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Recent reconstructions of Northern Hemisphere
temperatures and climate forcing over the past 1000 years allow the
warmingof the 20th century to be placed within a historical context
andvarious mechanisms of climate change to be tested. Comparisonsof
observations with simulations from an energy balance climatemodel
indicate that as much as 41 to 64% of preanthropogenic
(pre-1850)decadal-scale temperature variations was due to
changes in solarirradiance and volcanism. Removal of the forced
response fromreconstructed temperature time series yields residuals
that showsimilar variability to those of control runs of coupled
models,thereby lending support to the models' value as estimates of
low-frequencyvariability in the climate system. Removal of all forcing
exceptgreenhouse gases from the ~1000-year time series results in aresidual with a very large late-20th-century warming that closelyagrees with the response predicted from greenhouse gas forcing.The
combination of a unique level of temperature increase in thelate 20th
century and improved constraints on the role of naturalvariability
provides further evidence that the greenhouse effecthas already
established itself above the level of natural variabilityin the
climate system. A 21st-century global warming projectionfar exceeds
the natural variability of the past 1000 years andis greater than the
best estimate of global temperature changefor the last interglacial.
Department of Oceanography, Texas A&M University, College Station,
TX 77843, USA. E-mail: tcrowley{at}ocean.tamu.edu
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