Ocean mix­ing that drives cli­mate found in sur­prise spot

The Charlotte Observer (Sunday) - - News - BY SETH BOREN­STEIN As­so­ci­ated Press

One of the key driv­ers of the world’s cli­mate is an area in the North At­lantic Ocean where warmer and colder wa­ter mix and swirl. When sci­en­tists went for their first close look at this crit­i­cal un­der­wa­ter dy­namo, they found they were look­ing in the wrong place.

By hun­dreds of miles. The con­se­quences are not quite yet un­der­stood, but even­tu­ally it could change fore­casts of one of the worst-case global warm­ing sce­nar­ios – still con­sid­ered un­likely this

cen­tury – in which the mix­ing stops and cli­mate chaos en­sues.

It’s called the At­lantic Merid­ional Over­turn­ing Cir­cu­la­tion, and sci­en­tists de­scribe it as a gi­ant ocean con­veyor belt that moves wa­ter from Green­land south to be­yond the tip of Africa and into the In­dian Ocean.

Warm, salty wa­ter near the sur­face moves north and mixes with cold, fresher wa­ter near Green­land. As that wa­ter cools and sinks it drives a slow cir­cu­la­tion of the oceans that is crit­i­cal to global cli­mate, af­fect­ing the lo­ca­tion of droughts and fre­quency of hur­ri­canes. It also stores heat-trap­ping

car­bon diox­ide deep in the ocean. The faster it moves, the more warm wa­ter gets sent into the depths to cool.

The area where warm wa­ter turns over in the North At­lantic is con­sid­ered to be the en­gine of the con­veyor belt. Sci­en­tists thought it was in the Labrador Sea west of Green­land.

But then a new in­ter­na­tional science team measured tem­per­a­ture, salti­ness and the speed of ocean cur­rents through­out the North At­lantic to try to bet­ter un­der­stand the con­veyor belt. The pre­lim­i­nary results af­ter hun­dreds of mea­sure­ments in 21 months found that en­gine was sev­eral hun­dred of miles east of where they fig­ured, said study lead au­thor Su­san Lozier, an ocean sci­ences pro­fes­sor at Duke Univer­sity. The study, pub­lished in Thurs­day’s jour­nal Science, puts it east of Green­land, closer to Scot­land.

The com­puter sim­u­la­tions that pre­dict how the cli­mate could change in com­ing years didn’t

fac­tor in ex­actly where the con­veyor belt en­gine is, and now they may be able to. Lozier and sev­eral out­side ex­perts said this doesn’t change their trust in the mod­els, es­pe­cially be­cause when the mod­els are checked with what is hap­pen­ing in the real world, they are found to be gen­er­ally ac­cu­rate.

“It doesn’t mean that the mod­els are all wrong at all,” said Tom Del­worth, a se­nior sci­en­tist at the Na­tional Oceanic and At­mo­spheric Ad­min­is­tra­tion’s geo­phys­i­cal lab in Prince­ton, New Jer­sey.

MIT’s Carl Wun­sch and other out­side ex­perts said the study was help­ful, but pointed out that 21 months of study is not enough to know if this dif­fer­ent lo­ca­tion is tem­po­rary or per­ma­nent.

Sci­en­tists have long feared that the con­veyor belt could be slow­ing and, in a worst-case sce­nario, could even stop and cause abrupt and cat­a­strophic cli­mate change. It is con-

sidered a po­ten­tial cli­mate tip­ping point that was the premise of the sci­en­tif­i­cally in­ac­cu­rate 2004 dis­as­ter movie “The Day Af­ter To­mor­row.”

Based on com­puter model stud­ies, the United Na­tions’ In­ter­gov­ern­men­tal Panel on Cli­mate Change re­ported in an ear­lier study it is “very un­likely” that the con­veyor belt would col­lapse this cen­tury. But the No­bel Prize-win­ning sci­en­tific panel con­cluded it is likely to get about a third slower if green­house gas emis­sions con­tinue at its cur­rent pace.

A study last year found that global warm­ing is weak­en­ing the sys­tem, say­ing the con­veyor belt was mov­ing at its slow­est speed in nearly 140 years of records.

“Our ba­sic understanding that the col­lapse is un­likely still stands,” said Del­worth, who wasn’t part of the study. “Our un­cer­tainty about that pre­dic­tion is high.”

IS­ABELA LE BRAS AP

In Septem­ber 2018, a probe that col­lects wa­ter sam­ples and mea­sures tem­per­a­ture, salin­ity and pres­sure is pre­pared for de­ploy­ment on the con­ti­nen­tal shelf of Green­land. Sci­en­tists were study­ing the At­lantic Merid­ional Over­turn­ing Cir­cu­la­tion, a cir­cu­la­tion of warm and cold waters stretch­ing from Green­land to the In­dian Ocean.

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