R. T. Pierrehumbert
We offer some remarks on the greenhouse effect due to high clouds which
thermal infrared radiation, but do not absorb or emit it. Such clouds are an idealization of
the CO2 -ice clouds which are thought to have existed early in the history of Mars.
Clouds of this type enter also in the ability of the Earth to recover from a globally
glaciated "cold start," and in the determination of habitable zones of planetary systems.
A simplified model of cloud optical effects is used to estimate the effect of high CO2 -ice
clouds on the planetary radiation budget in the solar and infrared spectrum. It is argued
that the scattering greenhouse effect certainly cancels out a large part of the cooling effect
due to the cloud's visible albedo, and in some circumstances may even lead to a net
warming as compared to the no-cloud case. Speculative implications for the climate of
Early Mars are discussed.
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