AP Environmental Science Global Change Guide
Return to the AP Environmental Science exam guide.
Global Change in the AP Environmental Science blueprint
College Board assigns Unit 9, Global Change, 15–20% of AP Environmental Science multiple-choice questions. This range describes the multiple-choice section, not a percentage of the total exam score.
Global Change
- May 2027
- Bluebook
- Clean-room teaching
- Read the evidence surface.
- Trace the environmental mechanism.
- Check units, scale, and claim scope.
What Global Change covers
Within Global Change, use these routes to connect official topics to evidence, calculations, and environmental decisions.
Unit 9: Global Change
9.1 Stratospheric Ozone Depletion
When studying Global Change, stratospheric ozone absorbs harmful ultraviolet radiation; chlorine and bromine radicals from long-lived compounds catalyze ozone destruction.
9.2 Reducing Ozone Depletion
9.3 The Greenhouse Effect
9.4 Increases in the Greenhouse Gases
9.5 Global Climate Change
9.6 Ocean Warming
9.7 Ocean Acidification
9.8 Invasive Species
9.9 Endangered Species
9.10 Human Impacts on Biodiversity
Vocabulary anchors
Stratospheric Ozone Depletion; Reducing Ozone Depletion; The Greenhouse Effect; Increases in the Greenhouse Gases; Global Climate Change; Ocean Warming; Ocean Acidification; Invasive Species; Endangered Species; Human Impacts on Biodiversity are the official topic anchors used throughout this unit.
Evidence lab: Ocean carbonate response under two emissions pathways
Researchers used the same ocean model, initial chemistry, and 2080 climate forcing except for the carbon-dioxide pathway. Values are annual global surface-ocean means; the calcification index is relative to the present-day value of 100.
Pathway | Atmospheric CO2 (ppm) | Surface pH | Carbonate ion (µmol/L) | Calcification index Lower-emissions pathway | 510 | 8.02 | 205 | 91 Higher-emissions pathway | 780 | 7.86 | 164 | 73
What is the percent decrease in carbonate-ion concentration from the lower-emissions pathway to the higher-emissions pathway?
20.0% — The decrease is 205 − 164 = 41 µmol/L. Use the lower-emissions value as the comparison baseline: 41 / 205 × 100 = 20.0%. Dividing by 164 would answer a different relative-change question, and reporting 41% would confuse the concentration difference with a percentage.
Which chemical mechanism best explains why the higher-emissions pathway has lower pH and less carbonate ion?
Additional dissolved carbon dioxide forms carbonic acid, increases hydrogen-ion activity, and shifts carbonate equilibria away from carbonate ion. — This sequence links the atmospheric driver to both displayed chemical responses. Ocean acidification does not require seawater to become acidic below pH 7, and carbon dioxide does not remove carbonate by simple physical settling.
What does the calcification index support, and what does it not establish by itself?
It supports lower modeled calcification under the higher-emissions pathway, but it does not establish the response of every species or population. — The index is an aggregate modeled endpoint. A defensible claim preserves the model scope and does not convert a global mean into a universal organism-level observation.
Which design feature most directly isolates the effect of the carbon-dioxide pathway in this comparison?
The two runs hold the model, initial chemistry, and other 2080 climate forcing constant. — Keeping those features matched makes pathway carbon dioxide the stated manipulated difference. Repetition would help estimate numerical stability, but it would not repair a comparison in which multiple forcings changed together.
What additional observation would best test whether the modeled chemical shift affects a local shell-forming population?
Measure local carbonate chemistry together with growth, recruitment, and survival across replicated sites and seasons. — Coupled exposure and demographic measurements test the proposed pathway while representing spatial and temporal variation. A single shell photograph or one atmospheric carbon-dioxide value would not connect local chemistry to population performance.
Why should the two pathway rows not be interpreted as a prediction for one named coastline?
