is somewhat colder than Ts at the poles and somewhat warmer than Ts at the Equator. When Ts is very large, say greater than some threshold temperature To, the temperature is above freezing everywhere and there is no ice. In this temperature range, the planetary albedo reduces to the relatively low value (call it ao) characteristic of sea water. At the other extreme, when Ts is very, very low, the whole planet is below freezing, the ocean will become ice-covered everywhere, and the albedo reduces to that of sea ice, which we shall call a^. We suppose that this occurs for Ts < Tj, where Tj is the threshold temperature for a globally frozen ocean. In general Tj must be rather lower than the freezing temperature of the ocean, since when the mean temperature Ts = Tfreeze the equatorial portions of the planet will still be above freezing. Between Tj and To it is reasonable to interpolate the albedo by assuming the ice cover to decrease smoothly and monotonically from 100% to zero. The phenomena we will emphasize are not particularly sensitive to the detailed form of the interpolation, but the quadratic interpolation
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