Quantifying the water vapour feedback associated with post-Pinatubo global cooling
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There is an ongoing important debate about the role of water vapour in climate change. Predictions of future climate change depend strongly on the magnitude of the water vapour feedback and until now models have almost exclusively been relied upon to quantify this feedback. In this work we employ observations of water vapour changes, together with detailed radiative calculations to estimate the water vapour feedback for the case of the Mt. Pinatubo eruption. We then compare our observed estimate with that calculated from a relatively large ensemble of simulations from a complex coupled climate model. We calculate an observed water vapour feedback parameter of –1.6 Wm–2 K–1, with uncertainty placing the feedback parameter between –0.9 to –2.5 Wm–2 K–1. The uncertain is principally from natural climate variations that contaminate the volcanic cooling. The observed estimates are consistent with that found in the climate model, with the ensemble average model feedback parameter being –2.0 Wm–2 K–1, with a 5–95% range of –0.4 to –3.6 Wm–2 K–1 (as in the case of the observations, the spread is due to an inability to separate the forced response from natural variability). However, in both the upper troposphere and Southern Hemisphere the observed model water vapour response differs markedly from the observations. The observed range represents a 40%–400% increase in the magnitude of surface temperature change when compared to a fixed water vapour response and is in good agreement with values found in other studies. Variability, both in the observed value and in the climate model’s feedback parameter, between different ensemble members, suggests that the long-term water vapour feedback associated with global climate change could still be a factor of 2 or 3 different than the mean observed value found here and the model water vapour feedback could be quite different from this value; although a small water vapour feedback appears unlikely. We also discuss where in the atmosphere water vapour changes have their largest effect on surface climate.
KeywordsWater Vapour Ensemble Member Feedback Parameter International Satellite Cloud Climatology Project Surface Temperature Change
PMF was funded by an UK Natural Environment Research Council fellowship. Bill Read is thanked for providing the MLS data. MC was supported by the UK Natural Environment Research Council Coupled Ocean-Atmosphere and European Climate Thematic Programme. We are grateful to Gareth Jones and Peter Stott of the Hadley Centre for proving details of the model simulations. Brian Soden and an anonymous reviewer are thanked for helpful comments.
- Cess RD and 31 Co authors (1990) Intercomparison and interpretation of climate feedback processes in 19 atmospheric general circulation models. J Geophys Res 95: 16,601–16,615Google Scholar
- Cess RD and 36 Co authors (1996) Cloud feedback in atmospheric general circulation models: an update. J Geophys Res 101: 12,791–12,794Google Scholar
- Collins M (2003) Predictions of climate following volcanic eruptions. Volcanoes and the Earths Atmosphere. AGU monograph, (in press)Google Scholar
- Forster PM de F, Shine KP (1997) Radiative forcing and temperature trends from stratospheric ozone depletion. J Geophys Res 102: 10,841–10,855Google Scholar
- Hansen J, Sato M, Ruedy R (1997) Radiative forcing and climate response. J Geophys Res 102: 6831–6894Google Scholar
- Hansen J, Lacis A Ruedy R, Sato M, Wilson H (1993) How sensitive is the world’s climate? Natl Geog Res Explor 9: 142–158Google Scholar
- IPCC (1990) Climate change 1990: the IPCC scientific assessment. In: Houghton JT, Jenkins GJ, Ephraums JJ (eds) Cambridge University Press, Cambridge, UKGoogle Scholar
- IPCC (2001) Climate change 2001: the scientific basis. In: Houghton JT, Ding Y, Griggs D J, Noguer M, Van der Linden PJ, Dai X, Maskell K, Johnson CA (eds) Cambridge University Press, Cambridge, UKGoogle Scholar
- Joshi M Shine K P, Ponater M, Stuber N, Sausen R, Li L (2003) A comparison of climate response to different radiative forcings in three general circulation models: towards an improved metric of climate change. Clim Dyn (in press)Google Scholar
- Sato M, Hansen JE, McCormick MP, Pollack JB (1993) Stratospheric aerosol optical depths. 1850–1990, J Geophys Res 98: 22,987–22,994Google Scholar