Q13 - Up until about 2 billion years ago

[nanagyanewa]

Hello,

Could someone please explain why A is the answer and not E? I figured that A must be wrong because 3.8 billion years ago the atmosphere rather held in less heat and that was why the ocean froze. Or am I misunderstanding what is meant by "held in"?
Thanks.

[bbirdwell]

I think you're misunderstanding the question task, or misreading the choice, because you are correct about the meaning of "held in."

Your job on this question is to find a choice that allows both parts of the contradiction to be true. First you must find the contradiction.

Here:
Before 2Bill years ago, the sun was dimmer. If it were that dim now, oceans would freeze. But back then, the oceans did not freeze.

So the big question is WHY? If a dim sun would make the oceans freeze now, why didn't it make the oceans freeze back then?

(A) Today the atmosphere holds in less heat. Ah-ha! This gives an additional reason why the oceans would freeze today with a dim sun, and gives a reason why they didn't freeze in the past (when the atmosphere held in more heat).

(E) so what? Discussing what happens when large portions of the globe are covered in ice does not help us. We need something that explains why the globe was NOT covered in ice 3.8 billion years ago.

See what I mean?

[nanagyanewa]

I understand it better now. Thanks.

[peg_city]

I guessed A then erased it for C

Why is C wrong and A right?

Up to 2 billion years ago the sun was dimmer by 30% but there was still life and liquid water on earth. By today's standards the oceans would have been frozen.

What gives?

A) talks about 3.8 billion years ago. But who cares about 3.8 billion years ago. We want to know about 2 billion years ago which C does

frustrating

Thanks again

[lhermary]

Why is C wrong?

Thanks

[ManhattanPrepLSAT1]

Answer choice (C) can be tempting! It's really close too, but it doesn't get us liquid water as far back as 3.8 billion years ago. It only goes back 2 billion years, which is about the time the sun became brighter. Had answer choice (C) said 3.8 billion years ago, it would have been correct.

We need to resolve having a dimmer sun and liquid water 3.8 billion years ago. Answer choice (A) provides a means of extra warmth all the way back in the time when the sun was dimmer and yet the Earth had liquid water.

Let's look at the incorrect answers:

(B) doesn't tell us why! Telling us what happened cannot explain the means by which it happened.
(C) doesn't take us back far enough in time.
(D) doesn't contradict the phenomenon that there was liquid water as far back as 3.8 billion years and also fails to explain where the warmth came from. In fact, if anything, this answer choice makes the liquid water maybe a bit more puzzling.
(E) makes the liquid water even more puzzling, not less.

Hope that helps!

[shirando21]

By choosing B, we kind of assumed significant climate change happened, and this assumption should not be allowed. is this why?

[griffin.811]

The correct answer is A

[lym]

I also chose B !

Here's my thought process (in case anybody's interested in why anyone choose B) I think B resolves the discrepancy by assuming a climate change between 3.8B and 2B. In the 3.8, it was hot, and there was liquid and life. In 2B, it was cold(dim), and there's no liquid, no life.

As a matter of fact, I'm surprised by the fact all of you guys take it for granted that liquid water had NEVER CEASES TO BE PRESENT ever since 3.8B, cos that statement, to me, is an unwarranted inference from the original statement "life and liquid water both present as early as 3.8B years ago."

So, here, I, as a non-native speaker, have to ask you guys this :

is it really that explicit that "life and liquid water both present as early as 3.8B years ago" actually means "had NEVER CEASES TO BE PRESENT ever since 3.8B" ? If that's the case, clearly I've failed to detect the fullest/subtler meaning of these words.

[ManhattanPrepLSAT1]

Hey lym, I think you bring up a valid issue. Could water have existed 3.8 billion years ago, then disappeared for a while, only to reemerge again about 2 billion years ago. I think so. But the problem with this as for as the question goes, is that answer choice (B) doesn't explain how 3.8 billion years ago there was life and liquid water. Remember, we still have the issue that the sun was 30% dimmer 3.8 billion years ago. So if life did exist 3.8 billion years ago, disappear for a while, and then reemerge about 2 billion years ago, we need to know how 3.8 billion years ago the Earth was warm enough to have life and liquid water.

Answer choice (A) provides a mechanism - greenhouse gasses. But answer choice (B) doesn't offer a mechanism to heat the Earth 3.8 billion years ago. And it's critical on Explain/Resolve questions that the answer choice answers the question how/why?

Hope that helps!