Earlier (Section 9.3.1), it was noted that the climate response varies from model to model even when the radiative forcing used to drive the models is similar. This difference in the climate models’ response is mainly the result of differing climate sensitivities and differing rates of heat uptake by the oceans in each model, although differences in the AOGCM radiative forcing for a given CO2 concentration also have a small effect (see Chapter 6, Section 6.3).
The effective climate sensitivity and ocean heat uptake are compared by Raper et al. (2001b) using the CMIP2 data set (1%/yr CO2 increase to doubling). The effective climate sensitivities around the time of CO2 doubling (average for the years 61 to 80), when the signal is strongest, agree reasonably well with the mixed-layer equilibrium climate sensitivities given in Figure 9.20. Results are shown for various models in Figure 9.20 It is evident that the models with high effective climate sensitivity also tend to have a large net heat flux into the ocean. This oceanic heat flux causes a delay in the climate response. The relationship between the effective climate sensitivity and the oceanic heat uptake was first described by Hansen et al. (1984, 1985) using a box diffusion model. Raper et al. (2001b) show that an additional ocean-feedback is possibly associated with the warming and freshening of the high latitude surface waters that enhances this relationship. Details of the individual model’s sub-grid scale parametrizations also affect both the effective climate sensitivity and the oceanic heat uptake (Weaver and Wiebe, 1999). The evident relationship between effective climate sensitivity and ocean heat uptake leads to the transient climate response (TCR) having a smaller spread among the model results than the climate models’ climate sensitivity alone would suggest (see Section 9.3.1). Since the oceanic heat uptake is directly related to the thermal expansion, the range for thermal expansion is correspondingly increased due to the compensation noted above (see Chapter 11 for a complete discussion of sea level rise).
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Figure 9.20: Comparison of CMIP2 model results for 20-year average values centred on year 70, the time of CO2 doubling. Values are shown for the effective climate sensitivity, the net heat flux across the ocean surface multiplied by the ocean fraction and the global mean temperature change (TCR). |
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