Evaluation of different methods to assess model projections of the future evolution of the Atlantic meridional overturning circulation

Climate models predict a gradual weakening of the North Atlantic meridional overturning circulation (MOC) during the twenty-first century due to increasing levels of greenhouse gas concentrations in the atmosphere. Using an ensemble of 16 different coupled climate models performed for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), the evolution of the MOC during the twentieth and twenty-first centuries is analyzed by combining model simulations for the IPCC scenarios Twentieth-Century Climate in Coupled Models (20C3M) and Special Report on Emission Scenarios, A1B ( SRESA1B). Earlier findings are confirmed that even for the same forcing scenario the model response is spread over a large range. However, no model predicts abrupt changes or a total collapse of the MOC. To reduce the uncertainty of the projections, different weighting procedures are applied to obtain best estimates of the future MOC evolution, considering the skill of each model to represent present day hydrographic fields of temperature, salinity, and pycnocline depth as well as observation-based mass transport estimates. Using different methods of weighting the various models together, all produce estimates that the MOC will weaken by 25%-30% from present day values by the year 2100; however, absolute values of the MOC and the degree of reduction differ among the weighting methods.