Right now, ocean temperatures in the tropical Atlantic are running 2-3 degrees Celsius (3.6-5.4 degrees Fahrenheit) above average, said Michael Bell, an atmospheric scientist at Colorado State University’s Tropical Meteorology Project. But there’s more to a hurricane than ocean temperatures. There’s also little vertical wind shear — changes in air currents’ speed and direction that can tear apart a gathering storm.
And the 21st century has seen three years with more than one Category 5 storm, including the 2005 season that spawned four Cat 5s: Emily, Rita, Wilma and Katrina, which killed more than 1,800 people and left most of New Orleans underwater. None of those maintained that strength at landfall, however: The last hurricane to hit the US mainland at top strength was Andrew, which devastated the Miami area in 1992.
By comparison, only three years in the 20th century were known to have had more than one Category 5. But Bell said it’s hard to draw conclusions from that relatively short record, particularly since comprehensive hurricane data is a relatively recent development.
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In the early 20th century, major storms might have gone unnoticed if they didn’t hit land or cross common shipping lanes, and mariners tended to steer clear of them. And it’s still not clear what effect climate change will have on other ingredients of a hurricane.
“I think it’s still a little too early to say whether this is the start of a trend or just part of a natural cycle,” he said.
About 90 percent of the extra heat being trapped by greenhouse gases in the atmosphere is being absorbed by the oceans, Hayhoe said — meaning there’s more energy available to a building storm. It would be a surprise if that didn’t produce bigger storms, but it will take much more data to confirm that suspicion, she said.
“To really mark a climate trend, we have to have good data over a really long period of time. We’re talking multiple decades, not this one decade or even two,” Hayhoe said.
Scientists are also seeing signs that hurricanes are intensifying more rapidly. One 2012 study found that storms are growing from minimal hurricanes to major ones an average of 9 hours faster than they did a quarter-century ago, Hayhoe said. And since warmer air can carry more water vapor, storms also are expected to dump more rain as they hit.
“We have seen increases in heavy precipitation, not just across the US but around the world,” Hayhoe said. “And these observed increases in precipitation have been formally attributed to human-induced change.”
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Not only are future storms likely to be not only bigger, but wetter — and rising sea levels mean the storm surges that accompany those hurricanes can reach further inland than they would have before.
In the US Southeast, sea levels have been rising faster than the global average already, said Arnoldo Valle-Levinson, a professor of civil and coastal engineering at the University of Florida. And when Irma hit the state, “It is clear that the higher sea levels worsened the damage,” he said.
“Even though we're talking about a sea-level increase of 4-5 inches in the last five years, that is enough to make a big difference between flooding or not,” Valle-Levinson said.
Though Irma faded before it hit Florida, it broke a string of records on its way across the Atlantic, according to CSU. It was the strongest storm generated outside the Caribbean Sea or the Gulf of Mexico, and held its top winds of 185 mph for more than a day and half — longer than any tropical cyclone on record. That speed fell short of the all-time mark set by 1980’s Hurricane Allen, but tied for second in the record books with three earlier storms.
Its sheer size meant cities far from the eye felt its effects: Charleston, South Carolina, found downtown streets awash, while offshore winds on the opposite side of the storm pushed water out of Alabama's Mobile Bay – 500 miles away.
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CSU observers forecast a busier-than-average hurricane season, with 16 named storms. So far, with 13 named storms, seven of which turned into hurricanes and four became major hurricanes, “It’s certainly in that category,” said Bell, who co-authored that forecast. And with more people living on the coasts, those storms are causing more damage than they used to.
“Even if we have the same intensity of storms continuing into the future, the more houses and the more expensive property we put in those areas, the more likely we are to see some damage,” he said. “I think we need to be smart about our building codes and urban planning, regardless of whether the storms get more intense or more or less frequent.”
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