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A new way of quantifying GCM water vapour feedback

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Abstract

The water vapour feedback probably makes the largest contribution to climate sensitivity, and the second-largest contribution to its uncertainty, in the sense of disagreement between General Circulation Models (GCMs, the most physically detailed models of climate we have). Yet there has been no quantification of it which allows these differences to be attributed physically with the aim of constraining the true value. This paper develops a new breakdown of the non-cloud LW (longwave) response to climate change, which avoids the problems of the conventional breakdown, and applies it to a set of 4 GCMs. The basic physical differences are that temperature is used as the vertical coordinate, and relative humidity as the humidity variable. In this framework the different GCMs’ feedbacks look more alike, consistent with our understanding that their water vapour responses are physically very similar. Also, in the global mean all the feedback components have the same sign, allowing us to conveniently attribute the overall response fractionally (e.g. about 60% from the “partly-Simpsonian” component). The systematic cancellation between different feedback components in the conventional breakdown is lost, so now a difference in a feedback component actually contributes to a difference in climate sensitivity, and the differences between these GCMs in the non-cloud LW part of this can be traced to differences in formulation, mean climate and climate change response. Physical effects such as those due to variations in the formulation of LW radiative transfer become visible. Differences in the distribution of warming no longer dominate comparison of GCMs. The largest component depends locally only on the GCM’s mean climate, so it can in principle be calculated for the real world and validated. However, components dependent on the climate change response probably account for most of the variation between GCMs. The effect of simply changing the humidity variable in the conventional breakdown is also examined. It gives some of this improvement—the loss of the cancellations that leave the conventional breakdown of no use to understand differences between GCMs’ climate sensitivities—but not the link to mean climate.

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Notes

  1. Conventions vary—here “λ” means the net radiative response to global-mean temperature changes, in W m−2/K, inversely proportional to climate sensitivity, and negative for a net negative feedback, i.e. a stable system.

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Acknowledgments

This work was funded by the joint DECC/DEFRA (GA01101) and MoD (CBC/2B/0417_Annex C5) Integrated Climate Programme at the Met Office, with writing up partly funded by NERC contract NE/D012287/1 and EU contract GOCE-505539. Much of this work is obviously based on the vast amount of effort and expertise invested in various forms of the MetUM, so I am indebted to all the scientists who contributed to its development. More specifically I am grateful to Jean-Claude Thelen, John Edwards and James Manners for help with the Edwards-Slingo LW code in both its stand-alone and MetUM forms. Keith Williams advised me on the slab simulations, and had also run them, apart from the HadSM2 ones, run by Chris Hewitt. I am also particularly grateful to Catherine Senior and Mark Ringer for their support.

This was part of a doctoral project at AOPP, where I am again grateful to many colleagues, particularly my supervisor Professor David Andrews, Myles Allen, Mark Munro, Stu Teasdale, Neil Massey, Matt Rigby, Helene Muri, Helen Hanlon and Ruth Cerezo-Mota. The presentation has benefitted from comments from reviewers and many colleagues at both affiliations and elsewhere, and opportunities to give oral presentations.

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  1. Atmospheric, Oceanic and Planetary Physics, Department of Physics, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK

    William Ingram

  2. Hadley Centre for Climate Change, Met Office, FitzRoy Road, Exeter, EX1 3PB, UK

    William Ingram

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Correspondence to William Ingram.

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Ingram, W. A new way of quantifying GCM water vapour feedback. Clim Dyn 40, 913–924 (2013). https://doi.org/10.1007/s00382-012-1294-3

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