Failure to track a stable AMOC state under rapid climate change

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) is a tipping element of the climate system. The current estimate of the global warming threshold for the onset of an AMOC collapse is +4.0C (uncertainty range 1.4-8C). However, such a threshold may not be meaningful because AMOC stability rather depends on the rate of radiative forcing change. Here, we identify an AMOC stabilising mechanism that operates on timescales longer than present-day radiative forcing increase. Slow forcing permits coherent adjustment of surface and interior ocean properties, supported by enhanced evaporation and reduced sea-ice extent, counteracting destabilising feedbacks. We explicitly demonstrate this mechanism in a slow CO2 ramp (+0.5 ppm/yr) climate model simulation, in which the AMOC remains stable up to +5.5C of global warming. By contrast, under faster CO2 ramps, the AMOC collapses at substantially lower warming levels (+2C). Our findings demonstrate rate-induced AMOC tipping and imply that limiting the rate of greenhouse gas emissions is critical for reducing the near-term risk of an AMOC collapse.