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AP Environmental Science · Unit 9 of 9
Global Change
15–20% of the exam4 lessons · 50 min21 terms
What this unit covers
The topics below follow the published Env. Science course framework for Unit 9. This unit is worth 15–20% of the exam, so budget your time against that rather than against how long the unit takes to teach.
Ozone depletionGreenhouse gasesClimate changeInvasive species
Lessons in this unit
- Stratospheric Ozone Depletion12 min · 3 objectivesExplain the protective role of stratospheric ozone and how CFCs destroy it · Describe the effects of increased UV radiation reaching Earth’s surface · Identify the Montreal Protocol as the response to ozone depletion
- Greenhouse Gases & the Greenhouse Effect13 min · 3 objectivesDistinguish the natural greenhouse effect from the human-enhanced greenhouse effect · Identify the major greenhouse gases and their sources · Use global warming potential to compare the impact of different greenhouse gases
- Climate Change Impacts13 min · 3 objectivesDescribe the major physical and ecological impacts of a warming climate · Explain how CO₂ causes ocean acidification and why it harms marine life · Identify positive feedback loops that accelerate warming
- Invasive Species & Biodiversity Loss12 min · 3 objectivesExplain why invasive species succeed and the harm they cause · Use HIPPCO to organize the major causes of biodiversity loss · Describe conservation strategies that protect endangered species
Formulas in Unit 9
Catalytic ozone destruction
CFC + UV → free Cl· ; then Cl· destroys ozone repeatedly (one Cl → tens of thousands of O₃)
Chlorine is a catalyst: it is not used up, so a small amount of CFC destroys a vast amount of ozone. This is why even low CFC concentrations were so damaging.
CO₂-equivalent (using GWP)
CO₂-equivalent = mass of gas × GWP
GWP compares a gas to CO₂ (GWP = 1). Multiplying a gas’s mass by its GWP converts it to the mass of CO₂ that would trap the same heat.
Change over time (rate × time)
total change = rate of change × time
Useful for projecting impacts such as sea-level rise. If sea level rises at 3.3 mm per year, multiply by the number of years to estimate the total rise.
HIPPCO — causes of biodiversity loss
Habitat destruction · Invasive species · Population growth · Pollution · Climate change · Overexploitation
Habitat destruction is the #1 cause of extinction; invasive species are #2. Most threatened species suffer from several HIPPCO factors simultaneously.
Every term in Unit 9
All 21 terms we publish for Global Change, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.
- Stratospheric ozone depletion
- Chlorofluorocarbons release chlorine in the stratosphere, and one chlorine atom destroys thousands of ozone molecules catalytically.
- Montreal Protocol
- International agreement phasing out CFCs. The most successful environmental treaty; the ozone hole is measurably recovering.
- Consequences of ozone depletion
- More ultraviolet-B reaching the surface, raising skin cancer and cataract rates and damaging phytoplankton at the base of marine food webs.
- Greenhouse effect
- Greenhouse gases absorb outgoing infrared radiation and re-emit it downward, warming the surface. A natural process intensified by emissions.
- Major greenhouse gases
- CO₂ from combustion, methane from livestock, landfills and leaks, nitrous oxide from fertilizer, and water vapor. Methane traps far more heat per molecule but persists less long.
- Global warming potential
- A gas's heat-trapping ability relative to CO₂ over a set period, combining absorption strength with atmospheric lifetime.
- Positive feedback loops in climate
- Ice melt lowers albedo, permafrost thaw releases methane, and warming oceans hold less CO₂ — each amplifies the initial warming.
- Ocean acidification
- Absorbed CO₂ forms carbonic acid, lowering pH and reducing carbonate available for shells and coral skeletons.
- Coral bleaching
- Heat-stressed coral expel their symbiotic zooxanthellae, losing color and their main energy source. Prolonged bleaching kills the reef.
- Sea level rise mechanisms
- Thermal expansion of warming water plus melting land ice. Melting sea ice does not raise sea level, since it already displaces its own weight.
- Invasive species
- Introduced organisms spreading unchecked because their new range lacks their predators, parasites and competitors. Zebra mussels and kudzu are standard examples.
- Causes of extinction (HIPPCO)
- Habitat destruction, Invasive species, Pollution, Population growth, Climate change, Overexploitation.
- Endangered Species Act
- Prohibits harming listed species and requires recovery plans and designation of critical habitat.
- Sustainability
- Meeting present needs without compromising future generations' ability to meet theirs. Requires resource use at or below regeneration rate.
- Ecological footprint
- Land and water area needed to supply one person's resources and absorb their waste. Far larger in wealthy countries than in poor ones.
- Kyoto Protocol and Paris Agreement
- Kyoto set binding emission targets for developed countries; Paris uses voluntary nationally determined contributions with near-universal participation.
- Carbon sequestration
- Capturing and storing carbon in forests, soils, oceans or geological formations to keep it out of the atmosphere.
- Cap and trade
- A regulator caps total emissions and issues tradeable permits, so reductions happen where they are cheapest. Used successfully for SO₂ under the Clean Air Act.
- Carbon tax vs cap and trade
- A tax fixes the price and lets emissions vary; cap and trade fixes emissions and lets the price vary.
- Evidence for climate change
- Ice cores, tree rings, sea level records, glacial retreat, ocean heat content and instrumental temperature records — independent lines that agree.
- Range shifts and phenology
- Species are moving poleward and upslope, and seasonal events like flowering and migration are shifting earlier, decoupling species that depend on each other.
What examiners penalize here
- Anchor the ozone story: cause = CFCs → chlorine → catalytic O₃ destruction; effect = more UV-B → skin cancer, cataracts, harm to phytoplankton; solution = Montreal Protocol. Keep it firmly separate from the greenhouse-gas/climate story in Lessons 2–3.
- For CO₂-equivalent problems, multiply mass by GWP (a common trap is to add them). Know the main gases and sources: CO₂ (fossil fuels, deforestation), CH₄ (livestock, landfills, gas leaks), N₂O (fertilizers). CO₂ = largest total contributor despite its GWP of 1.
- Know the key positive feedback loops (ice-albedo, permafrost methane release) because they show up on free-response prompts about why warming accelerates. And keep ocean acidification (a CO₂ chemistry effect) distinct from warming (a heat effect) — both stem from CO₂ but by different mechanisms.
- Tie the units together on the exam: invasive species succeed for the same reasons r-strategists and generalists do (Unit 3, Unit 2), and corridors between protected areas apply island biogeography (Unit 2). HIPPCO with habitat loss at #1 is the framework graders look for on biodiversity free-response questions.
Practice Env. Science
Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.
Questions about this unit
How much of the AP Environmental Science exam is Unit 9?
Unit 9, Global Change, is worth 15–20% of the Env. Science multiple-choice section according to the published course framework. Across all 9 units that makes it one of the heaviest units on the exam, and worth front-loading.
What topics are covered in Env. Science Unit 9?
Global Change covers Ozone depletion, Greenhouse gases, Climate change and Invasive species. We publish 21 terms with definitions for this unit, all of them on this page.
How should I study Env. Science Unit 9?
Read the 4 lessons below first — about 50 minutes — then drill the 21 terms in cram mode until you can produce each definition from memory rather than just recognize it. Recognition is what makes a unit feel finished when it is not. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.
All 9 units of AP Environmental Science
Unit names, topics and exam weights follow the published College Board course framework for AP Environmental Science. AP® is a trademark registered by the College Board, which does not endorse this site.