Warming could disrupt Atlantic Ocean current

Spewing too much carbon dioxide into the atmosphere could shut down the major ocean current that ferries warm water to the North Atlantic, new climate simulations suggest. While not as extreme as the doomsday scenario portrayed in the movie The Day After Tomorrow, such a shutdown could cause wintertime temperatures to plummet by an estimated 7 degrees Celsius or more in northwestern Europe and shift rainfall patterns across the globe.

Many previous climate simulations predicted that the Atlantic circulation would remain largely stable under future climate change. But those simulations failed to accurately portray how relatively freshwater flows between the Atlantic and Southern oceans, an important mechanism as the climate warms. After fixing that inaccuracy, Yale University climate scientist Wei Liu and colleagues set up an extreme climate scenario to test the current’s robustness. Doubling CO2 concentrations in the atmosphere shuttered the Atlantic current in 300 years, the researchers’ simulation showed.
While such a rapid CO2 rise is unrealistic, the new simulation demonstrates that the current isn’t stable after all, the researchers conclude January 4 in Science Advances. “The next step is to use a more realistic warming scenario to predict what the future will look like,” Liu says.

Even with a more realistic scenario, the applicability to the real world will be hampered by a lack of direct long-term observations of the Atlantic circulation, says Gerald Meehl, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo. Observations help improve simulations, but such data for the Atlantic current don’t go back “for more than a decade or two,” he says.

Known as the Atlantic Meridional Overturning Circulation, the Atlantic current is a colossal conveyor belt. It carries warm water from the South Atlantic northward along the ocean surface into the North Atlantic. Near Greenland where the current makes a U-turn, cold water sinks and flows southward into the South Atlantic. These two halves of the AMOC form a loop that keeps northwestern Europe warm and drives rainfall across the tropical Atlantic.

Warming due to climate change in the North Atlantic makes the waters there less dense and less likely to sink. This change slows the AMOC. In many previous climate simulations, the current’s speed bounces back. That’s because these simulations incorrectly show that rain-freshened water flows from the Southern Ocean into the Atlantic Ocean. In these simulations, as the AMOC weakens, this influx of freshwater slackens and the Atlantic becomes saltier. Like cold water, salt-laden water is denser and more likely to sink, helping the AMOC recover.

But ocean observations show that freshwater flows from the Atlantic into the Southern Ocean, not the other way around. Liu and colleagues updated an existing simulation by manually correcting the flow direction. After doubling CO2 concentrations in the simulation compared with 1990 levels, the researchers found that the North Atlantic warmed and the AMOC slowed. With less warm water moving northward, countries such as England and Iceland cooled even when taking into account the greenhouse warming from the added CO2.

The researchers also found that as the AMOC slowed, less freshwater from the Atlantic flowed into the Southern Ocean. That decreased the Atlantic’s saltiness, further weakening and ultimately collapsing the AMOC. The same simulation without the flow-direction change did not show a disrupted current, the researchers found. Meltwater from shrinking Greenland ice may also freshen the Atlantic, suppressing the AMOC, though the researchers didn’t look at this effect.