netic fields produce not only dark spots but also bright faculae, whose brightening effects exceed (by a factor of two) the sunspot dimming. As it ages further, the Sun’s rotation rate will continue to decrease, but much more slowly because the solar wind is weak. More regular brightness cycles in phase with solar activity and dominated increasingly by bright faculae are expected (Baliunas et al., 1995).

The weak solar luminosity on the early Earth has posed a long-standing problem whose solution may implicate the greenhouse effect of CO₂ as a long-term temperature regulator. Climate models suggest that for a 30 percent weaker Sun and a modern greenhouse gas concentration, water would have been frozen from 4.5 Ga (billion years ago) until about 2.3 Ga (Sagan and Mullen, 1972), yet geological evidence from well before 2.3 Ga documents a vigorous hydrologic cycle and a diverse marine biota (Sagan and Mullen, 1972; Kasting; 1993; Kump et al., 2000). This contradiction has come to be known as the Faint Young Sun Paradox (Figure 3-1).

The most widely accepted solution to the paradox, based both on models and geological data, is that the early Earth was warmed by a high concentration of greenhouse gases, probably mainly CO₂, perhaps in the range of a few hundred to 1000 times present atmospheric levels (Kump et al., 2000). To explain such high levels of atmospheric CO₂, most attention has focused on the dominant process that draws down atmospheric CO₂ relative to production on geological timescales: the chemical weathering of continental silicate rocks illustrated in the following chemical reaction (Kasting, 1993; Kump et al., 2000; Ruddiman, 2001).

Because silicate weathering rates vary with temperature (by a factor of about two for each 10°C) and area of exposed continental silicate rocks (Ruddiman, 2001), it has been argued that on a cold early Earth with small continental mass (Kump et al., 2000), the negative feedback from severely reduced silicate weathering rates would have led to a build up of volcanic CO₂.

However, if the large CO₂ greenhouse effect remained unchanged as