Hur­ri­cane warn­ing: Cat­e­gory 6 is com­ing


FORT LAUDERDALE, FLA.— As a fe­ro­cious hur­ri­cane bears down on South Florida, wa­ter man­agers des­per­ately lower canals in an­tic­i­pa­tion of one me­tre of rain.

Ev­ery­one east of Dixie High­way is or­dered to evac­u­ate, for fear of a men­ac­ing storm surge. Fore­cast­ers de­bate whether the storm will gen­er­ate the 320 kilo­me­tres-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 — 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 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 119-152 kilo­me­tres an hour, to Cat­e­gory 5, which cov­ers winds of 250 kilo­me­tres an hour or more.

Since each cat­e­gory cov­ers a range of wind speeds, it would ap­pear that once wind speed reaches 300 kilo­me­tres an hour, the pat­tern may call for an­other cat­e­gory. Last sea­son saw two Cat­e­gory 5 hur­ri­canes, Irma and Maria, with Irma reach­ing 289 kilo­me­tres an hour. And in 2015, off Mex­ico’s Pa­cific coast, Hur­ri­cane Pa­tri­cia achieved a freak­ish sus­tained wind speed of 346 kilo­me­tres an hour.

“If we had twice as many Cat­e­gory 5s — at some point, sev­eral decades down the line — if that seems to be the new norm, then yes, we’d want to have more par­ti­tion­ing at the up­per part of the scale,” Hall said. “At that point, a Cat­e­gory 6 would be a rea­son­able thing to do.”

Many sci­en­tists and fore­cast­ers aren’t par­tic­u­larly in­ter­ested in cat­e­gories any- way, since they in­di­cate only wind speed, not the other dan­gers posed by hur­ri­canes.

“We’ve 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,” said Den­nis Felt­gen, a Na­tional Hur­ri­cane Cen­ter spokesper­son. “Cat­e­gory 5 on the Saf­fir-Simp­son scale al­ready cap­tures ‘cat­a­strophic dam­age’ from wind, so it’s not clear that there would be a need for an­other cat­e­gory even if storms were to get stronger.”

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’t mean a hur­ri­cane’s winds would slow down. It means the hur­ri­cane would be more likely to linger over an area — such as last year’s Hur­ri­cane Har­vey. It set­tled over the Hous­ton area and dropped more than one me­tre 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 done this year by sci­en­tists at the Na­tional Cen­ter for At­mo­spheric Re­search 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 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.

“Whether we’re 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,” said Brian So- den, pro­fes­sor of at­mo­spheric sciences at the Univer­sity of Mi­ami’s Rosen­stiel School of Marine and At­mo­spheric Sci­ence. “There’s so much nat­u­ral vari­abil­ity in the sys­tem, the typ­i­cal year-to-year vari­abil­ity in hur­ri­cane ac­tiv­ity, that the sig­nal re­ally doesn’t emerge from that back­ground vari­abil­ity un­til the lat­ter half of this cen­tury.”

As a hur­ri­cane ap­proaches Florida, the ques­tion ev­ery­one asks is when will it veer north? The answer could de­ter­mine whether it hits the Keys, slams into Fort Lauderdale or re­mains harm­lessly out in the ocean.

But the fa­mil­iar up­side-down comma shape that char­ac­ter­izes so many hur­ri­cane tracks could grad­u­ally be­come a thing of the past, as hur­ri­canes fol­low paths that are more me­an­der­ing and less pre­dictable. That’s be­cause cli­mate change could al­ter the jet stream, the high-al­ti­tude air cur­rent that pushes hur­ri­canes north and east.

“The hur­ri­cane track has less guid­ance steer­ing them, so are more prone to me­an­ders and un­usual turns,” NASA’s Hall said. That could yield weird turns and stalls, such as Har­vey’s screech to a halt over Texas or Hur­ri­cane Sandy’s sud­den, and cat­a­strophic, lurch to­ward New Jersey.

“If the jet stream were a lot far­ther north, then you could imag­ine a sit­u­a­tion where hur­ri­cane tracks could more eas­ily hit the North Amer­i­can con­ti­nent be­cause they have more abil­ity to con­tinue in the di­rec­tion of the con­ti­nent from their trop­i­cal for­ma­tion points,” he said.

Warm ocean wa­ter pro­vides the fuel for hur­ri­canes, but a hot­ter world would not nec­es­sar­ily pro­duce more of them. While many sci­en­tists for a long time did think an in­crease in tem­per­a­tures would pro­duce more storms, they have be­gun fo­cus­ing on fac­tors that could sup­press the for­ma­tion of hur­ri­canes.

Many mod­els for fu­ture cli­mate show an in­crease in wind shear, the criss­cross­ing high-al­ti­tude winds that tear up in­cip­i­ent trop­i­cal cy­clones. And they show less of the at­mo­spheric in­sta­bil­ity nec­es­sary for the gen­er­a­tion of thun­der­storms. But now the think­ing is swing­ing back. “We used to think 20 years ago that in a warmer cli­mate there would be more hur­ri­canes,” Columbia’ So­bel said. “Then the com­puter mod­els got bet­ter. Most of those started to show fewer hur­ri­canes, not more. No one knew why. Then some of the mod­els started to show in­creases with warm­ing. So I think we’re back to where we don’t know.”


Although there is de­bate over whether there will be more or fewer hur­ri­canes in the fu­ture, most re­searchers be­lieve the storms will be stronger.


Last sea­son saw two Cat­e­gory 5 hur­ri­canes, Irma and Maria. Hur­ri­cane Irma, shown above, reached 289 kilo­me­tres an hour.

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