A climate simulation of the last 500 years, with natural (orbital, solar and volcanic) forcings alone, is stable on multi-century timescales despite considerable forced multi-decadal variability. The simulated response is not in equilibrium with the external forcings suggesting that the rate at which the ocean uptakes heat, as well as climate sensitivity are factors that will determine the magnitude of the response to natural forcings. The natural forcing used in our experiment increases large scale precipitation and temperature variability relative to internal variability with decadal-mean temperature variability in the tropics being enhanced by a factor of two. However, it has no significant impact on the variability of the North Atlantic Oscillation or the meridional overturning circulation. No evidence of an orbital influence on simulated temperature was found.

A second simulation using both anthropogenic (well-mixed greenhouse gases, ozone, sulphate aerosol and land-surface) and natural forcings from 1750 to 2000 has been carried out. Comparing this simulation with the natural-only simulations suggests that anthropogenic forcings have had a significant impact on climate during the entire twentieth century. Comparison of recent observed trends with those simulated using natural forcings suggests that recent changes are outside the range of natural variability over large regions of Eurasia and the Indian Ocean. Thus it is likely that anthropogenic climate change has already affected natural systems in these regions.