Atmospheric carbon dioxide (CO2) has been increasing since 1700, but the amount of CO2 produced in that time by burning...
GMAT Reading Comprehension : (RC) Questions
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Atmospheric carbon dioxide (CO2) has been increasing since 1700, but the amount of CO2 produced in that time by burning fossil fuels should have resulted in a much greater increase than has been observed. Plant ecologist Allen Auclair claims that the woodlands of the Northern Hemisphere have been acting as a carbon sink, absorbing carbon from the atmosphere and turning it into wood. Auclair uses measurements of factors affecting the area and density of a forest-such as logging, fires, and pests - and estimates of tree growth rates to argue that increases in the growth rates of individual trees in these forests since 1920 have created a large volume of wood that accounts for the missing carbon.
To determine whether the woodlands as a whole are releasing or absorbing carbon, the volume of wood added to the woodlands must be compared with the wood lost. Auclair's analysis of the past hundred years shows the woodlands changing from a carbon source to a carbon sink. Before 1890, northern woodlands were a source of CO2, mainly because of forest fires and logging. Such deforestation releases CO2, because debris from the forest floor rots more quickly when the trees are cleared. After 1920, the steep increase in tree growth rates surpassed the losses stemming from fire and logging, turning the northern forests from a carbon source into a carbon sink and storing CO2 from fossil fuel over the next fifty years.
Ques. 1/3
It can be inferred from the passage that the northern woodlands would be more likely to function as a carbon source if which of the following were to occur?
Solution
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| Atmospheric carbon dioxide (CO2) has been increasing since 1700, but the amount of CO2 produced in that time by burning fossil fuels should have resulted in a much greater increase than has been observed. | What it says: CO2 levels have gone up since 1700, but not as much as scientists would expect based on how much fossil fuel we've burned. What it does: Introduces a scientific mystery/puzzle that the passage will attempt to solve Source/Type: Scientific fact/observation Connection to Previous Sentences: This is our opening sentence - no previous connections yet Visualization: If fossil fuels should have caused CO2 to rise by 150 units, but we only observe an increase of 100 units, where did those missing 50 units go? Reading Strategy Insight: This is a classic GMAT RC setup - identify the central puzzle that needs solving What We Know So Far: There's missing CO2 that should be in the atmosphere What We Don't Know Yet: Where the CO2 went or who has a theory about it |
| Plant ecologist Allen Auclair claims that the woodlands of the Northern Hemisphere have been acting as a carbon sink, absorbing carbon from the atmosphere and turning it into wood. | What it says: A scientist named Auclair thinks northern forests are soaking up the missing CO2 and storing it as wood. What it does: Introduces the main character and his theory that solves the mystery Source/Type: Researcher's claim/hypothesis Connection to Previous Sentences: This DIRECTLY answers the puzzle from sentence 1! Sentence 1 asked "where is the missing CO2?" and sentence 2 says "Auclair thinks it's in the trees." Visualization: CO2 from atmosphere → absorbed by trees → converted into wood fiber and stored there Reading Strategy Insight: Feel confident here - the author is giving us the solution immediately after the problem What We Know So Far: Missing CO2 + Auclair's theory that forests absorbed it What We Don't Know Yet: How Auclair proves this theory |
| Auclair uses measurements of factors affecting the area and density of a forest-such as logging, fires, and pests - and estimates of tree growth rates to argue that increases in the growth rates of individual trees in these forests since 1920 have created a large volume of wood that accounts for the missing carbon. | What it says: Auclair gathered data on what affects forests (logging, fires, pests) and tree growth speeds to show that since 1920, trees have been growing faster and creating more wood, which explains where the missing carbon went. What it does: Explains Auclair's methodology and key evidence Source/Type: Description of researcher's evidence and methods Connection to Previous Sentences: This builds on sentence 2 by explaining HOW Auclair supports his theory. Sentence 2 = the claim, Sentence 3 = the evidence for the claim Visualization: 1920: Trees growing at normal rate → 1970: Same trees growing 25% faster → More wood created = more carbon stored Reading Strategy Insight: This is still elaborating on the same basic idea - not new complexity What We Know So Far: The theory + the type of evidence used + the key timeframe (since 1920) What We Don't Know Yet: How to measure if forests are absorbing vs. releasing carbon overall |
| To determine whether the woodlands as a whole are releasing or absorbing carbon, the volume of wood added to the woodlands must be compared with the wood lost. | What it says: To figure out if forests help or hurt CO2 levels, you need to compare how much wood grows versus how much wood disappears. What it does: Introduces the analytical framework/method for evaluation Source/Type: Methodological principle (could be author's explanation or Auclair's approach) Connection to Previous Sentences: This sets up the logical framework for analyzing the data mentioned in sentence 3. It's saying "here's how you actually use that data to answer the question." Visualization: Wood Gained (from tree growth) vs. Wood Lost (from logging, fires, disease) = Net carbon absorption or release Reading Strategy Insight: This is setup language - the author is preparing us for the actual results coming next What We Know So Far: The theory + evidence + analytical method What We Don't Know Yet: What Auclair's analysis actually found |
