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How Sound Are Climate Model Simulations?
Reference
Stott, P.A., Mitchell, J.F.B., Allen, M.R., Delworth, T.L., Gregory, J.M., Meehl, G.A. and Santer, B.D. 2006. Observational constraints on past attributable warming and predictions of future global warming. Journal of Climate 19: 3055-3069.

Background
The authors set the stage for their study by emphasizing that "care should be taken not to over-interpret good agreement between climate models and past observed global mean warming," because, as they continue, "with large uncertainties in climate forcings, especially that due to aerosols, agreement when models include all the most important anthropogenic and natural forcings could be obtained fortuitously as a result of, for example, balancing too much (or too little) greenhouse gas warming by too much (or too little) aerosol cooling."

What was done
In an attempt to resolve this problem using three coupled climate models with very different sensitivities and aerosol forcings, Stott et al. performed an optimal detection analysis, wherein "the spatial and temporal nature of observed twentieth-century temperature change constrains the component of past warming attributable to anthropogenic greenhouse gases."

What was learned
The seven climate scientists report that "all three models, when constrained by observations, suggest that [20th-century] warming attributable to greenhouse gases is between 0.7 and 1.3 K (5 and 95 percentiles) and therefore probably greater than the observed warming of 0.6 K over the century." This being the case, they say that the "transient climate response under a 1% [yr-1] increase in carbon dioxide has a range of 2.2-4.0 K (5-95 percentiles)."

What it means
Is this the climate modeling community's final answer? Probably not, for Stott et al. appropriately acknowledge that "all our results are subject to unquantified [our italics] uncertainties arising from missing [our italics] forcings and imperfect [our italics] representation of climate physics," and since there is no way to know the impacts of "unquantified" uncertainties, "missing" forcings and "imperfect" representations of climate physics (and there will not be until the time these respective elements are quantified, no longer missing and at least close to perfected), there will be no way to know the climatic significance of either past or projected increases in the air's CO2 content via the use of climate models.

Reviewed 16 August 2006