Global warming is accelerating ocean currents. Here’s why | Science
Two years ago, oceanographers makes a surprising discovery: Not only have the oceans warmed due to human-caused climate change, but the currents through them have accelerated, by about 15% per decade from 1990 to 2013. At the time, many scientists suspected that faster ocean winds were causing the acceleration. . But a new modeling study points to another 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. The study suggests that climate change will continue to accelerate ocean currents, potentially limiting the heat the ocean can pick up and making it harder for already stressed marine life to migrate.
“This mechanism is important,” says Hu Shijian, an oceanographer at the Chinese Academy of Sciences’ Institute of Oceanology, who was the lead author of the 2020 paper.[The new paper] directly links surface warming and acceleration of the upper ocean circulation.
Currents like the Atlantic Ocean’s Gulf Stream are highways for marine life, heat harbingers, and engines of storms. Driven largely by the wind, each of them moves as much water as all the rivers in the world combined. And, despite the fact that the ocean absorbs more than 90% of the heat caused by global warming, until 2020 there was little evidence that these currents were changing.
When Shang-Ping Xie, a climatologist at the Scripps Institution of Oceanography, saw Hu’s study, he immediately suspected that ocean structure — not winds — played a major role in the acceleration. He knew that excess heat from climate change is not evenly distributed across the ocean, but rather concentrated on its surface. This makes surface waters more buoyant and more reluctant to mix with waters below. The shallower surface layers created by this process have been observed in all of the world’s oceans.
Xie and his colleagues also realized that, in shallower layers, the currents should naturally speed up: in effect, the winds were pushing the same amount of water through a narrower pipe. “If you assume that the total transportation cannot change, your business will speed up,” Xie says.
To test this hypothesis, Xie’s team turned to a climate model of all of the world’s oceans. The researchers increased winds, salinity or surface temperatures, while keeping all other variables stable. Rising temperatures alone have caused currents to accelerate more than 77% of the surface of the ocean. This was by far the largest increase, they found in a new study published today in Scientists progress. A notable exception was the Gulf Stream, which is probably slowing for an unrelated reason: as Arctic ice melts, it dilutes the salt water that flows into the North Atlantic and pulls the current north.
“This is an interesting study with a provocative discovery,” says Sarah Gille, physical oceanographer at Scripps. “We generally assume that if you warm the ocean evenly, there won’t be a major impact on ocean circulation.” Accounting for the bottom-up nature of ocean warming changes that picture, she adds.
The new findings also suggest that in much of the ocean, the lower waters, about 400 meters deep, would slow down as the warm upper waters pick up more and more motion, Xie says. Hu isn’t so sure, though. Unpublished measurements of the speed of Argo floats, a fleet of robotic instruments that have been drifting in the ocean for nearly 20 years, show a significant acceleration in surface currents and a modest increase at shallower depths. “I trust what the observations tell us,” Hu says. The new discovery, he adds, “might not be the full story.”
But if ocean currents do indeed get faster and shallower, there are many implications for the planet. For example, shallow, fast currents could ultimately limit the amount of heat the ocean can absorb, causing more of that excess heat to stay in the atmosphere. Marine microbes and fauna could be subjected to shallower, warmer and faster surface waters. And given that the acceleration is driven by the steady pace of warming, that means these trends are likely to continue into the future, as long as human emissions of greenhouse gases continue.