| Auclair's analysis of the past hundred years shows the woodlands changing from a carbon source to a carbon sink. | What it says: When Auclair crunched the numbers for the last 100 years, he found that forests switched from releasing CO2 to absorbing CO2. What it does: Provides the key finding/conclusion from Auclair's research Source/Type: Research results/findings Connection to Previous Sentences: This gives us the result of applying the method from sentence 4 to the data from sentence 3 - it's the payoff we've been building toward Visualization: Timeline: 1825-1900: Forests = CO2 source → 1920-2020: Forests = CO2 sink Reading Strategy Insight: This is the main conclusion - everything else will be supporting details What We Know So Far: Complete theory with main finding What We Don't Know Yet: Specific details about the timeline and what caused the change |
| Before 1890, northern woodlands were a source of CO2, mainly because of forest fires and logging. | What it says: Before 1890, forests were adding CO2 to the atmosphere, mostly due to fires and cutting down trees. What it does: Provides specific detail supporting the "carbon source" part of the previous sentence Source/Type: Supporting evidence/detail from Auclair's analysis Connection to Previous Sentences: This explains the FIRST part of the change described in sentence 5. Sentence 5 said "changed from source to sink" - this explains the "source" period. Visualization: Pre-1890: Forest fires + logging → trees cut down or burned → CO2 released to atmosphere Reading Strategy Insight: This is just filling in details of what we already learned - feel more confident, not overwhelmed |
| Such deforestation releases CO2, because debris from the forest floor rots more quickly when the trees are cleared. | What it says: When you cut down forests, CO2 gets released because the leftover plant material on the ground rots faster without tree cover. What it does: Explains the scientific mechanism behind the previous statement Source/Type: Scientific explanation/mechanism Connection to Previous Sentences: This simply explains WHY the previous sentence is true - it's not new information, just clarification Visualization: Trees cut down → forest floor exposed to sun/air → dead leaves and branches rot faster → rotting releases stored carbon as CO2 Reading Strategy Insight: Pure explanation - the author is helping us understand, not adding complexity |
| After 1920, the steep increase in tree growth rates surpassed the losses stemming from fire and logging, turning the northern forests from a carbon source into a carbon sink and storing CO2 from fossil fuel over the next fifty years. | What it says: After 1920, trees started growing so much faster that the new wood more than made up for losses from fires and logging, so forests switched to absorbing CO2 (including CO2 from fossil fuels) for the next 50 years. What it does: Provides specific detail supporting the "carbon sink" part and completes the timeline Source/Type: Supporting evidence/detail from Auclair's analysis Connection to Previous Sentences: This explains the SECOND part of the change from sentence 5. Sentence 5: "source to sink." Sentence 6: explained "source." This sentence: explains "sink." Visualization: Post-1920: Tree growth rate increases dramatically → new wood volume > wood lost to fires/logging → net carbon storage in forests Reading Strategy Insight: This completes the circle back to our original mystery - we now know where the missing fossil fuel CO2 went! Final Summary: The passage gave us a problem (missing CO2), a solution (Auclair's theory), evidence (growth data), and confirmation (forests changed from CO2 source to CO2 sink after 1920) |
2. Passage Summary:
Author's Purpose:
To explain how a scientist solved the mystery of missing atmospheric carbon dioxide by showing that northern forests have been absorbing it.
Summary of Passage Structure:
The author builds their explanation in clear steps:
- First, the author presents a scientific puzzle - atmospheric CO2 levels haven't risen as much as expected based on fossil fuel burning
- Next, the author introduces scientist Allen Auclair and his theory that northern forests have been acting as carbon sinks, absorbing the missing CO2
- Then, the author explains Auclair's research methods and his key finding that forests changed from releasing CO2 to absorbing it over the past century
- Finally, the author provides specific details about this transition, explaining how forests released CO2 before 1890 due to logging and fires, but after 1920 began absorbing CO2 as tree growth rates increased dramatically
Main Point:
Northern forests have solved the mystery of missing atmospheric CO2 by transforming from a source of carbon dioxide (before 1890) into a carbon sink (after 1920), absorbing and storing the excess CO2 from fossil fuel burning as increased tree growth.
3. Question Analysis:
This question asks us to identify what change would make northern woodlands "more likely to function as a carbon source" - meaning what would cause them to release CO2 into the atmosphere instead of absorbing it.
