A standard model of luminosity (L) variation of the sun (Sackmann et al. 1990) has been assumed in my simulations of the long-term climate of Earth. It is generally accepted that the zero age main sequence (ZAMS) solar luminosity, at the initial stage of core hydrogen burning, was about 70% of the present value and the sun's surface temperature was about 3% lower (Bahcall et al. 1982). Although a steadily increasing energy flux over time might seem counterintuitive, it results from the increase in the rate of fusion of H to He in the sun's core as it heats up as it contracts. However, some reports in the literature account for a warm early Earth without the intervention of a greenhouse effect. One of the most prominent is the discussion by Graedel et al. (1991) of a more luminous early sun (a similar argument is made by Doyle et al. 1993). The argument assumes that the mass of the sun was higher during roughly the first billion years of its tenure on the ZAMS. Subsequent mass loss is presumed to have reduced the mass to its present value. Because of the strong dependence of solar luminosity and radius on the star's mass, if the sun were about 10% more massive, it would have been about 50% brighter and about 10% hotter (note that surface temperature and energy flux are not directly proportional). This, they argue, is sufficient to completely account for the initially warm climate of the planet. The idea follows a more general suggestion by Willson et al. (1987) that this sort of mass loss is typical of stars when they initiate hydrogen core burning and slightly before.
However, the solar neutrino flux places severe constraints on any possible mass reduction after the onset on core nuclear processing. If the sun were indeed initially more massive, it would have a more massive core and an even larger deficit of neutrinos expected from fusion reactions than is presumed to be the case (Shore, personal communication). In addition, current models of the solar interior agree with the observed spectrum of surface and envelope oscillations (helioseismology) (Bahcall and Pinsonneault 1992). Thus, despite the claims made by Doyle et al. (1993), there is no empirical support for extended early main sequence mass loss. This scenario of early solar mass loss also has been offered as an explanation of warm temperatures on early Mars, implied by evidence for early liquid water on its surface (Whitmire et al. 1995). It is highly unlikely that the mass of the sun has changed appreciably during its lifetime (Shore, personal communication), and therefore the standard model of L variation has been used for modeling of the long-term carbon cycle.
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