This thesis concerns the equilibrium climate response to an anthropogenic change in aerosols since pre-industrial times compared to a 63 % increased CO2 level in the atmospheric GCM CAM-Oslo coupled to a slab ocean model. Recent studies suggest that changes in atmospheric circulation may be characterized as certain naturally occurring weather regimes becoming more or less prevalent in response to a given external forcing. The main purpose of this study is to investigate whether the CAM-Oslo model can simulate some of the weather regimes found in observed data. The output analysed consists of monthly mean values of the Northern Hemisphere winter (dec-mar) 500 hPa geopotential heights from 3 different experiments in the CAM-Oslo model; the first experiment (the control simulation) with present-day aerosols emissions and CO2 volume mixing ratio, the second experiment with pre-industrial emissions of aerosols and present-day CO2 and the third experiment with present-day emissions for aerosols and a 63 % increase in the CO2 mass mixing ratio. The data is detrended by removing a linear trend and the seasonal cycle from each simulation and the anomalies from each simulation is combined into one single data set. To define a reduced phase space based on the leading modes of variability a Empirical Orthogonal Function (EOF) analysis is applied to the height anomalies. The principal component probability density function (PDF) from each simulation is computed separately in the reduced phase space spanned by the two leading EOFs of the combined dataset in order for comparison. The CAM-Oslo model reproduces the Pacific/North-American pattern, but is less able to reproduce the leading mode of variability over the Atlantic ocean; the North Atlantic Oscillation or the Arctic Oscillation. Two aspects of the non-linear perspective of climate change is discussed; the climate response to a small imposed forcing is primarily manifested in terms of changes in the frequency of occurrence of flow regimes and the geographical structure of the flow regimes is relatively unaffected by the small imposed forcing. Compared to studies based on reanalysed meteorological fields, this study based on model calculations gives considerably less support to these two aspects of the non-linear perspective.