9.6.2.3 Constraints on Transient Climate Response

While ECS is the equilibrium global mean temperature change that eventually results from atmospheric CO₂ doubling, the smaller TCR refers to the global mean temperature change that is realised at the time of CO₂ doubling under an idealised scenario in which CO₂ concentrations increase by 1% yr^–1 (Cubasch et al., 2001; see also Section 8.6.2.1). The TCR is therefore indicative of the temperature trend associated with external forcing, and can be constrained by an observable quantity, the observed warming trend that is attributable to greenhouse gas forcing. Since external forcing is likely to continue to increase through the coming century, TCR may be more relevant to determining near-term climate change than ECS.

Stott et al. (2006c) estimate TCR based on scaling factors for the response to greenhouse gases only (separated from aerosol and natural forcing in a three-pattern optimal detection analysis) using fingerprints from three different model simulations (Figure 9.21) and find a relatively tight constraint. Using three model simulations together, their estimated median TCR is 2.1°C at the time of CO₂ doubling (based on a 1% yr^–1 increase in CO₂), with a 5 to 95% range of 1.5°C to 2.8°C. Note that since TCR scales linearly with the errors in the estimated scaling factors, estimates do not show a tendency for a long upper tail, as is the case for ECS. However, the separation of greenhouse gas response from the responses to other external forcing in a multi-fingerprint analysis introduces a small uncertainty, illustrated by small differences in results between three models (Figure 9.21). The TCR does not scale linearly with ECS because the transient response is strongly influenced by the speed with which the ocean transports heat into its interior, while the equilibrium sensitivity is governed by feedback strengths (discussion in Frame et al., 2005).

Estimates of a likely range for TCR can also be inferred directly from estimates of attributable greenhouse warming obtained in optimal detection analyses since there is a direct linear relationship between the two (Frame et al., 2005). The attributable greenhouse warming rates inferred from Figure 9.9 generally support the TCR range shown in Figure 9.21, although the lowest 5th percentile (1.3°C) and the highest 95th percentile (3.3°C) estimated in this way from detection and attribution analyses based on individual models lie outside the 5% to 95% range of 1.5°C to 2.8°C obtained from Figure 9.21.

Choosing lower and upper limits that encompass the range of these results and deflating significance levels in order to account for structural uncertainty in the estimate leads to the conclusion that it is very unlikely that TCR is less than 1°C and very unlikely that TCR is greater than 3.5°C. Information based on the models discussed in Chapter 10 provides additional information that can help constrain TCR further (Section 10.5.4.5).

Figure 9.21. Probability distributions of TCR (expressed as warming at the time of CO₂ doubling), as constrained by observed 20th-century temperature change, for the HadCM3 (Table 8.1, red), PCM (Table 8.1, green) and GFDL R30 (Delworth et al., 2002, blue) models. The average of the PDFs derived from each model is shown in turquoise. Coloured circles show each model’s TCR. (After Stott et al., 2006c).