Global Warming Speeds Up Ocean Currents: Oceanographers

Two years ago, oceanographers made a surprising discovery: Not only have oceans been warming because of human-driven climate change, but the currents that flow through them have accelerated—by some 15% per decade from 1990 to 2013, Paul Voosen reported for Science magazine.

Photo Insert: To test that hypothesis, Xie’s team turned to a climate model of all the world’s oceans.

At the time, scientists said faster ocean winds were driving the speedup. But a new study fingers the culprit: The ocean’s own tendency to warm from top to bottom, leading to constricted surface layers where water flows faster, like blood in clogged arteries.

Climate change will speed up ocean currents, potentially limiting the heat the ocean can capture and complicating migrations for stressed marine life. “This mechanism is important,” says Hu Shijian, an oceanographer at the Chinese Academy of Sciences’s Institute of Oceanology, who was the lead author of the 2020 paper.

“The new paper links directly the surface warming and acceleration of upper ocean circulation.” Currents like the Atlantic Ocean’s Gulf Stream are highways for marine life, ushers of heat, and drivers of storms.

Driven in large part by wind, each of them moves as much water as all the world’s rivers combined. And, despite the fact that the ocean absorbs more than 90% of the heat caused by global warming, until 2020, there had been little evidence that these currents were changing.

When Shang-Ping Xie, a climate scientist at the Scripps Institution of Oceanography, saw Hu’s study, he suspected that ocean structure—not winds—played a key role in the speedup. He knew the excess warmth from climate change is not distributed evenly through the ocean but is instead concentrated at its surface.

This causes surface waters to grow more buoyant—and more reluctant to mix with waters below. The shallower surface layers created by this process have been seen across the world’s oceans.

Xie and his colleagues found that in shallower layers, currents would naturally have to speed up: In effect, the winds were pushing the same amount of water through a narrower pipe. “If you assume the total transport can’t change, your stuff is going to accelerate,” Xie says.

To test that hypothesis, Xie’s team turned to a climate model of all the world’s oceans. The researchers increased winds, saltiness, and surface temperatures while holding all other variables steady. Increasing temperatures alone caused currents to speed up more than 77% of the ocean’s surface.

That was by far the largest increase, they found in a new study published in Science Advances. One notable exception was the Gulf Stream, which is likely slowing for an unrelated reason: As Arctic ice melts, it dilutes the sinking, salty water in the North Atlantic that pulls the current northward.

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