Weather of CON­SE­QUENCE

FOUR UN­DER-AP­PRE­CI­ATED WAYS THAT CLI­MATE CHANGE COULD MAKE HUR­RI­CANES WORSE

The Nation - - ENVIRONMENT - CHRIS MOONEY

WITH HUR­RI­CANE Har­vey’s rain­fall records and Irma’s stun­ning com­bi­na­tion of ex­treme strength and longevity, the hur­ri­cane-global warm­ing de­bate is back in full swing. And it has evolved a good bit since 2005, when the United States was se­verely dam­aged by su­per­strong hur­ri­canes (Katrina, Rita, Wilma).

Sci­en­tists are more con­fi­dent now in their mes­sages, more will­ing to link fac­tors like wors­ened storm surge flood­ing and hur­ri­cane rain­fall to cli­mate change (even while stat­ing the re­main­ing un­cer­tain­ties and caveats). En­vi­ron­men­tal Pro­tec­tion Agency ad­min­is­tra­tor Scott Pruitt, in con­trast, has called it “very in­sen­si­tive” to raise cli­mate pol­icy while storms bar­rel to­ward coasts.

Ac­tu­ally, there’s ac­tu­ally a case to be made that the cli­mate change-hur­ri­cane dis­cus­sion should be broad­ened – a move that will give a sense both of how con­se­quen­tial and how un­cer­tain the is­sue ac­tu­ally is.

The public de­bate over hur­ri­canes and global warm­ing gen­er­ally gets con­fined to a few is­sues: Will hur­ri­canes be in­creas­ingly in­tense – like Irma was? Will they rain more, like Har­vey did? Will they drive worse in­land storm surges be­cause of sealevel rise? Will they be more or less numer­ous?

That’s un­der­stand­able: These ques­tions have been more heav­ily stud­ied – and in some cases, es­pe­cially when it comes to sea-level rise (which is clearly hap­pen­ing), are more eas­ily an­swered.

But once you take a step back and think about how hur­ri­canes nes­tle into the broader cli­mate sys­tem – as re­searchers like MIT’s hur­ri­cane and cli­mate ex­pert Kerry Emanuel, a num­ber of whose pa­pers are cited be­low, tend to do – you re­alise there is a broader pic­ture, although it’s also a less cer­tain one.

Hur­ri­canes, Emanuel and others have shown, are mas­sive ther­mo­dy­namic sys­tems that with­draw en­ergy from the oceans and ex­pel it into the at­mos­phere. That means many of their pat­terns and traits could change with warmer seas – even if some of those traits are harder to talk about be­cause the re­search re­mains less de­fin­i­tive.

So let’s con­sider some less-dis­cussed storm at­tributes that could plau­si­bly change in a warm­ing world. We don’t know defini­tively that these things are hap­pen­ing – again, the re­search here is thin­ner – but they’re well worth won­der­ing about.

Sea­son length: Hur­ri­canes fol­low sea­sonal pat­terns. They oc­cur in the sum­mer and au­tumn, and this, too, re­flects the tem­per­a­ture of the oceans (among other fac­tors). But as the cli­mate warms, could hur­ri­canes be more likely to oc­cur out of sea­son – ei­ther in the late spring or late au­tumn and early win­ter?

There’s at least some sug­ges­tive ev­i­dence of sea­son length­en­ing. In a 2008 study on the At­lantic hur­ri­cane sea­son, for in­stance, James Kossin of NOAA and the Uni­ver­sity of Wis­con­sin at Madison found “an ap­par­ent ten­dency to­ward more com­mon early- and late-sea­son storms that cor­re­lates with warm­ing [sea sur­face tem­per­a­ture], but the un­cer­tainty in these re­la­tion­ships is high.”

How­ever, the ques­tion of sea­son length is com­plex, and other re­search has yielded more am­bigu­ous re­sults. “Ul­ti­mately, there is not yet a con­sen­sus on how the length of the [trop­i­cal cy­clone] sea­son will change as a re­sult of an­thro­pogenic ef­fects,” a 2015 com­puter mod­el­ling study by Columbia Uni­ver­sity’s John Dwyer and coau­thors con­cluded.

Mean­while, though the sci­ence re­mains un­re­solved, it’s hard not to miss real-world storms that ap­pear to fit the pat­tern. This very year, the first named storm in the At­lantic, Trop­i­cal Storm Ar­lene, formed in April, far out­side the bounds of the tra­di­tional hur­ri­cane sea­son.

Re­gions of for­ma­tion and in­ten­si­fi­ca­tion: If the globe’s oceans are warm­ing in gen­eral, that could also mean that the re­gions in which hur­ri­canes (or ty­phoons, or trop­i­cal cy­clones) can form – cur­rently, seven ma­jor “basins” across the globe – could shift. Or, it could mean that these storms – gen­er­ally crea­tures of the trop­ics – will be able to main­tain their strength in new places, fur­ther from the equa­tor.

Any gen­eral shift in hur­ri­cane for­ma­tion or ar­rival re­gions could have large im­pli­ca­tions be­cause it could sub­ject coast­lines that aren’t ac­cus­tomed to storms to their pun­ish­ment.

