Revision 1

Accepted · first revision · record

Title: At what warming or freshwater input does the AMOC collapse?
Plain: 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: 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.
Settled by: 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.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: contested
Status 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.
Posed since: 1961