Examkin

Question

Difficulty: HardIdentifying Core Claims and Hypotheses

### The Younger Dryas Cooling Event

The Younger Dryas (approx. 12,90012,900 to 11,70011,700 years ago) was a period of abrupt, severe global cooling that temporarily reversed the warming trend following the Last Glacial Maximum. Three hypotheses have been proposed to explain the trigger for this cooling event.

Hypothesis 1
Around 12,90012,900 years ago, a fragmented comet or asteroid entered Earth's atmosphere and exploded over North America. The thermal energy and pressure wave from this impact event caused widespread wildfires and destabilized the southern margin of the Laurentide Ice Sheet. The rapid melting of ice released a massive volume of freshwater into the North Atlantic. This freshwater influx disrupted the Atlantic Meridional Overturning Circulation (AMOC)—the oceanic conveyor belt that transports heat from the tropics to high latitudes—leading to rapid global cooling.

Hypothesis 2
The cooling event was triggered entirely by internal Earth climate system dynamics. As the Laurentide Ice Sheet naturally retreated due to post-glacial warming, it exposed new topographic outlets. Rather than flowing down the Mississippi River valley into the Gulf of Mexico, freshwater meltwater was suddenly diverted eastward through the St. Lawrence River valley into the North Atlantic. This sudden, non-catastrophic redirection of freshwater lowered the salinity of the North Atlantic, halting the AMOC and initiating the cooling cycle without the need for any extraterrestrial impact.

Hypothesis 3
The primary trigger for the cooling was a combination of reduced solar activity and intense, repeated volcanic eruptions. A prolonged period of low solar irradiance reduced global temperatures, which was exacerbated by stratospheric sulfate aerosols from volcanic eruptions. These aerosols reflected incoming solar radiation back into space. The initial cooling expanded Northern Hemisphere sea ice. Because sea ice has a high albedo, it reflected more sunlight, creating a self-sustaining feedback loop that suppressed the AMOC and sustained the Younger Dryas cold period.

---

Both Hypothesis 2 and Hypothesis 3 discuss the suppression or shutdown of the Atlantic Meridional Overturning Circulation (AMOC) during the Younger Dryas. Which of the following statements best describes a key difference between the two hypotheses regarding the role of the AMOC in the cooling event?

  1. A
    Hypothesis 2 claims that the shutdown of the AMOC was a feedback effect resulting from sea ice expansion, whereas Hypothesis 3 claims that the AMOC shutdown was the primary cause of cooling, triggered by redirected meltwater.
  2. B
    Hypothesis 2 claims that the shutdown of the AMOC was triggered by a cosmic impact, whereas Hypothesis 3 claims that the AMOC shutdown was triggered by a change in the river valley pathways of post-glacial meltwater.
  3. Hypothesis 2 claims that the shutdown of the AMOC was the primary cause of cooling, triggered by redirected meltwater, whereas Hypothesis 3 claims that the AMOC shutdown was a secondary feedback effect resulting from sea ice expansion.Answer
  4. D
    Hypothesis 2 claims that the shutdown of the AMOC had no influence on Younger Dryas cooling, whereas Hypothesis 3 claims that the AMOC shutdown directly caused volcanic eruptions.

Answer

The correct answer is the option stating that Hypothesis 2 claims that the shutdown of the AMOC was the primary cause of cooling, triggered by redirected meltwater, whereas Hypothesis 3 claims that the AMOC shutdown was a secondary feedback effect resulting from sea ice expansion.
The correct answer accurately contrasts the causal pathways of both viewpoints. According to Hypothesis 2, the redirection of meltwater directly halts the AMOC, which then triggers the Younger Dryas cooling cycle. In contrast, Hypothesis 3 claims that solar reduction and volcanic eruptions first caused cooling and sea ice expansion, and this expansion subsequently suppressed the AMOC as a secondary feedback mechanism.

Step-by-Step Solution

1
Analyze Hypothesis 2 to determine the causal role of the AMOC shutdown.
Hypothesis 2 states that post-glacial warming naturally redirected freshwater meltwater through the St. Lawrence River valley. This freshwater halted the AMOC, which directly initiated the Younger Dryas cooling cycle. Thus, the AMOC shutdown is the primary trigger of the cooling.
Understanding how the AMOC shutdown is positioned in the cause-and-effect chain of the second hypothesis.
2
Analyze Hypothesis 3 to determine the causal role of the AMOC shutdown.
Hypothesis 3 states that solar minimums and volcanic aerosols initiated cooling, which expanded sea ice. The expanded sea ice then suppressed the AMOC in a feedback loop. Thus, the AMOC shutdown is a secondary feedback effect resulting from initial cooling.
Understanding how the AMOC shutdown is positioned in the cause-and-effect chain of the third hypothesis.
3
Compare the roles identified in steps 1 and 2 to evaluate the options.
The correct option must state that Hypothesis 2 views the AMOC shutdown as the primary initiator, while Hypothesis 3 views it as a feedback effect of sea ice expansion.
Selecting the option that accurately captures the core differences in the claims.

Key Concept

Identifying and contrasting the causal claims and mechanisms in conflicting scientific hypotheses.
Estimated Time:2m 0s
Rate this question

Topics

Identifying Core Claims and Hypotheses

More questions from this topic