The Atlantic Meridional Overturning Circulation (AMOC) has not exhibited any decline since the 1950s, based on the observed heat exchange between the North Atlantic Ocean and the atmosphere during this period. This conclusion is at odds with previous research that suggested a decrease in AMOC activity, likely stemming from those earlier studies depending on sea surface temperature measurements to track changes in AMOC. However, according to the researchers, sea surface temperature is not an effective indicator for reconstructing AMOC dynamics. While the AMOC has not yet weakened, scientists are in consensus that it will eventually weaken; nevertheless, the possibility of an absolute collapse of this system, along with the timeline for such an event, remains a topic of active discussion. A collapse would lead to severe global consequences.
With the Earth covered by 71 percent water, ocean behavior significantly influences our planet. The Atlantic Ocean features a network of currents known as the Atlantic Meridional Overturning Circulation (AMOC), which circulates water across the world’s oceans, driven by wind and variations in ocean density. This movement is crucial as it distributes heat, moisture, and nutrients and helps regulate the Earth’s climate and weather patterns.
As climate change continues and the atmosphere warms, there are growing concerns among scientists that fresh water from melting polar ice sheets could dramatically disrupt—or potentially collapse—the AMOC. While a decline in the AMOC poses serious risks, a total collapse would be catastrophic. Nonetheless, predictions about the long-term future of the AMOC remain uncertain. Rather than speculating on what lies ahead, a research team from Woods Hole Oceanographic Institution (WHOI) has quantified historical data to provide insights into future scenarios.
A recent study published in Nature Communications reveals that the AMOC has maintained its strength for the past 60 years. The research team, including Nicholas P. Foukal, an adjunct scientist in Physical Oceanography at WHOI and a professor at the University of Georgia; Jens Terhaar, a scientist at WHOI and a senior researcher at the University of Bern; and Linus Vogt, a visiting student during the study who is now a scientist at LOCEAN, Sorbonne Université, asserts that these findings indicate the AMOC’s current stability is greater than previously thought.
“Our paper indicates that the Atlantic overturning hasn’t weakened,” stated Foukal, who conducted the study at WHOI. “This doesn’t provide information about the future, but it suggests that anticipated changes have not yet occurred.”
Their results differ from earlier research, particularly a 2018 study referenced in their report, which suggested a decline in the AMOC over the past 70 years. This earlier research depended on sea surface temperature data to assess changes in the AMOC; however, Terhaar commented, “we’ve learned that relying solely on sea surface temperature isn’t as effective as we once believed.”
To mitigate uncertainty, Terhaar and the team utilized new data from the Coupled Model Intercomparison Project (CMIP), a collection of climate models generated by the World Climate Research Program. They examined 24 distinct CMIP models and discovered that the latest surface temperature readings did not effectively recreate the AMOC situation. To enhance their analysis, the researchers explored another measure: air-sea heat fluxes, which involve the transfer of heat from the ocean to the atmosphere. An increase in AMOC strength corresponds to a greater release of ocean heat into the atmosphere over the North Atlantic.
The authors developed this AMOC proxy using CMIP models and then applied it to real-world data. The optimal sources for surface heat flux data in the North Atlantic stem from reanalysis products, which blend direct observations with model data, akin to weather forecasting methods. The study zeroed in on two reanalysis datasets that date back to the late 1950s to trace the AMOC’s historical changes.
“The results suggest that the AMOC is more stable than previously believed,” commented Vogt. “This means that it may not be as close to a tipping point as some have hypothesized.”
The study indicates that air-sea heat flux anomalies in the North Atlantic are closely connected to the AMOC, asserting that the “decadal averages of the AMOC have not weakened from 1963 to 2017.” There are numerous processes contributing to substantial annual fluctuations in the AMOC, but correlations are strongest at decadal timescales rather than annual averages.
“The consensus now is that the Atlantic overturning is likely to slow in the future, though whether it will fully collapse is still uncertain,” Foukal noted. “Our findings suggest that there is still time to intervene before reaching a critical tipping point.”
As with all reconstructions based on proxies, there are limitations and caveats. The authors highlight the scarcity of direct historical measurements for air-sea heat flux, which means the reanalysis products carry notable uncertainty. However, Terhaar concludes, “a decline in the AMOC over the last 60 years appears highly unlikely.”
Key Takeaways:
- The Atlantic Meridional Overturning Circulation (AMOC) has not weakened since the mid-20th century as indicated by North Atlantic air-sea heat flux data.
- This conclusion is in contrast to earlier studies that suggested a decrease in AMOC, which were likely based on unreliable sea surface temperature measurements.
- While the AMOC has not declined yet, scientists agree it is expected to slow in the future, but the total collapse of the system and the timing of such an event remain uncertain, with potentially dire global consequences.