Connecting to Our Passage Analysis:
From our passage analysis, we understand the key factors that determine whether forests are carbon sources or carbon sinks:
- Carbon Source Period (before 1890): Forests released CO2 mainly due to logging and fires, because "debris from the forest floor rots more quickly when trees are cleared"
- Carbon Sink Period (after 1920): "The steep increase in tree growth rates surpassed the losses stemming from fire and logging"
- The Critical Balance: "To determine whether the woodlands as a whole are releasing or absorbing carbon, the volume of wood added to the woodlands must be compared with the wood lost"
The passage shows us that forests become carbon sources when:
- Wood loss (from logging, fires, pests) exceeds wood growth
- Trees are removed, causing faster decomposition of forest floor debris
Prethinking:
For forests to return to being a carbon source, we need either:
- Increased wood loss through more logging, fires, or pest damage
- Decreased tree growth rates
- Something that tips the balance so that "wood lost" exceeds "wood added"
The correct answer should describe a scenario that increases forest destruction or decreases forest growth, disrupting the post-1920 pattern where growth surpassed losses.
Answer Choices Explained
A
Vegetation regrowing on land from which trees had been cleared grew back fast enough to absorb as much CO2 as was released by deforestation.
Why It's Wrong:
- This describes a scenario where regrowth compensates for deforestation by absorbing as much CO2 as was released
- This would maintain carbon neutrality, not create a carbon source
- The passage shows carbon sources occur when losses exceed growth, but this choice describes balanced absorption and release
Common Student Mistakes:
- Doesn't "fast regrowth" after deforestation mean more trees and more carbon absorption?
→ Yes, but this choice specifically states the regrowth only absorbs "as much" CO2 as was released - that's carbon neutral, not a carbon source - Isn't deforestation mentioned as causing carbon release in the passage?
→ True, but this choice describes regrowth that compensates for that release, creating balance rather than net CO2 release
B
Debris from the forest floor rotted less quickly after the rate of tree growth increased.
Why It's Wrong:
- Slower rotting of forest floor debris would reduce CO2 release, not increase it
- The passage states that faster rotting (when trees are cleared) contributes to carbon release
- This change would actually help forests remain carbon sinks by reducing decomposition-based CO2 emissions
Common Student Mistakes:
- The passage mentions increased tree growth rates - wouldn't this scenario with increased growth rates create similar effects?
→ This choice focuses on slower debris rotting, not faster tree growth, and slower rotting actually reduces CO2 release - Doesn't any change to the forest system potentially make it a carbon source?
→ Changes must specifically increase CO2 release or decrease CO2 absorption; slower decomposition does the opposite
C
A significant increase in the number of pests that destroy trees caused an increase in tree loss.
Why It's Right:
- Increased pest damage would significantly increase tree loss, disrupting the post-1920 balance where growth exceeded losses
- The passage specifically mentions "pests" as one of the factors Auclair measured that affects forest area and density
- This directly parallels the pre-1890 period when losses from "fires and logging" made forests carbon sources
- More tree death means the critical balance shifts from "wood added > wood lost" back to "wood lost > wood added"
Key Evidence: "To determine whether the woodlands as a whole are releasing or absorbing carbon, the volume of wood added to the woodlands must be compared with the wood lost" and "After 1920, the steep increase in tree growth rates surpassed the losses stemming from fire and logging" - pest damage would reverse this by dramatically increasing the "losses" side of the equation.
D
Pollution resulting from burning fossil fuels provided trees with extra nutrients, thus increasing the rate of their growth.
Why It's Wrong:
- Extra nutrients leading to increased tree growth would enhance the forest's role as a carbon sink
- The passage shows that increased growth rates after 1920 made forests into carbon sinks
- More rapid tree growth means more CO2 absorption and more carbon stored as wood
Common Student Mistakes:
- Doesn't pollution generally harm the environment and therefore harm forests?
→ While pollution often harms forests, this specific choice describes pollution that provides nutrients and increases growth - Could faster growth somehow lead to problems that make forests carbon sources?
→ The passage clearly establishes faster growth as the key factor that made forests carbon sinks after 1920
E
A decrease in temperature caused a significant decrease in the number of fires in the northern woodlands.
Why It's Wrong:
- Fewer fires would reduce forest losses, making forests more likely to remain carbon sinks
- The passage identifies fires as a major cause of carbon release in the pre-1890 period
- Reducing fires would help maintain the post-1920 pattern where "growth rates surpassed the losses stemming from fire and logging"
Common Student Mistakes:
- Don't forest fires release a lot of CO2 when they burn trees?
→ Yes, but this choice describes a decrease in fires, which would reduce CO2 release - Might temperature changes disrupt forest ecosystems in other ways?
→ Possibly, but this choice specifically focuses on decreased fires, which the passage clearly identifies as reducing CO2 release
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