Cli­mate Change Be­hind Sum­mer 2018’s Ex­tremes

Hartford Courant (Sunday) - - Ctopinion - By MICHAEL E. MANN

Sum­mer 2018 saw an un­prece­dented spate of ex­treme floods, droughts, heat waves and wild­fires break out across North Amer­ica, Europe and Asia. The scenes played out on our tele­vi­sion screens and in our so­cial me­dia feeds. This is, as I stated at the time, the face of cli­mate change.

It’s not rocket sci­ence. A warmer ocean evap­o­rates more mois­ture into the at­mos­phere — so you get worse flood­ing from coastal storms (think hur­ri­canes Har­vey and Florence). Warmer soils evap­o­rate more mois­ture into the at­mos­phere — so you get worse droughts (think Cal­i­for­nia or Syria). Global warm­ing shifts the ex­treme up­per tail of the “bell curve” to­ward higher tem­per­a­tures, so you get more fre­quent and in­tense heat waves (think sum­mer 2018 just about any­where in the North­ern Hemi­sphere). Com­bine heat and drought, and you get worse wild­fires (again, think Cal­i­for­nia).

Cli­mate sci­en­tists have be­come in­creas­ingly com­fort­able talk­ing about these con­nec­tions. Much like the way med­i­cal sci­ence has de­vel­oped key di­ag­nos­tic tools, we have de­vel­oped so­phis­ti­cated tools to di­ag­nose the im­pact cli­mate change is hav­ing on ex­treme weather events.

One of these tools, “ex­treme event at­tri­bu­tion,” can be thought of as cli­mate sci­ence’s ver­sion of an X-ray. In this case, a cli­mate model is run both with and with­out the hu­man ef­fect on cli­mate. One then com­pares how of­ten a par­tic­u­lar ex­treme event hap­pens in both the “with” and “with­out” cases. If it oc­curs suf­fi­ciently more of­ten (i.e., be­yond the “noise”) in the for­mer case, a study can “at­tribute” and quan­tify how cli­mate change af­fected the ex­treme­ness of the event.

The scorch­ing Euro­pean heat wave this sum­mer, ac­cord­ing to one such study, was made more than twice as likely by global warm­ing. The record rain­fall in North Carolina from Hur­ri­cane Florence was, ac­cord­ing to an­other study, in­creased by as much as 50 per­cent by warm­ing oceans.

The cli­mate mod­els used in these sorts of stud­ies rep­re­sent re­mark­able achieve­ments in the world of sci­ence. But no tool is per­fect. In our med­i­cal anal­ogy, some in­juries — such as soft tis­sue dam­age — are too sub­tle to be de­tected by an X-ray. So med­i­cal pro­fes­sion­als de­vel­oped even more so­phis­ti­cated tools, such as MRI. Sim­i­larly, some cli­mate-change im­pacts on ex­treme weather are too sub­tle to be cap­tured by cur­rent gen­er­a­tion cli­mate mod­els.

In a study my co-au­thors and I re­cently pub­lished in the jour­nal Sci­ence Ad­vances, we iden­ti­fied a key fac­tor be­hind the rise in ex­treme sum­mer weather events (such as the ones that played out in sum­mer 2018) that — as we demon­strate in our study — is not cap­tured by cur­rent gen­er­a­tion cli­mate mod­els. Us­ing an al­ter­na­tive ap­proach based on a com­bi­na­tion of mod­els and real-world ob­ser­va­tions, we showed that cli­mate change is caus­ing the sum­mer jet stream to be­have in­creas­ingly oddly. The char­ac­ter­is­tic con­ti­nen­tal-scale me­an­ders of the jet stream (its “wavi­ness”) as it trav­els from west to east are be­com­ing more pro­nounced and are tend­ing to re­main locked in place for longer stretches of time.

Un­der these cir­cum­stances — when, for ex­am­ple, a deep high-pres­sure “ridge” gets stuck over Cal­i­for­nia or Europe — we usu­ally see ex­treme heat, drought and wild­fire. And, typ­i­cally, there’s a deep low-pres­sure “trough” down­stream, stuck over, say, the eastern United States or Ja­pan, yield­ing ex­ces­sive rain­fall and flood­ing. That’s ex­actly what hap­pened in sum­mer 2018. The spate of ex­treme floods, droughts, heat waves and wild­fires we ex­pe­ri­enced were a con­se­quence of such jet stream be­hav­ior.

Our study shows that cli­mate change is mak­ing that be­hav­ior more com­mon, giv­ing us the dis­as­trous Euro­pean heat wave of 2003 (dur­ing which more than 30,000 peo­ple per­ished), the dev­as­tat­ing 2011 Texas drought (dur­ing which ranch­ers in Ok­la­homa and Texas lost 24 per­cent and 17 per­cent of their cat­tle, re­spec­tively), the 2016 Al­berta wild­fire (the costli­est nat­u­ral dis­as­ter in Cana­dian his­tory) and, yes, the ex­treme sum­mer of 2018.

Just as cli­mate mod­els al­most cer­tainly un­der­es­ti­mate the im­pact cli­mate change has al­ready had on such weather ex­tremes, pro­jec­tions from these mod­els also likely un­der­es­ti­mate fu­ture in­creases in these types of events. Our study in­di­cates that we can ex­pect many more sum­mers like 2018 — or worse.

Cli­mate-change de­niers love to point to sci­en­tific un­cer­tainty as jus­ti­fi­ca­tion for in­ac­tion on cli­mate. But un­cer­tainty is a rea­son for even more con­certed ac­tion. We al­ready know that pro­jec­tions his­tor­i­cally have been too op­ti­mistic about the rates of ice sheet col­lapse and sea-level rise. Now it ap­pears they are also un­der­es­ti­mat­ing the odds of ex­treme weather as well. The con­se­quences of do­ing noth­ing grow by the day. The time to act is now.

SCOTT SHARPE | TNS

AR­EAS OF KINSTON, N.C., were still flooded more than two weeks af­ter Hur­ri­cane Florence came ashore along the state’s coast. Sci­en­tists say in­creas­ingly se­vere storms are caused by cli­mate change.

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