#17 - Climatologists believe they know why Earth has

[Administrator]

Complete Question Explanation

Strengthen Except—CE. The correct answer choice is (D)

This stimulus tells us that the Earth has experienced a regular sequence of ice ages over the past 800,000
years, and that climatologists believe they have an explanation. They note that when Earth’s orbit
experiences certain fluctuations, an ice age tends to coincide, and they hypothesize a causal relationship
to explain this correlation. The climatologists believe that the fluctuations take Earth through a
dust cloud which enters the atmosphere and dims the Sun, thereby causing an ice age, noting that a
particularly dense cloud is required for the effect.

The argument as stated is weak. Although there has been a correlation between ice ages and orbital
fluctuations, one cannot logically conclude that a causal relationship even exists. Just because a dust
cloud might be sufficient to cause an ice age does not mean that an ice age indicates a dust cloud-that
presumption would be a Mistaken Reversal.

Test takers found this to be one of the most difficult in the section, probably because the “except” is
often missed. We are asked to eliminate the supporting choices, and select the response which fails to
strengthen the argument.

Answer choice (A): This response supports the climatologists’ hypothesis by establishing that no dust
clouds were passed through before the Earth’s first ice-age. If there had been such dust clouds appearing
without causing ice ages, the causal argument would be weakened, so ruling out this possibility lends
some strength to the hypothesis. This answer choice is therefore incorrect.

Answer choice (B): This answer choice establishes that dense dust clouds (like those hypothesized by
the scientists) do exist, and that the formation of one such dust cloud coincided with the Earth’s first ice
age. While this evidence is, of course, not conclusive, it does lend some credibility to the conclusion.
Since this would strengthen the argument, this is not the correct answer.

Answer choice (C): If dust from volcanic eruptions decreases Earth’s temperature slightly, that makes it
more likely that dust in Earth’s atmosphere, in sufficient concentrations, could cause an ice age.

Answer choice (D): This is the correct answer choice, because it offers no support for the hypothesis.
The climatologists’ hypothesis concern a dust cloud which enters the Earth’s atmosphere, not a dust
cloud created within the Earth’s atmosphere.

Answer choice (E): This response suggests that cosmic trace elements are found in sedimentary layers
that correspond with ice ages. That makes it more likely that a cosmic dust cloud was indeed involved.
284 PowerScore:

[cecilia]

Hi and thanks in advance for any insight you could should on this question about cosmic dust and the ice age.....I ultimately picked D, but had a really rough time in eliminating B. In my opinion, B seemed to introduce an alternative idea for why the dust came about - through asteroids as opposed to what the passage gave us (the Earth's fluctuations). Hence, if it's asteroids that cause the dust and not, as was initially proposed, the earth's fluctuations - wouldn't that tend to at minimum, NOT lend support to the passage?

[Jon Denning]

Hey cecilia,

Thanks for the question! This is a somewhat tricky Strengthen question, where four of the answers (the incorrect answers) will all help support the hypothesis about cosmic dust blocking sunlight and leading to ice ages, and the correct answer will either be irrelevant or suggest an alternative explanation.

Answer choice B strengthens by showing that there is, in fact, a lot of dense cosmic dust out there that could serve as the source of the ice ages. Think about how badly the argument would be harmed if you said something like "Earth's orbit does not take the planet through any zones of dust." This answer does the exact opposite, and helps as a result.

Answer choice D on the other hand presents an alternate cause: it's not clouds of cosmic dust that block sunlight, but rather it's the occasional presence of cosmic rocks that cause dust to be raised from the Earth's surface (i.e. not cosmic), and that blocks the sun. So that answer actually harms the argument, and is therefore correct.

I hope that helps!

[cecilia]

Thanks Jon! I totally get how the last half of answer choice B supports the climatologist's argument - about the dense cosmic dust indeed being present....But just to clarify, is it ok that the dust came as a result of the asteroids colliding and not , as in the passage - from the earth's fluctuations?? Should I not have gotten so hung up on that detail of the asteroids? I didn't diagram it but in my mind, this is how I represented it:

B. seemed to suggest: asteroids collide---> cosmic dust--->dimming of Sun---> Ice age

whereas the original argument said:
Earth's fluctuations--->cosmic dust--->dimming of Sun---> Ice age

Thanks again.

[Emily Haney-Caron]

Hi Cecilia,

I think what is tripping you up here is the idea of the Earth's fluctuations causing the cosmic dust. What the stimulus intended is actually that the fluctuations in the Earth's orbit cause the Earth to move through cosmic dust (and we don't get any information about how the dust might have been caused).

Does that help?

[cecilia]

Thanks Emily for pointing out the subtle distinction...That does help!

[amna.ali467]

Hi,

I'm having a hard time figuring out why answer choice D is correct. I easily eliminated answer choices A and B. Having a harder time figuring out clearly why C and E are wrong and why D is the right answer.


Thanks for the help.

[Nikki Siclunov]

Hi amna.ali467,

Thanks for your question. It would be tremendously helpful if you can provide a bit more information as to how you approached this problem. Did you break the argument down? What is the conclusion (if any), and how well is it supported by the premises? Did you prephrase an answer to the question? What specifically made (C), (D), and (E) equally attractive?

The more insight you can give us into your line of reasoning, the better we can help you out.

Thanks!

[amna.ali467]

It would be tremendously helpful if you can provide a bit more information as to how you approached this problem. Did you break the argument down? What is the conclusion (if any), and how well is it supported by the premises? Did you prephrase an answer to the question? What specifically made (C), (D), and (E) equally attractive?


(C) is attractive because I didn't think that cosmic dust is the same as dust from volcanic eruptions so how could this support the hypothesis.

(D) is attractive because the word "rock" through me off and I also didn't think there was a connection between raising large amounts of dust from Earth's surface and cosmic dust.

(E) I can see how this can be eliminated much clearer than C and D.

[Nikki Siclunov]

Alright, so the scientists blame the cosmic dust for the sequence of ice ages beginnign 800,000 years ago:

Cosmic dust (cause) Ice age (effect).

Four of the answer choices will strengthen this theory; one will not. Let's take a look at (C) and (D):

Answer choice (C) is quite relevant. If dust from volcanic eruptions can lower earth's average temperature slightly, then large clouds of cosmic dust can probably lower it even more. This answer choice strengthens the conclusion by analogy, because "dust from volcanic eruptions" is similar enough to "cosmic dust": if one produces a certain effect, then the other can probably produce a similar effect.

Answer choice (D) is completely irrelevant, because the climatic effect of the cosmic rock (and the dust they raise from Earth's surface) is left unclear. We have no idea if this periodic occurrence has any effect on Earth's average temperature. Since (D) does not strengthen the hypothesis, it is the correct answer choice.

Does that clear things up?

Thanks!