Simulating the recent Holocene

A climate simulation of the last 500 years, with natural (orbital, solar and volcanic) forcings alone, is generally stable on multi-century timescales despite considerable forced multi-decadal variability. Simulated natural forcing 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. 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 20^th century, and on the southern hemisphere during the 19^th 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.