Ocean currents — undersea conveyor belts that help regulate Earth’s climate and influence weather systems around the world — have been speeding up over the past two decades as the planet warms, according to new research.
This puzzling discovery is the subject of a study published Feb. 5 in the journal Science Advances. The research highlights that climate change could have wide-ranging effects that are either unexpected or severely understudied.
Climate models had predicted that ocean circulation would accelerate with unmitigated climate change, but these changes were not expected to occur until much later this century, according to Michael McPhaden, a senior scientist at the National Oceanic and Atmospheric Administration and a co-author of the study.
This disparity suggests that some climate models may be underestimating the effects of global warming.
“Our primary tool for looking into the future about how greenhouse gas forcing will impact the climate system is these models,” McPhaden said. “If we can’t have confidence in their projections, that raises very serious concerns about how we prepare for a future world different from the one today.”
McPhaden and his colleagues discovered that currents in three-quarters of the world’s oceans have accelerated over the last two decades, driven primarily by faster and more intense winds. On average, the researchers found that global ocean circulation has accelerated by 36 percent since the early 1990s.
“It surprised us how strong it was and how persistent it was over this time period — it was just too dramatic to be accounted for by natural fluctuations,” McPhaden said. “This is another manifestation of how the climate system is reacting to human activity.”
Ocean currents form a complex web of underwater highways that move water and heat around the globe. When currents funnel warm water from the equator to the poles, for example, this helps regulate land temperatures and drive weather systems.
“It’s like a global circulatory system that helps keep us healthy,” said Douglas Rader, chief ocean scientist at the Environmental Defense Fund, a nonprofit environmental advocacy group headquartered in New York. “It keeps the tropics cool enough to live in and northern regions warm enough to live in.”
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Rader, who was not involved with the new research, said changes to this dynamic process could “threaten the planet’s life-support system” and overwhelm the resilience of ecosystems and communities.
For the new study, the researchers tapped into a network of free-drifting instruments, known as Argo floats, to measure the movement of water up to a depth of 2,000 meters.
Though faster currents were observed in 76 percent of the world’s ocean waters, the most dramatic changes were seen in the tropics.
“There has been evidence that trade winds in the tropics have been strengthening over the past few decades, so we expected some response to that,” said Janet Sprintall, a co-author of the study and an oceanographer at the Scripps Institution of Oceanography at the University of California, San Diego. “But the acceleration was happening at a faster pace than what wind alone could explain.”
Scientists have observed an increase in the intensity of surface winds since the 1990s, combined with a steady rise in greenhouse gas emissions over the same time period. But McPhaden said the exact links between increased greenhouse gases and faster ocean currents are still unknown.
“We’re not 100 percent certain how all this is driven by climate change,” he said. “Our paper flagged that this acceleration is occurring, but why exactly this is happening, and the precise mechanisms, have not been ferreted out in great detail yet.”
It’s also not yet known what consequences could come from these sped-up currents. Since ocean circulation influences weather and plays a role in redistributing heat, it’s possible that changes in the ocean could also alter precipitation patterns, the behavior of the jet stream and atmospheric circulation, according to Sprintall.
“Warmer water will generate hurricanes and extreme weather like that, so there are definitely implications from our work,” she said.
There could also be enormous repercussions for ocean ecosystems.
“Warming oceans are causing a mass migration because fish are moving to areas where they can thrive,” said Lisa Suatoni, deputy director of the oceans division at the Natural Resources Defense Council, a nonprofit environmental advocacy group. “When I hear about changes in current patterns, I’m interested in how this will affect the productivity and distribution of fisheries,” said Suatoni, who was not involved with the research.
Any major impacts to fisheries could have cascading effects up and down the food chain, with subsequent impacts on countries and communities that depend on fishing, she added.
The scientists are planning to conduct additional research to address some of these unknowns.
The gaps in knowledge, McPhaden said, owe in part to the fact that there isn’t a massive archive of historical measurements of ocean circulation. This means having to rely heavily on models of past ocean health and how these bodies of water may respond to future climate change.
Rader said he hopes these types of findings could spur more funding for ocean research.
“The ocean remains a poor stepchild in terms of where money is spent,” he said, “but we’re finding more and more that impacts on people and ecosystems are delivered through changes in the ocean, so the need to understand these processes can’t be overstated.”
And as some of these changes occur faster than scientists previously anticipated, Rader said he hopes the needs of the scientific community will be met with urgency.
“The bottom line is: This is not a question for our children and grandchildren, but for everyone alive today,” Rader said. “It’s not too dramatic to say that if the ocean system changes significantly, it could directly threaten life on Earth.”