Passage Summary: Even though the sun was weaker long ago, the oceans didn't freeze. The author thinks this means there must have been way more carbon dioxide in the air back then to keep the planet warm.

Conclusion: Atmospheric carbon dioxide levels were likely much higher 3 billion years ago than they are today.

Reasoning: Because a dimmer sun would have frozen the oceans without greenhouse gases to trap heat, and liquid water existed, the author concludes CO2 must have been the specific gas responsible for that heat retention.

Analysis: The argument relies on a very strong 'only if' claim, asserting that higher greenhouse gas levels were the sole way to keep the oceans liquid. To weaken this, look for an alternative explanation for the warmth that doesn't involve high CO2 levels. For instance, if another gas like methane was the primary heat-trapper, the conclusion about CO2 becomes much less certain. The author mentions methane earlier but then conveniently ignores it to pin the blame—or credit—entirely on carbon dioxide.