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Uncertainty in temperature projections reduced using carbon cycle and climate observations


The future behaviour of the carbon cycle is a major contributor to uncertainty in temperature projections for the twenty-first century1,2. Using a simplified climate model3, we show that, for a given emission scenario, it is the second most important contributor to this uncertainty after climate sensitivity, followed by aerosol impacts. Historical measurements of carbon dioxide concentrations4 have been used along with global temperature observations5 to help reduce this uncertainty. This results in an increased probability of exceeding a 2 °C global–mean temperature increase by 2100 while reducing the probability of surpassing a 6 °C threshold for non-mitigation scenarios such as the Special Report on Emissions Scenarios A1B and A1FI scenarios6, as compared with projections from the Fourth Assessment Report7 of the Intergovernmental Panel on Climate Change. Climate sensitivity, the response of the carbon cycle and aerosol effects remain highly uncertain but historical observations of temperature and carbon dioxide imply a trade–off between them so that temperature projections are more certain than they would be considering each factor in isolation. As well as pointing out the promise from the formal use of observational constraints in climate projection, this also highlights the need for an holistic view of uncertainty.

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Figure 1: Time series of global-mean temperature change for selected SRES marker scenarios.
Figure 2: Probability of exceeding 2 °C global-mean temperature change relative to pre-industrial for A1FI, A1B and A2 emission scenarios.

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We thank M. Meinshausen and J. Kattge for supplying the MAGICC and MCMH code respectively. This research was supported by the Australian Research Council through the Discovery Projects funding scheme (project number FF0668679), Australian Research Council Centre of Excellence for Climate System Science (grant CE 110001028) and an ARC Professorial Fellowship (DP1096309).

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R.W.B, P.J.R. and D.J.K. designed the research. R.W.B. carried out the analysis, with P.J.R. adding the linear uncertainty analysis of the posterior covariance. R.W.B. wrote the paper. All authors discussed the results and edited the manuscript.

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Correspondence to Roger W. Bodman.

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The authors declare no competing financial interests.

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Bodman, R., Rayner, P. & Karoly, D. Uncertainty in temperature projections reduced using carbon cycle and climate observations. Nature Clim Change 3, 725–729 (2013).

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