density increases and the core shrinks slightly. This causes the core to heat up, which increases the Sun’s energy output. Running the standard models backward suggests that at the time of Earth’s formation, the Sun was only 70 percent as bright as it is now (Figure 3-1). At the same time however, the Sun was more active, possibly because it rotated three times faster than today, and the rotation helps create the dynamo that drives solar activity, thereby generating stronger magnetic fields (Noyes, 1982). Solar irradiance variations were likely irregular, with large dark spots dominating the surface. Total irradiance therefore likely varied inversely with activity. Although total brightness was lower overall, the ultraviolet (UV) irradiance of the early Sun may have been much higher than today. Solar rotation has slowed because of the loss of mass (and angular momentum) in the solar wind. Solar activity level has decreased because of the resultant decrease in dynamo action, and activity cycles have become more regular.

Today the Sun is middle aged with an anticipated remaining life of about 4 billion years. It rotates once every 27 days and it is brighter, not dimmer, when it is more active and has more spots. This is because mag-

FIGURE 3-1 The Faint Young Sun Paradox. Solid line is solar luminosity relative to present (S/S0). Ts is Earth’s surface temperature and Te is Earth’s effective radiating temperature. Thick vertical bars are glaciations. SOURCE: Modified from Kasting and Catling (2003).



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