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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.
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?
| 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) |
To explain how a scientist solved the mystery of missing atmospheric carbon dioxide by showing that northern forests have been absorbing it.
The author builds their explanation in clear steps:
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.
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.
From our passage analysis, we understand the key factors that determine whether forests are carbon sources or carbon sinks:
The passage shows us that forests become carbon sources when:
For forests to return to being a carbon source, we need either:
The correct answer should describe a scenario that increases forest destruction or decreases forest growth, disrupting the post-1920 pattern where growth surpassed losses.
Why It's Wrong:
Common Student Mistakes:
Why It's Wrong:
Common Student Mistakes:
Why It's Right:
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.
Why It's Wrong:
Common Student Mistakes:
Why It's Wrong:
Common Student Mistakes: