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Difficulty: MediumLogical Inferences

Paleoclimatologists often estimate ancient atmospheric carbon dioxide (CO2CO_2) levels by analyzing the stomatal index (the ratio of gas-exchange pores to total epidermal cells) of fossilized leaves, as many modern plants decrease their stomatal density under higher CO2CO_2 concentrations. In studying fossilized leaves of the conifer *Ginkgo adiantoides* from the Late Cretaceous epoch, researcher Dr. Helen Vance noted a significant decrease in stomatal index during a period of warming. However, Vance also observed that modern *Ginkgo* trees show no change in stomatal index when grown under elevated temperatures alone, maintaining a stable ratio unless the ambient CO2CO_2 level itself is altered. Which choice most logically completes the text?

  1. A
    a decline in the overall population density of *Ginkgo adiantoides* due to thermal stress.
  2. an elevation in atmospheric CO2CO_2 concentrations during the warming period.Answer
  3. C
    a decrease in atmospheric CO2CO_2 concentrations that occurred independently of the warming.
  4. D
    a migration of *Ginkgo adiantoides* populations to habitats with cooler baseline climates.

Answer

an elevation in atmospheric CO2CO_2 concentrations during the warming period.
The correct answer is correct because it directly follows from the premises. The passage states that modern plants, including *Ginkgo*, decrease their stomatal index in response to higher CO2CO_2 concentrations, but do not change this index in response to temperature increases alone. Since a decrease in stomatal index was observed during a historical warming period, the change cannot be attributed to the warming itself but must be attributed to an accompanying increase in atmospheric CO2CO_2 levels.

Step-by-Step Solution

1
Analyze the relationship between stomatal index and carbon dioxide (CO2CO_2) levels.
Stomatal index decreases as CO2CO_2 concentrations increase.
This is established by the first sentence of the text, which explains that plants decrease their stomatal density under higher CO2CO_2 concentrations.
2
Examine the observational data and the experimental control.
Fossilized leaves show a decreased stomatal index during warming, but modern *Ginkgo* trees do not change their stomatal index when temperature alone changes.
This isolates CO2CO_2 as the variable responsible for the stomatal index change, rather than temperature itself.
3
Synthesize the findings to draw a logical inference.
The historical warming must have been accompanied by an increase in atmospheric CO2CO_2 concentrations.
Since temperature alone cannot cause the stomatal index to decrease, and higher CO2CO_2 is known to cause a decrease in stomatal index, the observed decrease in the fossil leaves indicates that CO2CO_2 levels must have risen.

Key Concept

Logical Inferences
Estimated Time:1m 30s
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Logical Inferences

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