Causes of climate change over the past 1000 years

Recent reconstructions of Northern Hemisphere temperatures and climate forc ing over the past 1000 years allow the warming of the 20th century to be pl aced within a historical context and various mechanisms of climate change t o be tested. Comparisons of observations with simulations from an energy ba lance climate model indicate that as much as 41 to 64% of preanthropogenic (pre-1850) decadal-scale temperature variations was due to changes in solar irradiance and volcanism. Removal of the forced response from reconstructe d temperature time series yields residuals that show similar variability to those of control runs of coupled models, thereby lending support to the mo dels' value as estimates of low-frequency variability in the climate system . Removal of all forcing except greenhouse gases from the similar to 1000-y ear time series results in a residual with a very large late-20th-century w arming that closely agrees with the response predicted from greenhouse gas forcing. The combination of a unique level of temperature increase in the l ate 20th century and improved constraints on the role of natural variabilit y provides further evidence that the greenhouse effect has already establis hed itself above the level of natural variability in the climate system. A 21st-century global warming projection far exceeds the natural variability of the past 1000 years and is greater than the best estimate of global temp erature change for the last interglacial.