Question 150·Hard·Central Ideas and Details
Policymakers often tout large-scale tree-planting as a cost-effective strategy for mitigating climate change, claiming that vast new forests will absorb enough carbon dioxide to compensate for continued fossil-fuel use. Ecologist Andrea Liu contends that this optimism is misplaced. While acknowledging trees’ ability to store carbon in their trunks and leaves, Liu argues that most carbon in forest ecosystems is actually locked in soils, where its retention depends on cool temperatures and minimal disturbance. Because newly planted forests typically require ground preparation and are established in regions already experiencing warming, Liu maintains that the soil layer beneath them can become a net source—not a sink—of greenhouse gases. Consequently, she concludes, reforestation projects that ignore soil dynamics routinely exaggerate their climate benefits.
Which choice best states the main idea of the passage?
For main-idea questions, paraphrase the passage’s overall argument in one sentence by combining the topic (often from the start) with the author’s conclusion (often from the end). Choose the option that includes the author’s key reason, and eliminate choices that are too narrow (single detail) or that introduce a new recommendation not made in the passage.
Hints
Use the beginning and the end
Reread the first two sentences and the final sentence. What do policymakers claim, and what does Liu conclude about those claims?
Focus on what Liu says gets overlooked
What part of the ecosystem stores most carbon, and what happens to it when soils are disturbed and temperatures rise?
Watch for answers that become prescriptions
The correct main idea summarizes Liu’s critique; eliminate choices that mainly propose what projects “should” do instead.
Step-by-step Explanation
Recognize the question type
This is a main-idea question, so you’re looking for the choice that summarizes Liu’s overall argument (not a single detail and not a new recommendation).
Summarize the passage’s claim and support
Policymakers claim tree planting can offset emissions, but Liu argues this is overly optimistic because much forest carbon is in soils, and under common planting conditions (disturbance plus warming) soils can release greenhouse gases.
Match the best summary
The best answer must combine (1) the claim that benefits are overstated and (2) the key reason: soil dynamics, especially emissions from disturbed, warming soils.
Eliminate choices that are too narrow or shift the focus
- Choice 1 is too narrow: it focuses on where carbon is stored but not the key point that disturbance and warming can make soils emit greenhouse gases.
- Choice 2 turns the passage into a specific prescription (preserve soils rather than plant trees) that Liu does not explicitly argue.
- Choice 3 reframes the passage as conditional encouragement (“tree planting can work if…”), but Liu’s emphasis is that common claims are exaggerated because projects ignore soil emissions.
- Choice 4 captures both the overstatement and the overlooked problem of emissions from disturbed, warming soils, so it best states the main idea.