Further Implications to the CHZ for Complex Life

The surface temperature of a planetary surface is a function of the recepted solar flux, planetary albedo, and levels of greenhouse gas. The inner boundary of the CHZ for complex life is determined by a solar flux relative to the present, Seff, equal to 1.13, corresponding to a surface temperature (T) of 60°C at pCO2 = 0 (Caldeira and Kasting 1992b), because this is the limit to biotically mediated cooling (except perhaps cooling arising from a Love-lockian biotic effect on the planetary albedo). Thus, the inner boundary of the CHZ is close to that for water loss in a few hundred million years (Seff = 1.10; Kasting et al. 1993).

The results of an abiotic Earth/sun system is shown in figure 10-1, computed assuming our preferred model b for volcanic outgassing and land area as a function of time (see chapter 8 Appendix). This CHZ is limited by the boundaries for water loss (inner, D = 0.95 astronomical units [AU]) and first CO2 condensation (outer, D = 1.15) (Kasting, Whitmire, and Reynolds 1993). The scenario of cooling from dry ice clouds in CO2-rich atmosphere has been challenged (Forget and Pierrehumbert 1997). Note that t, the time (in billions of years) since the emergence of complex life (Ts < 60°C), varies inversely with the assumed B, the present biotic enhancement of weathering factor, for a given D, the distance of the planet from the sun (AU) (figure 10-2); i.e., the higher the assumed value of B on a biotic Earth, the higher

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