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9. Summary and Conclusions
Recent detection studies based on changes in surface temperature are consistent with the
findings reported in the IPCC TAR. Additional uncertainties, such as the effect of differences in
model fingerprints and simulations have meanwhile been addressed. It has been demonstrated
that averaging fingerprints from multiple models increases our confidence in detecting
anthropogenic climate change and that different implementations of the optimal detection
method show consistent results. New detection studies show that anthropogenic climate change
is detectable in the surface temperature records of individual continents and that it can be
distinguished from climate change due to natural forcing.
Important progress has also been made in our understanding of Northern Hemisphere mean
temperature over the last millennium. Paleo reconstructions of hemispheric mean temperature
show that the 20th century warming is unique in the last millennium both for its size and rapidity.
Recent evidence suggests that the borehole -record of climate change may be consistent with
that of conventional proxy records. However, reconstruction techniques are still being evaluated.
New detection studies based on proxy reconstructions yield generally consistent results with
studies based on the instrumental period. The combined response to volcanism and greenhouse
gases can be detected in most reconstructions and the individual responses to greenhouse gas,
volcanic and solar forcing can be distinguished from each other with only small differences in
results for different records. Simulations of the last millennium with coupled climate models are
becoming available, which should help in further improving our understanding of the climate of
the last millennium.
Progress has also been made towards understanding the temperature evolution in the free
atmosphere. Recently, an independently processed version of the Microwave Sounding Unit38
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Barnett, T.; Zwiers, F.; Hegerl, G.; Allen, M.; Crowley, T.; Gillett, N. et al. Detecting and Attributing External Influences on the Climate System: A Review of Recent Advances, article, January 26, 2005; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc1418795/m1/42/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.