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Difficulty: Very hardDistinguishing Main Idea from Supporting Evidence

For decades, paleoclimatologists held that the abrupt cooling of the Younger Dryas period (circa 12,900 years ago) was triggered exclusively by a massive release of glacial meltwater from Lake Agassiz into the North Atlantic, which disrupted the Atlantic Meridional Overturning Circulation (AMOC) and suppressed heat transport to the Northern Hemisphere. Proponents of this traditional ocean-centric model cited high-resolution sediment cores showing marked drops in sea surface temperatures alongside contemporaneous reductions in deep-water formation rates as definitive proof that ocean circulation failure was the fundamental initiator of global cooling.

However, recent high-precision ice-core analyses and atmospheric modeling indicate that the suppression of AMOC, while empirically verified by sediment data, was a secondary feedback mechanism rather than the primary catalyst. Instead, shifts in atmospheric circulation patterns—specifically, a persistent southward displacement of the northern polar jet stream driven by subtle insolation anomalies—initiated the Northern Hemisphere cooling trend prior to the major meltwater discharge. The AMOC slowdown undoubtedly amplified and prolonged the frigid conditions across the North Atlantic basin, but elevating this oceanographic response to the role of primary driver misinterprets an amplifying feedback loop for the initial climate trigger. Consequently, while oceanic circulation plays a vital role in regional thermal modulation, the overarching impulse driving the Younger Dryas cooling originated in atmospheric dynamics.

Which of the following best states the central thesis of the passage?

  1. Although disruption of Atlantic ocean circulation amplified the Younger Dryas cooling, atmospheric circulation shifts were the true primary trigger of the climate event.Answer
  2. B
    High-resolution sediment cores demonstrate that reductions in North Atlantic deep-water formation rates coincided with marked drops in sea surface temperatures during the Younger Dryas.
  3. C
    The suppression of the Atlantic Meridional Overturning Circulation acted as a vital feedback mechanism that prolonged frigid conditions across the North Atlantic basin.
  4. D
    A massive discharge of glacial meltwater from Lake Agassiz into the North Atlantic was the primary catalyst that initiated the abrupt cooling of the Younger Dryas period.
  5. E
    Paleoclimatologists now agree that ocean-centric models of climate change are completely invalid because marine sediment data cannot reliably measure historical sea surface temperatures.

Answer

The main idea of the passage is that although disruption of Atlantic ocean circulation amplified Younger Dryas cooling, atmospheric circulation shifts were the true primary trigger of the climate event.
The passage follows a classic GMAT Reading Comprehension structure: Paragraph 1 outlines a traditional scientific hypothesis (ocean circulation failure as primary trigger) and its supporting evidence, while Paragraph 2 presents a pivot ('However') introducing the author's main point. The author argues that while ocean circulation (AMOC) suppression did occur and amplify cooling, it was a secondary feedback mechanism; the initial and primary driver was atmospheric jet stream displacement. The correct answer accurately synthesizes this main thesis by distinguishing the primary atmospheric cause from the secondary oceanographic feedback.

Step-by-Step Solution

1
Analyze the passage structure and identify the main structural pivot.
Paragraph 1 introduces a traditional view (AMOC disruption caused by meltwater was the primary trigger) along with supporting sediment core evidence. Paragraph 2 introduces a pivot ('However') presenting recent findings that relegate AMOC disruption to a secondary feedback and establish atmospheric shifts as the primary trigger.
Main idea questions require distinguishing between the author's main thesis and secondary claims or refuted background views.
2
Differentiate the primary thesis from supporting evidence and sub-arguments.
The primary thesis is that atmospheric dynamics initiated the cooling, whereas AMOC suppression was merely an amplifying feedback mechanism.
Distractors frequently use true details or secondary sub-arguments from the passage to trap readers.
3
Evaluate the choices against the identified primary thesis.
The statement identifying atmospheric shifts as the primary trigger while recognizing ocean circulation as an amplifying factor accurately captures the complete main argument.
The correct choice must encompass the full scope of the author's central claim without narrowing down to a single supporting detail.

Key Concept

Distinguishing Main Idea from Supporting Evidence
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Distinguishing Main Idea from Supporting Evidence