Re­searchers Say Slow­ing At­lantic ‘Con­veyor Belt’ Will Bring Ris­ing Sea Lev­els, Fish­ery Changes

Soundings - - Focus - By Kim Kavin

Two re­cently pub­lished stud­ies in the jour­nal Na­ture make it seem that the 2004 film “The Day Af­ter To­mor­row” was, in fact, more sci­ence than fic­tion. You re­mem­ber the premise: Re­searcher Den­nis Quaid’s son, played by Jake Gyl­len­haal, is trapped in Man­hat­tan when the North At­lantic cur­rent goes hay­wire and ev­ery­thing north of Mex­ico ends up in an eco-apoc­a­lypse. Snow blan­kets sky­scrapers. A tsunami floods New York City. Peo­ple who step out­side freeze to death. “When this storm is over,” Quaid’s char­ac­ter says, “we’ll be in a new Ice Age.”

To­day’s real-life re­searchers aren’t be­ing quite so melo­dra­matic, of course, but the stud­ies pub­lished in April say a sim­i­lar ocean phe­nom­e­non is now well un­der­way — and is al­ready caus­ing “species shift” in fish­eries, ris­ing sea lev­els along the U.S. East Coast and more.

The stud­ies, by two sets of re­searchers, are about At­lantic merid­ional over­turn­ing cir­cu­la­tion, or AMOC. It is a sys­tem of cur­rents in the At­lantic Ocean. The flow of those cur­rents hasn’t gone com­pletely out of whack, like the ocean cur­rent did in the movie, but AMOC has slowed, the re­search shows, to a record 1,000-year low.

AMOC’s slow­down, or weak­en­ing, is sig­nif­i­cant for nu­mer­ous rea­sons — not the least of which is that AMOC is the con­veyor belt of the ocean. It car­ries warm water from the equa­tor up to the north­ern At­lantic, and then brings colder, north­ern water back down south, af­fect­ing ev­ery­thing from the cli­mate in West­ern Europe to the water tem­per­a­tures off the U.S. East Coast.

The colder water from the north usu­ally moves along the AMOC con­veyor belt to the south at deeper ocean lev­els, be­cause cold, salty water is dense. It sinks. But be­cause melt­ing ice in the Arc­tic and Green­land is adding fresh water to the wa­tery mix up north, more of the colder water is re­main­ing at the ocean’s sur­face and, it seems, jam­ming up the AMOC con­veyor belt.

Both sets of re­searchers found AMOC’s weak­en­ing to be real, us­ing two meth­ods of study. One team used an ex­pen­sive, high­res­o­lu­tion map­ping tool to look more clearly at the ocean than re­searchers gen­er­ally are able to see.

“It’s the ul­tra- 4K TV of mod­els,” says Vin­cent Saba of the Na­tional Oceanic and At­mo­spheric Ad­min­is­tra­tion’s Marine Fish­eries Ser­vice, and a co-au­thor of one of the stud­ies. “There’s very high de­tail of the whole East Coast. We were re­ally able to look at de­tailed changes in the Gulf Stream and the Labrador Cur­rent as they re­late to AMOC.”

The re­search team that pro­duced the other study looked at sed­i­ment sam­ples off North Carolina, to see how thick the de­posits of sand grains were over the years. Their the­ory was that a stronger AMOC in decades or cen­turies past would have been able to carry and de­posit thicker grains.

“We were look­ing at the last 160 or 170 years,” Saba says of the high­res­o­lu­tion team. “They were look­ing at more of a 1,500-year record.”

Saba’s team showed a weak­en­ing AMOC dur­ing the time since about the In­dus­trial Rev­o­lu­tion in the 18th and 19th cen­turies, sug­gest­ing that man­made cli­mate change is a cause of the slow­down. The team that looked at sand grains, he says, showed a weak­en­ing AMOC from nat­u­ral causes start­ing dur­ing the 14th to 19th cen­turies, a pe­riod that sci­en­tists call the Lit­tle Ice Age.

Taken to­gether, the stud­ies sug­gest that hu­man-driven cli­mate change is in­ten­si­fy­ing what started hun­dreds of years ago as a nat­u­ral AMOC-slow­ing phe­nom­e­non.

“If their re­sults are cor­rect and our re­sults are cor­rect,” Saba says, “then ours sug­gest an AMOC weak­en­ing in the al­ready-weak­ened state caused by that Lit­tle Ice Age.”

