Modelling the equilibrium and transient responses of global temperature to past and future trace gas concentrations

Abstract

Recent works with energy balance climate models and oceanic general circulation models have assessed the potential role of the world ocean for climatic changes on a decadal to secular time scale. This scientific challenge is illustrated by estimating the response of the global temperature to changes in trace gas concentration from the pre-industrial epoch to the middle of the next century. A simple energetic formulation is given to estimate the effect on global equilibrium temperature of a fixed instantaneous radiative forcing and of a time-dependent radiative forcing. An atmospheric energy balance model couple to a box-advection-diffusion ocean model is then used to estimate the past and future global climalic transient response to trace-gas concentration changes. The time-dependent radiative perturbation is estimated from a revised approximate radiative parameterization, and the recent reference set of trace gas scenarios proposed by Wuebbles et al. (1984) are adopted as standard scenarios. Similar computations for the past and future have recently been undertaken by Wigley (1985), but using a purely diffusive ocean and slightly different trace gas scenarios. The skill of the socalled standard experiment is finally assessed by examining the model sensitivity of different parameters such as the equilibrium surface air temperature change for a doubled CO2 concentration [ΔT ae (2×CO2)], the heat exchange with the deeper ocean and the trace gas scenarios. For ΔT ae (2×CO2) between 1 K and 5 K, the following main results are obtained: (i) for a pre-industrial CO2, concentration of 270 ppmv, the surface air warming between 1850 and 1980 ranges between 0.4 and 1.4 K (if a pre-industrial CO2 concentration of 290 ppmv is chosen, the range is between 0.3 and 1 K); (ii) by comparison with the instantaneous equilibrium computations, the deeper ocean inertia induces a delay which amounts to between 6 years [for lower ΔT ae (2×CO2)] and 23 years [for higher ΔTae(2×CO2)] in 1980; (iii) for the standard future CO2 and other trace gas scenarios of Wuebbles et al., the surface air warming between 1980 and 2050 is calculated to range between 0.9 and 3.4 K, with a delay amounting to between 7 years and 32 years in 2050 when compared to equilibrium computations.