Radiative Forcing Path Dependent Temperature Thresholds for AMOC Tipping

The Atlantic Meridional Overturning Circulation (AMOC) is a tipping element of the climate system, capable of transitioning from a strong overturning state to a substantially weaker one. AMOC collapse can occur through the destabilising salt-advection feedback, which may be triggered by freshwater input into the North Atlantic Ocean. Alternatively, the AMOC may become unstable under 21st century climate change. This risk was recently reassessed in the Global Tipping Points Report (2025), which suggests that the AMOC could become unstable above 1.5°C of global warming. By contrast, other studies report stable AMOC states even under extreme climate change conditions (e.g., 4xCO₂). Consequently, it remains unclear whether a global warming threshold for AMOC tipping exists.

Here, we analyse transient CO₂ forcing experiments performed with the Community Earth System Model (CESM) at different rates of CO₂ increase. For slow ramping (+0.5 ppm yr^-1), we show that the AMOC remains stable under extreme climate change, up to +5.5°C of global warming. In contrast, under more rapid forcing in the RCP4.5 and RCP8.5 scenarios, the AMOC collapses at much lower warming levels of +2.2°C and +2.8°C, respectively. These results demonstrate that AMOC tipping is strongly radiative path-dependent rather than governed by a specific global temperature threshold. Slow forcing permits a coherent adjustment of surface and interior ocean properties, supported by enhanced evaporation and reduced sea-ice extent, which together stabilise the AMOC. A similar stabilising response is found in several CMIP6 models under extended SSP scenarios. Our findings imply that limiting the rate of radiative forcing increase is crucial for reducing the near-term risk of AMOC collapse and other climate tipping elements.