Review AP Environmental Science Unit 9 to understand how human activities drive global-scale environmental change, from ozone depletion and the greenhouse effect to ocean acidification, invasive species, and biodiversity loss. This unit carries 15-20% of the exam and connects directly to concepts from nearly every earlier unit.
Use this page to review all 10 topics in Unit 9, check key terms, and find topic guides and practice questions available through Fiveable.
Unit 9 asks you to trace human-caused changes from the atmosphere all the way through ocean chemistry and living communities. The topics build on each other: greenhouse gas increases warm the planet, warming changes ocean temperature and chemistry, and those changes combine with direct human pressures to threaten species and biodiversity.
CFCs destroy stratospheric ozone through catalytic chlorine reactions, increasing UV-B at Earth's surface. Separately, CO2, methane, nitrous oxide, and CFCs trap outgoing infrared radiation and enhance the greenhouse effect. GWP ranks CFCs highest, followed by nitrous oxide, then methane, with CO2 as the baseline of 1.
Rising greenhouse gases cause oceans to absorb excess heat, stressing marine species through habitat loss, metabolic disruption, and coral bleaching when zooxanthellae are expelled. Simultaneously, oceans absorb CO2, forming carbonic acid, lowering pH, and reducing calcium carbonate availability, which harms shell-building organisms and corals.
Invasive species are often generalist, r-selected organisms that outcompete natives. Species become endangered through overhunting, habitat loss, limited diet, or invasive competition. HIPPCO summarizes the six main human-driven biodiversity threats: habitat destruction, invasive species, population growth, pollution, climate change, and overexploitation.
Every topic in Unit 9 traces back to a human action, whether burning fossil fuels, releasing CFCs, fragmenting habitat, or introducing non-native species. The exam expects you to explain the mechanism behind each change, not just name it. For example, knowing that CO2 dissolves to form carbonic acid (CO2 + H2O forms H2CO3, which releases H+ ions) is more useful than simply stating that oceans become acidic.
The ozone layer absorbs UV-B radiation. CFCs release chlorine radicals that catalytically destroy ozone. The Antarctic ozone hole forms each spring. Reduced ozone increases UV-B, raising skin cancer and cataract risk.
CFCs are replaced by HFCs, which do not deplete ozone because they lack chlorine. However, many HFCs are potent greenhouse gases. The Montreal Protocol phased out CFC production internationally.
CO2, methane, water vapor, nitrous oxide, and CFCs are the principal greenhouse gases. The natural greenhouse effect maintains surface temperatures for life. GWP ranks CFCs highest, then nitrous oxide, then methane, with CO2 as the baseline.
Rising greenhouse gas concentrations enhance the greenhouse effect, causing sea-level rise from ice melt and thermal expansion, and shifting disease vector ranges toward the poles.
Climate change alters sea level, ocean circulation, atmospheric circulation (Hadley cells, jet stream), soil, and polar ecosystems. The ice-albedo positive feedback loop causes polar regions to warm faster than the rest of Earth.
Greenhouse gases cause oceans to absorb excess heat. Warmer water causes coral bleaching by expelling zooxanthellae, disrupts marine species metabolism and reproduction, and shifts species ranges poleward.
Oceans absorb CO2, forming carbonic acid and lowering pH. Lower pH reduces calcium carbonate availability, harming corals, oysters, and other shell-building organisms. Fossil fuel combustion and deforestation are the main anthropogenic causes.
Invasive species thrive outside their native habitat and threaten native species. They are often generalist, r-selected species. Control strategies include prevention, physical removal, chemical treatment, and biological control.
Species become endangered through overhunting, limited diet, invasive competitors, or specific habitat needs. Selective pressures reduce fitness. Strategies include the Endangered Species Act, CITES, captive breeding, and habitat protection.
HIPPCO summarizes the six main biodiversity threats. Habitat fragmentation isolates populations and reduces gene flow. Some species are domesticated for economic use, reducing genetic diversity. Mitigation includes protected areas, corridors, and legislation.
Review major AP Environmental Science disasters from Chernobyl to coral bleaching, with causes, effects, and prevention strategies you can use on FRQs.
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Across 17k multiple-choice practice attempts for this unit.
Practice activity included in this snapshot.
Across 31 scored free-response attempts for this unit.
Review The Greenhouse Effect with attention to how the concept appears in AP-style source and evidence questions.
Review Global Climate Change with attention to how the concept appears in AP-style source and evidence questions.
Review Reducing Ozone Depletion with attention to how the concept appears in AP-style source and evidence questions.
