Reading Comprehension

PT150 · S1 · P1 · Q5 The Greenhouse Effect

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The following passage was written in the mid-1990s.

Topic

The evidence for global warming, the role of greenhouse gases, and evaluating alternative explanations for the recent unprecedented rise in Earth's atmospheric temperature.

Framework

Answer / Explain (with a touch of Present Debate: alternative explanations vs. greenhouse effect)

Main Point

Recent scientific evidence shows that the Earth’s average temperature has risen to levels not seen in the past 1,000 years, and while some alternative explanations exist, the best explanation for this unprecedented warming is the effect of greenhouse gases. The Most Valuable Sentence: Appears at the end of the last paragraph —

P1: Evidence for Unprecedented Warming and the Greenhouse Effect

The passage opens by telling us that compelling new evidence shows Earth’s atmosphere has recently warmed in a way that hasn’t happened in the last 1,000 years. This is based on analyzing things like ice samples, tree rings, and coral. The idea that greenhouse gases are causing this rise is getting stronger, even though it’s still debated.

P2: Addressing Problems with Early Models & the Role of Sulfates

Early computer models predicted even more warming than we’re seeing, which critics used to question the greenhouse theory. But new research shows that airborne sulfates (from volcanoes and pollution) actually reflect sunlight and offset some of the warming, explaining the mismatch and closing the gap between past predictions and what has actually happened.

P3: Considering Solar Variations as an Alternative Explanation

Another possible reason for the warming is changes in solar energy, but even though these solar cycles seem to track with temperature ups and downs over the decades, solar models can’t account for the full extent of the recent warming. The current rise exceeds what solar variation alone could produce, leading the author to conclude that greenhouse gases are still the main culprit.

Which one of the following most accurately states the author's primary purpose in the last two sentences of the second paragraph?

to provide an example of
Out of Scope: Observations
This highlighted portion doesn't contain any observations predicted by a theory. The word 'observations' in the context of a theory's predictions means 'empirical data'. "Were the observed data aligned with the theory's predictions?" We're not getting a set of data points. We're getting an explanation about how scientists fixed their theory (to account for sulfates) and an update that the theory's predictions now do match our observations about the temperatures in recent decades. This sort of trap answer is stuck within the Detail, whereas the correct answer on Local Purpose is all about the Big Idea language that bookends the detail.
6% picked this
to argue that a new
Opposite
This section is arguing that the Greenhouse Theory is looking stronger than ever, now that they realized their failure to account for sulfates was throwing off the numbers. They've tweaked the theory and now it makes predictions that do accommodate data from recent decades.
2% picked this
to demonstrate the degree to
Opposite
It was never said that Greenhouse Theory was previously accepted. Some proponents believed it but its early models were not matching observed data. And this section is arguing that the Greenhouse Theory is looking stronger than ever, now that it accounts for sulfates, which were throwing off its numbers.
8% picked this
Correct
to show that a certain
Why this is right
This is supported by the previous two sentences (which is where we look for a match on Local Purpose questions). The first sentence of the 2nd was saying that the greenhouse theory was previously in doubt because its models did not match the observed data; they were too alarmist. The second sentence of the 2nd is saying that new methods are allowing greenhouse theorists to refine their models. The calculable cooling effect of sulfates is the additional data they needed, which is explained in the highlighted portion.
Skill tested: Local Purpose · how this choice captures the passage's function is the move to repeat next time.
82% picked this
to propose a way of
Out of Scope: two competing theories
There aren't two theories being discussed in this paragraph. Just one theory, in two different time periods: Old greenhouse, which didn't account for sulfates and thus made bad predictions, and New greenhouse, which has rectified the situation.
2% picked this

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