We already know that greenhouse gases were higher 3 billion years ago, which is what helped keep the Earth warmer. The original source of the heat doesn’t change the fact that we know greenhouse gases were higher.

This raises the possibility that the greater levels of greenhouse gases 3 billion years ago were due to higher levels of methane. Carbon dioxide could have been equal or lower than it is today, because methane was higher.

We already know that the ocean was liquid and that this proves greenhouse gases were higher 3 billion years ago. The freezing point of water doesn’t change how we interpret these facts.

How the sun’s brightness correlates with the complexity of life is irrelevant. The argument concerns levels of greenhouse gases and whether higher levels of greenhouse gases imply higher levels of carbon dioxide.

We already know the Earth was liquid and that this proves greenhouse gases were higher 3 billion years ago. Levels of radiation don’t change these facts or how we interpret them. The argument concerns whether higher levels of greenhouse gases imply higher carbon dioxide levels.