Recent reconstructions of northern hemisphere temperatures and climate
forcing over the last 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. Comparison of observations with
simulations from an energy balance climate model indicate that as much
as 41-64% of pre-anthropogenic (pre-1850) decadal-scale ... temperature
variations were 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 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 last 1000 years and is greater
than the best estimate of global temperature change for the last
Data files refer to figures in PDF at:
Decadally smoothed time series of Crowley-Lowery time series spliced
into smoothed Jones et al instrumental record after 1860, the 11-point
smoothed Mann et al. record, a slight modification (labeled CL2) of
the original Crowley and Lowery reconstruction to 1965, and B1 2
s.d. values from Mann et al. The autocorrelation of the Mann et al
time series has been used to adjust (adj) the standard deviation units
for the reduction in variability for decadal time scales.
Forcing time series (W/m**2, note scale changes for different panels)
used in model runs: (A) ice core millennial volcanism time series from
this study; ice-core Robcock and Free(19) reconstruction from 1400 to
the present after adjustments discussed in (9) and (25); and Sato et
al. (28) Northern Hemisphere radiative forcing, updated to 1998.
April 2001 file forc-total-4.12.01.txt contains estimates of volcano,
solar, greenhouse gas, and tropospheric (1000-1998) total forcing,
prior to accounting for the planetary albedo. All units are in
W/m**2. hl in volcanic time series refers to the fact that eruptions
of unknown origin have been assigned a high latitude (hl) origin.
There are "tails" to most of the large eruptions that were determined
based on the estimated e-folding time of the aerosols as being about 1
year. Sol.Be10 refers to the Beryllium 10 measurements of Bard et al.
scaled to the Lean et al. changes over the last 400 years.
A) Model response to different forcings, calculated at a sensitivity
of 2.0B0C for a doubling of CO2; B) Example of the combined effect of
volcanism volcanism and solar variability (with 11-point smoothing),
using the Bard et al. (30) 14C index.
Comparison of model response using all forcing terms (with a
sensitivity of 2.0 C) against (A) the CL (12) data set spliced into
the 11-point smoothed Jones et al. (16) Northern Hemisphere
instrumental record, with rescaling as discussed in the text and in
the Fig. 1 caption; and
(B) the smoothed Mann et al. (11) reconstruction. Both panels include
the Jones et al. instrumental record for reference. To illustrate
variations in the modeled response, the 14C calculation from Bard et
al. (30) has been used in (A) and the 10Be estimates from (30) have
been used in (B).