Climatologists believe they know why Earth has undergone a

Question

Climatologists believe they know why Earth has undergone a regular sequence of ice ages beginning around 800,000 years ago. Calculations show that Earth’s orbit around the Sun has fluctuations that coincide with the ice-age cycles. The climatologists hypothesize that when the fluctuations occur, Earth passes through clouds of cosmic dust that enters the atmosphere; the cosmic dust thereby dims the Sun, resulting in an ice age. They concede, however, that though cosmic dust clouds are common, the clouds would have to be particularly dense in order to have this effect.

Each of the following, if true, would lend support to the climatologists’ hypothesis EXCEPT:

(A) Earth did not pass through clouds of cosmic dust earlier than 800,000 years ago.
(B) Two large asteroids collided 800,000 years ago, producing a tremendous amount of dense cosmic dust that continues to orbit the Sun.
(C) Earth’s average temperature drops slightly shortly after volcanic eruptions spew large amounts of dust into Earth’s atmosphere.
(D) Large bits of cosmic rock periodically enter Earth’s atmosphere, raising large amounts of dust from Earth’s surface.
(E) Rare trace elements known to be prevalent in cosmic debris have been discovered in layers of sediment whose ages correspond very closely to the occurrence of ice ages.

Source : LSAT PrepTest 44, Q17

prasi55 · Accepted Answer

To support the climatologists' hypothesis, you need to connect the Earth's passage through dust to the lowering of temperatures that characterises the ice age(s). C says that large amounts of dust (from wherever) have in the past caused drops in the Earth's average temperature. So there's reason to believe that this pattern might be evidenced with cosmic dust as well.

D, on the other hand, provides no such correlation. D doesn't suggest that sometimes those large rocks create dense enough clouds to cause an ice age and that sometimes they do not. At the same time, D doesn't suggest that the periodic appearance of cosmic dust DOES NOT cause corresponding drops in temperature (characteristic of the ice ages) either. D basically provides no link between the clouds or their density and the ice ages.

To your statement about the cloud density:
If non-dense clouds do not cause an ice age, it doesn't mean that dense clouds will cause one.

--Prasad

prasi55 · Answer

Sure, let me try.

What is the climatologists' main hypothesis? -- Cosmic dust (resulting from the fluctuations caused during the ice age cycles) caused the ice ages.

What could support this hypothesis? Evidence of the existence of the cause (cosmic dust) or evidence that a similar cause has produced a similar effect would support it.

What could weaken this hypothesis? Evidence that things other than cosmic dust were present at that time (confounding the cosmic dust => ice age hypothesis) or evidence that unrelated things have produced a similar effect would weaken it.

A says that the clouds of dust did not appear before the regular sequence of ice ages started. If the clouds had appeared before then, then you should have expected the regular sequence of the ice ages to start before as well.

B provides evidence that the arrival of the cosmic dust coincided with the start of the regular sequence of the ice ages.

C suggests that cosmic dust has caused near-immediate drops in temperatures in the past.

E suggests that cosmic dust was likely present during the times the ice ages occurred.

Thus, A, B, C, and E do support the hypothesis (however indirectly).

What about D? D says that cosmic dust periodically enters the Earth's atmosphere. Fair enough. If D provided evidence that these periods corresponded to the occurrence of the ice ages, the hypothesis would be strengthened. But D goes on to say that the cosmic dust causes large amounts of dust from the Earth's own surface to appear.

All this dust has indeed made the picture a tad murky.

. We are no longer sure "which" dust (or whether any dust) caused the ice age cycles.

I leave you with this gem from the celebrated poet, Henry Longfellow:
"Dust thou art, to dust returnest, was not spoken of the soul."

--Prasad

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. We are no longer sure "which" dust (or whether any dust) caused the ice age cycles. I leave you with this gem from the celebrated poet, Henry Longfellow: "Dust thou art, to dust returnest, was not spoken of the soul." --Prasad

ronr34 · Answer

If I'm not mistaking, C is about volcanic dust and not cosmic dust, thus not really having anything to do with our question.
D, on the other hand supports the conclusion...
Although cosmic dust enters the atmosphere sometimes, it doesn't create an ice-age. This supports that the clouds need to be dense and just them being there isn't enough...
What am I missing?

AkshdeepS · Answer

VeritasKarishma  : Please help to clarify between C and D.

Can a slight edge of one option over the other be a reason of eliminating one and keeping the other.

Or we need some significant difference/s to choose between right and wrong.

KarishmaB · Answer

Ice ages began 800,000 years ago. 
Earth’s orbit around the Sun has fluctuations that coincide with the ice-age cycles.

Hypothesis: When the fluctuations occur, Earth passes through clouds of cosmic dust that enters the atmosphere; the cosmic dust thereby dims the Sun, resulting in an ice age.
Though cosmic dust clouds are common, the clouds would have to be particularly dense in order to have this effect.

Climatologists’ hypothesis is regarding cosmic dust causing ice ages beginning 800,000 years ago.

Which of the following does not support the hypothesis?

(A) Earth did not pass through clouds of cosmic dust earlier than 800,000 years ago.
Helps establish link between ice age and cosmic dust. Ice ages started 800,000 years ago. So did cosmic dust clouds.

(B) Two large asteroids collided 800,000 years ago, producing a tremendous amount of dense cosmic dust that continues to orbit the Sun.
Again, helps establish link between ice age and cosmic dust. Ice ages started 800,000 years ago. So did cosmic dust clouds.