Both teams had to use in­di­rect mea­sure­ments, he says, be­cause more di­rect ones are harder to come by.

“The prob­lem with AMOC is that we only have about 14 years of di­rect mea­sure­ments of only one com­po­nent of AMOC,” Saba says. “It isn’t long enough to have enough ob­ser­va­tion to sug­gest trends, so we have to rely on these prox­ies.”

Saba says that, un­like in “The Day Af­ter To­mor­row” film, none of us has to make an emer­gency mass mi­gra­tion to warmer lat­i­tudes just yet — but we do have to re­al­ize that ev­ery­thing from East Coast sea lev­els to fish habi­tats are al­ready chang­ing, and are pro­jected to change more, as AMOC con­tin­ues to weaken.

There is con­sen­sus, he says, among 25 to 30 pre­dic­tion mod­els that are run glob­ally about ev­ery half- dozen years, that AMOC will keep weak­en­ing, es­pe­cially with con­tin­ued in­dus­trial emis­sions re­lated to cli­mate change and freshwater ice melt. Sci­en­tists dis­agree on the fu­ture amount of AMOC weak­en­ing, he says, but there is ad­di­tional ev­i­dence to sup­port the con­sen­sus about AMOC’s ef­fects right now.

For in­stance, AMOC’s im­pact on East Coast fish­eries is al­ready be­ing seen from about Cape Hat­teras, North Carolina, up through the Gulf of Maine, he says.

Re­search shows that as AMOC weak­ens, the pres­ence of warmer, salty water shifts to the northwest. To un­der­stand the phe­nom­e­non, think about a rush­ing river. Its water is mov­ing so fast that the whole of the river stays in a nar­row lane, com­pared to a lazy, me­an­der­ing river, which has time to set­tle in and broaden out. Ba­si­cally, as AMOC slows, the warm water mov­ing up from the Gulf Stream spreads, and re­search shows that the di­rec­tion of the warm-water spread­ing is to the northwest.

Saba says there’s di­rect re­search of this phe­nom­e­non from the past decade in the Gulf of Maine, as an ex­am­ple. That gulf gen­er­ally gets its water from one of two sources: the cold Labrador Cur­rent wa­ters flow­ing down from its north, and the warm Gulf Stream wa­ters com­ing up from its south.

“We know, for the past 10 years or so, the ma­jor­ity of water com­ing into the Gulf of Maine is from the Gulf Stream,” he says. “It’s pretty sub­stan­tial. If you look at the last 10 years, the wa­ters in the Gulf of Maine have warmed faster than 99 per­cent of the global ocean.”

And warmer wa­ters af­fect the types of an­i­mals that can thrive in them.

For in­stance, Saba says, cold-water species are dwin­dling in the Gulf of Maine, while lob­sters seem to be mov­ing off­shore to colder water now.

The lob­sters are an in­ter­est­ing case study; they also used to be happy in more mid-At­lantic climes, such as off the coast of New York.

“If you look at Long Is­land Sound, the lobster pop­u­la­tion crashed and hasn’t re­cov­ered since 1999, whereas in the Gulf of Maine, they’re thriv­ing — but we don’t think that’s sus­tain­able be­cause of what’s hap­pen­ing with the water,” Saba says. “My sense is that what’s hap­pen­ing is a bad thing for lob­sters. They’re mov­ing off­shore. Cold-water species like cod, there are very few left off the Gulf of Maine. Near New Jer­sey, we’re see­ing species like win­ter floun­der mov­ing north. If you have a colder- water species and the water is warm­ing, they ei­ther have to go to deeper water or they have to move north, where the water is colder.”

A young fish­er­man holds a small floun­der, a fish that may ex­pand its habi­tat on the east coast as At­lantic cur­rents slow down.

Saba cau­tions that species be­hav­ior is based on a lot more than just water tem­per­a­ture. Fish, lob­sters and all kinds of sea dwellers have to find food and places to spawn. They have to avoid predators. Fish­eries ex­perts have trou­ble pre­dict­ing the next year’s stock, let alone what’s go­ing to hap­pen 25 or 50 years into the fu­ture.

