The capstone unit, tying the entire course together. Stratospheric ozone depletion, the greenhouse effect and global climate change, ocean warming and acidification, invasive and endangered species, and the broader human impacts on biodiversity worldwide.
How chlorofluorocarbons (CFCs) break down stratospheric ozone, thinning the layer that protects life from UV radiation.
Topic 9.2
Reducing Ozone Depletion
International agreements like the Montreal Protocol that phased out ozone-depleting chemicals.
Topic 9.3
The Greenhouse Effect
How greenhouse gases trap outgoing infrared radiation, warming Earth's surface and atmosphere.
Topic 9.4
Increases in Greenhouse Gases
The human activities — fossil fuel combustion, agriculture, deforestation — driving rising CO2 and methane levels.
Topic 9.5
Global Climate Change
The wide-ranging effects of a warming planet, including shifting weather patterns, sea level rise, and ecosystem disruption.
Topic 9.6
Ocean Warming
Rising ocean temperatures and their effects on marine ecosystems, including coral bleaching.
Topic 9.7
Ocean Acidification
How absorbed atmospheric CO2 lowers ocean pH, threatening shell- and skeleton-building marine organisms.
Topic 9.8
Invasive Species
Non-native species introduced to an ecosystem that outcompete native species and disrupt food webs.
Topic 9.9
Endangered Species
Species at risk of extinction, and the human activities (habitat loss, overharvesting) that drive species toward endangerment.
Topic 9.10
Human Impacts on Biodiversity
A synthesis of how habitat destruction, pollution, climate change, and invasive species together drive global biodiversity loss.
About Unit 9
Unit 9 is the capstone of AP Environmental Science, synthesizing concepts from every prior unit into a global perspective. You'll learn how human-released chemicals deplete stratospheric ozone, how greenhouse gases drive global climate change, how warming and acidifying oceans threaten marine life, and how invasive species, habitat loss, and climate change together drive biodiversity loss worldwide.
This unit is roughly 15–20% of the AP Environmental Science exam — the single largest unit — and takes about 14–17 class periods. Because it draws on energy (Unit 6), pollution (Units 7–8), and biodiversity (Unit 2) concepts, expect cumulative, connection-heavy questions.
The College Board ties Unit 9 to two Big Ideas:
Big Idea: STB
Sustainability — ozone depletion, climate change, and ocean warming and acidification
Big Idea: EIN
Interactions Between Different Species & the Environment — invasive species, endangered species, and biodiversity loss