AP Environmental Science Unit 9 Essentials

Rising atmospheric CO2 enhances the greenhouse effect, trapping infrared radiation and warming the atmosphere. Separately, oceans absorb a large share of that same CO2, which reacts with seawater to lower ocean pH — ocean acidification. These are two independent consequences of one cause, and the exam frequently tests whether you understand they're not the same mechanism.

The Montreal Protocol's successful phase-out of CFCs demonstrates that coordinated global action can reverse an atmospheric crisis when the problem is narrowly defined and viable substitutes exist. Climate change has proven harder to address through similar international agreements, partly because greenhouse gas emissions are far more economically embedded across every sector of the global economy than CFCs ever were.

Habitat loss, pollution, climate change, and invasive species rarely threaten a species through just one mechanism — a warming climate can shift a species' viable range right as habitat fragmentation blocks its ability to migrate there, while invasive species exploit the resulting disrupted ecosystem. Understanding this compounding effect, rather than treating each threat in isolation, is essential for both exam questions and real conservation strategy.