According to standard models of the sun, the sun's luminosity has
increased by 40 percent since the origin of the earth (the "faint young sun
paradox"). This would mean that the early earth could not have supported
life. Thus an old earth is impossible; the earth must be young.
The change in luminosity is not as drastic as it sounds. Much of the
change would have occurred before the origin of life; the luminosity
increase since the origin of life is about 25 percent. And this
translates to a 7 percent increase in temperature when the earth's heat
outflow is taken into account.
The 7 percent change described above assumes no feedback system, but
the earth's climate feedback systems are complex. In particular, the
greenhouse effect and albedo could moderate the temperature further.
On the early earth, it is likely that greenhouse gases such as carbon
dioxide and methane were commoner than they are today.
Life has survived fairly large changes in climate over its history,
from a near-global glaciation in the late Precambrian, to a global
temperature warmer than today's in the Carboniferous.
