The Atlantic Meridional Overturning Circulation is a system of ocean currents that transports heat to the Northern Hemisphere. (Image courtesy of NOAA)
Scientists say there is a high probability that the Atlantic Ocean's major currents will weaken in the coming decades due to climate change, but whether this has already happened remains hotly debated. A new study now shows that the Atlantic circulation has remained stable since the 1960s, suggesting the system may be more resilient to warming than researchers thought.
However, the results are controversial. Previous studies have provided mixed evidence, with some studies suggesting that the Atlantic Meridional Overturning Circulation (AMOC) is weaker than at any time in the last thousand years, while other studies find no compelling evidence of a decline in the current's strength.
“I would say the jury is still out on whether the AMOC is weakening,” Maya Ben-Yami, a climate scientist at the Technical University of Munich in Germany who was not involved in the new study, told Live Science in an email.
The AMOC is a system of Atlantic Ocean currents, including the Gulf Stream, that circulate in a huge loop from Antarctica to Greenland, transporting heat to northern latitudes. If it weakens, it could cause significant cooling in northern Europe and trigger dramatic changes in weather patterns around the world.
Direct observations of the AMOC's strength only go back to 2004. So to analyze long-term trends in the Atlantic's circulation, researchers rely on climate parameters, or “fingerprints,” that they believe are associated with the AMOC. These include sea surface temperature, salinity, sea level height, water density, and marine sediment data, Ben-Yami said.
“The reason we're not confident about AMOC weakening is because all these different fingerprints could be giving us different results,” she added.
The results of a new study, published January 15 in the journal Nature Communications, indicate that the AMOC has not weakened over the past 60 years.
“The data is quite imprecise”
The scientists assessed the strength of the AMOC using newer climate models, which are more sensitive to increases in atmospheric greenhouse gas concentrations than their predecessors. The researchers tested 24 models and found that the most commonly used fingerprint, sea surface temperature, had a weaker relationship with the AMOC than previously thought.
As a result, the team focused on a measure called the sea-air heat flux, which simulates the exchange of heat between the ocean and the atmosphere. If the AMOC weakens, less heat will be transferred from the ocean to the atmosphere in the North Atlantic. However, the models have shown no decrease in heat transfer over the past 60 years, indicating that the AMOC has been stable over this period.
“Our paper suggests that Atlantic overturning has not yet diminished,” said study co-author Nicholas Fucal, an associate research scientist at the Woods Hole Oceanographic Institution. “That doesn’t say anything about its future, but it appears that the expected changes have not yet occurred.”
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