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support the proposition that hemispheric- or larger-scale temperature fluctuations
on timescales from multiyear to centuries are largely determined by changes in
radiative forcing.
In fact, the AR4 comparisons between the reconstructions (largely tree-ring
based) and climate model runs yielded a result that supported and extended the con-
clusion of Mann et al.'s (1998) statistical comparison of the Northern Hemisphere
temperature reconstruction with estimates of solar, volcanic, and greenhouse gas
forcing. Simulations were run on EBM and EMIC models, using differing histo-
ries of forcing. One set was run using all available forcing, whereas another had
the anthropogenic forcings rendered flat from 1765 on. Each set included runs with
different strengths of solar irradiance forcing. All were rather similar for the period
before 1765, but only the simulations in which the observed history of variation
in the anthropogenic factors was applied after 1765 tracked the reconstructed and
observed Northern Hemisphere temperature out to the late twentieth century (AR4,
Fig. 6.14d ). As in Mann et al. ( 1998 ) , natural forcings, primarily solar and vol-
canic, account for the reconstructed hemispheric temperature history for most of the
past millennium, but it is not possible to account for late-nineteenth- and twentieth-
century values without the anthropogenic forcings. This is a particularly compelling
finding because it is based on current knowledge of the mechanisms of the climate
system as incorporated in the models rather than a solely statistical analysis.
11.5 Climate Sensitivity
There is a challenge that high-resolution paleoclimatology has yet to meet. Bradley
( Chapter 1 , this volume) expressed the expectation that meaningful estimates of cli-
mate sensitivity in pre-industrial times will be made in the next decade. Climate
sensitivity is the global surface temperature change associated with a doubling in
the atmospheric carbon dioxide equivalent. Specifically, Jansen et al. ( 2007 ) wrote
in AR4 (Section 6.6.4) that, 'It is difficult to constrain the climate sensitivity from
the proxy records of the past millennium.' As they stand, reconstructions of global
temperature and estimates of forcings are too uncertain to permit this. In partic-
ular, Jansen et al. ( 2007 ) note that the magnitudes of low-frequency variations in
hemispheric temperature reconstructions differ by 'up to about a factor of two for
different reconstructions' and that the reconstructions of natural forcings (solar and
volcanic) are uncertain for this period. As Bradley points out, dendroclimatology
can play a role in reducing the uncertainty of these temperature reconstructions by
increasing the number, quality, temporal extent, and geographic coverage of the
global network of tree-ring chronologies useful in dendroclimatology research. It
should be added that dendrochronology in the broader sense plays a role in improv-
ing estimates of forcing; for example, by improving the chronology of climatically
effective volcanic eruptions (e.g., Larsen et al. 2008 ; Salzer and Hughes 2007 ) and
as a source of information on cosmogenic isotopes as proxies for solar activity (e.g.,
Bond et al. 2001 ) .
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