Reducing the Uncertainty in Projections of the North Atlantic Thermohaline Circulation for the 21st century

Most climate models predict a weakening of the North Atlantic thermohaline circulation (THC) for the 21st century when forced by increased levels of greenhouse gas concentrations. The model spread, however, is rather large, even when the forcing scenario is identical, indicating a large uncertainty in the response to forcing. In order to reduce the model uncertainties a weighting procedure is applied that considers the skill of each model in simulating the climatological hydrographic properties. This procedure yields an `estimate' for the evolution of the North Atlantic THC during the 21st century by taking into account a measure of model quality when computing the ensemble mean. The analysis predicts a gradual weakening of the North Atlantic THC by about 30% until 2100.