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A hemispheric general circulation model set up for annual mean conditions has been used to investigate the consequences of varying the obliquity from 23.5* to 0* and 65*. Such obliquity values have been claimed to have occurred in past eras. The 0* obliquity climate is more vigorous than the existing climate, but also drier and colder at the surface at high latitudes. However, overall a slight warming of the troposphere occurred in the model. Two experiments were performed for 65* obliquity, both had low albedoes at high latitudes, while one had glacial albedoes at low latitudes. For annual mean conditions there was virtually no tropospheric latitudinal temperature gradient in the first experiment, and consequently greatly reduced mean zonal winds and baroclinic activity. The second experiment had minimum surface temperatures in the tropics, but still sufficiently high to prevent glacial conditions prevailing, and consequently a tropical easterly jet maintained by a "reversed Hadley cell" . Other aspects such as the simulated hydrology, energy exchanges, etc. provide considerable enlightenment concerning climatic conditions at such extreme obliquity. It is concluded that the Earth's habitable zone would be reduced for both 0* and 65* obliquity. These experiments illustrate the considerable unexploited potential of general circulation models in evaluating hypothetical climatic states which have veen proposed at various times.

The Impact of Large Variations of the Earth's Obliquity on the Climate