Emissions corridors preserving the Atlantic ocean thermohaline circulation

The Atlantic thermohaline circulation (THC) transports large amounts of heat northward, acting as a heatingsystem for the northern North Atlantic and north-western Europe. Paleo-reconstructions and a large numberof model simulations have shown the THC to be stable only within certain limits beyond which the circulationshuts down. In this paper we derive emission corridors for the 21st century preserving the Atlanticthermohaline circulation. To this end a multi-gas reduced-form climate model has been coupled to a dynamicfour-box model of the Atlantic thermohaline circulation. Both models allow for the relevant uncertainties (i.e.,climate and hydrological sensitivity) to be taken into account. The emission corridors are calculated along theconceptual and methodological lines of the tolerable windows approach. The corridor boundaries demarcatetime-dependent limits beyond which either the THC collapses or the mitigation burden to avoid such an eventbecomes intolerable. Accordingly, the corridors represent the maneuvering space for any climate policy committedto preserve the THC without endangering future economic growth. Results show a large dependence ofthe width of the emission corridors on hydrological sensitivity, which is a measure for the amount of additionalfreshwater entering the North Atlantic, and on climate sensitivity.