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
Global Change focus: Use these routes to connect official topics to evidence, calculations, and environmental decisions.
Unit 9: Global Change
9.1 Stratospheric Ozone Depletion
Global Change focus: Stratospheric ozone absorbs harmful ultraviolet radiation; chlorine and bromine radicals from long-lived compounds catalyze ozone destruction.
9.2 Reducing Ozone Depletion
Global Change focus: The Montreal Protocol and substitutions for ozone-depleting substances reduce future stratospheric chlorine and bromine loading.
9.3 The Greenhouse Effect
Global Change focus: The natural greenhouse effect warms Earth's surface because certain gases absorb and reemit outgoing infrared radiation.
9.4 Increases in the Greenhouse Gases
Global Change focus: Human activities increase greenhouse gases through fossil-fuel use, land change, agriculture, industry, and waste management.
9.5 Global Climate Change
Global Change focus: Climate change alters temperature, precipitation, extremes, ice, sea level, ecosystems, and human systems, with regional differences.
9.6 Ocean Warming
Global Change focus: Ocean warming causes thermal expansion, stresses organisms, shifts ranges, and can intensify stratification and coral bleaching.
9.7 Ocean Acidification
Global Change focus: Ocean acidification occurs as dissolved carbon dioxide forms carbonic acid and reduces carbonate availability for calcifying organisms.
9.8 Invasive Species
Global Change focus: Invasive species establish outside their native range and cause harm, often aided by transport, disturbance, and release from natural enemies.
9.9 Endangered Species
Global Change focus: Endangered species face a high risk of extinction from factors such as habitat loss, overuse, invasive species, pollution, and climate change.
9.10 Human Impacts on Biodiversity
Global Change focus: Human activities reduce biodiversity through habitat change, overexploitation, invasive species, pollution, and climate change.
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.
Continue in the A+ teaching layer for the remaining evidence labs, figure, and final audit.
How the AP Environmental Science assesses Global Change
Global Change focus: Use this contract to connect unit study to the current APES exam.
| Item | Weight / count | What it means |
|---|---|---|
| Official topic denominator | 10 | Global Change focus: Every listed CED topic is taught on this page and remains owned by one frozen taxonomy leaf. |
| Multiple choice | 80 questions in 90 minutes | Global Change focus: The section includes discrete and shared-stimulus sets using models, data, maps, calculations, and text sources. |
| Free response | 3 questions in 70 minutes | Global Change focus: The task families are investigation design, quantitative-data analysis, and an environmental problem with calculations. |
| Calculation credit | Global Change focus: Setup and answer can be separate rubric lines | Global Change focus: Write the model, substitute with units, and preserve a reasonable final magnitude. |
| Administration | May 2027 | Global Change focus: This page is scoped to the current fully digital Bluebook administration and dated public facts. |
Radioactive half-life
- Step 1Thirty-six years contains 36 / 12 = 3 half-lives.
- Step 2Apply three halvings: 160 → 80 → 40 → 20 mg.
- Step 3The remaining mass is 20 mg; half-life is exponential decay, not subtraction of a fixed mass.
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.