Review Stratospheric Ozone Depletion with attention to how the concept appears in AP-style source and evidence questions.
The stratospheric ozone layer absorbs UV-B radiation, protecting life on Earth. CFCs released at the surface eventually reach the stratosphere, where UV light breaks them apart and releases chlorine radicals. Those radicals catalytically destroy ozone molecules. The Antarctic ozone hole forms each spring when polar stratospheric clouds provide surfaces for these reactions after winter darkness ends. Natural factors such as melting ice crystals also contribute to seasonal ozone loss.
| Factor | Type | Mechanism |
|---|---|---|
| CFCs | Anthropogenic | Release Cl radicals that catalytically destroy O3 |
| Polar stratospheric clouds | Natural/amplified | Provide surfaces for ozone-destroying reactions in Antarctic spring |
| Melting ice crystals | Natural | Release trapped reactive compounds at start of Antarctic spring |
Ozone depletion is reduced by replacing CFCs with chemicals that break down before reaching the stratosphere. Hydrofluorocarbons (HFCs) are the main CFC substitute because they contain no chlorine and do not deplete ozone. However, many HFCs are potent greenhouse gases, creating a trade-off between ozone protection and climate impact. The Montreal Protocol is the international agreement that phased out CFC production.
| Compound | Depletes Ozone? | Strong Greenhouse Gas? |
|---|---|---|
| CFCs | Yes | Yes |
| HFCs | No | Yes (many) |
| HFOs (newer alternatives) | No | Low GWP |
The natural greenhouse effect keeps Earth warm enough for life by allowing solar radiation in while greenhouse gases absorb outgoing infrared radiation. The principal greenhouse gases are CO2, methane, water vapor, nitrous oxide, and CFCs. Human activities have increased concentrations of these gases, enhancing the greenhouse effect and driving global warming. Water vapor has a short residence time and does not directly drive climate change the way long-lived gases do. GWP ranks the potency of each gas relative to CO2 (GWP = 1). Consequences of rising greenhouse gases include sea-level rise from melting ice and thermal expansion, and disease vectors such as mosquitoes spreading toward the poles.
| Greenhouse Gas | GWP Rank | Key Human Source |
|---|---|---|
| CO2 | Baseline (1) | Fossil fuel combustion, deforestation |
| Methane (CH4) | High (about 25x CO2) | Livestock, landfills, natural gas leaks |
| Nitrous oxide (N2O) | Very high (about 265x CO2) | Fertilizers, agriculture |
| CFCs | Highest | Refrigerants, aerosols (now phased out) |
| Water vapor | Greenhouse gas but short residence time | Evaporation (natural, not a driver) |
Climate change reshapes ecosystems through multiple interconnected pathways. Polar regions warm fastest because of the ice-albedo positive feedback loop: melting ice exposes darker ocean or land, which absorbs more solar energy, causing further warming. Atmospheric circulation changes include Hadley cell expansion and jet stream shifts. Ocean circulation, including the thermohaline conveyor belt, can slow as freshwater from melting ice reduces salinity-driven density differences. Marine ecosystems face both gains (new shallow habitats on flooded continental shelves) and losses (deeper communities pushed below the photic zone). Soil erosion and permafrost thaw releasing methane are additional land-based consequences.
| System Affected | Change | Consequence |
|---|---|---|
| Polar ice | Melting | Sea-level rise, reduced albedo, positive feedback |
| Ocean circulation | Slowing conveyor belt | Regional climate shifts, especially in North Atlantic |
| Atmosphere | Hadley cell expansion, jet stream shifts | Altered precipitation, more extreme weather |
| Marine ecosystems | Sea-level rise | New shelf habitats gained, deep communities lose photic zone access |
Oceans absorb most of the excess heat from the enhanced greenhouse effect, raising sea surface temperatures. Warmer water causes coral bleaching when corals expel their symbiotic zooxanthellae algae; some corals recover if temperatures drop, but prolonged bleaching leads to coral death. Warmer water also disrupts marine species metabolism and reproduction and shifts species ranges poleward. Separately, oceans absorb atmospheric CO2, which reacts with water to form carbonic acid: CO2 + H2O forms H2CO3, which dissociates to release H+ ions, lowering pH. Lower pH reduces carbonate ion availability, making it harder for corals, oysters, and other calcifying organisms to build calcium carbonate shells and skeletons.
| Process | Primary Cause | Effect on Coral |
|---|---|---|
| Ocean warming | Greenhouse gas-driven heat absorption | Bleaching from zooxanthellae expulsion |
| Ocean acidification | CO2 absorption forming carbonic acid | Reduced calcium carbonate, weaker skeletons |
Invasive species are organisms living outside their native habitat that threaten native species. They are often generalist, r-selected species that reproduce quickly, tolerate a wide range of conditions, and outcompete native specialists for resources. Examples include zebra mussels filtering out phytoplankton in the Great Lakes, kudzu overgrowing native vegetation in the southeastern US, and the brown tree snake eliminating native birds on Guam. Control strategies include prevention (inspections, ballast water treatment), physical removal, chemical treatment, and biological control using natural predators or pathogens.