(C) Earth’s average temperature drops slightly shortly after volcanic eruptions spew large amounts of dust into Earth’s atmosphere.
Helps establish that dust clouds can cause lowering of temperature. So if clouds are thick enough, sun may get blocked and result in massive temperature drop causing ice ages.

(D) Large bits of cosmic rock periodically enter Earth’s atmosphere, raising large amounts of dust from Earth’s surface.
This only says that periodically, dust enters Earth's atmosphere. What impact it has, we don't know. Does it lower temperature periodically, it is not mentioned so it doesn't support our hypothesis. Does it have no impact? If so, our hypothesis may become less likely. 
Hence, (D) does not support our hypothesis.

(E) Rare trace elements known to be prevalent in cosmic debris have been discovered in layers of sediment whose ages correspond very closely to the occurrence of ice ages.
It links cosmic dust to ice ages. Supports our hypothesis.

Answer (D)

KarishmaB · Answer

It isn't about slight edge. It is about only one correct option. Though we do talk about "best option" in Verbal questions, it's mostly relevant for SC and RC questions.

Mavisdu1017 · Answer

Hello expert,
I read through the thread above, but unable to be convinced. I still can not understand why C is wrong.
Some one said “C says that large amounts of dust (from volcano eruption) have caused drops in the Earth's average temperature. So there's reason to believe that this pattern might be evidenced with cosmic dust as well”, but I think this is far fetched. Although they are both dust, they are totally different things, so how could we infer an effect of cosmic dust from that of volcanic dust? Unless this question requires us to know additional knowledge...

While I think D means: when Earth passes through clouds of cosmic dust, the cosmic dust clouds maybe NOT dense enough to dims the Sun, but cosmic rock occuringnat the same time help to raise large amounts of dust from Earth’s surface, so strengthen the conclusion “have to be particularly dense in order to have this effect”.
Hope experts shed Somme light on this question. Much thanks.

RonTargetTestPrep · Answer

It's  supporting  evidence—not proof. The connection doesn't have to be completely rigorous.

If we establish that one type of dust cloud causes a lowering of the Earth's surface temperature, that makes it more likely that other types of dust clouds will have the same effect. Yes, they're different types of dust, but physically they're extremely similar. (In terms of the mechanism hypothesized here—partial blocking of the Sun's light—they're identical. Solid particles of a given size will block the same amount of light regardless of their composition.)

In any case, if you have a hard time with choice C, you may be able to solve the problem more easily by directly identifying D as the choice that goes the wrong way.

The hypothesis is that Earth flies through  clouds of cosmic dust that are floating around outer space  while it's orbiting around the Sun.

If choice D is true, it actually disproves this hypothesis! If D is what's actually happening, then Earth isn't flying through clouds of dust in outer space. Instead, Earth occasionally flies through the path of a BIG rock in outer space, which then crashes into Earth and kicks up  bust from Earth itself . That contradicts the idea that the dust is flying around in outer space.

Mavisdu1017 · Answer

RonTargetTestPrep  thanks for your response expert, now I can understand D. 
But for C - The connection doesn't have to be completely rigorous, such kind of a choice in GMAT would be an irrelevant choice, thus neuther strengthens nor weakens the conclusion. Is it a different rule since this is a LAST question? Thanks.

TanyaVarshney7 · Answer

the question asks if true, would lend support except,
so the dilemma here is that "is it looking for weakening the conclusion or is this another way of asking a strengthning question.
I selected my practice question only for strengthen ques in cr.
Please give your expert reply on this.

RonTargetTestPrep · Answer

That choice could work on the GMAT. In fact, this problem fits in pretty well with GMAT problems.
Critical reasoning problems on the GMAT rely substantially more on real-world common sense, and far less on formal/rigorous logic, than do their LSAT counterparts. (This is true for very good reasons! Legal reasoning, especially in the use of precedential cases, often does call upon formal logical argumentation. The job of a big business executive, on the other hand, rarely or never uses formal logical reasoning.)

RonTargetTestPrep · Answer

For this type of problem statement, the correct answer is whichever choice  does NOT support/strengthen  the given argument. That choice could be something that weakens the argument; it could also just be something that has no effect on the argument at all (e.g., because it merely repeats something the argument already says, or because it's irrelevant altogether).

jim441 · Answer

How to choose between C & D.

GMATNinja · Answer

Let's start by outlining the argument:

Climatologists believe that the Earth has undergone ice ages because of "fluctuations" that casued it to pass through "clouds of cosmic dust that enter the atmosphere." This dust then dims the sun and causes an ice age.   The climatologists concede that the clouds of dust would need to be "particularly dense" to cause these ice ages.

Let's now consider (C):

This confirms that dust in the atmosphere can decrease Earth's average temperature. This supports the idea that dust in the atmosphere could lead to an ice age. Eliminate (C).

Let's consider (D):

The argument says that cosmic  dust  can trigger an ice age. Answer choice (D), by contrast, talks about bits of cosmic  rock , which isn't relevant to the argument. While (D) does talk about dust from the Earth's surface, it doesn't tie this dust to lower temperatures, or anything else that might support the idea that dust can cause an ice age.

So unlike (C), (D) doesn't support the idea that dust can lower the earth's temperature, or lead to an ice age. For that reason, (D) is correct.

I hope that helps!

VerbalBot · Answer

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Climatologists believe they know why Earth has undergone a

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