Quantifying anthropogenic influence on recent near-surface temperature change

We assess the extent to which observed large-scale changes in near-surface temperatures over the latter half of the twentieth century can be attributed to anthropogenic climate change as simulated by a range of climate models. The hypothesis that observed changes are entirely due to internal climate variability is rejected at a high confidence level independent of the climate model used to simulate either the anthropogenic signal or the internal variability. Where the relevant simulations are available, we also consider the alternative hypothesis that observed changes are due entirely to natural external influences, including solar variability and explosive volcanic activity. We allow for the possibility that feedback processes, other than those simulated by the models considered, may be amplifying the observed response to these natural influences by an unknown amount. Even allowing for this possibility, the hypothesis of no anthropogenic influence can be rejected at M. R. Allen (&) Atmospheric, Oceanic and Planetary Physics, University of Oxford Clarendon Laboratory, Parks Road OX1 3PU, UK e-mail: [email protected] N. P. Gillett School of Earth and Ocean Sciences, University of Victoria, Victoria, Canada Present Address: N. P. Gillett Climate Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, UK J. A. Kettleborough Space Science and Technology Department, Rutherford Appleton Laboratory, Didcot, UK Present Address: J. A. Kettleborough Met. Office, FitzRoy Road, Exeter, UK G. S. Jones Æ J. F. B. Mitchell Met. Office, FitzRoy Road, Exeter, UK 123 Surv Geophys (2006) 27:491–544 DOI 10.1007/s10712-006-9011-6 ORI GI N A L P A PE R Quantifying anthropogenic influence on recent near-surface temperature change M. R. Allen Æ N. P. Gillett Æ J. A. Kettleborough Æ G. Hegerl Æ R. Schnur Æ P. A. Stott Æ G. Boer Æ C. Covey Æ T. L. Delworth Æ G. S. Jones Æ J. F. B. Mitchell Æ T. P. Barnett Received: 30 December 2005 / Accepted: 19 August 2006 / Published online: 15 November 2006 Springer Science+Business Media B.V. 2006 the 5% level in almost all cases. The influence of anthropogenic greenhouse gases emerges as a substantial contributor to recent observed climate change, with the estimated trend attributable to greenhouse forcing similar in magnitude to the total observed warming over the 20th century. Much greater uncertainty remains in the response to other external influences on climate, particularly the response to anthropogenic sulphate aerosols and to solar and volcanic forcing. Our results remain dependent on model-simulated signal patterns and internal variability, and would benefit considerably from a wider range of simulations, particularly of the responses to natural external forcing.