Species become endangered when threats exceed their ability to adapt or relocate. Specialist species with limited diets, specific habitat needs, or low reproductive rates are most vulnerable. Selective pressures including overhunting, invasive competitors, and habitat loss reduce fitness and population size. HIPPCO summarizes the six main human-driven biodiversity threats: habitat destruction, invasive species, population growth, pollution, climate change, and overexploitation. Habitat fragmentation breaks large habitats into isolated patches, reducing gene flow and increasing edge effects. Strategies to protect species include the Endangered Species Act, CITES, captive breeding programs, habitat corridors, and protected areas.
| Strategy | How It Helps | Example |
|---|---|---|
| Endangered Species Act | Legal protection for listed species and their habitats | Bald eagle recovery in the US |
| CITES | Regulates international trade in endangered species | Bans ivory trade to protect elephants |
| Captive breeding | Increases population size outside the wild | California condor reintroduction |
| Habitat corridors | Connects fragmented patches for gene flow | Wildlife overpasses across highways |
| Protected areas | Preserves habitat from development | National parks and wildlife refuges |
Try stimulus-based AP practice questions and written prompts after you review the notes.
| Term | Definition |
|---|---|
| Chlorofluorocarbons (CFCs) | Synthetic compounds containing carbon, chlorine, and fluorine; primary anthropogenic cause of stratospheric ozone depletion and also potent greenhouse gases; phased out under the Montreal Protocol. |
| Ozone Layer | Region of the stratosphere with high ozone (O3) concentration that absorbs UV-B radiation, protecting life on Earth from increased skin cancer and cataract risk. |
| Montreal Protocol | International agreement that phased out production of ozone-depleting substances, primarily CFCs; widely regarded as a successful global environmental policy. |
| Hydrofluorocarbons (HFCs) | CFC substitutes that do not deplete the ozone layer because they contain no chlorine, but many have high global warming potential, creating a climate trade-off. |
| Global warming potential (GWP) | A measure of how much heat a greenhouse gas traps relative to CO2 (GWP = 1) over 100 years; CFCs rank highest, followed by nitrous oxide, then methane. |
| residence time | How long a gas remains in the atmosphere; water vapor has a short residence time and does not accumulate, while CO2 and methane persist for decades to centuries and drive long-term warming. |
| ice and snow albedo feedback | A positive feedback loop in which melting ice exposes darker surfaces that absorb more solar energy, causing further warming and more melting; explains why polar regions warm faster than the tropics. |
| ocean conveyor belt | Global thermohaline circulation that distributes heat through the world's oceans; can slow when freshwater from melting ice reduces the salinity-driven density differences that drive it. |
| Coral Bleaching | Thermal stress causes corals to expel their symbiotic zooxanthellae algae, turning white; prolonged bleaching leads to coral death because zooxanthellae provide most of the coral's energy. |
| Zooxanthellae | Photosynthetic algae living symbiotically inside coral tissue; their expulsion during bleaching deprives corals of nutrients and is caused by elevated sea surface temperatures. |
| Calcium Carbonate | The mineral used by corals, oysters, and other calcifying organisms to build shells and skeletons; less available as ocean pH drops due to acidification. |
| Generalist species | Species that tolerate a wide range of environmental conditions and use varied food sources; this flexibility helps invasive species establish and outcompete native specialists. |
| habitat corridors | Connected strips of habitat linking fragmented patches, allowing organisms to move between areas, maintaining gene flow and reducing local extinction risk. |
| Sea-Level Rise | Long-term increase in average ocean height caused by two mechanisms: melting ice sheets and glaciers adding water volume, and thermal expansion of warming seawater. |
These are two separate problems. CFCs deplete stratospheric ozone by releasing chlorine radicals, increasing UV-B at the surface. The greenhouse effect is about infrared radiation being trapped by greenhouse gases. CFCs contribute to both, but the mechanisms are completely different.
Water vapor is a greenhouse gas, but it has a very short residence time in the atmosphere and does not accumulate the way CO2 or methane do. It amplifies warming as a feedback but is not the primary driver of anthropogenic climate change.