How­ever, he adds, what’s hap­pen­ing with AMOC in­di­cates that the en­vi­ron­ments avail­able to var­i­ous species are chang­ing, and will con­tinue to change, tem­per­a­ture- wise.

“If we only as­sume ocean tem­per­a­ture will af­fect fish, what do those mod­els look like?” he says. “Sum­mer floun­der, a warmer- weather fish, you’re go­ing to see more habi­tat avail­able 50 years from now.”

And where the fish and shell­fish go, boaters tend to want to fol­low.

“Recre­ational fish­er­men, they like to go to a cer­tain wa­ter­ing hole where they al­ways catch sea bass or sum­mer floun­der,” Saba says. “Twenty- five years from now, they may find that they can’t go to those same holes be­cause the water is warm­ing. They may have to go 25 or 30 miles north to catch those fish, and all they can find at the old place is some­thing they don’t want, like a slimy dog­fish. There are go­ing to be win­ners and losers.”

Chang­ing fish­ing grounds are ex­pected to af­fect ev­ery­thing from the amount that boaters have to spend on fuel to reach the new habi­tats, as well as fees in places where an­glers must catch cer­tain species only within the ter­ri­to­rial wa­ters of the state that is­sued their fish­ing li­censes. For boaters who want to fish for spe­cific va­ri­eties, pay­ing for fish­ing li­censes in mul­ti­ple states may be­come the norm.

“That’s go­ing to af­fect com­mer­cial and recre­ational fish­ing,” Saba says. “We have no doubt about that. We’re see­ing those changes to­day.”

Re­searchers also ex­pect boaters who don’t fish to see con­tin­u­ing ef­fects from AMOC’s weak­en­ing, pri­mar­ily in terms of ris­ing sea lev­els along the

whole of the East­ern Seaboard.

In 2009, re­searchers looked at the rate of sea level rise from Florida up to Maine and be­yond into Canada, Saba says. That year was sig­nif­i­cant, be­cause it’s also the year that re­searchers found the weak­est- ever AMOC mea­sure­ments from a point where read­ings are reg­u­larly taken.

“Dur­ing that year,” he says, “sea-level rise across the whole East­ern Seaboard was un­usu­ally high.”

The 2009 link be­tween a weaker AMOC and ris­ing sea lev­els could be a co­in­ci­dence, but a 2010 re­search pa­per posited that there may be a cor­re­la­tion — a the­ory that Saba thinks is cor­rect.

“It makes sense,” he says. “If the Gulf Stream is weaker, you get a buildup of water along the U. S. to the west of the Gulf Stream. The water re­mains there un­til AMOC re­cov­ers, and then sea level goes back to av­er­age again. It’s like a see­saw.”

More re­search is needed to show a cor­re­la­tion be­tween AMOC’s weak­en­ing and ris­ing East­ern Seaboard sea lev­els, he says, adding that such re­search could pro­duce re­sults even more har­row­ing than ex­ist­ing pre­dic­tions for coastal mari­nas and com- mu­ni­ties in the low­est-ly­ing ar­eas.

“If there is that re­la­tion­ship, and if AMOC con­tin­ues to weaken like we’re pre­dict­ing, then that could mean even higher sea level rise than what we’re pre­dict­ing in the mod­els,” Saba says.

Re­searchers, in gen­eral, can’t pre­dict whether Mi­ami or Man­hat­tan will see the great­est sea- level rise over time. Glob­ally, the oceans aren’t all ex­pected to rise in an equal man­ner. Coastal com­mu­ni­ties in In­dia may get hit harder than those in Africa. The same is true for towns up and down the East Coast of the United States.

But there will be an im­pact, Saba says, es­pe­cially from about North Carolina to Maine.

“We don’t see these en­hanced warm­ing sig­nals south of Cape Hat­teras,” he says. “The sea level rise would af­fect the en­tire East Coast, but in terms of the warm­ing tem­per­a­tures, we think the most dra­matic changes in terms of ecosys­tem change may be be­tween North Carolina and the Gulf of Maine — and that may have a lot to do with what AMOC is do­ing now, and what it might be do­ing in the fu­ture.”

As ocean wa­ters warm due to slow­ing cur­rents, and lob­sters seek colder water off­shore, lob­ster­men will have to make longer and costlier trips to catch them.

A cod dives for deeper and colder water. As the Gulf of Maine con­tin­ues to warm, cod may be­come even harder to find and catch.

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