Influence of various forcings on global climate in historical times using a coupled atmosphere-ocean general circulation model

The results of a simulation of the climate of the last five centuries with a state-of-the-art coupled atmosphere-ocean general circulation model are presented. The model has been driven with most relevant forcings, both natural (solar variability, volcanic aerosol) and anthropogenic (greenhouse gases, sulphate aerosol, land-use changes). In contrast to previous GCM studies, we have taken into account the latitudinal dependence of volcanic aerosol and the changing land cover for a period covering several centuries. We find a clear signature of large volcanic eruptions in the simulated temperature record. The model is able to simulate individual extreme events such as the "year without a summer" 1816. Warm periods in the early seventeenth century and the second half of eighteenth century occur in periods of increased solar irradiation. Strong warming is simulated after 1850, in particular over land, going along with an increase of the positive North Atlantic Oscillation (NAO) phase. Consistent circulation anomalies are simulated in multidecadal means with similarity to observed and reconstructed anomalies, for example during the late seventeenth and early eighteenth century. The model is able to reproduce some of the observed or reconstructed regional patterns. We find that cooling around 1700 and at the end of the eighteenth century is less than in other studies, due to the relatively small variations in solar activity and the relatively modest volcanic forcing applied here. These cooling events are not restricted to Europe and North America, but cover most of the Northern Hemisphere. Colder than average conditions, for example during the late seventeenth and early eighteenth centuries, go along with a decrease in pressure difference between low and high latitudes and a decrease of the North Atlantic Oscillation. This favours positive sea ice anomalies east of Greenland and around Iceland, leading to widespread negative temperature anomalies over Europe. We also find characteristic blocking patterns over Western Europe, in particular during autumn, which contribute to the advection of cold air.