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Overestimated global warming over the past 20 years

Nature Climate Change volume 3, pages 767–769 (2013)Cite this article

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Recent observed global warming is significantly less than that simulated by climate models. This difference might be explained by some combination of errors in external forcing, model response and internal climate variability.

The evidence, therefore, indicates that the current generation of climate models (when run as a group, with the CMIP5 prescribed forcings) do not reproduce the observed global warming over the past 20 years, or the slowdown in global warming over the past fifteen years. This interpretation is supported by statistical tests of the null hypothesis that the observed and model mean trends are equal, assuming that either: (1) the models are exchangeable with each other (that is, the 'truth plus error' view); or (2) the models are exchangeable with each other and with the observations (see Supplementary Information). Differences between observed and simulated 20-year trends have p values (Supplementary Information) that drop to close to zero by 1993–2012 under assumption (1) and to 0.04 under assumption (2) (Fig. 2c). Here we note that the smaller the p value is, the stronger the evidence against the null hypothesis. On this basis, the rarity of the 1993–2012 trend difference under assumption (1) is obvious. Under assumption (2), this implies that such an inconsistency is only expected to occur by chance once in 500 years, if 20-year periods are considered statistically independent. Similar results apply to trends for 1998–2012 (Fig. 2d). In conclusion, we reject the null hypothesis that the observed and model mean trends are equal at the 10% level.

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Figure 1: Trends in global mean surface temperature.
Figure 2: Global mean surface temperature trends and p values.
Figure 3: Trends in global mean surface temperature and in associated natural and residual time series.

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Acknowledgements

We thank Greg Flato, Bill Merryfield and Slava Kharin for their comments on an early draft, Knutti Reto for input on our statistical analysis and Piers Forster and Jochem Marotzke for many helpful discussions. We acknowledge the Program for Climate Model Diagnosis and Intercomparison and the World Climate Research Programme's Working Group on Coupled Modelling for their roles in making the WCRP CMIP multi-model datasets available.

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Affiliations

  1. Canadian Centre for Climate Modelling and Analysis, Environment Canada, Victoria, V8W 2Y2, British Columbia, Canada

    John C. Fyfe & Nathan P. Gillett

  2. Pacific Climate Impacts Consortium, University of Victoria, Victoria, V8W 3R4, British Columbia, Canada

    Francis W. Zwiers

Authors
  1. John C. Fyfe
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  2. Nathan P. Gillett
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  3. Francis W. Zwiers
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Contributions

J.C.F. helped to carry out the analysis and wrote the initial draft. N.P.G. and F.W.Z. helped with the analysis and edited the manuscript.

Corresponding author

Correspondence to John C. Fyfe.

Supplementary information

Supplementary Information

CMIP5 Models (PDF 241 kb)

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Fyfe, J., Gillett, N. & Zwiers, F. Overestimated global warming over the past 20 years. Nature Clim Change 3, 767–769 (2013). https://doi.org/10.1038/nclimate1972

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