The Intertropical Convergence Zone (ITCZ), a band of clouds that encircles Earth in the tropics and is a major rainfall...
GMAT Information and Ideas : (Ideas) Questions
The Intertropical Convergence Zone (ITCZ), a band of clouds that encircles Earth in the tropics and is a major rainfall source, shifts position in response to temperature variations across Earth's hemispheres. Data from Huagapo Cave in Peru suggest the ITCZ shifted south during the Little Ice Age (circa 1300–1850), but a shift as far into South America as Huagapo should have led to dry conditions in Central America, which is inconsistent with climate models. To resolve the issue, geologist Yemane Asmerom and colleagues collected data from Yok Balum Cave in Central America and compared them with the Huagapo data. They concluded that during the Little Ice Age, the ITCZ may have expanded northward and southward rather than simply shifted.
Which finding from Asmerom and colleagues' study, if true, would most directly support their conclusion?
Neither the Yok Balum data nor the Huagapo data show significant local variations in temperature during the Little Ice Age.
Both the Yok Balum data and the Huagapo data show increased temperatures and prolonged dry conditions during the Little Ice Age.
The Yok Balum data show prolonged dry conditions during the same portions of the Little Ice Age in which the Huagapo data show heightened levels of rainfall.
The Yok Balum data and the Huagapo data show strongly correlated patterns of high rainfall during the Little Ice Age.
Looking at this Command of Evidence question, I need to determine which finding would best support Asmerom's conclusion about ITCZ expansion. Let me work through this systematically.
Step 1: Decode and Map the Passage
Part A: Passage Analysis Table
| Text from Passage | Analysis |
|---|---|
| "The Intertropical Convergence Zone (ITCZ), a band of clouds that encircles Earth in the tropics and is a major rainfall source, shifts position in response to temperature variations across Earth's hemispheres." |
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| "Data from Huagapo Cave in Peru suggest the ITCZ shifted south during the Little Ice Age (circa 1300–1850)," |
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| "but a shift as far into South America as Huagapo should have led to dry conditions in Central America, which is inconsistent with climate models." |
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| "To resolve the issue, geologist Yemane Asmerom and colleagues collected data from Yok Balum Cave in Central America and compared them with the Huagapo data." |
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| "They concluded that during the Little Ice Age, the ITCZ may have expanded northward and southward rather than simply shifted." |
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Main Point: Researchers concluded that the ITCZ expanded in both directions during the Little Ice Age, rather than simply shifting south.
Argument Flow: The passage starts with background on ITCZ behavior, then presents evidence of southward movement that creates a logical problem. To resolve this contradiction, Asmerom compared data from two locations and concluded the ITCZ expanded rather than shifted.
Step 2: Interpret the Question Precisely
What's being asked? Which finding would most directly support Asmerom's conclusion that the ITCZ expanded rather than shifted
What type of answer do we need? Evidence that would confirm the expansion theory over the simple shift theory
Any limiting keywords? "most directly support" means we need the strongest, clearest evidence for their specific conclusion
Step 3: Prethink the Answer
- The key insight is the difference between "shifting" and "expanding."
- If the ITCZ simply shifted south, then Peru would get more rain but Central America would get less (dry conditions).
- But if the ITCZ expanded both north and south, then both regions could experience high rainfall simultaneously.
- So the right answer should show evidence that both Peru and Central America had similar patterns during the Little Ice Age—specifically, both showing high rainfall when the traditional shift theory would predict Central America should be dry.
Neither the Yok Balum data nor the Huagapo data show significant local variations in temperature during the Little Ice Age.
✗ Incorrect
- This focuses on temperature data, but Asmerom's conclusion is about rainfall patterns and ITCZ behavior.
- Temperature variations aren't the key evidence needed to distinguish between shifting vs. expanding.
Both the Yok Balum data and the Huagapo data show increased temperatures and prolonged dry conditions during the Little Ice Age.
✗ Incorrect
- Shows both locations had dry conditions, which doesn't help distinguish between shifting and expanding theories.
- Actually contradicts the idea that the ITCZ was a major rainfall source for either location.
The Yok Balum data show prolonged dry conditions during the same portions of the Little Ice Age in which the Huagapo data show heightened levels of rainfall.
✗ Incorrect
- This shows opposite patterns (Peru wet, Central America dry), which would actually support the simple shift theory, not the expansion theory.
- Students might think contrasting patterns support the conclusion, but this actually supports the original shift hypothesis that Asmerom was trying to replace.
The Yok Balum data and the Huagapo data show strongly correlated patterns of high rainfall during the Little Ice Age.
✓ Correct
- Shows both locations had high rainfall during the same periods of the Little Ice Age.
- This directly supports the expansion theory: if the ITCZ expanded both north and south, both Peru and Central America could simultaneously receive high rainfall.
- Perfectly matches our prethinking about what evidence would distinguish expansion from simple shifting.