Critical ocean current may be more stable than previously thought

The Earth is a water planet, with over 71% of its surface covered by oceans. These vast bodies of water play a crucial role in regulating the planet’s climate, influencing weather patterns and distributing heat.

One of the most significant systems within this global ocean network is the Atlantic Meridional Overturning Circulation (AMOC), a massive conveyor belt of currents that moves warm and cold water between the tropics, poles, and continents.

Impact of AMOC on climate

The AMOC has a profound impact on global weather and climate. It helps keep European winters milder, affects monsoon patterns in Africa and Asia, and supports marine ecosystems that millions of people depend on for food and livelihoods.

However, scientists have long feared that climate change, particularly the melting of polar ice sheets, could weaken or even collapse the AMOC.

A decline in the AMOC would lead to extreme weather shifts, including more severe storms, colder winters in some regions, and dramatic sea level rise along the U.S. East Coast. A complete collapse would be even more catastrophic, potentially triggering irreversible changes to the global climate.

Recent research, however, suggests that the AMOC may be more stable than previously thought. A study by scientists from the Woods Hole Oceanographic Institution (WHOI) challenges earlier claims that the AMOC has been weakening over the past several decades.

The findings indicate that, contrary to previous studies, the AMOC has not declined over the last 60 years.

New perspective on AMOC stability

For years, scientists have attempted to track changes in the AMOC by analyzing sea surface temperature (SST) data.

A 2018 study, which concluded that the AMOC had significantly weakened over the past 70 years, relied heavily on this approach.

However, researchers from WHOI now argue that SST-based reconstructions are less reliable than initially thought. Instead of using sea surface temperature as a proxy, they examined air-sea heat fluxes, which measure the exchange of heat between the ocean and the atmosphere.

Nicholas P. Foukal, adjunct scientist at WHOI and assistant professor at the University of Georgia, emphasized that the AMOC’s status today is different from what was previously believed.

“Our paper says that the Atlantic overturning has not declined yet. That doesn’t say anything about its future, but it doesn’t appear the anticipated changes have occurred yet.”

Analyzing AMOC behavior

To improve the accuracy of their study, the researchers used data from the Coupled Model Intercomparison Project (CMIP), a collection of advanced climate models developed by the World Climate Research Program.

The team analyzed 24 different climate simulations, which allowed them to refine their understanding of how the AMOC has behaved over time. The findings indicate that the AMOC remains stable and has not shown any long-term weakening over the past six decades.

Jens Terhaar, a senior scientist at the University of Bern, explained why the new method was necessary. “We’ve learned that sea surface temperature doesn’t work as well as initially thought,” he said.

By focusing on air-sea heat fluxes instead, the researchers were able to build a more reliable picture of how the AMOC has evolved.

Why air-sea heat fluxes matter

The AMOC plays a key role in moving warm water from the tropics to the North Atlantic, where it releases heat into the atmosphere. This process helps regulate the climate of many regions.

When the AMOC is strong, more heat is transferred from the ocean to the air. By measuring these air-sea heat exchanges, the researchers could track changes in AMOC strength with greater confidence.

Linus Vogt, a scientist at Sorbonne Université, highlighted the significance of these findings. “Based on the results, the AMOC is more stable than we thought. This might mean that the AMOC isn’t as close to a tipping point as previously suggested.”

The research team concluded that the AMOC has remained steady between 1963 and 2017, reinforcing the idea that its collapse is not imminent.

What this means for the future

Despite these findings, the study does not suggest that the AMOC is invariable. Nearly all climate models agree that the AMOC will slow down in the future due to global warming. The real question is whether it will weaken gradually or collapse suddenly.

Foukal emphasized that the AMOC’s future remains uncertain. “It’s almost unanimous at this point that the Atlantic overturning will slow in the future, but whether or not it will collapse is still up for debate. This work indicates that there is still time to act before we reach this potential tipping point.”

While this study provides reassurance that we are not yet at a crisis point, it does not eliminate the long-term risks.

As the planet continues to warm, melting polar ice sheets will introduce large amounts of freshwater into the North Atlantic, disrupting the delicate balance that keeps the AMOC functioning.

If too much freshwater enters the system, the AMOC could weaken rapidly, leading to dramatic shifts in global weather patterns.

Uncertainties and limitations

As with all climate research, this study has limitations. The findings rely on proxy data, meaning they are based on indirect measurements rather than direct observations of AMOC strength.

Air-sea heat flux measurements from past decades are limited, which introduces some uncertainty into the results. However, the researchers believe their approach provides a more accurate reconstruction of AMOC behavior than previous methods.

Jens Terhaar acknowledged these limitations but remained confident in the study’s conclusions. “A decline in AMOC over the last 60 years seems very unlikely,” he said.

The bigger picture

If the AMOC were to collapse, the consequences would be devastating. Scientists predict that such a shutdown could lead to harsher winters in Europe, more intense hurricanes in the Atlantic, and major disruptions to monsoon systems in Africa and Asia.

Additionally, the U.S. East Coast would experience rapid sea level rise, threatening major cities like New York and Miami.

Given these potential risks, continued monitoring of the AMOC is essential. While this study suggests that we still have time to act, it reinforces the urgency of addressing climate change.

Reducing carbon emissions and slowing global warming remain critical to ensuring that the AMOC does not reach an irreversible tipping point.

Significance of the study

The AMOC is one of the most important components of the global climate system. While past research suggested it was already in decline, this new study challenges that idea.

Instead, the AMOC appears to have remained stable over the past 60 years. This does not mean that the AMOC is safe forever, but it does suggest that we may have more time than previously thought to prevent a worst-case scenario.

The future of the AMOC – and the climate as a whole – depends on human action. The world must continue to study, monitor, and mitigate climate change to ensure that this crucial ocean current remains stable for generations to come.

The study is published in the journal Nature Communications.

Image Credit: NOAA AOML

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