher winds. There’s a solid scientific consensus that hurricanes are becoming more powerful.

Hurricanes are complex, but one of the key factors that determines how strong a given storm ultimately becomes is ocean surface temperature, because warmer water provides more of the energy that fuels storms. Higher surface temperatures allow hurricanes to reach higher levels of maximum sustained wind.

“Potential intensity is going up,” said Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology. “We predicted it would go up 30 years ago, and the observations show it going up.”

Rapid intensification technically refers to an increase of at least 30 knots, or 35 m.p.h., in the maximum sustained winds over a 24-hour period, according to the National Hurricane Center. Researchers have found that the likelihood of a hurricane undergoing rapid intensification has increased to 5 percent from 1 percent since the 1980s.

Stronger winds mean downed power lines, damaged roofs and, when paired with rising sea levels, worse coastal flooding.

2. More rain. Warming also increases the amount of water vapor that the atmosphere can hold. In fact, every degree Celsius of warming allows the air to hold about 7 percent more water.

That means we can expect future storms to unleash more rain.

3. Slower storms. Researchers do not yet know why storms are moving more slowly, but they are. Some say a slowdown in global atmospheric circulation, or global winds, could be partly to blame.

In a 2018 paper, Dr. Kossin wrote that hurricanes over the United States had slowed 17 percent since 1947. Combined with the increase in rain rates, storms are causing a 25 percent increase in local rainfall in the United States, he said.

Slower, wetter storms also worsen flooding. Dr. Kossin likened the problem to walking around your back yard while using a hose to spray water on the ground. If you walk fast, the water won’t have a chance to start pooling. But if you walk slowly, he said, “you’ll get a lot of rain below you.”

4. Wider-ranging storms. Because warmer water helps fuel hurricanes, climate change is enlarging the zone where hurricanes can form.

There is a “migration of tropical cyclones out of the tropics and toward subtropics and middle latitudes,” Dr. Kossin said. That could mean more storms making landfall in higher latitudes, like in the United States and Japan.

5. More volatility. As the climate warms, researchers also expect storms to intensify more rapidly, they say. They are still unsure about why it’s happening, but the trend appears to be clear.

In a 2017 paper based on climate and hurricane models, Dr. Emanuel wrote that storms that intensify rapidly — the ones whose wind speed increases by 70 miles per hour or more in the 24 hours before landfall — were rare from 1976 through 2005. On average, he estimated, their likelihood in those years was equal to about once per century.

By the end of the 21st century, he found, those storms might form once every five or 10 years.

The rapid intensification of hurricanes can bedevil forecasters, whose assessments affect a community’s preparedness.

The window of time to make a decision gets smaller, Dr. Emanuel said. For example, if officials, working with forecasters, issue an evacuation order too early, then they risk unnecessarily sending hundreds of thousands, and sometimes millions, scrambling, jamming highways and snarling transit systems. In some cases, that could be more dangerous, disruptive and costly than staying in place.

“It’s a forecaster’s nightmare,” Dr. Emanuel said. If a tropical storm or Category 1 hurricane develops into a Category 4 hurricane overnight, he said, “there’s no time to evacuate people.”