Atlantic Overturning Circulation and Climate Tipping Points

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) transports approximately 1.3 PW of heat northward, sustaining the relative warmth of western Europe and influencing precipitation patterns across the tropics and subtropics. Multiple observational and proxy lines of evidence suggest that the AMOC has weakened over recent decades, raising concerns about a potential approach to a critical tipping point beyond which collapse could become irreversible. This study analyzes AMOC projections from eight CMIP6 Earth system models under the SSP5-8.5 scenario, finding a multi-model mean decline of 38 percent by 2100, from 17.7 Sv to 10.9 Sv. Bifurcation analysis applied to three models with freshwater hosing experiments reveals estimated collapse thresholds at 0.28 to 0.32 Sv of anomalous freshwater forcing, with hysteresis widths of 0.12 to 0.18 Sv indicating that recovery from a collapsed state would require substantially greater cooling than the warming that triggered the transition. Two of three analyzed models indicate a high risk of crossing the tipping point by 2100 under unmitigated emissions, with cascading consequences including northern European cooling, southward shift of the Intertropical Convergence Zone, disruption of West African and South Asian monsoons, and accelerated sea-level rise along the North American east coast.