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Difficulty: HardNecessary Assumption

Geochemist: Ice-core records from the 8.2-kiloyear cooling event show a pronounced drop in atmospheric methane concentrations. Because atmospheric methane is produced predominantly in tropical wetlands, several climatologists hypothesize that a sudden disruption of ocean currents triggered widespread tropical drying, thereby reducing wetland extent and lowering methane emissions. However, this hypothesis is mistaken. High-resolution speleothem data from tropical South America indicate that precipitation in that region increased substantially during this period. Therefore, the drop in atmospheric methane was not caused by a reduction in tropical wetland extent.

Which one of the following is an assumption required by the geochemist's argument?

  1. A
    A decline in atmospheric methane concentrations cannot occur without a concurrent reduction in the global extent of wetlands.
  2. The increased precipitation in tropical South America was not offset by drying in other tropical regions sufficient to produce an overall reduction in tropical wetland area.Answer
  3. C
    Speleothem records provide consistently more accurate paleoclimate data than do high-resolution ice-core samples.
  4. D
    Atmospheric methane emissions from non-tropical sources remained completely constant throughout the 8.2-kiloyear event.
  5. E
    If oceanic circulation had remained undisrupted, precipitation levels in tropical South America would have stayed at baseline levels.

Answer

The argument assumes that the increase in precipitation in tropical South America was not offset by drying in other tropical regions sufficient to produce a net decrease in global tropical wetland area.
The correct answer identifies an essential defender assumption required to link regional rainfall evidence to global wetland area. The geochemist infers that total tropical wetland extent did not decrease based solely on evidence from tropical South America. For this inference to hold, it is strictly necessary that drying in other tropical regions (such as tropical Africa or Asia) was not severe enough to outweigh the South American increase and cause a net global decrease in tropical wetlands. Negating this statement destroys the argument.

Step-by-Step Solution

1
Analyze the premises and the main conclusion of the stimulus.
Premise: Speleothem data show increased precipitation in tropical South America during the 8.2-kiloyear cooling event. Conclusion: The drop in global atmospheric methane was not caused by a reduction in tropical wetland extent.
Identifying the premise and conclusion reveals the scope shift between a single region (tropical South America) and the global phenomenon (overall tropical wetland extent and global atmospheric methane).
2
Identify the unstated logical gap.
The author concludes that total tropical wetland area did not decline solely because one specific tropical region became wetter, implicitly assuming that other tropical regions did not experience severe drying that offset this regional increase.
A necessary assumption must bridge the gap between regional precipitation data and the conclusion regarding global tropical wetland extent.
3
Apply the Negation Test to confirm necessity.
Negating the statement that other regions did not offset the increase yields: drying in other tropical regions DID offset the South American rainfall, causing a net decrease in global tropical wetlands. This directly validates the climatologists' hypothesis and refutes the geochemist's conclusion.
A statement is a necessary assumption if and only if its logical negation destroys the argument.

Key Concept

Necessary Assumption in Logical Reasoning (Defender Assumption / Negation Test)
Estimated Time:2m 0s
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Topics

Necessary Assumption

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