Ocean Current That Keeps Europe Warm Is Weakening

Two new studies show the Atlantic Meridional Overturning Circulation has decreased 15 to 20 percent over the last 150 years

Ocean Currents

Great Britain and Ireland can be a little chilly, but they’re surprisingly balmy for their latitude. These regions have an ocean current to thank for that warm(ish) weather. Known as the Atlantic Meridional Overturning Circulation (AMOC), the current works like a conveyor belt, drawing warm water up from the Gulf Stream to North America’s east coast and then shunting it toward Europe.

But, as Victoria Gill at the BBC reports, two new studies suggest that the AMOC is the weakest it’s been in over 1,600 years, with the most drastic changes taking place in the last 150 years.

The first study, published in the journal Nature, addresses the history of the AMOC. Researchers studied the size of the grains in cores of sediment from the ocean floor. As Andrea Thompson at Scientific American reports, the stronger a current is, the larger the grains of sediment it can move, allowing researchers to map changes in current strength by sediment size. The team also looked for tiny fossil critters, known as foraminifera of “forams,” to get a sense of ocean temperatures. Since some species of foram thrive in warm waters while others prefer cooler temps, researchers can use foram species as a rough thermometer for past ocean temperature.

In the second study in Nature, the team used state-of-the-art climate models and a century of ocean sea surface temperatures to study AMOC changes. The results of both studies suggest that the AMOC is weak, but when that change started is up for debate.

As Summer K. Praetorius writes for Nature, the sediment core study suggests that the AMOC began weakening in 1850, the beginning of the industrial era. She also points out that the change corresponds with the end of the Little Ice Age, a climate dip that lasted from the 1300s to 1850s. As the climate warmed up, it’s possible more fresh water flowed into the oceans, which disrupted the AMOC.

The sea temperature model, however, suggests that the AMOC flow has weakened since the mid-twentieth century as a result of human-induced climate warming. As Thompson notes, however, this record did not extend as far back as the sediment study.

Despite the difference in timing, both studies show similar pattern of current decline, weakening about 15 to 20 percent in the last 150 years. “We think it’s quite remarkable that all the evidence is converging,” David Thornalley of University College London tells Thompson.

“What is common to the two periods of AMOC weakening—the end of the Little Ice Age and recent decades—is that they were both times of warming and melting,” Thornalley says in a press release. “Warming and melting are predicted to continue in the future due to continued carbon dioxide emissions.”

Does that mean London will soon come to resemble Nome, Alaska?

“The [current] climate models don’t predict [an AMOC shutdown] is going to happen in the future,” Thornalley tells Damian Carrington at The Guardian, “the problem is how certain are we it is not going to happen? It is one of these tipping points that is relatively low probability, but high impact.”

Murray Roberts, who studies ocean temperatures at the University of Edinburgh tells Gill that even if AMOC changes don’t meddle with overall climate, these changes could wreak havoc on delicate ecosystems in the Atlantic.

“The deep Atlantic contains some of the world's oldest and most spectacular cold-water coral reef and deep-sea sponge grounds,” he says. “These delicate ecosystems rely on ocean currents to supply their food and disperse their offspring. Ocean currents are like highways spreading larvae throughout the ocean and we know these ecosystems have been really sensitive to past changes in the Earth's climate.”

Researchers expect future changes in global climate will cause further slowdowns of the Atlantic overturning. But there is still much more to learn about these complex systems. As Alexander Robinson, co-author of the sea temperature, tells Carrington: “We are only beginning to understand the consequences of this unprecedented process – but they might be disruptive.”

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