They are global annual surface-ocean means from a model comparison, so regional circulation, freshwater input, temperature, upwelling, and biological uptake can produce a different local trajectory. — The rows are useful for testing the direction and relative magnitude of the modeled pathway response. They do not resolve seasonal extremes or habitat exposure at one site. A local management decision would therefore pair the global scenario with site-specific chemistry, repeated biological observations, and a stated threshold for revising the response plan.
Which displayed variable is the direct chemical measure rather than a biological response index?
Carbonate-ion concentration, reported in micromoles per liter, is the direct chemical quantity. — The calcification index summarizes a modeled biological process relative to a reference value. Keeping those endpoints separate prevents a chemical change from being described as if it were a directly counted population decline. Surface pH is also chemical evidence, but it represents hydrogen-ion activity on a logarithmic scale rather than carbonate-ion abundance.
Why is the pH difference of 0.16 units not properly described as only a two-percent change in acidity?
The pH scale is logarithmic, so subtracting pH values and dividing by the original pH does not calculate the relative change in hydrogen-ion activity. — The table permits a direct comparison of reported pH, but a percentage claim about acidity would require converting both pH values to hydrogen-ion activity first. Treating pH as a linear concentration is a dimensional and conceptual error.
Which intervention addresses the scenario driver, and which intervention chiefly addresses exposure?
Reducing carbon-dioxide emissions addresses the forcing pathway, while protecting local refuges or reducing other stressors chiefly manages exposure and vulnerability. — Emissions mitigation can limit additional chemical change across long time horizons. Local adaptation can improve persistence under conditions already experienced, but it does not remove the global carbon-dioxide driver. A combined plan should label both functions and monitor separate outcomes.
What result would challenge the claim that carbonate chemistry alone explains the lower calcification index?
A replicated run with matched carbonate chemistry but a substantially different calcification index would indicate that another modeled factor or interaction contributes. — Temperature, nutrient supply, species composition, acclimation, and model structure may alter the biological endpoint. This does not erase the displayed association; it narrows the causal claim and identifies the controlled comparison needed to test the proposed mechanism.
What is the relative decrease in the calcification index between pathways?
About 19.8%. — The index falls by 91 − 73 = 18 points. Relative to the lower-emissions pathway, 18 / 91 × 100 ≈ 19.8%. The 18-point difference is not itself an eighteen-percent decrease, and using 73 as the denominator would measure the difference relative to the higher-emissions endpoint instead of the stated baseline.
Continue in the A+ teaching layer for the remaining evidence labs, figure, and final audit.
How the AP Environmental Science assesses Global Change
| Item | Weight / count | What it means |
|---|---|---|
| Official topic denominator | 10 | |
| Multiple choice | 80 questions in 90 minutes | |
| Free response | 3 questions in 70 minutes | |
| Calculation credit | ||
| Administration | May 2027 |
Radioactive half-life
Read each question as it appears on the exam paper, choose your answer, then tap it to see the feedback.
Practice 1: Apply radioactive half-life
A radioactive tracer begins at 160 mg and has a 12-year half-life.
How much of the tracer remains after 36 years?
Select the best answer.
A. Subtract 50 mg during each 12-year interval to obtain 10 mg.
❌ Not quite 😭 Half-life removes half of the current mass, not a fixed mass each interval. Linear subtraction does not represent exponential decay.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
B. Count three half-lives and halve 160 mg three times to obtain 20 mg.
✅ Correct! 😊 Thirty-six years contains three 12-year intervals. The sequence 160 → 80 → 40 → 20 mg applies the same proportional loss each time.
C. Halve 160 mg twice to obtain 40 mg.
❌ Not quite 😭 Two halvings represent only 24 years. The final 12-year interval requires one more halving.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
D. Halve 160 mg once to obtain 80 mg because the half-life is constant.
❌ Not quite 😭 A constant half-life means the same time per halving, not that only one halving occurs. Three intervals have elapsed.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
Full explanation
- Step 1Determine elapsed half-lives: 36 years ÷ 12 years = 3.
- Step 2Apply three successive halvings: 160 → 80 → 40 → 20 mg.
- Step 3Report 20 mg remaining; do not subtract a fixed mass.