Sea-level rise has two distinct mechanisms: melting ice sheets and glaciers add water volume, and thermal expansion occurs because warmer water takes up more space. Both must be mentioned for full credit on a free-response question.
Bleaching is caused by thermal stress that expels zooxanthellae. Acidification reduces calcium carbonate availability, weakening coral skeletons. Both harm coral but through different mechanisms, and the exam may ask you to distinguish them.
HFCs do not deplete ozone, which is why they replaced CFCs. But many HFCs have very high global warming potential, so switching to HFCs solves one environmental problem while potentially worsening another.
AP Environmental Science free-response questions frequently ask you to trace a human activity through multiple environmental consequences. In Unit 9, practice building chains such as: fossil fuel combustion increases CO2, which enhances the greenhouse effect, which warms oceans, which causes coral bleaching and disrupts marine food webs. Being able to write these chains with specific mechanisms (not just general statements) is the core skill tested.
The exam often asks you to propose a realistic solution to an environmental problem and explain why it works or identify a limitation. Unit 9 has several built-in trade-offs to practice with: HFCs solve ozone depletion but worsen climate change; biological control of invasive species can introduce new ecological risks; captive breeding helps endangered species but does not address the habitat loss driving endangerment.
Stimulus-based questions may present graphs of atmospheric CO2 concentration, ocean pH trends, temperature anomalies, or species population data. You should be able to identify the trend, connect it to a mechanism from Unit 9, and predict a consequence. Knowing the carbonic acid reaction and the GWP rankings gives you the quantitative and chemical vocabulary to answer these questions precisely.
Open the individual guides for Unit 9 when you want a closer review of one topic.
browse guidesPractice free-response reasoning and compare your answer with scoring guidance.
practice FRQsWatch past review streams filtered to Unit 9 when you want a video walkthrough.
open videosUse unit cheatsheets for a quick visual review after you work through the notes.
open cheatsheetsEstimate your broader AP score goal after you review the course and exam format.
open calculatorAPES Unit 9: Global Change covers 10 topics: Stratospheric Ozone Depletion, Reducing Ozone Depletion, The Greenhouse Effect, Increases in Greenhouse Gases, Global Climate Change, Ocean Warming, Ocean Acidification, Invasive Species, Endangered Species, and Human Impacts on Biodiversity. Together they connect human activity to large-scale environmental consequences. See the full topic breakdown at /ap-enviro/unit-9.
APES Unit 9 makes up 15-20% of the AP exam, making it one of the heavier-weighted units. It covers global climate change, the greenhouse effect, ozone depletion, ocean warming, ocean acidification, invasive species, and human impacts on biodiversity. Expect several multiple-choice questions and possible FRQ components drawn from these topics.
The APES Unit 9 progress check includes both MCQ and FRQ parts that pull from all 10 topics in the unit. The MCQ section tests concepts like the greenhouse effect, stratospheric ozone depletion, ocean acidification, and invasive species. The FRQ part typically asks you to explain causes and consequences of global climate change or human impacts on biodiversity, and may ask you to propose solutions. For matched practice questions that mirror the progress check format, visit /ap-enviro/unit-9.
APES Unit 9 FRQs most often focus on global climate change, the greenhouse effect, biodiversity loss, and ocean acidification. These questions typically ask you to identify causes, describe environmental consequences, and propose realistic solutions or policies. To practice, write out full responses to past prompts, check that you use precise vocabulary like 'greenhouse gases' and 'stratospheric ozone', and time yourself at about 22 minutes per FRQ. Find Unit 9 FRQ practice at /ap-enviro/unit-9.
The best place to find APES Unit 9 practice questions, including multiple-choice and practice test sets, is /ap-enviro/unit-9. That page has MCQ practice covering the greenhouse effect, ozone depletion, ocean warming, invasive species, and biodiversity. Working through unit-specific MCQs before a full practice test helps you spot which of the 10 topics still need attention.
Start APES Unit 9 by building a cause-and-effect map that links human activities to outcomes like global climate change, biodiversity loss, and ocean acidification. Then study each of the 10 topics in order, since topics like The Greenhouse Effect (9.3) and Increases in Greenhouse Gases (9.4) directly set up Global Climate Change (9.5). Use diagrams for the greenhouse effect and ozone depletion cycles, make a comparison chart for invasive vs. endangered species, and practice explaining solutions out loud since FRQs reward clear, specific reasoning. Finish each study session with a short MCQ set to check retention. All topic guides and practice are at /ap-enviro/unit-9.