Ecocentric

Oceans: Resetting the Ocean Conveyor Belt

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When we think about the climate, we think about the atmosphere. Changes in the atmosphere—winds, clouds, precipitation, even thunderstorms—seem to give us weather, and it’s the accumulation of carbon and other greenhouse gases in the atmosphere that is gradually warming the planet. But the atmosphere is just one part of the climate story, and it’s not even the biggest one. The vast oceans—which cover some 70% of the planet, a familiar figure but one that’s difficult to really grip—hold 50 times more carbon than the atmosphere, and since the mid-20th centuries the oceans have absorbed some 18 times more of the excess heat from global warming than the atmosphere has. As Tony Knap, the director of the Bermuda Institute of Ocean Sciences (BIOS), told me recently: “Climate change is all about the oceans, not the atmosphere.”

That’s why a new paper in Nature Geoscience on global ocean currents could have a major impact on climate change modelers. As warm tropical water from the Equator flows north through the Atlantic Ocean, it cools down and becomes denser. (High school chemistry reminder—colder water is denser, warmer water less so.) Some of the surface water also evaporates during the trip north, leaving the remaining water saltier and even denser. (High school chemistry reminder the second—the saltier the water, the denser it is, which is why it’s easier to float in the sea than in a freshwater lake.) As the current reaches the Arctic, the dense water sinks, and then moves southward, where it gradually warms up and returns to the surface—a process called the Atlantic meridional overturning circulation (MOC), or ocean conveyor belt.

But the new Nature Geoscience paper—the lead author is the oceanographer M. Susan Lozier of Duke University—indicates that conveyor belt model might be too simplistic. The MOC has always been a rough model because it’s difficult and expensive to take physical measurements of the deep ocean—just ask Tony Knap at BIOS. But Lozier and her colleagues were able to improve the model by tapping measurements of water temperature and salinity taken by more than 500,000 research vessels between 1950 and 2000. (Because water density increases with higher salinity and colder temperatures, Lozier could use their data points as a way to track the turnover of the ocean currents.) They found that ocean circulation was far from the uniform, smooth motion a “conveyor belt” model might indicate, and that over the past 50 years ocean circulation had grown weaker near the Equator but stronger in the northern regions. As Lozier told Adam Mann of Nature: “The more we look, the more complicated the ocean is.”

Mann went on:

The idea of the seas moving as a smooth belt is being changed by the accessibility of satellite data, says oceanographer Joël Hirschi, also at the National Oceanography Centre and a co-author of the earlier study. The essential outline remains, he says, “but on top of that conveyor picture, there is a lot of variability going on”.

Indeed, our understanding of the physics and chemistry of the ocean remains frustratingly limited—which is one more reason why we need a global ocean monitoring system. But the Nature Geoscience study will help bring detail to our picture of global ocean circulation—and that in turn will help climate change experts improve their models. Until we know the oceans, we can’t know our planet.

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