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Climate sensitivity constrained by temperature reconstructions over the past seven centuries


There is a Brief Communications Arising (01 March 2007) associated with this document

The magnitude and impact of future global warming depends on the sensitivity of the climate system to changes in greenhouse gas concentrations. The commonly accepted range for the equilibrium global mean temperature change in response to a doubling of the atmospheric carbon dioxide concentration1, termed climate sensitivity, is 1.5–4.5 K (ref. 2). A number of observational studies3,4,5,6,7,8,9,10, however, find a substantial probability of significantly higher sensitivities, yielding upper limits on climate sensitivity of 7.7 K to above 9 K (refs 3–8). Here we demonstrate that such observational estimates of climate sensitivity can be tightened if reconstructions of Northern Hemisphere temperature over the past several centuries are considered. We use large-ensemble energy balance modelling and simulate the temperature response to past solar, volcanic and greenhouse gas forcing to determine which climate sensitivities yield simulations that are in agreement with proxy reconstructions. After accounting for the uncertainty in reconstructions and estimates of past external forcing, we find an independent estimate of climate sensitivity that is very similar to those from instrumental data. If the latter are combined with the result from all proxy reconstructions, then the 5–95 per cent range shrinks to 1.5–6.2 K, thus substantially reducing the probability of very high climate sensitivity.

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Figure 1: Palaeoclimatic records compared to a climate model simulation.
Figure 2: Northern Hemisphere mean radiative forcing.
Figure 3: Estimated probability density functions (PDFs) for equilibrium climate sensitivity to CO2 doubling (in K).


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We thank C. Forest for suggesting merging PDF estimates from results based on past-millennium climate changes with those based on twentieth-century climate change; J. Kenyon for technical support; M. Allen, F. Zwiers, A. Gelfand and M. Lavine for discussions; and C. Forest, N. Andronova, R. Knutti and J. Gregory for providing data. G.C.H. and T.J.C. were supported by NOAA, NOAA's office of global programmes and the DOE's office of biological and environmental research. G.C.H. was additionally supported by NSF. Author Contributions G.C.H. developed and implemented the method to estimate sensitivity and to calibrate proxy records, T.J.C. provided the reconstruction of past forcing and developed the CH-blend reconstruction, W.T.H. performed the EBM simulation, and D.J.F. derived the prior estimate of sensitivity from instrumental data 1950–2000. G.C.H., T.J.C. and W.T.H analysed the results.

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Correspondence to Gabriele C. Hegerl.

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Supplementary information

Supplementary Notes

This file discusses detailed Supplementary Methods, with estimates and models and references. It contains Supplementary Figures 1–5 and Supplementary Tables 3 and 4, and also legends for Supplementary Tables 1 and 2. (PDF 973 kb)

Supplementary Table 1

These files contain our new reconstructions and their 2.5% and 97.5% uncertainty ranges, and thus contain data necessary to reproduce our result. Both the baseline reconstruction CH-blend and the longer, less densely sampled reconstruction CH-blend (long) are attached here. The files are organized as Year/ Central value/ Upper 97.5th percentile/Lower 2.5th percentile. The uncertainty ranges reflect both uncertainty in the amplitude of the reconstruction, and sampling uncertainty combined. *Supplementary Table 1 and its heading have been replaced on 5 February 2007. This is a corrected version of the previous Table S1, in which only the scaling uncertainty was included; in the corrected Table S1, scaling and sampling uncertainties are now included. (XLS 15 kb)

Supplementary Table 2

Gives the forcing data for greenhouse gas, solar, volcanic and aerosol forcing used in the simulations. It also contains data necessary to reproduce our result. (XLS 86 kb)

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Hegerl, G., Crowley, T., Hyde, W. et al. Climate sensitivity constrained by temperature reconstructions over the past seven centuries. Nature 440, 1029–1032 (2006).

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