There is no official Category 6 for hurricanes, but scientists say they're pondering whether there should be as evidence mounts that hurricanes around the world have sharply worsened over the past 30 years -- and all but a handful of hurricane experts now agree this worsening bears the fingerprints of man-made global warming.
In fact, say scientists, there have already been hurricanes strong enough to qualify as Category 6s. They'd define those as having sustained winds over 175 or 180 mph. A couple told me they'd measured close to 200 mph on a few occasions.
The Saffir-Simpson hurricane category scale is based on wind speed: A Category 1 hurricane has sustained winds from 74 to 95 mph, Category 2 has sustained winds from 96 to 110 mph, Category 3 has sustained winds from 111 to 130 mph, Category 4 has sustained winds from 131 to 155, and a Category 5 storm has sustained winds greater than 155 mph.
The categories run in roughly 20 mph increments, so a Cat 6 would be greater than 175 or 180 mph.
To put this all in perspective, Katrina was a Category 5 hurricane out over some hot spots in the Gulf. But when it hit New Orleans, scientists now know, Katrina had winds at a low Category 3, and much of them Category 2, including the "left side winds" that then came down from the north and pushed the surge-swollen waters of Lake Pontchartrain over and through NOLA's levees. (Hurricanes spin counterclockwise in the northern hemisphere, so when Katrina came ashore just east of New Orleans, its winds hit the city from the north.)
Only three Category 5s have come ashore in the United States in the past century -- the 1935 Labor Day Hurricane, Camille in 1969 and Andrew in 1992.
But because of man-made global warming, most hurricane scientists say now we will probably be getting Category 4 and 5 hurricanes more frequently in the coming decades.
That's on top of the natural multi-year cycles of hurricane intensity the scientists already know about.
In fact, says atmosphere scientific Greg Holland, the world already has seen far more frequent Cat 4s and 5s. He points to several studies published over the past 12 months which "indicated the frequency of Category 4 and 5 hurricanes had almost doubled around the world in the period since 1970."
The fact that these patterns (on top of the natural cycles) have been seen in not just one ocean but all tropical and subtropical waters around the world is what worries many hurricane experts -- and, they say, it is why they now calculate that they are due to man-made global warming, not regional natural weather patterns.
"We're actually looking at an entire world that is heating up," says Holland, "not just the Atlantic Ocean -- which is why we are absolutely convinced that there is a very large greenhouse warming signal in what we're seeing."
In the past, say these scientists, when one region of the globe concentrated more heated water or air (both of which can intensify hurricanes), other regions would cool in compensation because the total heat available on the planet at any one time is limited; now, with the average global temperatures going up, such related cooling is happening less and less.