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.
The puzzling discovery, detailed in a study published last week in the journal Science Advances, highlights that climate change could have wide-ranging effects that are unexpected or severely understudied.
Climate models had predicted that ocean circulation would accelerate with unmitigated climate change, but the changes had not been expected until much later this century, said a co-author of the study, Michael McPhaden, a senior scientist at the National Oceanic and Atmospheric Administration.
The disparity suggests that some climate models may underestimate 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, 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. Warm water funneled by currents from the equator to the poles, for example, 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 based in New York. "It keeps the tropics cool enough to live in and northern regions warm enough to live in."
Rader, who was not involved with the new research, said changes in the 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.
Although 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 another co-author of the study, Janet Sprintall, 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, combined with a steady rise in greenhouse gas emissions, since the 1990s. 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 speeded-up currents. Because 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.
Download the NBC News app for breaking news
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 of 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 on fisheries could have cascading effects up and down the food chain, with impacts on countries and communities that depend on fishing, she added.
The scientists plan to conduct more research to address some of these unknowns.
The gaps in knowledge, McPhaden said, are due in part to the fact that there isn't a massive archive of historical measurements of ocean circulation. That means scientists have to rely heavily on models of past ocean health and how those bodies of water might respond to future climate change.
Rader said he hopes such 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 had expected, 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."