Manabe and Stouffer's 1993 coupled-model study found that quadrupled atmospheric CO2 drives the Atlantic thermohaline circulation — now called the Atlantic meridional overturning circulation (AMOC) — into a collapsed steady state over centuries, with a deepened thermocline and suppressed deep-ocean ventilation, while doubled CO2 produces weakening followed by slow recovery. The authors explicitly flagged that anomaly-independent surface-flux adjustments, the omission of ice-sheet meltwater (which could induce collapse even under doubled CO2), parameterized mesoscale eddies, a climate sensitivity in the upper half of the then-assessed range, and absent biogeochemistry left the real-world likelihood, thresholds, and consequences of such circulation collapse undetermined. The open problem is to determine, with quantified uncertainty, whether and under what forcing pathways, emission rates, and ice-sheet meltwater inputs the real AMOC crosses a collapse threshold; how robust that threshold is to model formulation; and what the coupled carbon-cycle and biogeochemical feedbacks of suppressed deep ventilation are.

Background

The Atlantic branch of the ocean's overturning circulation — called the thermohaline circulation (THC) in the older literature and the Atlantic meridional overturning circulation (AMOC) today — carries warm, salty upper waters northward across the equator; in the Nordic and Labrador seas and around Antarctica these waters cool, become dense, sink, and return southward at depth. The circulation moves heat, freshwater, carbon, and nutrients on basin scale, so its strength shapes regional and global climate; it is measured in sverdrups (Sv, 10610^{6} m3^{3} s−1^{-1}). Stommel (1961) showed in an idealized box model that the feedback between freshwater transport and surface density can give the circulation two stable regimes, an 'on' and an 'off' state, which motivates the concepts of multiple equilibria, thresholds, and hysteresis (the failure of the circulation to return to its original state when the forcing is reversed). Broecker (1987) argued that abrupt climate changes recorded in ice and marine cores reflect rapid reorganizations of this conveyor and warned that greenhouse warming could trigger one.

Coupled atmosphere–ocean general circulation models (GCMs) of the early 1990s needed 'flux adjustments': spatially and seasonally varying prescribed corrections of heat and water fluxes at the ocean–atmosphere interface to stop slow drift of the simulated climate; because the corrections are prescribed independently of evolving sea-surface temperature and salinity anomalies, they were suspected to distort the modeled overturning sensitivity. Mesoscale eddies — swirling currents of tens of kilometres that dominate ocean heat and salt transport — were not resolved on the roughly 4-degree grids of the time and were represented by parameterized isopycnal diffusion, i.e. mixing along constant-density surfaces. 'Climate sensitivity' denotes the equilibrium global-mean surface warming for doubled atmospheric CO2; the source paper cites the then-assessed range of 1.5–4.5 °C of the Intergovernmental Panel on Climate Change (IPCC, the United Nations body that periodically assesses climate science). Against this background Manabe and Stouffer (1993) integrated the coupled model of the Geophysical Fluid Dynamics Laboratory (GFDL) for 500 years under CO2 rising at 1% per year compounded to doubled and then quadrupled levels, each held constant thereafter. Global-mean surface air temperature rose by about 3.5 °C and 7 °C; thermal expansion alone raised sea level by about 1 and 2 m. In the quadrupled-CO2 integration the model ocean settled into a new stable state in which the THC had ceased entirely and the thermocline — the upper-ocean layer of rapid vertical temperature transition separating the ventilated from the isolated deep ocean — deepened substantially, cutting off deep ventilation; in the doubled-CO2 integration the circulation weakened to less than half its strength and then began a slow recovery (Manabe and Stouffer 1994). The authors conjectured that suppressed deep ventilation would have profound carbon-cycle and biogeochemical consequences but their model computed none, and they noted that omitted meltwater from disintegrating continental ice sheets might push the threshold for THC disappearance down to the doubled-CO2 case.

Determining whether, where, and with what consequences the real ocean can cross such a threshold has remained a central open problem of climate science, pursued since through freshwater-forcing ('hosing') experiments, coordinated model intercomparisons such as CMIP (the Coupled Model Intercomparison Project, which standardizes climate-model experiments across modeling centers), direct AMOC monitoring arrays, and 'early-warning signal' analyses that look for critical slowing down and loss of resilience in observed indicators. These approaches have resolved parts of the question, but they disagree on whether and where such a threshold lies, with collapse-warning analyses and resilience analyses reaching opposite conclusions.

Problem Statement

Determine, with quantified uncertainty for the real climate system rather than for any single model, whether sustained anthropogenic CO2 increase and the associated warming can push the Atlantic meridional overturning circulation past a collapse threshold into a persistently weakened or 'off' state on century-to-multi-century timescales. The single objective — a defensible, observationally constrained assessment of the likelihood, thresholds, and consequences of anthropogenic AMOC collapse — requires three jointly supporting results:

(1) Threshold location and crossing probability in forcing space: the combinations of CO2 stabilization level (including the doubled- and quadrupled-CO2 cases of the source study and realistic scenario pathways), rate of CO2 increase or emission pathway, and input of ice-sheet meltwater (Greenland and Antarctic, at observationally constrained rates) that do and do not collapse the circulation; in particular whether meltwater can induce collapse at forcing levels, such as doubled CO2 or present-day-to-2100 pathways, that would not otherwise do so; whether any collapsed state is irreversible or hysteretic; and the probability of crossing under realistic scenarios.

(2) Robustness of the answer to model formulation: how the threshold, the collapsed state, and recovery depend on the absence of surface-flux adjustments, explicit versus parameterized mesoscale eddies, model climate sensitivity, and present-day salinity and stratification biases — that is, which of the specific caveats raised in the source study do and do not change the conclusion.

(3) Coupled carbon-cycle and biogeochemical consequences: the magnitude, sign, and timescale of changes in ocean carbon uptake or release and atmospheric CO2, export production (the flux of organic carbon from the surface to the deep ocean), and nutrient and oxygen distributions caused by suppressed deep ventilation, determined from the strongest available evidence, and their feedback on climate.

The criterion applied to the 'collapse threshold' and to the 'persistently weakened or off' state is specified in terms of the overturning index or observable, the reference baseline against which change is measured, the amplitude and persistence required, and how the threshold is located in forcing space.

Determine which physical mechanisms control the outcome (such as high-latitude surface freshening versus surface heat-flux changes, Southern Ocean wind-driven upwelling, or a compensating Pacific overturning), and distinguish forced changes from internal multidecadal variability.

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