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Quantifying the uncertainty in forecasts of anthropogenic climate change


Forecasts of climate change are inevitably uncertain. It is therefore essential to quantify the risk of significant departures from the predicted response to a given emission scenario. Previous analyses of this risk have been based either on expert opinion1, perturbation analysis of simplified climate models2,3,4,5 or the comparison of predictions from general circulation models6. Recent observed changes that appear to be attributable to human influence7,8,9,10,11,12 provide a powerful constraint on the uncertainties in multi-decadal forecasts. Here we assess the range of warming rates over the coming 50 years that are consistent with the observed near-surface temperature record as well as with the overall patterns of response predicted by several general circulation models. We expect global mean temperatures in the decade 2036–46 to be 1–2.5 K warmer than in pre-industrial times under a ‘business as usual’ emission scenario. This range is relatively robust to errors in the models' climate sensitivity, rate of oceanic heat uptake or global response to sulphate aerosols as long as these errors are persistent over time. Substantial changes in the current balance of greenhouse warming and sulphate aerosol cooling would, however, increase the uncertainty. Unlike 50-year warming rates, the final equilibrium warming after the atmospheric composition stabilizes remains very uncertain, despite the evidence provided by the emerging signal.

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Figure 1: Predicted anthropogenic warming by about 2040 under the IS92a scenario before and after reconciling model simulations with the recent climate record.
Figure 2: Transfer functions relating past and future climate change in a simple climate model.
Figure 3: Global temperatures under IS92a consistent with the recent climate record.
Figure 4: Forecast anthropogenic warming 1996 to 2046 under IS92a.


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We thank T. Barnett, C. Forest, N. Gillett, K. Hasselmann, G. Hegerl, W. Ingram, G. Jones, S. Raper, B. Ripley, S. Smith, A. Sokolov, P. Stone, S. Tett, I. Tracey and A. Weaver for suggestions. This work was supported by the UK Natural Environment Research Council (M.R.A.); The UK Department of Environment, Transport and Regions (P.A.S.); the UK Meteorological Office's Research and Development Programme (J.F.B.T.); the European Commission (R.S.); and the US National Oceanic and Atmospheric Administration (T.L.D.); with additional support from the US Department of Energy and the British Council.

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Correspondence to Myles R. Allen.

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Allen, M., Stott, P., Mitchell, J. et al. Quantifying the uncertainty in forecasts of anthropogenic climate change . Nature 407, 617–620 (2000).

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