At what warming or freshwater input does the AMOC collapse?
In plain words
The Atlantic meridional overturning circulation (AMOC, the ocean conveyor carrying warm surface water north) can in theory switch to a weak state if the North Atlantic gets too fresh. Nobody knows how much warming or meltwater would cause that switch.
Precise statement
Determine whether the present AMOC (about $17\,\mathrm{Sv}$ at $26.5\,\mathrm{N}$, $1\,\mathrm{Sv} = 10^{12}\,\mathrm{cm}^3/\mathrm{s}$) is bistable and, if so, the critical North Atlantic freshwater flux or global warming level at which the strong state disappears, including the timescale of the transition. An answer is a threshold value with uncertainty from models whose salt-advection feedback and mean salinity biases are constrained by observations.
What would settle it
Observationally constrained estimates of the freshwater transport by the AMOC at the southern Atlantic boundary combined with bias-corrected coupled models that locate the bifurcation.
Status in the literature
Unverified note
Drijfhout et al. 2025 (Environmental Research Letters) found northern overturning shuts down after 2100 in extended high-emission CMIP6 runs, while Baker et al. 2025 (Nature) found wind-driven Southern Ocean upwelling keeps a weakened AMOC running under extreme forcing.