Climate of the Past www.clim-past.net 1814-9324 1814-9332 6 4 2010 10.5194/cp-6-411-2010 http://www.clim-past.net/6/411/2010/ http://www.clim-past.net/6/411/2010/cp-6-411-2010.html http://www.clim-past.net/6/411/2010/cp-6-411-2010.pdf 411 414 2010-07-02 Comment on "Using multiple observationally-based constraints to estimate climate sensitivity" by J. D. Annan and J. C. Hargreaves, Geophys. Res. Lett., 2006 S. V. Henriksson svante.henriksson@fmi.fi E. Arjas M. Laine J. Tamminen A. Laaksonen Finnish Meteorological Institute, 00101, Helsinki, Finland Department of Mathematics and Statistics, University of Helsinki, 00014, Helsinki, Finland Department of Physics, University of Kuopio, 70211, Kuopio, Finland In their article from 2006, Annan and Hargreaves present a probabilistic estimate of climate sensitivity obtained by using Bayes' theorem to combine information from different sources. In this comment article we critisize two aspects of their reasoning, namely using probability density functions and likelihood functions interchangeably and the assumed independence of evidence from the different sources. The derivation of their result rests on key assumptions, some stated explicitly and some left implicit, which could be unrealistic. Thus their study does not convincingly reduce the large uncertainty of climate sensitivity remaining in previous observationally-based studies. Andreae, M. O., Jones, C. D., and Cox, P. M.: Strong present-day aerosol cooling implies a hot future, Nature, 435, 1187–1190, 2005. Andronova, N. G. and Schlesinger, M. E.: Objective estimation of the probability density function for climate sensitivity, J. Geophys. Res., 106, 22605–22611, doi:10.1029/2000JD000259, 2001. Annan, J. D., Hargreaves, J. C., Ohgaito, R., Abe-Ouchi, A., and Emori, S.: Efficiently constraining climate sensitivity with paleoclimate simulations, SOLA, 1, 181–184, 2005. Annan, J. D. and Hargreaves, J. C.: Using multiple observationally-based constraints to estimate climate sensitivity, Geophys. Res. Lett., 33, L06704, doi:10.1029/2005GL025259, 2006. Annan, J. D. and Hargreaves, J. C.: On the generation and interpretation of probabilistic estimates of climate sensitivity, Climatic Change, doi:10.1007/s10584-009-9715-y, 2009. Folland, C. K., Rayner, N. A., Brown, S. J., Smith, T. M., Shen, S. S. P., Parker, D. E., Macadam, I., Jones, P. D., Jones, R. N., Nicholls, N., and Sexton D. M. H.: Global temperature change and its uncertainties since 1861, Geophys. Res. Lett., 28, 2621–2624, doi:10.1029/2001GL012877, 2001. Forster, P., Ramaswamy, V., Artaxo, P., Berntsen, T., Betts, R., Fahey, D. W., Haywood, J., Lean, J., Lowe, D. C., Myhre, G., Nganga, J., Prinn, R., Raga, G., Schulz, M., and Van Dorland, R.: Changes in Atmospheric Constituents and in Radiative Forcing, in: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M., and Miller, H. L., Cambridge University Press, Cambridge, UK and New York, NY, USA, 2007. Frame, D. J., Booth, B. B. B., Kettlebrough, J. A., Stainforth, D. A., Gregory, J. M., Collins, M., and Allen, M. R.: Constraining climate forecasts: The role of prior assumptions, Geophys. Res. Lett., 32, L090702, doi:10.1029/2004GL022241, 2005. Hegerl, G. C., Zwiers, F. W., Braconnot, P., Gillett, N. P., Luo, Y., Marengo Orsini, J. A., Nicholls, N., Penner, J. E., and Stott, P. A.: Understanding and Attributing Climate Change, in: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M., and Miller, H. L., Cambridge University Press, Cambridge, UK and New York, NY, USA, 2007. Kiehl, J. T.: Twentieth century climate model response and climate sensitivity, Geophys. Res. Lett., 34, L22710, doi:10.1029/2007GL031383, 2007. Knutti, R., Stocker, T. F., Joos, F., and Plattner, G.-K.: Constraints on radiative forcing and future climate change from observations and climate model ensembles, Nature, 416, 716–723, 2002. Levitus, S., Antonov, J. I., Boyer, T. P., and Stephens, C.: Warming of the world ocean, Science, 287, 2225–2229, 2000. Ramaswamy, V., Boucher, O., Haigh, J., Hauglustaire, D., Haywood, J., Myhre, G., Nakajima, T., Shi, G.-Y., and Solomon, S.: Radiative forcing of climate change, in: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK and New York, NY, USA, 349–416, 2001. Wigley, T. M. L., Amman, C. M., Santer, B. D., and Raper, S. P.: Effect of climate sensitivity on the response to volcanic forcing, J. Geophys. Res., 110, D09107, doi:10.1029/2004JD005557, 2005.