Contributed by Benjamin D. Santer, June 22, 2012
We perform a multimodel detection and attribution study with climate model simulation output and satellite-based measurements of tropospheric and stratospheric temperature change. We use simulation output from 20 climate models participating in phase 5 of the Coupled Model Intercomparison Project. This multimodel archive provides estimates of the signal pattern in response to combined anthropogenic and natural external forcing (the fingerprint) and the noise of internally generated variability. Using these estimates, we calculate signal-to-noise (S/N) ratios to quantify the strength of the fingerprint in the observations relative to fingerprint strength in natural climate noise. For changes in lower stratospheric temperature between 1979 and 2011, S/N ratios vary from 26 to 36, depending on the choice of observational dataset. In the lower troposphere, the fingerprint strength in observations is smaller, but S/N ratios are still significant at the 1% level or better, and range from three to eight. We find no evidence that these ratios are spuriously inflated by model variability errors. After removing all global mean signals, model fingerprints remain identifiable in 70% of the tests involving tropospheric temperature changes. Despite such agreement in the large-scale features of model and observed geographical patterns of atmospheric temperature change, most models do not replicate the size of the observed changes. On average, the models analyzed underestimate the observed cooling of the lower stratosphere and overestimate the warming of the troposphere. Although the precise causes of such differences are unclear, model biases in lower stratospheric temperature trends are likely to be reduced by more realistic treatment of stratospheric ozone depletion and volcanic aerosol forcing.
This article is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected in 2011.
Author contributions: B.D.S., C.A.M., S.S., K.E.T., L.T., F.J.W., and T.M.L.W. designed research; B.D.S., J.F.P., C.A.M., C.D., P.C., J.P., and L.J.W. performed research; B.D.S., J.F.P., C.A.M., C.D., P.C., P.J.C.-S., P.J.G., J.P., S.S., L.T., and L.J.W. analyzed data; C.A.M., F.J.W., and C.-Z.Z. contributed key observational datasets; and B.D.S., J.F.P., C.A.M., C.D., P.C., J.A., P.J.C.-S., N.P.G., P.J.G., J.L., J.P., S.S., P.A.S., K.E.T., L.T., P.W.T., M.F.W., F.J.W., T.M.L.W., L.J.W., and C.-Z.Z. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
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This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1210514109/-/DCSupplemental.
Freely available online through the PNAS open access option.
