Reducing the risk of abrupt climate change: emissions corridors preserving the Atlantic thermohaline circulation

The aim of this paper is to present a modeling framework for deriving emissions corridors that preserve the Atlantic thermohaline circulation (THC). The framework consists of a multi-gas reduced-form climate model coupled to a four-box THC model and allows for the main physical uncertainties (i.e., climate and North Atlantic hydrological sensitivity) to be taken into account. The emissions corridors are calculated along the conceptual and methodological lines of the tolerable windows approach (TWA). The corridor boundaries demarcate emissions limits for the 21 century beyond which either the THC collapses or the mitigation burden becomes intolerable. Accordingly, the corridors represent the maneuvering space for climate policies committed to preserve the THC without endangering future economic growth. Results indicate a large dependence of the width of the emissions corridors on climate and North Atlantic hydrological sensitivity: for low values of climate and/or hydrological sensitivity the upper corridor boundary is far from being transgressed by any of the SRES emissions scenario for the 21 century, while for high values of both quantities even low non-intervention scenarios leave the corridor.