Contrary to previous predictions, the circulation of water in the world’s oceans appears to be accelerating, according to a study published in Science Advances today (February 5). Although the implications for this trend are, as yet, unknown, the discovery will be critical for informing future models of climate change, researchers say.
“This is quite an exciting paper,” says Joellen Russell, a geoscientist at the University of Arizona who was not involved in the study. “I think the results are robust, I think they’re important, and I think they are a little shocking,” she says.
The water of the Earth’s oceans is continuously circulating around the planet via currents, gyres, and eddies. These movements regulate the Earth’s climate by dispersing heat—they transport warm water from the tropics to the polar regions and drive cooler water back via the oceans’ depths. They also transport dissolved atmospheric gases, such as oxygen and carbon dioxide, and churn up nutrients from below. Ocean circulation is thus a fundamental process for life on Earth.
According to certain models—based on water temperature measurements and other factors—global warming has been predicted to weaken these currents. “Most people are expecting the global ocean circulation to slow,” says Russell, and consequently, for the seas to be more stratified in temperature gradient from the surface to their depths and therefore to be more stable with less mixing.
But, in contrast to these predictions, the new study that examines circulation on a global scale shows “a clear increase over the last twenty years . . . [in] the strengths of the currents,” says the University of Reading’s David Ferreira, who studies the dynamics of the oceans but who was not involved in the research. And that’s “pretty interesting,” he says.
There have been “numerous studies on various important but regional or basin-scale ocean circulations,” writes coauthor Shijian Hu of the Chinese Academy of Sciences’s Institute of Oceanology in an email to The Scientist. “However, it has not been well understood what the trend of the Earth’s large-scale ocean circulation is under the background of climate warming, partly because of a lack of systematic and continuous direct observations of the Earth’s oceans.”
Hu’s team examined historical observations and assimilated datasets from a wide range of sources, including satellite measurements, acoustic Doppler current profilers, and assorted temperature and salinity measurements, including those obtained by the global Argo system—a fleet of thousands of free-drifting floats that have been deployed in the worlds oceans since the early 2000s and that provide continuous monitoring of the temperature, salinity, and velocity of the upper ocean layer.
From these data, the team discovered that while ocean circulation in some regions shows a decelerating trend—for example, in the North Atlantic—across the planet as a whole there was a “surprising significant acceleration of global mean circulation during the past two decades,” Hu writes. Particularly prominent acceleration was apparent in the tropical oceans, the team notes.
The likely culprit behind the acceleration, the team suggests, is increasing surface wind speeds. Indeed, “All the [data assimilation] products show a remarkable increase in the global mean sea surface wind speed over the past two decades,” Hu writes.
So, what are the likely impacts of accelerated ocean movements? “If the large-scale planetary transports are increasing, that could play a role in modulating climate heat uptake,” says oceanographer and climatologist Gael Forget of MIT who was not involved in the study. “As a general trend,” he explains, more turbulence would “probably increase the uptake of anthropogenic heat.”
“The most important influence of changes in ocean circulation probably is on the biology,” says marine systems modeler George Nurser of the National Oceanography Centre in the UK who also was not part of the research team. For example, some fish species only spawn at particular temperatures. The accelerated movement of water masses might therefore alter spawning grounds, or it may impact the connectivity between marine ecosystems.
Aside from altered geographical distributions, food sources may be affected, says Forget. “Marine ecosystems rely on nutrients that in many ways come from below, so if you increase the mixing near the surface where marine ecosystems reside, you might expose them to more nutrients.”
Acceleration of the ocean’s circulation is not necessarily good or bad news regarding climate change, says Russell. “There are always winners and losers in these situations.” And there are likely to be ongoing debates about the effects, the major drivers of the trend, and how long it might last, she says. “But to have this paper as the launching spot for those debates is marvelous.”
S. Hu et al., “Deep-reaching acceleration of global mean ocean circulation over the past two decades,” Sci Adv, 6:eaax7727, 2020.