Category 6? Scientists warn hurricanes could keep getting stronger

Hurricanes have a ceiling, a limit to how strong they can get.

Scientists say that ceiling is going up.

It's why it's so common these days to use superlative language when describing storms: Superstorm Sandy in 2012 was the largest diameter storm in the Atlantic since the record on that statistic began in the early 1980s. Hurricane Harvey in 2017 was the wettest storm on record in the U.S. after it dumped more than 60 inches of rain onto parts of the Houston metro area. Irma, just weeks after Harvey, was the strongest storm ever to form in open Atlantic waters and maintained Category 5 strength for a nearly unprecedented 66 hours.

Even hurricanes Florence and Michael — together the legacy of the 2018 hurricane season, which ends today — were record-breaking in their own way. Florence inundated the Carolinas with record rain there, dropping the eighth most rain of any storm on record in the continential U.S. And Michael, which reduced Mexico Beach to rubble with bomb-like efficiency, was the strongest storm on record ever to strike the Panhandle, and had the third lowest central pressure and fourth highest wind speed of any storm ever recorded to hit the continental U.S.

Those records will all likely be broken again, as scientists say the "potential intensity" of storms — the theoretical maximum strength storms can achieve — is increasing. That means storms will get worse, a disastrous scenario for coastal states like Florida.

"The strongest storms are getting stronger — the science is clear on this," said Penn State University atmospheric science professor Michael Mann. "We can expect more intense hurricanes, greater flooding, worse storm surge which, combined with sea level rise, presents an existential threat to the Florida coastline."

What's raising the ceiling? A familiar culprit, and a surprising one.

The first is global warming from greenhouse gas emissions.

Hurricanes feed off warm water. A warming planet means warmer oceans.

"The linkage there is very, very simple and direct," said National Oceanic and Atmospheric Administration research scientist Jim Kossin.

The other is more counterintuitive: clean air. Normally a bad thing, air pollution does prevents sunlight from reaching the ocean, which tamps down sea surface temperatures just enough to reduce the hurricane threat. Since congress passed clean air legislation in the 1960s and 70s, pollutant levels have dropped, allowing more solar energy to hit the oceans.

The pair is a perfect storm for potential intensity.

"In terms of our immediate future, like this century, it will just continue to increase," Kossin said.

It's already happening. More storms are packing winds that are literally off the charts. Scientists put storms into categories based on their wind speeds. Category 5, the highest, includes all storms with wind at least 157 mph. Category 6, which doesn't exist, would theoretically begin at 195 mph, Kossin said. Pacific Ocean typhoons Haiyan and Meranti, in 2012 and 2016, respectively, both had winds that sustained 195 mph. Hurricane Patricia, which formed in 2015 off the Pacific coast of Mexico, achieved sustained winds of 215 mph.

Category 7 would begin at 230 mph, according to Kossin.

Mann has advocated for a formalization of higher wind categories. Some scientists are reluctant out of fear that it sends the message that lower-rated storms, though still deadly, can be dismissed.

Another effect of a warming climate is that storms will likely be wetter, since there's more water vapor in hot air. That could lead to more cases of disastrous flooding like hurricanes Harvey and Florence.

The flooding risk is enhanced by rising sea levels. Swollen rivers won't drain as well. And take Sandy, which inundated New York City with storm surge.

"If Sandy had happened 100 years earlier, it probably wouldn't have flooded Manhattan the way it did because sea level was a foot lower 100 years ago," said MIT atmospheric science professor Kerry Emanuel.

Another consequence global warming is that hurricanes are moving toward the poles and away from the traditional tropical development zones.

"Basically what seems to be happening is that the subtropics are becoming more hospitable for hurricanes," Kossin said, "and the deep tropics are becoming slightly more hostile. So as they move north, they are able to maintain their intensity longer."

That scenario played out in the 2018 season, where seven storms started in the subtropic latitudes.

The 2018 season ends with 15 named storms, eight hurricanes, two of which — Florence and Michael — reached major hurricane status. A major hurricane is a Category 3 or higher with wind speeds of at least 111 mph.
An average season has 12 storms, six hurricanes, two of which are major.

Three factors contributed to the high level of cyclone activity this year: the lack of an El Nino, a strong African monsoon and warmer water in the Atlantic, according to the National Oceanic and Atmospheric Administration's end-of-season summary.

El Nino, which is warmer-than-normal water in the tropical Pacific Ocean, produces high-altitude wind across the tropical Atlantic that has a tendency to sheer storms apart before they can coalesce into dangerous cyclones. Without the El Nino, storms were free to develop.

The African monsoon is the wind that comes off the Atlantic coast of Africa. Strong winds are favorable to hurricane development.

Another factor that scientists believe influences hurricanes is a salinity cycle in the northern Atlantic. The cycle takes several decades to run its course, and it has just recently died down, suggesting a quieter period to come.

That hasn't yet panned out.

Contact Josh Solomon at jsolomon@tampabay.com or (813) 909-4613. Follow @ByJoshSolomon.