LSAT 131 | Section 2 | Logical Reasoning: Q25

LR Question 25 Explanation

QUESTION TEXT: About 3 billion years ago, the Sun was only 80 percent

QUESTION TYPE: Weaken

CONCLUSION: The level of carbon dioxide was likely much higher three billion years ago than it is today.

REASONING: The sun was weaker 3 billion years ago, which would normally freeze the oceans. But the oceans weren’t frozen.

The earth could only be warmer if there were higher levels of greenhouse gases. Greenhouse gases include carbon dioxide and methane.

ANALYSIS: The evidence shows that greenhouse gas levels must have been higher. But then the conclusion says that carbon dioxide levels must have been higher. The author completely forgot about methane, even though they mentioned it!

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1. The issue isn’t necessarily generating the heat. The issue is retaining the heat. The stimulus is clear that the oceans could only have avoided freezing if greenhouse gas levels were higher.
   Also, this says “could not have been”. That’s a hypothetical. We care about what did happen, not what could have happened.
2. CORRECT. If there is less methane now, then there was more methane back then. Methane is a greenhouse gas. That means methane could have accounted for the increased heat retention: more carbon dioxide may not have been necessary.
3. Well, this is useless. This basically tells us: “an additional factor existed, but it had no impact.”
4. This tells us nothing about heat retention three billion years ago. Instead, it just tells us that life got more complex over the past three billion years.
5. This explains nothing about heat retention three billion years ago. It just tells us that the earth has been receiving more energy from the sun recently (which the stimulus already strongly implied.)