Quantifying anthropogenic influence on recent near-surface temperature change
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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 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.
KeywordsClimate change Detection Attribution
This work was originally motivated by Professor David Ritson’s critical analysis of detection and attribution work prior to the IPCC Third Assessment Report, for which we are duely grateful. This work was undertaken in support of the TAR, and originally accepted for publication in 2001. The delay in finalising this paper was due entirely to the personal circumstances of the lead author, and we are deeply grateful to the editors for their forbearance. We would also like to thank Francis Zwiers and Ben Santer for two rounds of exceptionally thorough and thoughtful reviews.
This synthesis of detection and attribution results from a range of climate models was primarily supported by the European Commission QUARCC project, ENV4-96-0250, and the US Department of Energy/NOAA ad hoc advisory committee on detection and attribution of climate change. Detection code development and model runs were supported by the UK Department of Environment, Transport and the Regions under contract no. PECD 7/12/37, the UK Natural Environment Research Council, the Deutsches Klimarechenzentrum, the US Department of Energy, the US National Oceanographic and Atmospheric Administration and the Canadian Centre for Climate Modelling and Analysis.
Code for the analyses presented in this study is available from http://www.climateprediction.net/detection.
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