1. Division of Earth and Ocean Sciences, Nicholas School of the Environment and Earth Sciences, Duke University, Durham, North Carolina, 27708, USA
2. Climate Dynamics Group, Department of Physics, University of Oxford, Oxford OX1 3PU, UK

Correspondence to: Gabriele C. Hegerl¹ Correspondence and requests for materials should be addressed to G.C.H. (Email: hegerl@duke.edu).

Abstract

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 studies^{3, 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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