Once again, there is at least some ev­i­dence this is hap­pen­ing or could hap­pen. Kossin and two col­leagues (MIT’s Emanuel and Prince­ton and NOAA’s Gabriel Vec­chi) pub­lished a 2014 study in Na­ture find­ing “a pro­nounced pole­ward mi­gra­tion in the aver­age lat­i­tude at which trop­i­cal cy­clones have achieved their life­timemax­i­mum in­ten­sity over the past 30 years.”

Hur­ri­canes were mov­ing out of the trop­ics “a rate of about one de­gree of lat­i­tude per decade,” the re­searchers added. The study linked the change to a broader “trop­i­cal ex­pan­sion” that, in turn, ap­pears tied to hu­man al­ter­ation of the planet’s at­mos­phere.

Rapid in­ten­si­fi­ca­tion: Hur­ri­cane Har­vey epit­o­mised a num­ber of dan­ger­ous storm traits, one of which was in­creas­ing in strength very quickly as it ap­proached the Texas coast. This is a nightmare sce­nario for fore­cast­ers and emer­gency plan­ners be­cause it gives lit­tle time for peo­ple to evac­u­ate.

In gen­eral, rapid in­ten­si­fi­ca­tion is some­thing that we have seen a lot of lately, in­clud­ing in su­per-in­tense storms Wilma (2005) and Pa­tri­cia (2015). Wilma’s max­i­mum sus­tained winds in­creased by 110 mph in just 24 hours, and Pa­tri­cia’s beat that record, in­creas­ing by 120 mph in the same time frame.

So will storms be more likely to rapidly in­ten­sify as the cli­mate warms? Emanuel, at least, thinks the an­swer is yes. He just pub­lished a study in the Bul­letin of the Amer­i­can Me­te­o­ro­log­i­cal So­ci­ety find­ing that the “in­ci­dence of storms that in­ten­sify rapidly just be­fore land­fall in­creases sub­stan­tially as a re­sult of global warm­ing.” He reached this re­sult by cre­at­ing thou­sands of syn­thetic hur­ri­canes in a com­puter sim­u­la­tion and then com­par­ing how they be­haved with and with­out a chang­ing cli­mate.

This area re­mains lit­tle re­searched, so this one com­puter mod­el­ling study shouldn’t be taken as a fi­nal an­swer. But it’s still worth pon­der­ing.

Storm size: And then we come to one of the most un­cer­tain changes of all.

Sep­a­rate from the mat­ter of their wind speeds, over­all hur­ri­cane sizes also vary greatly, from the rel­a­tively small An­drew up to the mas­sive Katrina and be­yond. So would a chang­ing cli­mate have any ef­fect on this?

It’s very un­clear. But a trend to­ward big­ger storms, like Katrina and Sandy, could be just as much of a prob­lem as a trend to­ward stronger storms when mea­sured by wind speeds.

In Emanuel’s work you find a hint of this idea. A 2014 pa­per that he wrote with MIT’s Daniel Chavas won­dered “how the dis­tri­bu­tion of storm size may dif­fer in other cli­mate states”. But the study made clear that be­fore an­swer­ing that ques­tion, it would first be nec­es­sary for sci­en­tists to achieve a bet­ter un­der­stand­ing of what con­trols hur­ri­cane size in the first place.

That hasn’t stopped some top hur­ri­cane gurus from spec­u­lat­ing, though. Con­tem­plat­ing the mas­sive size of Hur­ri­cane Sandy, the Weather Un­der­ground’s Jeff Mas­ters wrote, “We have pushed our cli­mate sys­tem to a fun­da­men­tally new, higher-en­ergy state where more heat and mois­ture is avail­able to power stronger storms, and we should be con­cerned about the pos­si­bil­ity that Hur­ri­cane Sandy’s freak size and power were par­tially due to hu­man-caused cli­mate change.”

Still, this should be con­sid­ered a fron­tier – while it’s not ab­surd to think that storm sizes could change in some way as the cli­mate does, we don’t know what’s go­ing to ac­tu­ally hap­pen. And it’s pos­si­ble that here, and else­where, there could be a trade- off – larger storms but fewer of them, per­haps. (We shouldn’t as­sume ev­ery change is an un­mit­i­gated neg­a­tive.)

So, in sum, there is much more to be said about chang­ing hur­ri­cane traits than the usual mantra that they will prob­a­bly be more in­tense, will rain more, will ride atop higher seas, but could be less numer­ous over­all.

As sci­en­tists dig into these other ques­tions, we will prob­a­bly con­tinue to see large storms, rapidly in­ten­si­fy­ing storms, out-of-sea­son storms and sus­pi­ciously placed storms. Those should be re­garded as anec­dotes – not proof of any­thing. But we should re­mem­ber that as the cli­mate changes, all of the dif­fer­ent ways that hur­ri­canes ex­tract en­ergy from the trop­i­cal oceans could change, too.

Hur­ri­cane Sandy slammed into New Jersey in Oc­to­ber 2012.

Hur­ri­cane Har­vey brought cat­a­strophic flood­ing to much of Texas last month.

Hur­ri­cane Katrina dev­as­tated New Or­leans in Au­gust 2005.

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