Q25 - About 3 billion years ago

by **noah** Mon Jan 11, 2010 6:41 pm

The conclusion of this argument is that the levels of carbon dioxide back then (3 billion years ago) we're probably quite high. Why? Because the oceans remained liquid, showing that the temperature was rather high, even though the sun was not as strong as it used to be. It's a fact, we're told, that the only way to bring about such a temperature back then would have been through a high level of greenhouse gases. . . So, there must have been a lot of CO2.

What's the gap? Why does it have to be CO2? Couldn't it have been methane?

(B) capitalizes on this assumption by suggesting that there used to be more methane (though it's a bit confusing since it seems to be discussing how much methane is in the atmosphere now).

(A) simply rules out another reason that might explain the heat -- if anything, this strengthens the argument.
(C) is irrelevant. The effect mentioned is not a possible alternate explanation.
(D) is out of scope as it relates the sun's strength with life forms.
(E) is also out of scope. We do not know what role increased radiation plays in this argument; furthermore, we don't know when this increase has reached a level that may or may not effect the earth's temperature as the increase is only limited to some time over the last 3 billion years.

I hope that helps.

by **futuretoshow08** Tue Sep 14, 2010 7:00 pm

I understand why all of the wrong answers are wrong. But, I do not understand why the right answer is right.

The question asserts that long ago, the sun was less luminous. Even so, Earth's oceans were unfrozen. This is explained by the fact that the level of greenhouse gases (specifically carbon dioxide) was much greater in the past, which trapped heat in the atmosphere. The question also indicates that methane and carbon dioxide are both greenhouse gases.

Why is (B) correct? B indicates that there is less methane in Earth's atmosphere today than there was in the past.

How does this weaken the argument? Wouldn't this really strengthen the argument? The argument needs to prove that the level of carbon dioxide was higher in the past than it is today. Answer B shows that there was more methane in the past than today.

Methane and Carbon are both greenhouse gases. If greenhouse gases were higher in the past, Earth's oceans would not have frozen.

How is there being more methane 3 billion years ago weakening the argument?

I understand why A C, D, and E are not correct. I just fail to comprehend how B makes the argument weaker.

by **cyruswhittaker** Wed Sep 15, 2010 12:24 am

The conclusion of the argument is limited in such a way as to make it open to attack. The premises indicate that carbon dioxide and methane are both greenhouse gases. That is the only explicit information given about methane.

But then the conclusion states that it is "likely, therefore, that the level of carbon dioxide...was signficantly higher than it is today."

But since the argument just said methane is a greehouse gas and since all we were told was that greehouse gases in general could cause the heat affect, then the author overlooks the fact (assumes) that there wasn't a lot more methane 3 billion years ago compared to today.

If that assumption turns out to be false (which is what choice B explicitly makes clear), then his conclusion does not follow.

by **pinkdatura** Tue Sep 28, 2010 11:52 pm

Thank you, Noah. One one question about C:
Is it wrong because "but not enough to alter their freezing point significantly", the dissolved minerals do not constitute a stronger enough alternative than CO?
If this answer is 'dissolved minerals 3 billion years ago can significantly alter the freezing point", will it be a good candidate as weakener?
thx

by **noah** Thu Sep 30, 2010 12:55 pm

Right - if anything, (C) strengthens by dismissing a possible alternate explanation.

by **mitchliao** Thu Jan 20, 2011 5:26 pm

pinkdatura Wrote:Thank you, Noah. One one question about C:
Is it wrong because "but not enough to alter their freezing point significantly", the dissolved minerals do not constitute a stronger enough alternative than CO?
If this answer is 'dissolved minerals 3 billion years ago can significantly alter the freezing point", will it be a good candidate as weakener?
thx

Also note that it specifically states that "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."

The condition above is necessary. Therefore even if the minerals altered the freezing point significantly, this additional piece of information would not strengthen or weaken the author's line of reasoning.

by **lhermary** Wed Sep 21, 2011 5:04 pm

I'm having a hard time understand why B is right here.

Methane is only mentioned once in the stimulus and all it says is that it is a greenhouse gas similar to carbon dioxide.

If there is a much less methane in the atmosphere today then that doesn't necessarily mean that there is less greenhouse gasses in the atmosphere today.

Thanks geeks for bestowing your wisdom upon us once again 8-)

by **LSAT-Chang** Sat Sep 24, 2011 1:23 pm

lhermary Wrote:I'm having a hard time understand why B is right here.

Methane is only mentioned once in the stimulus and all it says is that it is a greenhouse gas similar to carbon dioxide.

If there is a much less methane in the atmosphere today then that doesn't necessarily mean that there is less greenhouse gasses in the atmosphere today.

