Objective estimation of the probability density function for climate sensitivity

Abstract

Abstract. The size and impacts of anthropogenically induced climate change (AICC) strongly depend on the climate sensitivity, AT2x. If AT2x is less than the lower bound given by the Intergovernmental Panel on Climate Change (IPCC), 1.5øC, then AICC may not be a serious problem for humanity. If AT2x is greater than the upper bound given by the IPCC, 4.5øC, then AICC may be one of the most severe problems of the 21st century. Here we use a simple climate/ocean model, the observed near-surface temperature record, and a bootstrap technique to objectively estimate the probability density function for AT2x. We find that as a result of natural variability and uncertainty in the climatic radiative forcing, the 90% confidence interval for AT2x is 1.0øC to 9.3øC. Consequently, there is a 54% likelihood that AT2x lies outside the IPCC range.

Efficiently Constraining Climate Sensitivity with Ensembles of Paleoclimate Simulations

“…For our ensemble of highest quality models, which have comparable skill to the control run, more than half of the members have sensitivity greater than 6°C. Although our ensembles cover a substantial range of sensitivities, the median values contrast strongly with the bulk of previous work, which suggests a most likely value for climate sensitivity of around or below 4°C (Andronova and Schlesinger 2001;Knutti et al 2002;Murphy et al 2004).…”

“…Evaluating the uncertainty in model forecasts of anthropogenically-forced climate change was identified as a high priority in the IPCC TAR (Houghton et al 2001) but despite much effort, substantial uncertainty remains. Analyses of recent climate change data appear to indicate that climate sensitivity (the equilibrium temperature response to a doubling of atmospheric carbon dioxide) has a substantial probability of being well in excess of 6°C (Andronova and Schlesinger 2001;Gregory et al 2002;Knutti et al 2002), and a recent massive ensemble of GCM simulations illustrated that such extreme behaviour can also be exhibited by stateof-the-art models which simulate the present annuallyaveraged climate state reasonably well (Stainforth et al 2005). The behaviour of a coupled ocean-atmosphere model on climatological time scales is highly dependent on the details of parameterisations which cannot be accurately determined from theory or direct observations.…”

The Relationship Between Radiative Forcing and Temperature: What Do Statistical Analyses of the Instrumental Temperature Record Measure?

“…These estimates indicate that doubling atmospheric carbon dioxide would raise surface temperature by 1.7-3.5 • C (Kaufmann and Stern, 2002;Kaufmann et al, 2006). This range is consistent with estimates for T 2x of 3.5 ± 0.9 • C reported by Cubasch and Meehl (2001), but are considerably smaller than the range reported by Forest et al (2002) who argue that the 5 to 95% confidence interval for T 2x lies between 1.4 • and 7.7 • K and Andronova and Schlesinger (2001) who argue that the 90 percent confidence interval for T 2x is 1 to 9.3 • C.…”

Linking climate change modelling to impacts studies: recent advances in downscaling techniques for hydrological modelling

“…Other studies (e.g. Andronova and Schlesinger, 2001;Forest et al, 2002;Knutti et al, 2002;Stott and Kettleborough, 2002) considered results from only one climate model and assessed uncertainties in emissions scenario (radiative forcing), 'climate sensitivity' (the equilibrium global-mean warming for a doubling of the CO 2 level) and the rate of oceanic mixing or heat uptake.…”