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Climate sensitivity constrained by CO2 concentrations over the past 420 million years


A firm understanding of the relationship between atmospheric carbon dioxide concentration and temperature is critical for interpreting past climate change and for predicting future climate change1. A recent synthesis2 suggests that the increase in global-mean surface temperature in response to a doubling of the atmospheric carbon dioxide concentration, termed ‘climate sensitivity’, is between 1.5 and 6.2 °C (5–95 per cent likelihood range), but some evidence is inconsistent with this range1,2,3,4,5. Moreover, most estimates of climate sensitivity are based on records of climate change over the past few decades to thousands of years, when carbon dioxide concentrations and global temperatures were similar to or lower than today1,6, so such calculations tend to underestimate the magnitude of large climate-change events7 and may not be applicable to climate change under warmer conditions in the future. Here we estimate long-term equilibrium climate sensitivity by modelling carbon dioxide concentrations over the past 420 million years and comparing our calculations with a proxy record. Our estimates are broadly consistent with estimates based on short-term climate records, and indicate that a weak radiative forcing by carbon dioxide is highly unlikely on multi-million-year timescales. We conclude that a climate sensitivity greater than 1.5 °C has probably been a robust feature of the Earth’s climate system over the past 420 million years, regardless of temporal scaling.

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Figure 1: Comparison of CO 2 calculated by GEOCARBSULF for varying Δ T (2×) to an independent CO 2 record from proxies.
Figure 2: Calculation of the long-term equilibrium climate sensitivity to CO2.
Figure 3: Sensitivity analysis for the effect of nine additional parameters in the GEOCARBSULF carbon cycle model on the fit between model-derived and proxy CO 2 values for varying Δ T (2×).

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We thank L. D. Harvey, E. W. Bolton and T. J. Crowley for discussions, and G. C. Hegerl for data. This work was supported in part by the US Department of Energy (R.A.B.).

Author Contributions D.L.R. and R.A.B. initiated the project, and all authors developed it. R.A.B. performed the carbon cycle calculations and J.P. performed most of the statistical analyses. D.L.R. managed the project and wrote most of the paper.

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Correspondence to Dana L. Royer.

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Reprints and permissions information is available at The authors declare no competing financial interests.

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Supplementary Figure 1

This file contains Supplementary Figure 1 with Legend. The Supplementary Figure 1 illustrates sensitivity analysis for the effect of variation of four critical parameters in the GEOCARBSULF carbon cycle model on the fit between model-derived and proxy CO2 values for varying ΔT(2x). (PDF 160 kb)

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Royer, D., Berner, R. & Park, J. Climate sensitivity constrained by CO2 concentrations over the past 420 million years. Nature 446, 530–532 (2007).

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