Thanks geeks for bestowing your wisdom upon us once again

Hmm.. I'm not quite sure what you mean by your third sentence, but let me try and explain clearly why answer choice (B) weakens the conclusion. The problem with this argument is that the stimulus explicitly states that both carbon dioxide and methane are types of greenhouse gases, then the author says how greenhouse gases (could be both carbon dioxide and methane or some other type) should have been higher 3 billion years ago than today for it to keep the ocean from not freezing. Then the author concludes from this that it was the CO2 that was significantly higher than it is today and which accounted for the defreezing of the ocean. Well.. why is the author all of a sudden ignoring methane??? He/she even stated that methane is also a type of greenhouse gas -- so what makes it more likely that it was the carbon dioxide that accounted for this defreezing and not the methane??? Do you see it? So if we know (B) to be true, which basically says that the level of methane was higher 3 billion years ago then it is now, then it makes it less likely that carbon dioxide was the cause for the defreezing. It could have been just as likely that it was the methane. Does this make sense?

by **abond06** Mon Nov 28, 2011 11:45 am

The questio asks what option weakens the argument... Option B simply does not weaken the argument. Given the info in the set up, it strengthens the argument.

by **kaseyb002** Tue Sep 11, 2012 6:18 pm

To me, this is a false choice flaw. Why does the author think carbon dioxide is the only thing that could cause the increase in greenhouse gases? Why not methane? Why not a million other things?

(B) introduces another possible cause of the higher levels in greenhouse gases (which is a required condition for the oceans not to freeze) 3 billion years ago.

by **nthakka** Wed May 22, 2013 5:13 pm

I chose B by process of elimination but really took a long time on this question during my PT. Really a good example of how you can eliminate 4 crappy AC's to arrive at the right answer, even if you do not fully understand it on your first go through.

I will try my hand at explaining this question.

Background: Sun 80% as luminous back then as today, which would result in freezing of oceans. Instead however water filled them. Heat is trapped within atmosphere through Co2 like Methane, which is defined as GG.

core: Only if level of GG higher 3 million years ago would Earth retain enough heat to keep oceans from freezing (RH ~~> GG higher) therefore, more Co2 back then than today.

We know the suff. condition trigger is already activated because we are told that the oceans did retain heat preventing it from freezing. therefore we know that the level of GG higher back then than today.

Notice how in the BG information we are told that "co2, like methane is a GG". Take a look again at the necessary condition in the supporting premise. Does it HAVE TO BE Co2? What about methane?

That is the flaw of this argument, the author assumes one path (Co2) when some other greenhouse gas could have accounted for preventing the Earth from freezing. This is admittedly hard to see when you are taking this timed and pressed for time at the end of a section, but a strong grasp of eliminating 4 horrible answer choices will lead you to the correct answer on this one.

(A): This strengthens the argument by ruling out an alternative explanation for something that could have accounted for the oceans not freezing.

(B): This is what we are looking for.

(C): "not enough to alter their freezing points significantly". This has no bearing on the argument. We don't care about phenomena that do not affect the freezing points. We are trying to determine whether it was ^Co2 that accounted for the oceans remaining in liquid form.

(D): Increasing life forms? What does this have to do with anything? This doesn't tell us whether ^Co2 was the reason for oceans not freezing. Out of scope, eliminate.

(E): Same reason as D. What does radiation have to do with anything? The LSAT author wants you to make a faulty assumption that increased radiation levels is somehow related to the oceans not freezing. We know better than this. Eliminate.

Hope this helps.

by **aznriceboi17** Sun Mar 30, 2014 11:33 pm

Is the sentence

Heat is trapped within Earth's atmosphere through the presence of carbon dioxide, which, like methane, is a "greenhouse gas."

essentially not a required part of the argument? I was somewhat thrown off because it seems to suggest that carbon dioxide, but not methane, traps heat within Earth's atmosphere.

But then the sentence after it is what the argument actually seems to depend on:

Only if the level of greenhouse gases were higher ...

since here is where we are told that higher levels of greenhouse gases is a necessary condition for the oceans not being frozen 3 billion years ago.

by **maryadkins** Sat Apr 05, 2014 8:49 am

You're right that the first sentence you quote is telling us that carbon dioxide traps the heat in the atmosphere, but that doesn't mean methane DOESN'T trap it. I agree that the next sentence is where we really learn that methane also can trap heat"”and that greenhouse gasses generally were required to keep the oceans in liquid form.

I hope this answers your question.

by **KenM242** Wed Mar 28, 2018 4:05 pm

I got this one wrong but now I can see how ridiculously easy this would have been to some people.

By including 'like methane' in the stimulus, the author basically is saying that

"It is either CO2, or methane (wink, wink), or other greenhouse gases that kept up the earth's temperature.

Blah blah therefore, it must have been the CO2 that did the job."

Of course, phrasing the stimulus like that would have been a dead give-away, so the author wrote the stimulus the way it is and got some of us going 'well they all seem like wrong answers'.