|Early Faint Sun Paradox|
(Aka Faint Sun Paradox) The paradoxical geological evidence that liquid water existed at a time early in earth's history when the Sun's luminosity was so low that the planet should have been frozen. This paradox is usually resolved by assuming that there must have been a very large amount of CO2 in the atmosphere, and that this caused a heightened greenhouse effect.
The skeptic interpretation
According to skeptics, this phenomenon suggests that there is at least one very important mechanism for how the earth's temperature is regulated that we don't understand, e.g., cloud formation in the tropics might put a limit on how hot or cold the earth can get.
Here's Richard Lindzen on the phenomenon:
"Our recent work on the early faint sun may prove particularly important. 2.5 billion years ago, when the sun was 20% less bright (compared to the 2% change in the radiative budget associated with doubling CO2), evidence suggests that the oceans were unfrozen and the temperature was not very different from today's. No greenhouse gas solution has worked, but a negative cloud feedback does."
Example Usages -
1. "The notion that the earth's climate is dominated by positive feedbacks is intuitively implausible, and the history of the earth's climate offers some guidance on this matter. About 2.5 billion years ago, the sun was 20%-30% less bright than now (compare this with the 2% perturbation that a doubling of CO2 would produce), and yet the evidence is that the oceans were unfrozen at the time, and that temperatures might not have been very different from today's. Carl Sagan in the 1970s referred to this as the Early Faint Sun Paradox."
(Source: Richard Lindzen, Wall Street Journal, Nov 30, 2009 )