The Carbon Cycle - Introduction to Carbon Capture and Sequestration - page 118

Faint young sun paradox
Figure 3.4.1 compiles important events in the history of the earth. Its age
is about 4.6 Ga (giga annum) and geological evidence has shown that for
4.4 Ga, liquid water has existed on the earth. This implies that the surface
temperature of the earth must have been in the 0-100°C range for over 4
billion years. At fi rst glance the presence of water on the earth this long
ago appears to be inconsistent with the fact that the sun was still young,
and thus much cooler, all those billions of years ago.
As a star gets older it gets denser and hence brighter. A brighter star
has a higher temperature. As we have seen in the previous chapter, the
amount of energy the earth receives from the sun depends on the tem-
perature at which this radiation is emitted. A faint young star emits less
energy than an otherwise identical older star. In Figure 3.4.2 the conse-
quence of this difference in energy is plotted for the history of the earth
[3.10]. This graph shows that at the beginning of the earth the sun had
about 70% of its current energy output. It also shows how this change in
luminosity of the sun would be expected to infl uence the surface
Figure 3.4.1 The history of the earth
Figure from Andrée Valley and the University of Wisconsin-Madison, with permission.