#25 - About 3 billion years ago, the Sun was only 80

[smile22]

In this question, I know that I must weaken the fact that the level of CO2 in the atmosphere 3 billion years ago was higher than it is today. My prephrase was to show that the level of CO2 in the atmosphere 3 billion years ago was not higher (so lower or equal) to what it is today. Using that prephrase did not allow me to pick the right answer, however. Is there a better prephrase that I could have used that would have allowed me to correctly select answer B?

[Emily Haney-Caron]

My prephrase for this question would be, "There is some other explanation for the oceans not being frozen, other than CO2 levels being higher." When I got to answer B, I would think, "hey, if methane is a greenhouse gas, maybe we used to have higher methane levels rather than higher CO2 levels." Going back to the question, I find that methane is a greenhouse gas. So, there used to be a lot more methane, which is a greenhouse gas, and that is why the oceans didn't freeze.

Does that help answer the question?

[NeverMissing]

I have a bit of an oddball question about one of the premises of this argument. I noted that the key premise of the argument used to justify its conclusion noted that only if "greenhouse gasses" were higher 3 billion years ago than today would the oceans not have froze. The plural form of gas--gasses--tripped me up a bit. Does this refer to two or more distinct types of greenhouse gasses (ex. methane and carbon dioxide), or does this refer to two or more particles of any greenhouse gas (ex. a bunch of particles of carbon dioxide in the air)?

I assumed at first that "gasses" plural referred to two or more distinct types of gasses, but it seems from the conclusion of the argument that perhaps it wants us to interpret "greenhouse gasses" to mean two or more particles of a greenhouse gas, in this case carbon monoxide.

In other words, is the premise saying this: Only if the level of two or more distinct types of greenhouse gasses were higher 3 billion years ago than it is today would the Earth retain enough heat to prevent ocean freezing.

Or this: Only if the level of greenhouse gas particles from any greenhouse gas were higher 3 billion years ago than it is today would the Earth retain enough heat to prevent ocean freezing.

[Kristina Moen]

Hi there NeverMissing,

This is a good example of an Error of Composition/Division flaw.
"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" can be diagrammed as NO FREEZE higher level of greenhouse gases.

We're told that the oceans did not freeze. So we can infer that the level of greenhouse gasses were higher.

But the author doesn't stop there. The author infers that carbon dioxide was significantly higher. But carbon dioxide is just one greenhouse gas, like methane. We can't infer that because the overall level of greenhouse gases was higher, that any particular element was higher. So it's an Error of Composition/Division.

We're being asked to weaken this conclusion, which we can do in a couple of ways. We can show that there's ANOTHER element of greenhouse gases that was higher back then (or is lower now, which means the same thing). You can show that carbon dioxide is a very small percentage of the total level of greenhouse gases. Or you can have an answer choice that shows that carbon dioxide was lower or the same level back then. Basically, you want an answer choice that shows how even though the level of greenhouse gases was higher 3 billion years ago, that doesn't mean carbon dioxide was higher back then.

Answer choice (B) does this by showing that methane was higher back then, which would have increased the total level of greenhouse gases without any change in levels of carbon dioxide.