Practice a College Board-style free response question on Global Change. Write your response, then reveal the model answer to see exactly what earns each point.
Free Response Question · Unit 9 · The Greenhouse Effect & Ocean Acidification
Scientists have tracked atmospheric CO2 concentration and average ocean surface pH at a monitoring station over a 40-year period.
Year
Atmospheric CO2 (ppm)
Ocean surface pH
1985
345
8.11
1995
360
8.09
2005
380
8.06
2015
400
8.04
2025
425
8.01
A
Using the data, describe the relationship between atmospheric CO2 concentration and ocean surface pH over the 40-year period, and explain the chemical mechanism responsible for this relationship.
✓ Model answer (earns the point)
As atmospheric CO2 concentration increased from 345 ppm to 425 ppm, ocean surface pH decreased from 8.11 to 8.01 — an inverse relationship. This occurs because the ocean absorbs a significant portion of atmospheric CO2, and that CO2 reacts with seawater to form carbonic acid, which releases hydrogen ions and lowers the water's pH — the process known as ocean acidification.
Why it scores: Describes the inverse relationship using specific data values AND names the specific chemical mechanism (CO2 + water → carbonic acid → lower pH) — not just "CO2 went up and pH went down" without the underlying chemistry.
B
Explain why this same rising CO2 concentration is also linked to global climate change, and identify the specific atmospheric mechanism involved — making clear how it differs from the ocean acidification mechanism in Part A.
✓ Model answer (earns the point)
Rising atmospheric CO2 also enhances the greenhouse effect: CO2 molecules absorb and re-emit outgoing infrared radiation, trapping heat in the atmosphere and warming Earth's surface. This is a distinct mechanism from ocean acidification — the greenhouse effect involves CO2's interaction with infrared radiation in the atmosphere, while ocean acidification involves CO2's direct chemical reaction with seawater. The same gas causes two separate, independent global problems through two different physical/chemical processes.
Why it scores: Names the specific mechanism (greenhouse effect — infrared absorption) AND explicitly distinguishes it from the acidification mechanism — not just stating both exist without clarifying they're different processes.
C
Propose one specific policy or technology that could address the root cause of both trends shown in the data, and explain why addressing CO2 emissions would help mitigate both problems simultaneously.
✓ Model answer (earns the point)
Transitioning electricity generation from fossil fuels to renewable energy sources like solar and wind would directly reduce CO2 emissions at the source. Because both the greenhouse effect and ocean acidification trace back to the same root cause — rising atmospheric CO2 from fossil fuel combustion — reducing emissions addresses both problems simultaneously: less atmospheric CO2 means both less enhanced greenhouse warming and less CO2 available for the ocean to absorb and convert into carbonic acid.
Why it scores: Proposes a specific, realistic solution AND explicitly explains why reducing the shared root cause (CO2 emissions) addresses both problems — not just naming a solution without connecting it to both mechanisms from Parts A and B.
How to score points on AP Environmental Science FRQs
Use the data, not just general knowledge. Cite specific values and describe trends directly using the table.
Name the specific mechanism, not just the phenomenon. "Carbonic acid formation" and "greenhouse effect/infrared absorption" are precise vocabulary graders check for.
Distinguish related-but-different mechanisms explicitly. Unit 9 FRQs often reward clarifying that the greenhouse effect and ocean acidification are separate processes sharing a common cause.
Answer the verb in the prompt. "Describe" needs the trend. "Explain" needs the mechanism. "Propose and explain" needs a specific solution PLUS the reasoning connecting it to the problem.
Be concise but complete. 2–4 sentences per part is usually enough. Every sentence should add something a grader can mark as a scoring point.