A model study of the time evolution of climate at the secular time scale



Numerical experiments are carried out with a two-dimensional zonally averaged climate model in order to investigate the transient response of the climate of the Northern Hemisphere to the solar and greenhouse-gas forcings at the secular time scale. The atmospheric component of the model is based on the two-level quasi-geostrophic potential vorticity system of equations. At the surface, the model has land-sea resolution and incorporates detailed snow and sea-ice mass budgets. The upper ocean is represented by an integral mixed-layer model in which meridional convergence of heat is parameterized by a diffusive law. For the simulation of the transient response of climate to continuously changing forcings, the uptake of heat perturbations by the deep ocean is approximated as vertical diffusion. A comparison between the computed and observed present climates shows that the model does reasonably well in simulating the seasonal cycle of various climatic variables. In the experiments performed here, we consider the solar irradiance changes, parameterized from recent satellite observations using the Wolf number as a basis, as well as the variations in solar radiation caused by the changes in the Earth's orbital elements. The former induces in our model a warming of about 0.005 K between the time intervals 1765-1875 and 1876-1990, while the latter is responsible for a cooling of about 0.003 K. These changes appear weak compared to the greenhouse-gas-induced warming simulated by the model.

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