By Ru­airi Casey - AL JAZEERA

For thou­sands of years, the cir­cu­lat­ing cur­rents of the At­lantic Ocean have con­tin­u­ous­ly reg­u­lat­ed the tem­per­a­tures of Eu­rope and North Amer­i­ca, pro­duc­ing a warm­ing ef­fect that al­lows them to en­joy rel­a­tive­ly mod­er­ate weath­er con­di­tions.

But the ef­fects of an­thro­pogenic cli­mate change have di­min­ished the flow of this vast con­vey­or belt sys­tem, known as the At­lantic Merid­ion­al Over­turn­ing Cir­cu­la­tion (AMOC), and re­cent sci­en­tif­ic re­search sug­gests it may even be head­ed for col­lapse.

The un­prece­dent­ed slow­down of the vast sys­tem has been mea­sured di­rect­ly since 2004, but analy­sis of in­di­rect da­ta sug­gests a longer de­cline, be­gin­ning in the mid to late 19th cen­tu­ry and ac­cel­er­at­ing af­ter 1950.

One study, which looked at ice cores and ocean sed­i­ments, de­ter­mined the AMOC was “in its weak­est state in over a mil­len­ni­um”.

“Every­thing points to a weak­en­ing of the AMOC,” said Sybren Dri­jfhout, an oceanog­ra­ph­er at the Uni­ver­si­ty of Southamp­ton.

The time­line of a po­ten­tial col­lapse of the AMOC re­mains un­clear, but the con­se­quences for the Earth’s cli­mate would be im­mense.

Tem­per­a­tures in Eu­rope and the east of North Amer­i­ca would drop by as much as 5 de­grees Cel­sius (9 de­grees Fahren­heit), lead­ing to more ex­treme win­ter weath­er.

Coastal cities in North Amer­i­ca would be flood­ed by ris­ing sea lev­els. It would al­so dis­rupt the West African and Asian mon­soons, which sup­ply vi­tal rain­fall for crops that tens of mil­lions of peo­ple de­pend up­on.

How AMOC works

An enor­mous sys­tem of ocean cur­rents, the AMOC is dri­ven by chang­ing wa­ter den­si­ty, which is de­ter­mined by the wa­ter’s salt con­tent and tem­per­a­ture.

Un­der a process known as “ther­mo­ha­line cir­cu­la­tion”, warm wa­ter moves north through the Gulf of Mex­i­co to­wards Eu­rope – the stretch known as the Gulf Stream – with the sur­face tem­per­a­ture de­creas­ing as evap­o­ra­tion oc­curs and salin­i­ty in­creas­es.

Be­com­ing denser, the wa­ter then sinks in the north At­lantic and whisks south along the ocean floor be­fore “up­welling” to the sur­face again far in­to the south­ern hemi­sphere.

The ef­fects of glob­al warm­ing on the AMOC are twofold. Warmer wa­ter is less dense, and fresh­wa­ter runoff from ice melt­ing in the po­lar re­gion re­duces salin­i­ty, which re­duces den­si­ty even fur­ther. These fac­tors slow the sink­ing mech­a­nism that pro­pels the cir­cu­la­tion.

The last time the AMOC shut down was to­wards the end of the last ice age, about 14,500 years ago. Then glacial melt flood­ed the North At­lantic with fresh wa­ter, col­laps­ing the sys­tem and caus­ing tem­per­a­tures in Eu­rope to plunge.

The In­ter­gov­ern­men­tal Pan­el on Cli­mate Change (IPCC) re­port pub­lished in Au­gust found with high con­fi­dence that the AMOC will like­ly weak­en over the com­ing decades, but a to­tal col­lapse be­fore 2100 is un­like­ly.

“Even though the AMOC is very un­like­ly to col­lapse over the 21st cen­tu­ry, its weak­en­ing may be sub­stan­tial, which may there­fore in­duce strong and large-scale cli­mat­ic im­pacts with po­ten­tial far-reach­ing im­pacts on nat­ur­al and hu­man sys­tems,” it said.

‘Very mas­sive im­pact’

Whether the de­cline of the AMOC will con­tin­ue in a lin­ear fash­ion, or reach some tip­ping point, af­ter which the de­cline could ac­cel­er­ate pre­cip­i­tous­ly, re­mains a point of dis­cus­sion among sci­en­tists.

“That’s the mil­lion-dol­lar ques­tion,” said Niklas Boers, a re­searcher at the Pots­dam In­sti­tute for Cli­mate Im­pact Re­search. “Whether it’s just lin­ear, slow­ing down, or whether there’s ac­tu­al­ly a loss of sta­bil­i­ty.”

A pa­per pub­lished by Boers in the Na­ture Cli­mate Change jour­nal in Au­gust analysed eight sep­a­rate in­di­ca­tors, mak­ing use of sea sur­face tem­per­a­ture and wa­ter salin­i­ty da­ta that stretch­es back to the 19th cen­tu­ry.

It found the AMOC may have evolved from a pe­ri­od of rel­a­tive sta­bil­i­ty to­wards a “crit­i­cal” tran­si­tion that would sig­nal a pro­found change in the glob­al cli­mate sys­tem. Such a tip­ping point could see the AMOC halt en­tire­ly over a rel­a­tive­ly short pe­ri­od of decades.

“We have a sit­u­a­tion where there’s a thresh­old … If we reached that thresh­old, then we will have a very, very mas­sive im­pact that is pos­si­bly prac­ti­cal­ly ir­re­versible,” said Boers.

‘Re­duce emis­sions as fast as pos­si­ble’

Dis­crep­an­cies be­tween ob­served da­ta and ex­ist­ing cli­mate mod­els re­main, and there is still no con­sen­sus on how long a full shut­down could take. Some es­ti­mates sug­gest as long as sev­er­al hun­dred years.

“All mod­els agree that in warmer cli­mates that the AMOC will be­come weak­er and weak­er,” said Dri­jfhout. “That doesn’t have to mean a col­lapse. It could go very, very grad­u­al­ly.”

In ei­ther case, West­ern Africa will have to adapt to de­clin­ing rain­fall and Eu­rope to in­creas­ing­ly un­pre­dictable win­ter weath­er, on top of oth­er ef­fects al­ready pro­duced by cli­mate change.

Fur­ther ad­vances in cli­mate mod­el­ling could pro­vide a more ac­cu­rate pic­ture of things to come, but the ev­i­dence is al­ready clear that re­duc­ing hu­man-caused glob­al heat­ing will be cru­cial to main­tain­ing sta­bil­i­ty in the At­lantic sys­tem.

The most im­por­tant fac­tor in how the AMOC de­vel­ops is the amount of green­house gas­es that will be re­leased in­to the at­mos­phere in the com­ing years and decades, said Boer.

“There’s not so much room for com­pro­mis­es. So you have to re­al­ly re­duce emis­sions as much as pos­si­ble – and as fast as pos­si­ble.”