Scientists say unusually warm surface temperatures in the Atlantic have contributed to increased storm activity. “It is very likely that human-induced climate change contributed to this unusually warm ocean,” said James P. Kosin, a climate scientist at the National Oceanic and Atmospheric Administration. “Climate change makes it more likely for hurricanes to behave in certain ways.” Here are some of those ways.

  1. Greater winds. There is a solid scientific consensus that hurricanes are getting stronger. Hurricanes are complicated, but one of the key factors that determines how strong a given storm will ultimately become is ocean surface temperature, because warmer water provides more energy that powers storms. “The potential intensity is increasing,” said Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology. “We predicted it would go up 30 years ago, and observations show it’s going up.” Stronger winds mean downed power lines, damaged roofs and, when combined with rising sea levels, worse coastal flooding. “Even if the storms themselves didn’t change, the storm surge leads to raised sea levels,” Dr. Emanuel said. He used New York City, where sea levels have risen about a foot over the past century, as an example. “If Sandy had happened in 1912 and not 2012,” he said, “Lower Manhattan probably wouldn’t have flooded.”
  2. More rain. Warming also increases the amount of water vapor the atmosphere can hold. In fact, each degree Celsius of warming allows the air to hold about 7 percent more water. This means we can expect future storms to unleash more rain.
  3. Slower storms. Researchers don’t yet know why storms move more slowly, but they do. Some say slowing 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 by 17 percent since 1947. Coupled with the increase in rain rates, storms are causing a 25 percent increase in local precipitation in the United States, he said. Slower, wetter storms also make flooding worse. Dr. Kossin likened the problem to walking around your backyard while using a hose to spray water on the ground. If you walk quickly, the water won’t have a chance to start pooling. But if you walk slowly, he said, “you’re going to get a lot of rain underneath you.”
  4. Wider Range Storms. Because warmer water helps fuel hurricanes, climate change is expanding the zone where hurricanes can form. There is a “migration of tropical cyclones out of the tropics and into the subtropics and mid-latitudes,” Dr Kossin said. That could mean more storms making landfall at higher latitudes, such as in the United States and Japan.
  5. More volatility. As the climate warms, researchers also expect storms to intensify more quickly, they say. They’re still not sure why it happens, but the trend seems to be clear. In a 2017 paper based on climate and hurricane models, Dr. Emanuel wrote that rapidly intensifying storms — those whose wind speeds increase by 70 miles per hour or more in the 24 hours before landfall — were rare from 1976 to 2005. He estimated that their probability in those years was about once per century. By the end of the 21st century, he found, these storms might form once every five or ten years. “It’s a forecasters’ nightmare,” Dr. Emanuel said. If a Category 1 tropical storm or hurricane develops into a Category 4 hurricane overnight, he said, “there’s no time to evacuate people.”