"Evidence now supports astronomers’ belief that the sun’s power comes from the fusion of hydrogen into helium deep in the sun’s core, but there is a huge problem. As the hydrogen fuses, it should change the composition of the sun’s core, gradually increasing the sun’s temperature. If true, this means that the earth was colder in the past. In fact, the earth would have been below freezing 3.5 billion years ago, when life supposedly evolved.
The rate of nuclear fusion depends upon the temperature. As the sun’s core temperatures increase, the sun’s energy output should also increase, causing the sun to brighten over time. Calculations show that the sun would brighten by 25% after 3.5 billion years. This means that an early sun would have been fainter, warming the earth 31°F (17°C) less than it does today. That’s below freezing.
Over the years scientists have proposed several mechanisms to explain away this problem. These suggestions require changes in the earth’s atmosphere. For instance, more greenhouse gases early in earth’s history would retain more heat, but this means that the greenhouse gases had to decrease gradually to compensate for the brightening sun.
None of these proposals can be proved, for there is no evidence. Furthermore, it is difficult to believe that a mechanism totally unrelated to the sun’s brightness could compensate for the sun’s changing emission so precisely for billions of years."
AIG