Practice 2: Interpret an ocean-acidification model
Using the same ocean model and initial chemistry, a lower-emissions pathway gives 205 µmol/L carbonate ion and a calcification index of 91; a higher-emissions pathway gives 164 µmol/L and an index of 73.
Calculate the percent decrease in carbonate-ion concentration, explain the chemical mechanism, state one supported biological conclusion and one limitation, and propose a local field test.
Show your work.
A. Report a 41% decrease and conclude that every shell-forming population will decline by the same amount.
❌ Not quite 😭 Forty-one is the concentration difference, not the percentage. A global modeled index also cannot establish an identical response for every population.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
B. Report a 20.0% decrease, link dissolved CO₂ to increased hydrogen-ion activity and lower carbonate, and limit the biological claim to the modeled index.
✅ Correct! 😊 This uses the lower-emissions value as the baseline, states the carbonate-chemistry mechanism, and keeps the conclusion within the model's measured scope.
C. Divide 41 by 164 and treat the global annual means as direct observations from one coastline.
❌ Not quite 😭 The denominator answers a different relative-change question. Global modeled means do not resolve local circulation, seasons, or habitat exposure.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
D. Explain the change as carbonate physically settling out and infer complete ecosystem collapse.
❌ Not quite 😭 The stated mechanism is a shift in carbonate equilibria, not simple settling. The evidence does not support a universal collapse claim.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
Full explanation
- Step 1Calculate the decrease: 205 − 164 = 41 µmol/L; then 41 ÷ 205 × 100 = 20.0%.
- Step 2Additional dissolved CO₂ forms carbonic acid, increases hydrogen-ion activity, and shifts equilibria away from carbonate ion.
- Step 3The lower index supports reduced modeled calcification, but not the response of every species or local population.
- Step 4Test locally by pairing repeated carbonate measurements with growth, recruitment, and survival across replicated sites and seasons.
Key terms for Global Change
- Stratospheric Ozone Depletion
- Reducing Ozone Depletion
- The Greenhouse Effect
- Increases in the Greenhouse Gases
- Global Climate Change
- Ocean Warming
- Ocean Acidification
- Invasive Species
- Endangered Species
- Human Impacts on Biodiversity
Global Change FAQ
How do CFCs damage stratospheric ozone?
Ultraviolet radiation releases chlorine radicals from stable source molecules in the stratosphere. Those radicals participate in catalytic cycles that destroy ozone while the chlorine can be regenerated.
Why are ozone depletion and climate change not the same problem?
They involve different atmospheric layers, chemicals, and mechanisms. Some substances affect both, but ultraviolet shielding by stratospheric ozone is distinct from greenhouse trapping of outgoing infrared energy.
How does carbon dioxide cause ocean acidification?
Dissolved carbon dioxide forms carbonic acid and shifts carbonate equilibria, increasing hydrogen-ion activity and reducing carbonate availability. The process concerns chemistry even when seawater remains above neutral pH.
Why does sea level respond slowly to mitigation?
Ocean heat uptake, thermal expansion, land-ice dynamics, and long-lived greenhouse gases create inertia. Cutting emissions limits additional forcing but does not instantly reverse changes already committed.
How should an invasive-species solution be judged?
Identify the introduction pathway, affected interaction, feasibility, non-target risks, and monitoring endpoint. Prevention, early detection, containment, removal, or biological control have different tradeoffs and timing.
Continue through the APES system
How to study Global Change
Keep the stratospheric-ozone and climate mechanisms on separate pages, then add only the justified links. Label ultraviolet photolysis and chlorine catalysis on one; infrared absorption, feedbacks, ocean heat, and carbon reservoirs on the other.
Practice climate evidence across timescales: forcing, atmospheric concentration, temperature, ice, sea level, ocean chemistry, ecosystems, and people respond at different rates. State whether each number is observed, projected, local, or global.
Assess biodiversity responses with prevention first, then early detection, containment, removal, restoration, and monitoring. Write a measurable trigger for adapting the plan and explicitly check non-target effects, reinvasion, habitat connectivity, and community burden.
