The Faint Young Sun Paradox
Our star, the Sun, started its life when hydrogen first ignited to make helium, at its center where pressure and temperature are greatest. The explosion attempted to tear the star apart. But the inward pressure generated by gravity prevented this. Since then, the two opposing forces from the ongoing fusion and from gravity have been more or less balanced. However, things change through time, as the hydrogen is used up. A middle-aged star such as our Sun has built up a core of helium. Thus, the hydrogen fusion takes place farther out from the center, which increases energy production and produces a hotter surface. The effect can be calculated according to the laws of physics, and from this it is widely accepted that the early sun was some 70% cooler than now. If this was so, the Earth's average temperature should have been 25°C (45°F) colder and the surface of our planet should have been covered by thick ice. (For a discussion of the "Faint Young Sun" Paradox, see:
Courtesy: University of Bristol
The paradox lies in the fact that we know from the geological and fossil record that the Earth was not iced over in its early stages of history. Sedimentary rocks are layered showing the presence of flowing water, for example. Ice-age deposits are no more abundant, apparently, than later in Earth history.
There is one notable exception to the above statement. Some Earth scientists have proposed that the planet was subject to major icing over some 600 million years ago, that lasted about 10 million years. This theory is known as "Snowball Earth". It suggests that the ice cover on the ocean was about 1 km thick, sealing off the ocean from the atmosphere. Extensive vulcanism, releasing copious amounts of greeen house gases is postulated to have brought an end to this frozen era. Perhaps, to cope with the rigors of this awful climate, multicellular organisms had to evolve rapidly, setting the stage for the subsequent proliferation of fossils observed in the Cambrian era. The Sun was no longer faint 600 million years ago. So if there was a snowball planet, we have to look for other reasons to drop global temperature. The "Snowball Earth" hypothesis is by no means established. While it is generally accepted that Earth suffered widespread glaciation in the late Precambrian, there is no evidence for a thick ice cover on the ocean.
The conventional resolution to the Faint Sun paradox is that greenhouse gases were much more abundant in the atmosphere, in the distant past. High levels of carbon dioxide (and early on perhaps methane) would have kept the Earth warm even though much less energy came in from the Sun. The Faint Sun paradox is the main impetus for the "Gaia" concept originated by the British engineer and science writer James Lovelock (b. 1919). (Lovelock first detected chlorofluorocarbons in the atmosphere, which helped start the process leading to the Montreal Protocol for the protection of the ozone layer.) The point is, greenhouse gases are regulated by life processes, which are thereby responsible for keeping Earth at a habitable temperature.
(The concern now is that humans are interfering with these regulating processes and raising global temperatures at a time when it is not needed and possibly quite harmful. See our companion course on climate change.)