Ocean currents are slowing down, and that’s bad news

View of boats and a pier in East Kalimantan, Indonesia. CIFOR/Augusta
8 June 2018

BONN, Germany (Landscape News) — Two recent articles in the journal Nature reached the same disconcerting conclusion: One of the planet’s main ocean-current systems is slowing down, which will have large effects on the global climate.

Both studies focus on the Atlantic meridional overturning circulation, or AMOC, a system of ocean currents in the Atlantic, which influences weather patterns in Europe, North America and parts of Africa. The AMOC is like a conveyor belt, bringing warm water from the tropics to the north, while transporting cold water back to the South through the deep ocean.

SLUGGISH CURRENTS

The articles — in focus on Landscape News ahead of World Oceans Day on June 8 — were written by two groups of independent researchers, using very different techniques. The first article, spearheaded by Levke Caesar from the Potsdam Institute for Climate Impact Research, reports on an analysis of sea surface temperature data. The authors found that the AMOC has slowed down by 15 percent since the mid twentieth century—the lowest point in the last 1,000 years. This slow-down implies a decrease of 3 million cubic meters of water per second, the equivalent of nearly 15 Amazon rivers.

The results of the study are largely supported by a second article that appeared in the same issue of Nature. A research team around David Thornalley from University College London analyzed the size of sediments on the ocean floor, which provide information about the strength of the current, as stronger currents deposit larger grains. They also analyzed how ocean temperatures changed with variations in the current, by looking at the fossils of small organisms, some of which thrive in warmth, while others require colder conditions. Their results suggest that the AMOC started to slow down 160 or 170 years ago.

HUMAN IMPACT

Although both research groups found that the strength of the AMOC has been weakening, their time-lines differ. This has implications concerning what influences can be attributed to humans.

The findings discovered by Caesar and her co-authors suggest that the AMOC started changing around the middle of the 20th century, and they point at human-induced climate change as the main suspect.

According to the second study, however, the turning point was at the end of the little ice age—a period of relatively low temperatures throughout the northern hemisphere, that lasted from about 1350 to 1850 A.D. As this was before the massive release of industrial carbon emissions into the atmosphere, the authors believe the changes started due to natural circumstances, while human activities may have continued to suppress the AMOC.

THE EFFECTS

The slowing down of the AMOC leads to changes in the temperature of the Atlantic Ocean, which in turn has a direct influence on the weather in Europe. The European heatwave of the summer of 2015, for example, has been associated with the record cold in the North Atlantic that year, Levke Caesar told Landscape News. This paradoxical situation arose because the cold North Atlantic favored an air pressure pattern conducting warm air from the south to Europe, she explained.

According to Caesar and her co-authors, continued global warming is likely to further weaken the system of ocean currents. If this happens, the effects will be manifold. It is expected to result in an increased intensity of winter storms and heatwaves in Europe, a rising sea level on the east coast of the U.S., and more prolonged drought in the Sahel.

A slower AMOC will thus exacerbate the effects of global warming, to which people will need to adapt. Farmers in Europe and the Sahel, for example, may have to change their agricultural practices to adjust to more extreme weather conditions and prolonged droughts, while such major coastal cities as New York and Boston will need to become water-proof.

TIPPING POINT

As the AMOC is crucial to maintaining relatively stable weather conditions on the planet, the observed changes are worrying. The biggest risk is that the AMOC reaches a tipping point, when the currents slow down to such an extent that the system itself changes, radically changing the way that heat is distributed.

Reaching a tipping point could have drastic consequences for the whole planet, Caesar explained. Model studies suggest that a strong weakening of the AMOC would reduce the absorption of atmospheric carbon dioxide by the ocean in that area, which would increase the concentration of carbon dioxide in the atmosphere more rapidly and thus increase the greenhouse effect and global warming.

Exactly how close we are to the tipping point is hard to tell. It depends, for example, on freshwater influx into the North Atlantic due to the melting of the Arctic sea ice and the Greenland ice sheet. It is difficult for models to accurately predict the future behavior of the currents, warranting more research, Caesar said.

Will ocean life survive climate change? Five takeaways from a study of marine protected areas