Figure 4.27   Snowball Earth scenario. (a) Continents are near the equator, increasing precipitation removes CO2 from the atmosphere, and with falling temperatures ice begins to spread from the poles. (b) Ice continues to spread with temperatures further reduced by the albedo (refl ection of solar energy) effect. (c) Atmospheric CO2 increases due to volcanic activity, prompting a reversal in temperatures. (d) Greenhouse conditions return and the ice sheets recede. (Courtesy of Jørgen Christiansen and Svend Stouge.)

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