Geological evidence indicates that the oceans contained greater amounts of dissolved minerals 3 billion years ago, but not enough to alter their freezing points significantly. Geological studies indicate that there is much less methane in Earth's atmosphere today than there was 3 billion years ago. Sufficient heat to keep the oceans liquid 3 billion years ago could not have been generated through geological processes such as volcanic activity. Because the distance from Earth to the Sun has not changed significantly over the last 3 billion years, the increase in the Sun's luminosity has resulted in more radiation reaching Earth.
About 3 billion years ago, the Sun was only 80 percent as luminous as it is currently. Such conditions today would result in the freezing of Earth' s oceans, but geological evidence shows that water rather than ice filled the oceans at that time. Heat is trapped within Earth' s atmosphere through the presence of carbon dioxide, which, like methane, is a "greenhouse gas. " Only if the level of greenhouse gases were higher 3 billion years ago than it is today would Earth have retained enough heat to keep the oceans from freezing. It is likely, ttherefore, that the level of carbon dioxide in the atmosphere was significantly higher then than it is today.
Which one of the following, if true, weakens the argument?