FORT LAUDERDALE, Florida 鈥� As a ferocious hurricane bears down on South Florida, water managers desperately lower canals in anticipation of more than a metre 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 320-kilometre-per-hour 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 鈥� such as the slowing of hurricanes鈥� forward motion and the worsening of storm surges from rising sea levels 鈥� are happening now. Other effects, such as their increase in strength, might 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.
鈥淭here鈥檚 almost unanimous agreement that hurricanes will produce more rain in a warmer climate,鈥� said Adam Sobel, professor of applied physics at Columbia University and director of its Initiative on Extreme Weather and Climate. 鈥淭here鈥檚 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鈥檚 more debate about whether we can detect that already.鈥�
No one knows how strong they could get, as they鈥檙e fuelled by warmer ocean water. Timothy Hall, senior scientist at the NASA Goddard Institute for Space Studies, said top wind speeds of up to 370 km/h 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 (although tornadoes, because of their rapid changes of wind direction, are considered more destructive).
Does that mean the current five-category hurricane scale should be expanded to include a Category 6, or even Category 7?
The Saffir-Simpson hurricane wind scale, developed in the early 1970s, ranks hurricanes from Category 1, which means winds of 120 to 150 km/h, to Category 5, which covers winds of 250 km/h or more.
Since each category covers a range of wind speeds, it would appear that once wind speed reaches 320 km/h, the pattern might call for another category. Last season saw two Category 5 hurricanes, Irma and Maria, with Irma reaching 290 km/h. And in 2015, off Mexico鈥檚 Pacific coast, Hurricane Patricia achieved a freakish sustained wind speed of 346 km/h.
鈥淚f we had twice as many Category 5s 鈥� at some point, several decades down the line 鈥� if that seems to be the new norm, then yes, we鈥檇 want to have more partitioning at the upper part of the scale,鈥� Hall said. 鈥淎t that point, a Category 6 would be a reasonable thing to do.鈥�
Many scientists and forecasters aren鈥檛 particularly interested in categories anyway, since they indicate only wind speed, not the other dangers posed by hurricanes.
鈥淲e鈥檝e tried to steer the focus toward the individual hazards, which include storm surge, wind, rainfall, tornadoes and rip currents, instead of the particular category of the storm, which only provides information about the hazard from wind,鈥� said Dennis Feltgen, spokesman for the U.S. National Hurricane Center. 鈥淐ategory 5 on the Saffir-Simpson scale already captures 鈥渃atastrophic damage鈥� from wind, so it鈥檚 not clear that there would be a need for another category even if storms were to get stronger.鈥�
Among the most solid predictions is that storms will move more slowly. In fact, that has already happened. A new study in the journal Nature found that tropical cyclones have decreased their forward speed by 10 per cent since 1949, and many scientists expect the trend to continue.
That doesn鈥檛 mean a hurricane鈥檚 winds would slow down. It means the hurricane would be more likely to linger over an area 鈥� like last year鈥檚 Hurricane Harvey. It settled over the Houston area and dropped more than a metre and a half of rain on some areas, flooding thousands of houses.
In addition to moving slower, future hurricanes are expected to dump a lot more rain. A study by scientists at the U.S. National Center for Atmospheric Research this year looked at how 20 Atlantic hurricanes would change if they took place at the end of the century, under the average projection for global warming. Warm air holds more water than cold air. The study found that hurricanes would generate an average of 24 per cent more rain, an increase that guarantees the more storms would produce catastrophic flooding.
The production of horrifying amounts of rain shows another way in which Harvey is a window into the future. One study, which looked at how much rain Harvey would have produced if it had formed in the 1950s, found that global warming had increased its rainfall by up to 38 per cent.
Other scientists see Harvey less as a symptom of climate change than an indication of what we can expect in the future.
鈥淲hether we鈥檙e talking about a change in the number of storms or an increase in the most intense storms, the changes that are likely to come from global warming are not likely to be detectable until 50 years from now,鈥� said Brian Soden, professor of atmospheric sciences at the University of Miami鈥檚 Rosenstiel School of Marine and Atmospheric Science.
