This is the only way forward with scientific advice on a subject as complex as the global climate. Policy makers and the general public will have to possess their souls in patience: to do otherwise would be foolhardy and almost inevitably would be regretted.. Moreover, the models do a reasonable job of simulating both natural variability and other spatial patterns, including temperature differences between the hemispheres, land and oceans, and the troposphere and stratosphere. This suggests, then, that recent changes are not solely natural variability but, in part, evidence of the fingerprint of human activities.The IPCC view represents the painstaking compilation and analysis of a huge amount of work, but there has been no sea-change in our understanding.

Moreover, the combination of greenhouse gases and sulphate aerosols matches observed global temperature trends rather well. In addition, the predictions of the regional effects of sulphate aerosols produce temperature trends around the world more in tune with recent developments.The fact that the inclusion of aerosols produces a better match between model predictions and observed surface temperature trends is a significant step towards attributing global warming to human activities. But these predictions did not tally well with observed changes this century.Recent models of the climate provide improved treatment of the atmosphere and the oceans and consider other human activities, notably production of sulphate aerosols. The impact of aerosols is of particular interest and the Hadley Centre model, developed by a team under John Mitchell, indicates more modest warming than earlier predictions.

By the late 1980s the broad consensus was that the effect of greenhouse gases, equivalent to doubling pre-industrial carbon dioxide levels, was to increase the global temperature between about 1.5 and 4.5C with the greatest warming occurring in polar regions in the winter half of the year. Measurements from space show a much smaller warming than surface-based observations.The potential of the global climate to change of its own accord, however, is the real joker in the pack. The more we find out about past changes the greater its capacity to spring surprises on us. Not only have we discovered that before 10,000 years ago the climate was capable of sudden large shifts, but also that more recent relatively orderly changes such as the warm period around 1,000 years ago (the "Medieval Climatic Optimum") and the subsequent cool period (the "Little Ice Age") are less clear-cut than previously assumed.This changing view of natural variability makes it more difficult to test whether computer models are providing a realistic representation of the longer-term behaviour of the global climate.