9.6.2.2 Estimates Based on Individual Volcanic Eruptions

Some recent analyses have attempted to derive insights into ECS from the well-observed forcing and response to the eruption of Mt. Pinatubo, or from other major eruptions during the 20th century. Such events allow for the study of physical mechanisms and feedbacks and are discussed in detail in Section 8.6. For example, Soden et al. (2002) demonstrate agreement between observed and simulated responses based on an AGCM with a climate sensitivity of 3.0°C coupled to a mixed-layer ocean, and that the agreement breaks down if the water vapour feedback in the model is switched off. Yokohata et al. (2005) find that a version of the MIROC climate model with a sensitivity of 4.0°C yields a much better simulation of the Mt. Pinatubo eruption than a model version with sensitivity of 6.3°C, concluding that the cloud feedback in the latter model appears inconsistent with data. Note that both results may be specific to the model analysed.

Constraining ECS from the observed responses to individual volcanic eruptions is difficult because the response to short-term volcanic forcing is strongly nonlinear in ECS, yielding only slightly enhanced peak responses and substantially extended response times for very high sensitivities (Frame et al., 2005; Wigley et al., 2005a). The latter are difficult to distinguish from a noisy background climate. A further difficulty arises from uncertainty in the rate of heat taken up by the ocean in response to a short, strong forcing. Wigley et al. (2005a) find that the lower boundary and best estimate obtained by comparing observed and simulated responses to major eruptions in the 20th century are consistent with the TAR range of 1.5°C to 4.5°C, and that the response to the eruption of Mt. Pinatubo suggests a best fit sensitivity of 3.0°C and an upper 95% limit of 5.2°C. However, as pointed out by the authors, this estimate does not account for forcing uncertainties. In contrast, an analysis by Douglass and Knox (2005) based on a box model suggests a very low climate sensitivity (under 1°C) and negative climate feedbacks based on the eruption of Mt. Pinatubo. Wigley et al. (2005b) demonstrate that the analysis method of Douglass and Knox (2005) severely underestimates (by a factor of three) climate sensitivity if applied to a model with known sensitivity. Furthermore, as pointed out by Frame et al. (2005), the effect of noise on the estimate of the climatic background level can lead to a substantial underestimate of uncertainties if not taken into account.

In summary, the responses to individual volcanic eruptions provide a useful test for feedbacks in climate models (Section 8.6). However, due to the physics involved in the response, such individual events cannot provide tight constraints on ECS. Estimates of the most likely sensitivity from most such studies are, however, consistent with those based on other analyses.