Detection of anthropogenic climate change using an atmospheric GCM |
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| Authors: | D M H Sexton D P Rowell C K Folland D J Karoly |
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| Institution: | (1) Hadley Centre for Climate Prediction and Research, Met Office, Bracknell, United Kingdom E-mail: dsexton@meto.gov.uk, GB;(2) Department of Mathematics and Statistics, Monash University, Clayton, Victoria, Australia, AU |
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| Abstract: | Atmosphere-only general circulation models are shown to be a useful tool for detecting an anthropogenic effect on climate
and understanding recent climate change. Ensembles of atmospheric runs are all forced with the same observed changes in sea
surface temperatures and sea-ice extents but differ in terms of the combinations of anthropogenic effects included. Therefore,
our approach aims to detect the `immediate' anthropogenic impact on the atmosphere as opposed to that which has arisen via
oceanic feedbacks. We have adapted two well-used detection techniques, pattern correlations and fingerprints, and both show
that near-decadal changes in the patterns of zonal mean upper air temperature are well simulated, and that it is highly unlikely
that the observed changes could be accounted for by sea surface temperature variations and internal variability alone. Furthermore,
we show that for zonally averaged upper air temperature, internal `noise' in the atmospheric model is small enough that a
signal emerges from the data even on interannual time scales; this would not be possible in a coupled ocean-atmosphere general
circulation model. Finally, although anthropogenic forcings have had a significant impact on global mean land surface temperature,
we find that their influence on the pattern of local deviations about this mean is so far undetectable. In order to achieve
this in the future, as the signal grows, it will also be important that the response of the Northern Hemisphere mid-latitude
westerly flow to changing sea surface temperatures is well simulated in climate model detection studies.
Received: 3 December 1999 / Accepted: 30 October 2000 |
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