General circulation model experiments on the climatic effects due to a doubling and quadrupling of carbon dioxide concentration

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Abstract
A global, spectral atmospheric general circulation model (GCM) with an energy balance ocean formulation, sometimes referred to as a swamp ocean, and realistic geography is run for doubled and quadrupled the present amount of atmospheric carbon dioxide, CO2, with fixed and computed clouds. All experiments are compared to control runs with the present amount of CO2. The experiments use annually averaged solar forcing; thus, there is not a seasonal cycle. It is found that globally averaged surface air temperature in the doubled CO2 experiments increases 1.3°C for the fixed cloud formulation and 1.3°C for the computed cloud case. In the quadrupled CO2 experiment with fixed clouds the surface air temperature increase is 2.7°C, and, in the computed cloud it increases 3.4°C. Warming throughout the troposphere is of the order of 1°–2°C for the doubled CO2 cases, and the stratosphere at 30 km cools up to 6°C. In the quadrupled experiment the maximum tropospheric warming is 4°C, and the model stratospheric cooling is 11°C. An analysis of geographic areas of persistent change in snow cover and soil moisture due to a doubling of CO2 for the fixed cloud case shows a general retreat of the snow line and drying of the soil in tropics and mid‐latitudes with increases of snow depth in the polar regions. These results indicate warming due to increased CO2 is smaller than that found in other studies with swamp‐ocean models coupled to atmospheric GCMs. One possible explanation may be attributed to large differences in the various models used, especially with regard to snow‐sea ice albedo parameterizations.