What can present climate models tell us about climate change?

Climate models are evaluated in terms of their ability to describe the past climatic changes. Past climatic trends are inferred from fitting a truncated Taylor series to the observational record and an ensemble of downscaled results from climate models. Analytical expressions are derived for the warming rates associated with long-term temperature trends using simple calculus. Different trend models are compared, and a third-order polynomial gives the best description of the past winter warming over southwestern Scandinavia. The coefficients from the regression analysis are used in an objective comparison of the past climatic evolution in the models and the observations. Comparisons between the temperature trends from observations and the output from an ensemble of various climate models suggest that single climate model scenarios do not provide a reliable description of the climatic evolution. Ensembles of state-of-the-art climate models, on the other hand, capture the main features of the past climatic evolution. However, there has been an interval with pronounced local winter warming over Scandinavia in the past, which is not reproduced by the majority of climate models. It is difficult to say whether this accelerated warming event was part of natural decadal variations or induced by external factors. The climate models may not yet be able to predict similar local episodes for the future if they are related to events unaccounted for, such as solar activity or volcanism.