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Model structure in observational constraints on transient climate response

Abstract

The transient climate response (TCR) is a highly policy-relevant quantity in climate science. We show that recent revisions to TCR in the IPCC 5th Assessment Report have more impact on projections over the next century than revisions to the equilibrium climate sensitivity (ECS). While it is well known that upper bounds on ECS are dependent on model structure, here we show that the same applies to TCR. Our results use observations of the planetary energy budget, updated radiative forcing estimates and a number of simple climate models. We also investigate the ratio TCR:ECS, or realised warming fraction (RWF), a highly policy-relevant quantity. We show that global climate models (GCMs) don’t sample a region of low TCR and high RWF consistent with observed climate change under all simple models considered. Whether the additional constraints from GCMs are sufficient to rule out these low climate responses is a matter for further research.

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Acknowledgments

RJM is supported by a NERC and Met Office CASE studentship. JAL and WJI were supported by the Joint DECC/Defra Met Office Hadley Climate Centre Programme (GA01101). MRA was supported by the Oxford Martin School and DECC contract TRN 307/11/2011. WJI and MRA were also supported by NERC under projects NE/I00680X/1. PMF was supported by EPSRC grant EP/I014721/1 and a Royal Society Wolfson Merit award.

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Correspondence to Richard J. Millar.

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Millar, R.J., Otto, A., Forster, P.M. et al. Model structure in observational constraints on transient climate response. Climatic Change 131, 199–211 (2015). https://doi.org/10.1007/s10584-015-1384-4

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Keywords

  • Climate Sensitivity
  • Climate Response
  • Heat Uptake
  • Ocean Heat Uptake
  • Global Mean Surface Temperature