About 3 billion years ago, the Sun was only 80 percent as luminous as it is currently. Such conditions today would re...

on October 22 at 04:31AM

B

So even though there is significantly less methane in the earth's atmosphere today than there was 3 billion years ago, that does not mean that there were not more carbon dioxide in thee earths' atmosphere 3 billion years ago than there is today. Shouldn't the correct answer choice also mention the reduction in carbon dioxide?

1 Reply

Irina on October 22 at 09:12PM

@tomgbean,

This is a weaken question meaning the correct answer undermines the argument rather than renders it invalid as with necessary assumption questions. The fact that there was more methane - which is also a greenhouse gas - opens up a possibility that oceans could be liquid even without higher level of carbon dioxide in the atmosphere, but given the significantly lower sun luminosity, it is also possible that higher levels of both methane and CO2 were required to keep the oceans liquid. We have no additional information to make this determination, but the fact that methane levels were higher alone would make the conclusion less certain, thereby weakening the argument.