Climate models of the type used here incorporate very simplified representations of the physical processes involved. Nevertheless, they seem to simulate well the responses to external forcing produced by more complex models. They account for the lag eff ect of oceanic thermal inertia, and they allow one to assess the sensitivity of the results to model uncertainties, in particular to uncertainties in the climate sensitivity. The present model additionally differentiates between the hemispheres and so pe rmits the consideration of forcing changes specific to one hemisphere, as is necessary if we are to account for the effects of sulphate aerosol forcing.
The most important model parameter is the climate sensitivity which is specified by the equilibrium global-mean temperature change for a CO2 doubling. Largely on the basis of general circulation model experiments, this temperature change, [Del ta] T2x, is thought to lie in the range 1.5-4.5°C, with a 'best-guess' value of 2.5°C (ref. 1). The other parameters affecting the model's response are the mixed-layer depth (h), the oceanic vertical diffusivity (K), the upwelling ra te (w) and the temperature change of high-latitude sinking water relative to the global-mean change (¼). Sensitivity to these parameters is less than to [Delta] T2x (ref. 35). We assume h = 90 m, K = 1 cm2s-1, w = 4 m yr-1 and ¼ = 0.2. Slightly different values were used in IPCC90. The set of values used here is justified more fully in ref. 36. The greatest difference from IPCC is in our choice of ¼ (IPCC9037 used ¼ = 1). Our choice of a lower value is supported by Schlesinger and Jiang38. Smaller ¼ produces larger surface warming and smaller oceanic thermal expansion. The net effect on sea level change is a small reduction.
Figure 3 shows the radiative forcing to 1990 and the corresponding modelled temperature changes, both for the global mean and for the Northern Hemisphere. Modelled temperature changes are shown only for [Delta] T2x = 2.5°C. For comparison, observed temperature changes, as recently updated by IPCC39, are also shown.
If ozone-depletion feedback and best-guess sulphate aerosol effects are included, then the total forcing change to 1990 is reduced markedly (by ~1 Wm-2 or 40%) from the IPCC90 estimate14. This brings the predictions of temperature c hange using [Delta] T2x = 2.5°C into closer agreement with observations. In the absence of these two factors, the modelled changes with [Delta] T2x = 2.5°C are appreciably larger than the observed changes; the value of [De lta] T2x giving the best fit to the 1880-1990 warming is only 1.5°C (compared with 1.4°C in ref. 36, which used slightly different temperature data). For the case with ozone-depletion feedback and best-guess aerosol forcing, the mod elled changes using [Delta] T2x = 2.5°C are noticeably less than the observed changes. The required sensitivity to obtain the best fit to the observations is [Delta] T2x = 3.4°C. If only best-guess aerosol forcing is incl uded (without ozone-depletion feedback), our best-guess empirical value for the sensitivity is 3.0°C (compare with ref. 40 for similar assumed forcing). The inclusion of the aerosol forcing flattens the model-projected warming trend over 1950-70, es pecially in the Northern Hemisphere, in qualitative agreement with observations.
Figure 3 also indicates the possible range of values for the negative aerosol forcing. Our largest negative values, which are at the smaller end of the range given by Charlson et al.10, lead to an overall negative forcing trend over 186 0-1980 in the Northern Hemisphere. As a consequence, the modelled temperature change for the Northern Hemisphere over this period is much less than observed, and, more important, there is a marked difference in warming between the Northern and Southern h emispheres which is not evident in the observations. It is possible that differential warming has been masked by natural variability, but this seems unlikely. On empirical grounds, therefore, and in accord with refs 29 and 40 and three-dimensional model ling results8, our choice for the range of likely aerosol forcing is more defensible than the values suggested in ref. 10 which were based on a simpler one-dimensional analysis.
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