INFLUENCE OF CO2 EMISSION RATES ON THE STABILITY OF THE THERMOHALINE CIRCULATION

Present estimates of the future oceanic uptake of anthropogenic CO2 an d calculations of CO2-emission scenarios(1) are based on the assumptio n that the natural carbon cycle is in steady state, But it is well kno wn from palaeoclimate records(2-5) and modelling studies(6-9) that the climate system has more than one equilibrium state, and that perturba tions can trigger transitions between them, Anticipated future changes in today's climate system due to human activities have the potential to weaken the thermohaline circulation of the North Atlantic Ocean(10- 12), which would greatly modify estimates of future oceanic CO2 uptake (13). Here we use a simple coupled atmosphere-ocean climate model to s how that the Atlantic thermohaline circulation is not only sensitive t o the final atmospheric CO2 concentration attained, but also depends o n the rate of change of the CO2 concentration in the atmosphere. A mod elled increase to 750 parts per million by volume (p.p.m.v.) CO2 withi n 100 years (corresponding approximately to a continuation of today's growth rate) leads to a permanent shut-down of the thermohaline circul ation. If the final atmospheric concentration of 750 p.p.m.v. CO2 is a ttained more slowly, the thermohaline circulation simply slows down. T he reason for this rate-sensitive response of the climate system lies with the transfer of buoyancy in the form of heat and fresh water from the uppermost layers of the ocean into the deep waters below. This se nsitivity of the simulated thermohaline circulation to the rate of cha nge of atmospheric CO2 concentration has potentially important implica tions for the choice of future CO2-emission scenarios(1).