Three April 2026 Papers in ‘Science Advances’ Address AMOC: 1) Overturning Transport Change at AMOC’s Western Boundary May Indicate Weakening; 2) Volcanic Activity at the Onset of the Younger Dryas May Have Disrupted AMOC; and 3) Ridge-Regularized Linear Regression Model Shows AMOC Weakening of 51%, Higher Than Climate Models, Which Predict a Weakening of 32%: Thermohaline Circulation Post #5

  

   This post reviews three recent papers involving the AMOC and thermohaline circulation, all published by Science Advances in April 2026.

 

1) Overturning Transport Change at AMOC’s Western Boundary May Indicate Weakening

     This paper identifies a “meridionally consistent decline in deep western overturning transport across these latitudes over the past two decades” at AMOC’s western boundary and is put forth as evidence for AMOC weakening, something which is widely acknowledged to be occurring. 

     In their introduction, the authors note that the hard data available “are relatively short compared to simulations and reconstructions, making it difficult to discern whether we are observing decadal-scale fluctuating variability or a declining trend of the AMOC.” That is another uncertainty in whether or by how much the AMOC is weakening. They also address measurement uncertainties, which they say they resolve in the current study.

“Additional uncertainty in direct AMOC observations arises from their associated methodologies. The existing observational arrays, located at various latitudes within the Atlantic Ocean, have used different observation schemes and equipment, as well as methods of computation. Consequently, there is no consistent method for continuously estimating the strength of the AMOC across the Atlantic basin in modern times. Hence, we apply a common treatment to the now available observational data, to obtain consistent overturning transport estimates, which can then be used collectively to address research questions about the long-term variability of the AMOC.”

     The paper is concerned with developing consistent and accurate methodologies to measure and monitor AMOC weakening.

“…accurately monitoring the AMOC across various latitudes of the Atlantic basin presents a considerable challenge for the physical oceanography community.

“As proposed by Hughes et al. (36), long-term observations of OBP is a promising approach for monitoring large-scale ocean circulation, as they provide high accuracy while minimizing the influence of mesoscale variability. Our results further indicate that the estimation of the deep western overturning transport, derived from the western OBP gradient or equivalently the western geostrophic transport shear, can act as a reasonable method to capture interannual variability of the AMOC and indicate trends related to signals occurring at the western boundary.”

 

2) Volcanic Activity at the Onset of the Younger Dryas May Have Disrupted AMOC

     The Younger Dryas period occurred from 12,900 years ago to 11,700 years ago, and during that 1200-year period, the Earth returned to a glaciated state after it had begun an interglacial period. The question often asked is what triggered it? Comets or asteroids were one possible answer, but no evidence of that has been found. This study suggests that volcanic activity was the cause. The paper relies on geochemical and stratigraphic data. A well-dated, continuous sedimentary record spanning the Younger Dryas onset is utilized. Osmium isotope ratios (187 Os/188 Os) were used to detect volcanic eruptions. The eruptions were determined to occur from 12,980 to 12,870 years ago, which occurred just before, during, and after the Younger Dryas onset. The continuous stratigraphic sections utilized are located in Texas and Florida.

     The paper notes:

“…the YD is traditionally interpreted in terms of changes in North Atlantic circulation, particularly disruptions to the Atlantic Meridional Overturning Circulation (AMOC) following meltwater input.”

     This paper supports that assumption and offers geochemical and stratigraphic evidence that previous attempts to evaluate a possible volcanic cause failed to provide. They also serve to disprove extraterrestrial impact as a cause. As the abstract notes:

“The magnitude and hemispheric asymmetry of this volcanic activity imply forcing of sufficient magnitude capable of disrupting the Atlantic Meridional Overturning Circulation and triggering rapid Northern Hemisphere cooling. These findings provide multiproxy, regionally consistent evidence for a volcanically driven perturbation at the onset of the YD, offering a robust alternative to impact-based explanations.”

     If they are correct, then climate forcing via volcanic eruptions emitting large quantities of aerosols can affect the climate enough to initiate changes in the AMOC and initiate cooling.

