Danish Climate Centre

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.

By using the most recent models and a large ensemble, climate change signal can be separated from noise in the 21st century despite the large natural variability. This chapter provides results from a transient climate simulation at an (for Greenland) unprecedented horizontal resolution of 25 km, which has been forced with a rather high-resolution coupled Atmosphere-Ocean General Circulation Model. Compared to the driving global model, the regional model shows considerably stronger temperature increase in regions where sea ice retreats. The assessment of extreme events reveals considerably less cold (and warmer) days than under present-day conditions. The largest changes can be expected along the east coast, particularly in the Zackenberg region, where days with a positive average temperature are projected to become the rule rather than the exception. Most of Greenland, especially the northeast, will experience more precipitation. At lower elevations, an increasing percentage of this precipitation can be expected to fall as rain instead of as snow. On the contrary, more extreme snowfall events than at present have been projected on the ice sheet.

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.

To understand recent climate change in the North Atlantic region and to produce better climate forecasts with uncertainty estimates it is important to determine the atmospheric ‘response’ to Atlantic sea-surface temperature (SST) forcing. There have been conflicting results regarding the strength, character and tropical-versus-extratropical origin of this response. For model-based studies, this may indicate differing sensitivities to Atlantic SST, but the comparison is complicated by changes in experimental design. Here, a highly controlled experiment with five atmospheric models is undertaken. The influence of realistic (if reasonably strong) and optimally chosen North Atlantic (equator to 70°N) SST anomalies is isolated. Unexpected global agreement between the models is found (e.g. the North Atlantic Oscillation (NAO), Eurasian temperatures, rainfall over the Americas and Africa, and the Asian monsoon). The extratropical North Atlantic region response appears to be associated with remote Caribbean and tropical Atlantic SST anomalies, and with local forcing. Some features such as the European winter-temperature response would be stronger than atmospheric ‘noise’ if the prescribed SST anomalies persisted for just two years. More generally, Atlantic air–sea interaction appears to be important for climate variability on the 30-year timescale and, thus, to be important in the climate-change context. The multi-model mean response patterns are in reasonable agreement with observational estimates, although the model response magnitudes may be too weak. The similarity between their responses helps to reconcile models. Inter-model differences do still exist and these are discussed and quantified. © Crown copyright, 2004.