Assessing the probability of CO₂-driven AMOC transitions using a rare event algorithm in PlaSim-LSG

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) is a key climate tipping element, regulating the meridional transport of heat and freshwater. Its future evolution depends on the interaction between external forcings, such as greenhouse gas increases, and internal climate variability. Given the limitations of deterministic predictions, we apply the trajectory-adaptive multilevel splitting algorithm within the intermediate-complexity model PlaSim-large scale geostrophic to estimate AMOC transition probabilities under different CO₂ concentration levels and Shared Socioeconomic Pathway (SSP) scenarios. Results show a non-zero (1%–7.5%) probability of a strong AMOC weakening within 150 years for CO₂ levels between 500 and 600 ppm. While the transition is exceptionally unlikely within the 21st century under SSP5–8.5, it becomes unlikely (20%) by 2150 and very likely (95%) by 2200. Notably, the model excludes Greenland meltwater input, suggesting these probabilities may be underestimated. These findings highlight the importance of probabilistic approaches and the value of rare-event techniques in assessing AMOC stability and potential climate tipping points.