In addition, one model8 reveals that the thermohaline circulation initially weakens but eventually recovers if the forcing is held fixed once the atmospheric concentration of CO2 reaches a level of twice the present day. This is not the case in the response under 4xCO2 forcing8, although it appears that the deep ocean may not have fully-equilibrated in this latter experiment. The effects on the ocean environment are probably far-reaching but not quantifiable at this stage. On page xxxx of this same issue Manabe and Stouffer11 undertake a study to try and quantify the response of the coupled atmosphere-ocean system to large perturbations of freshwater associated with the melting of continental ice sheets during the transition from the last glacial period to the present interglacial period. By adding a large perturbation of 1 Sv of freshwater uniformly released over the 50deg.
Closer inspection reveals subtle differences between the independent experiments.
Rahmstorf maintains his much smaller freshwater flux perturbation for a long period of time while Manabe and Stouffer apply their perturbation over a rapid 10 year period which is too short a time to, in the words of Rahmstorf, invoke an advective spindown of the circulation. This spindown timescale is of the order of a few centuries and is linked to the timescale of the overturning of the North Atlantic conveyor. An extremely interesting experiment would be to rerun the glacial runoff experiment of Manabe and Stouffer by reducing the perturbation by a factor of 10 but increasing the time over which it is applied by a factor of 10. The net freshwater perturbation would have the same size but it would persist over a longer period perhaps causing a shutdown of the North Atlantic Conveyor as suggested by Rahmstorf's results. The work of Rahmstorf has put many pieces of the climate puzzle together.