Abstract

6 [1] The problem of deglaciating a globally ice-covered (‘‘hard snowball’’) Earth is 7 examined using a series of general circulation model simulations. The aim is to determine 8 the amount of CO 2 that must be accumulated in the atmosphere in order to trigger 9 deglaciation. Prior treatments of this problem have been limited to energy balance models, 10 which are incapable of treating certain crucial physical processes that turn out to 11 strongly affect the conditions under which deglaciation can occur. CO2 concentrations up 12 to.2 bars are considered in the general circulation model simulations, and even at such 13 high CO 2 content the model radiation code is found to perform well in comparison 14 with codes explicitly designed for high CO2. In contrast to prevailing expectations, the 15 hard snowball Earth is found to be nearly 30 K short of deglaciation, even at.2 bars. The 16 very cold climates arise from a combination of the extreme seasonal and diurnal cycle, 17 lapse rate effects, snow cover, and weak cloud effects. Several aspects of the atmospheric 18 dynamics are examined in detail. The simulations indicate that the standard scenario, 19 wherein snowball termination occurs after a few tenths of a bar of CO2 has built up 20 following cessation of weathering, is problematic. However, the climate was found to be

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