AP Environmental Science Unit 9 Cheat Sheet

The basics

What it covers: The capstone unit — ozone depletion, the greenhouse effect, climate change, ocean warming/acidification, and biodiversity loss at a global scale.

Exam weight: About 15–20% of the AP Environmental Science exam — the single largest unit.

The big question: How is human activity altering Earth's atmosphere, oceans, and biodiversity at a planetary scale?

Big Ideas covered: Sustainability (STB), Interactions Between Different Species & the Environment (EIN).

The key terms you must know

• CFCs (chlorofluorocarbons) — chemicals that catalytically destroy stratospheric ozone.
• Montreal Protocol — international treaty that phased out ozone-depleting substances.
• Greenhouse effect — gases trapping infrared radiation, warming the atmosphere.
• Radiative forcing — change in Earth's energy balance from a given factor (e.g., a greenhouse gas).
• Coral bleaching — corals expelling symbiotic algae under heat stress, turning white.
• Ocean acidification — falling ocean pH from absorbed CO2.
• Invasive species — non-native species disrupting a new ecosystem.
• Endangered species — a species at high risk of extinction.
• Feedback loop — a self-reinforcing or self-dampening cycle (e.g., melting ice → less reflectivity → more warming).
• Endangered Species Act — U.S. law protecting at-risk species and their habitats.

Key themes to remember

• This unit is the synthesis of the whole course. Energy choices (Unit 6), pollution (Units 7–8), and biodiversity (Unit 2) all converge here at a global scale.
• CO2 causes two separate problems. The greenhouse effect (atmospheric warming) and ocean acidification (absorbed CO2 lowering pH) are distinct mechanisms, both driven by the same gas.
• International cooperation can work. The Montreal Protocol's success with ozone contrasts with the greater difficulty of global climate agreements.
• Biodiversity threats compound each other. Climate change, habitat loss, pollution, and invasive species rarely act in isolation.
• Feedback loops can accelerate change. Climate change isn't necessarily linear — some effects (like ice melt) reinforce further warming.

Common exam traps

• Don't confuse ozone depletion with the greenhouse effect. Different chemicals (CFCs vs. CO2/CH4), different layers (stratosphere vs. whole atmosphere), different mechanisms.
• The greenhouse effect itself is natural and necessary — the problem is the human-caused enhancement, not its existence.
• Ocean acidification and ocean warming are different problems with different mechanisms (CO2 absorption lowering pH vs. heat stress), even though both stem from rising atmospheric CO2.
• Habitat loss, not hunting, is the leading cause of most species endangerment — don't default to overharvesting as the primary driver.
• The Montreal Protocol addressed ozone, not climate change — don't conflate the two international agreements or problems.
• Methane has a different time profile than CO2. It traps more heat per molecule but breaks down faster — both facts can be tested separately.