 

 

3) Ridge-Regularized Linear Regression Model Shows AMOC Weakening of 51%, Higher Than Climate Models, Which Predict a Weakening of 32%

     This paper utilizes statistical methods to arrive at an AMOC weakening that is more intense than the middle and most likely climate scenario, SPP2-4.5. However, they also show and emphasize the uncertainties and the huge differences in the prediction of AMOC weakening for the different climate scenarios, something I pointed out in my previous post. The graph below shows all the climate models used and includes the SSP5-8.5 scenario now abandoned by the IPCC. The graph also visually exemplifies that the historical data, only available since around 2000, is very small compared to the modeled data. As mentioned in the previous post, the changes seen in the small amount of real data available could be caused by decadal changes rather than or in addition to anthropogenic climate forcing.  

“Because of the large differences between the historical simulations of absolute AMOC strength in the respective climate models, the model uncertainty of AMOC is very large for each scenario in Fig. 1A.”

“This suggests that most of the total uncertainty in the future AMOC is due to differences between climate models (model uncertainty).”

     This is illustrated in the graphs below:

“According to a recent IPCC report, a slowdown of more than 50% can be called a “substantial weakening of the AMOC.” This substantial weakening could have important implications for future adaptation plans in various regions affected by the AMOC, around the Atlantic and in teleconnected regions. This refinement toward stronger AMOC decline is mainly due to SSS fresh biases in the South Atlantic regions and SST cold biases in the North Atlantic regions in the CMIP6 MMM. This highlights the importance of biases in climate models for their ability to represent future AMOC fate.”    

     Perhaps it seems a little odd that the results of their statistical analysis of the model data arrive at a point just above the point where the weakening becomes “substantial” compared to previous MMM methods, which are well below that “substantial” threshold. I am a bit skeptical of their statistical methods, but I am not qualified to evaluate them. I am not suggesting deliberate bias, just noting conclusions that conveniently support a more catastrophic position, though not much more.

“Overall, the best OC method in terms of leave-one-out error, using the information from multiple observable variables, suggests an AMOC slowdown that is 60% stronger than estimated by the MMM. This could result in significant modifications to the climate change projections for various regions worldwide and introduce additional risks that stakeholders must consider, such as the potential for extensive drying in the Sahel region.”

     I think the main issue with this paper is that the historical data is quite sparse, which makes the projections more dependent on the models used and the model assumptions. As noted in the abstract, the change in projections is mainly a result of correcting a bias in sea surface salinity (SSS) in the South Atlantic region.

“This refinement mainly results from correcting a bias in South Atlantic surface salinity, consistent with recent studies emphasizing its role in the proximity to an AMOC tipping point. This more substantial AMOC weakening has key implications for future adaptation strategies.”

     This study is interesting but does not seem to resolve uncertainties nor prove that a tipping point is imminent or even possible, despite sort of suggesting it in the above quote.

 

References:

 

Volcanic forcing of global climate cooling at the Younger Dryas onset preserved in North American sediments. Lucien Nana Yobo, Alan D. Brandon, Sydney O’Brien, Jessi J. Halligan, and Michael R. Waters. Science Advances. 29 Apr 2026. Vol 12, Issue 18. Volcanic forcing of global climate cooling at the Younger Dryas onset preserved in North American sediments | Science Advances

Meridionally consistent decline in the observed western boundary contribution to the Atlantic Meridional Overturning Circulation. Qianjiang Xing, Shane Elipot, William E. Johns, David A. Smeed, Ben I. Moat, and John W. Loder. Science Advances. 8 Apr 2026. Vol 12, Issue 15. Meridionally consistent decline in the observed western boundary contribution to the Atlantic Meridional Overturning Circulation | Science Advances

Observational constraints project a ~50% AMOC weakening by the end of this century. Valentin Portmann, Didier Swingedouw, Omar Khattab, and Marie Chavent. Science Advances. 15 Apr 2026. Vol 12, Issue 16. Observational constraints project a ~50% AMOC weakening by the end of this century | Science Advances