There has never been a Category 6 hurricane, but the day may be coming

As a ferocious hurricane bears down on South Florida, water managers desperately lower canals in anticipation of 4 feet of rain.

Everyone east of Dixie Highway is ordered evacuated, for fear of a menacing storm surge. Forecasters debate whether the storm will generate the 200 mph winds to achieve Category 6 status.

That is one scenario for hurricanes in a warmer world, a subject of fiendish complexity and considerable scientific research.

Some changes — such as the slowing of hurricanes’ forward motion and the worsening of storm surges from rising sea levels — are happening now. Other effects, such as their increase in strength, may have already begun but are difficult to detect, considering all of the other climate forces at work.

But more certainty has developed over the past few years. Among the conclusions: Hurricanes will be wetter. They are likely to move slower, lingering over whatever area they hit. And although there is debate over whether there will be more or fewer of them, most researchers think hurricanes will be stronger.

“There’s almost unanimous agreement that hurricanes will produce more rain in a warmer climate,” said Adam Sobel, professor of applied physics at Columbia University and director of its Initiative on Extreme Weather and Climate. “There’s agreement there will be increased coastal flood risk, at a minimum because of sea-level rise. Most people believe that hurricanes will get, on average, stronger. There’s more debate about whether we can detect that already.”

No one knows how strong they could get, as they’re fueled by warmer ocean water. Timothy Hall, senior scientist at the NASA Goddard Institute for Space Studies, said top wind speeds of up to 230 mph could occur by the end of the century, if current global warming trends continue. That would be the strength of an F-4 tornado, which can pick up cars and throw them through the air.

Does that mean the current five-category hurricane scale should be expanded to include a Category 6, or even a Category 7?

The Saffir–simpson hurricane wind scale, developed during the early 1970s, ranks hurricanes from Category 1, which means winds of 74 to 95 mph, to Category 5, which covers winds of 157 mph or more.