Causes of Climate Change Over the Past 1000 Years
Thomas J. Crowley
Recent reconstructions of Northern Hemisphere
temperatures and climate forcing over the past 1000 years allow the
warming of the 20th century to be placed within a historical context
and various mechanisms of climate change to be tested. Comparisons of
observations with simulations from an energy balance 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 reconstructed temperature time series yields residuals
that show similar variability to those of control runs of coupled
models, thereby lending support to the models' value as estimates of
low-frequency variability in the climate system. Removal of all forcing
except greenhouse gases from the ~1000-year time series results in a residual with a very large late-20th-century warming that closely agrees with the response predicted from greenhouse gas forcing. The
combination of a unique level of temperature increase in the late 20th
century and improved constraints on the role of natural variability
provides further evidence that the greenhouse effect has already
established 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 temperature change for the last interglacial.
Department of Oceanography, Texas A&M University, College Station,
TX 77843, USA. E-mail: tcrowley{at}ocean.tamu.edu
