Sci­en­tists find Har­vey’s idling a sticky is­sue

Study says cli­mate change may help storms stay locked in place. Not all agree.

Sun Sentinel Palm Beach Edition - - NATION & WORLD - By Eric Ros­ton Bloomberg News

Me­te­o­rol­o­gists ex­plain that Hur­ri­cane Har­vey stalled off the Texas coast be­cause two high-pres­sure at­mo­spheric masses — huge book­ends made out of air — squeezed it in place, and there weren’t any high-level cur­rents to help steer it away.

Har­vey is another of sev­eral re­cent weather dis­as­ters marked by such stay­ing power, pun­ish­ing whole re­gions for days or weeks and longer. Oth­ers in­clude a mas­sive 2010 heat wave over Rus­sia and flood­ing in Pak­istan; the Texas drought of 2011 and the Cal­i­for­nia drought that be­gan around the same time and con­tin­ued through this year; and the flood­ing last year in Louisiana.

Slug­gish­ness in storms is a big deal, par­tic­u­larly if they’re in­creas­ing in fre­quency. “It turns a gar­den­va­ri­ety dis­as­ter into a catas­tro­phe,” said Paul Dou­glas, a broad­cast me­te­o­rol­o­gist and weather entrepreneur. As Har­vey stays put, it func­tions as a fire­hose that sucks warm wa­ter from the Gulf of Mex­ico and the at­mos­phere, dump­ing it in­land.

Ex­perts don’t agree on whether — or how much — hu­man-caused cli­mate change is re­spon­si­ble for the me­te­o­ro­log­i­cal stick­i­ness that kept Har­vey over South Texas.

Sci­en­tists aren’t ask­ing ques­tions such as, “Did cli­mate change cause the hur­ri­cane?” Cli­mate change didn’t cause the hur­ri­cane, though to­day’s warmer wa­ter and more hu­mid air pro­vided it with rocket fuel, mak­ing it more in­tense.

In­stead, ex­perts are ask­ing how huge, “dy­namic” at­mo­spheric sys­tems may be chang­ing, at least in­di­rectly, be­cause of hu­man­ity’s prodi­gious green­house gas pol­lu­tion. So, a bet­ter ques­tion might be, “Does hu­man­ity have any­thing to do with sum­mer­time North­ern Hemi­sphere storms that are pre­vented from mov­ing off be­cause of changes in the jet stream?” The an­swer is there’s not enough ev­i­dence in yet.

Michael Mann, dis­tin­guished pro­fes­sor of at­mo­spheric science at Penn State Univer­sity, pub­lished an es­say Mon­day in which, among items bet­ter un­der­stood about hur­ri­canes and warm­ing, he in­cluded:

“More ten­u­ous, but pos­si­bly rel­e­vant still, is the fact that very per­sis­tent, nearly ‘sta­tion­ary’ sum­mer weather pat­terns of this sort, where weather anom­alies ... stay locked in place for many days at a time, ap­pears to be fa­vored by hu­man­caused cli­mate change.”

In March, Mann and sev­eral col­leagues pub­lished a study in the jour­nal Sci­en­tific Re­ports that demon­strates a re­la­tion­ship be­tween ex­treme events, such as the 2011 Texas drought and 2010 Pak­istan flood­ing, and a rare sta­tion­ary phase that up­per at­mo­spheric cur­rents some­times go through in the mid-lat­i­tudes.

Ste­fan Rahm­storf, a coau­thor of that pa­per and head of Earth Sys­tems Anal­y­sis at the Pots­dam In­sti­tute for Cli­mate Im­pact Re­search, ex­plained that there may be sev­eral things go­ing on. In gen­eral, the jet stream, the high-fly­ing river of air that flows west-toeast, has slowed and gone all wavy in re­cent sum­mers, with pro­nounced north­south me­an­ders. That’s one thing that may have helped hold Har­vey in place. Re­searchers have sparred since 2012 over whether Arc­tic warm­ing, which is oc­cur­ring at twice the global av­er­age, is driv­ing this at­mo­spheric wob­ble, con­se­quently cre­at­ing more op­por­tu­ni­ties for per­sis­tent weather far­ther south.

In a num­ber of ex­treme cases an­a­lyzed by their pa­per, the north-south me­an­der of the jet-stream sta­bi­lizes for pe­ri­ods in some places, cre­at­ing an in­sur­mount­able wave­like band. The re­searchers looked for some kind of mis­be­hav­ior in at­mo­spheric cir­cu­la­tion after re­al­iz­ing that heat-re­lated ef­fects alone couldn’t ex­plain the ex­treme na­ture of some dis­as­ters.

Not ev­ery­body’s sold on ei­ther the gen­eral jet­stream wob­bli­ness from a warm­ing Arc­tic, or the sta­bi­liz­ing at­mo­spheric waves de­scribed by Mann, Rahm­storf and col­leagues.

“It’s still con­tro­ver­sial,” said Adam So­bel, a pro­fes­sor at Columbia Univer­sity who spe­cial­izes in ex­treme weather.

Even Mann, the lead au­thor, in­tro­duced the pat­tern as “ten­u­ous,” and Rahm­storf said that more re­al­world case stud­ies and more years of ob­ser­va­tions are needed.

With Hous­ton still re­cov­er­ing, with flood­wa­ters ris­ing Wed­nes­day east of there and with hur­ri­cane season still churn­ing, the thought of more case stud­ies is the last thing any­body wants to con­sider.

And yet, just as it took decades to prove cli­mate change, time and more stud­ies will likely show whether hu­mans have gone be­yond global warm­ing and in fact changed which way the wind blows.

DAVID J. PHILLIP/AP

Ex­perts don’t agree on whether — or how much — hu­man-caused cli­mate change is re­spon­si­ble for me­te­o­ro­log­i­cal stick­i­ness that kept Har­vey sta­tion­ary over South Texas.

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