Gulf stream threat to world weather

Sci­en­tists say dis­rup­tion of At­lantic cur­rents could ex­ac­er­bate cli­mate shifts

The Guardian Weekly - - International News - Damian Car­ring­ton

Se­ri­ous dis­rup­tion to the Gulf Stream ocean cur­rents that are cru­cial in con­trol­ling global cli­mate must be avoided “at all costs”, se­nior sci­en­tists have warned. The alert fol­lowed the rev­e­la­tion last week that the sys­tem is at its weak­est ever recorded.

Past col­lapses of the giant net­work have seen some of the most ex­treme im­pacts in cli­mate his­tory, with west­ern Europe par­tic­u­larly vul­ner­a­ble to a de­scent into freez­ing win­ters. A sig­nif­i­cantly weak­ened sys­tem is also likely to cause more se­vere storms in Europe, faster sea level rise on the east coast of the US and in­creas­ing drought in the Sa­hel in Africa.

The new re­search wor­ries sci­en­tists be­cause of the huge im­pact global warm­ing has al­ready had on the cur­rents and the un­pre­dictabil­ity of a fu­ture “tip­ping point”.

The cur­rents that bring warm At­lantic wa­ter north­wards to­wards the pole, where they cool, sink and re­turn south­wards, is the most sig­nif­i­cant con­trol on north­ern hemi­sphere cli­mate out­side the at­mos­phere. But the sys­tem, for­mally called the At­lantic Merid­ional Over­turn­ing Cir­cu­la­tion (Amoc), has weak­ened by 15% since 1950, thanks to melt­ing Green­land ice and ocean warm­ing mak­ing sea wa­ter less dense and more buoy­ant.

This rep­re­sents a mas­sive slow­down – equiv­a­lent to halt­ing all the world’s rivers three times over, or stop­ping the great­est river, the Ama­zon, 15 times. Such weak­en­ing has not been seen in at least the last 1,600 years, which is as far back as re­searchers have an­a­lysed so far. Fur­ther­more, the new analy­ses show the weak­en­ing is ac­cel­er­at­ing.

“From the study of past cli­mate, we know changes in the Amoc have been some of the most abrupt and im­pact­ful events in the his­tory of cli­mate,” said Prof Ste­fan Rahm­storf, at the Pots­dam In­sti­tute for Cli­mate Im­pact Re­search in Ger­many and one of the world’s lead­ing oceanog­ra­phers, who led some of the new re­search. Dur­ing the last ice age, win­ter tem­per­a­tures changed by up to 10C within three years in some places.

“We are deal­ing with a sys­tem that in some as­pects is highly non-lin­ear, so fid­dling with it is very dan­ger­ous, be­cause you may well trig­ger some sur­prises,” he said. “I wish I knew where this crit­i­cal tip­ping point is, but that is un­for­tu­nately just what we don’t know. We should avoid dis­rupt­ing the Amoc at all costs. It is one more rea­son why we should stop global warm­ing as soon as pos­si­ble.”

Oceanog­ra­pher Peter Spooner, at Univer­sity Col­lege Lon­don, shares the con­cern: “The ex­tent of the changes we have dis­cov­ered comes as a sur­prise to many, in­clud­ing my­self, and points to sig­nif­i­cant changes in the fu­ture.”

A col­lapse in the Amoc would mean far less heat reach­ing west­ern Europe and plunge the re­gion into very se­vere win­ters, the kind of sce­nario de­picted in an ex­treme fash­ion in the movie The Day Af­ter To­mor­row. A wide­spread col­lapse of deep-sea ecosys­tems has also been seen in the past.

But as the Amoc weak­ens, it might ac­tu­ally in­crease sum­mer heat­waves. That is be­cause it takes time for the cool­ing of the north­ern wa­ters to also cause cool­ing over the ad­ja­cent lands. How­ever, the cooler wa­ters af­fect the at­mos­phere in a way that helps warm air to flood into Europe from the south, a sit­u­a­tion al­ready seen in 2015.

Other new re­search last week showed Green­land’s mas­sive ice cap is melt­ing at the fastest rate for at least 450 years. This in­flux will con­tinue to weaken the Amoc into the fu­ture un­til hu­man-caused cli­mate change is halted, but sci­en­tists do not know how fast the weak­en­ing will be or when it reaches the point of col­lapse.

How­ever, Rahm­storf said the in­ter­na­tional cli­mate deal agreed in 2015 of­fers some hope if its am­bi­tion is in­creased and achieved: “If we can keep the tem­per­a­ture rise to well be­low 2C as agreed in the Paris agree­ment, I think we run a small risk of cross­ing this col­lapse tip­ping point.”

Source: Na­ture Ocean cir­cu­la­tion in the At­lantic is driven by warm sur­face cur­rents and cold deep-wa­ter re­turn flows

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