AP Environmental Science, AP exams, 15–20%

Unit 9: Global Change

Unit 9 carries the largest published weighting in the course. It covers stratospheric ozone depletion, the greenhouse effect and climate change, ocean warming and acidification, invasive species, and human impacts on biodiversity. Almost every earlier unit feeds into it, which is why it is worth reviewing last and most thoroughly.

Position: 9 of 9 units
Published weighting: 15–20% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Environmental Science. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Stratospheric ozone depletion: How chlorofluorocarbons destroy ozone, the resulting effects, and the treaty response that reduced them.
The greenhouse effect and greenhouse gases: How the effect works, the major gases, and their differing residence times and warming potentials.
Global climate change: Observed changes, feedback loops such as albedo and permafrost, and the consequences for ecosystems.
Ocean warming and acidification: Coral bleaching, thermal expansion and sea level, and the carbonate chemistry behind acidification.
Invasive species: Why introduced species can spread rapidly, the damage they cause, and the control options available.
Human impacts on biodiversity: Habitat loss, overharvesting, pollution, and the conservation strategies used to slow species decline.

Study tips

How to practice Global Change.

Section-specific guidance that follows from how this material is usually tested.

Keep ozone depletion and the greenhouse effect strictly separate in your notes; mixing them is the most common exam error in this unit.
Practice explaining one feedback loop in full, naming the trigger, the amplifying step, and the result.
Review this unit last and most often; it carries the largest published weighting of the nine.

Where this fits

Unit 9: Global Change is 9 of 9 units in the AP Environmental Science guide, and it carries a published weighting of 15–20% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Environmental Science guide Official exam information

Sample Global Change questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
Which class of compounds, once widely used as refrigerants and aerosol propellants, releases chlorine atoms that destroy stratospheric ozone?
1. ANitrogen oxides
2. BSulfur dioxide
3. CChlorofluorocarbons
4. DVolatile organic compounds
Show answer
Answer: C. Chlorofluorocarbons
Chlorofluorocarbons are so stable that they drift intact into the stratosphere, where ultraviolet light splits off chlorine atoms that each destroy many ozone molecules before being removed. Sulfur dioxide is the tempting distractor because it is a famous air pollutant, but it drives acid deposition rather than ozone loss. The catalytic behavior of chlorine is why small quantities did so much damage.
Key insight: One chlorine atom destroys thousands of ozone molecules because it is regenerated after each reaction.
Question 2Foundational
Which international agreement phased out the production of ozone-depleting substances such as chlorofluorocarbons?
1. AThe Kyoto Protocol
2. BThe Clean Water Act
3. CThe Paris Agreement
4. DThe Montreal Protocol
Show answer
Answer: D. The Montreal Protocol
The Montreal Protocol committed its parties to phasing out chlorofluorocarbons and related compounds, and stratospheric ozone has since begun a slow recovery. The Kyoto Protocol is the tempting distractor because it is the other well-known atmospheric treaty, but it addressed greenhouse gas emissions rather than ozone-depleting chemicals. Keeping the two treaties straight is a reliable way to keep the two atmospheric problems straight.
Key insight: Montreal is for the ozone layer; Kyoto and Paris are for greenhouse gases.
Question 3Intermediate
True or false: stratospheric ozone depletion is the main cause of global climate change.
1. ATrue
2. BFalse
Show answer
Answer: B. False
Ozone depletion and climate change are separate problems with different causes and different effects. Ozone loss is driven by chlorine and bromine compounds and lets more ultraviolet radiation reach the surface, while warming is driven by greenhouse gases that absorb outgoing infrared radiation. The only real overlap is that several ozone-depleting compounds happen to be potent greenhouse gases as well.
Key insight: Ozone depletion lets ultraviolet in; the greenhouse effect keeps infrared from getting out.
Question 4Intermediate
As the ocean absorbs more carbon dioxide from the atmosphere, what happens to seawater pH and to the carbonate ions that shell-building organisms need?
1. ApH decreases and carbonate ion availability decreases
2. BpH decreases and carbonate ion availability increases
3. CpH increases and carbonate ion availability decreases
4. DpH increases and carbonate ion availability increases
Show answer
Answer: A. pH decreases and carbonate ion availability decreases
Dissolved carbon dioxide forms carbonic acid, which releases hydrogen ions and lowers pH. Those extra hydrogen ions also combine with carbonate ions to form bicarbonate, so less carbonate remains available for corals, oysters, and plankton to build calcium carbonate shells and skeletons. The tempting answer pairs the correct pH change with the wrong carbonate response.
Key insight: Acidification is a double blow: more acid to resist and less carbonate to build with.
Question 5Advanced
Melting Arctic sea ice exposes darker open ocean beneath it. What kind of feedback does this create for warming, and why?
1. ANegative feedback, because open water reflects more sunlight than ice does
2. BPositive feedback, because open water reflects more sunlight than ice does
3. CPositive feedback, because lower albedo means more solar energy is absorbed
4. DNegative feedback, because open water absorbs heat and cools the atmosphere
Show answer
Answer: C. Positive feedback, because lower albedo means more solar energy is absorbed
Bright ice reflects most of the sunlight that strikes it, while dark ocean absorbs most of it, so replacing ice with water lowers the region's albedo and increases absorbed energy. That extra energy warms the water and melts still more ice, which reinforces the original change and makes the loop a positive feedback. The distractors invert either the sign of the feedback or the reflectivity of open water.
Key insight: A positive feedback amplifies the change that started it; it is not a change that is good.

Continue through AP Environmental Science

Practice Global Change deliberately.

Create your Arcade Academy workspace to turn this topic list into spaced-repetition reviews, linked notes, and progress you can act on.

Create your account

Unit 9: Global Change