Atlantic currents won’t collapse this century

According to new research from Caltech, the key marine systems that regulate Earth’s climate will be moderately weakened over the next 75 years, but will not experience the dramatic collapse predicted by some climate models.

Atlantic meridian capsulation (AMOC) (AMOC) arrives in the Atlantic Basin to transport warm water northward and cold water southward – reaching the last century, it will drop by 18-43%, far less than the nearly completely shutdown expected for some models. The discovery involves one of the most pressing uncertainties in climate science and suggests that despite the inevitable regional climate impacts, the most catastrophic situation seems impossible.

AMOC acts like a huge conveyor belt, moving warm tropical water to Europe and the Arctic in a cold current, back south to the deep sea. This cycle pattern helps maintain a relatively mild climate in Europe and affects rainfall patterns throughout Africa, South America and North America.

Hot air physics reveals the facts

The key breakthrough comes from applying fundamental physics principles to understand why climate models produce such distinct AMOC predictions. Dave Bonan, a leading researcher who recently completed his PhD at Caltech, developed a simplified model based on hot wind balance – the relationship between density differences throughout the ocean basin and the depth of cycles occurring.

Current climate models show confusing changes in their AMOC predictions. Some predicted that only 2 SVerdrups (a unit that measures currents), while others weakened the decline of 15 Sverdrups – enough to fundamentally change global climate patterns. This seven-fold difference makes it nearly impossible to plan for future climate impacts.

“Our results suggest that AMOC is more likely to experience a limited decline than a limited decline in the 21st century, but is still somewhat weak, but less dramatic than previous predictions suggest.”

The study shows that the relationship between today’s AMOC intensity and future weakened attenuation is due to the depth of the cycle. Today, models that simulate stronger, deeper ocean cycles can also predict dramatic declines in the future. This happens because deeper cycles allow surface warming and freshwater changes to penetrate further into the ocean, thus making the density gradient of the driving current larger.

Layered key holding

A key insight that emerged in the analysis involves ocean stratification – how water density varies with depth. Today’s models with stronger AMOC cycles usually simulate weaker models of the North Atlantic, meaning less resistance to vertical mixing between the surface and deep water.

These changes penetrate deeper in weakly stratified oceans as global warming increases surface temperatures and increases freshwater from melting ice. The result is that the density of the depth varies greatly and the circulation speed is greater. Instead, the more layered model limits the depth changes in surface changes that can penetrate naturally to protect the circulation system from severe damage.

The researchers found that North Atlantic stratification and AMOC intensity showed a strong negative correlation with a correlation coefficient of -0.89. This relationship explains why some models predict extreme weakening, while others propose modest changes – they are essentially modeling different background marine countries.

Observe reality check

To move beyond model divisions, the team combined real-world measurements from two key sources: a fast surveillance array that has tracked AMOC strength since 2004, and an ECCO state estimate that combines ocean models with observation data from 1992-2015.

These observations suggest that today’s AMOC intensity falls around 15-17 SVerdrups until the lower end of the climate model simulation. When the researchers applied their hot air relationship using observed rather than modeled AMOC intensity, these projections shifted sharply to a more limited aspect of weakening.

The constrained projection shows that by 2071-2100, AMOC weakened approximately 3-6 Sverdrups, regardless of greenhouse gas emissions. This represents moderate weakness, which still has regional climate effects, but is far from enough for the system crashes warned by some studies.

Why does the model disagree

Analysis shows that the uncertainty of AMOC prediction is more about how the model represents today’s ocean conditions than the differences in future emission scenarios. Models with deeper circulatory systems always predict larger declines in the future, while those with shallower cycling will change more modestly.

This explains the confusing feature of climate prediction: AMOC weakening predictions are still very similar between different emission pathways within the same model, but using the same emission scenarios varies greatly between different models. The answer lies in the unique representation of the background sea layer by each model.

The findings suggest that improving today’s ocean simulations should be a priority for reducing uncertainty in climate predictions. Models that better capture observed stratified patterns may provide more reliable estimates for future changes.

Beyond simple linear relationships

Hot air analysis shows that the relationship between today’s AMOC intensity and future weakening is not only linear. Instead, it includes linear and square root terms, which create curved relationships, and stronger circulatory systems experience greater declines.

This nonlinear behavior emerges from the physics that subverts deep changes. The researchers found that under warming, models with deeper cycles (larger H values) would be more shallow, with the correlation coefficients about -0.61 between current depth and future depth changes.

The nonlinear relationship partly explains why some emerging constraint studies using pure statistical methods may produce different results. The physically-based hot air method captures this curvature and provides a more robust projection when extrapolated to the observed conditions.

Climate impact

Although the study excludes the AMOC collapse this century, moderate attenuation still has significant regional implications. A 20-40% drop may affect European temperatures, African monsoon patterns and North American storm tracks, although although less than the overall shutdown.

This study cannot address the potential conversion points that exceed 2100 long-term risks or may emerge due to continuous warming. However, this suggests that often cited research warns that imminent AMOC collapse may exaggerate near-term risks.

What does this mean for climate adaptation programs? These findings suggest that communities should prepare for progressive changes in AMOC-related climate patterns over the next few decades.

Tapio Schneider of Tapio Schneider of Caltech, Theodore Y. Wu professor of environmental science and engineering and co-author of research, stressed that this work highlights how fundamental physics principles can help address major uncertainties in climate science.

As Bonan points out, research proves the value of basic scientific research: “There is great value to conduct basic research – as our research shows, it can better show how the future looks.”