AP Environmental Science Unit 9 ReviewGlobal Change

AP Environmental Science Unit 9, Global Change, is about how human activity reshapes Earth's systems on a global scale, from the greenhouse effect and climate change to ocean warming, ozone depletion, and biodiversity loss. The single biggest idea: excess greenhouse gases from burning fossil fuels and other human activities are warming the planet, and that warming ripples out into rising seas, acidifying oceans, shifting ecosystems, and disappearing species. It is worth 15-20% of the AP exam, making it one of the heaviest-weighted units and the place where everything you learned all year comes together.

What this unit covers

Ozone depletion and the greenhouse effect

These are two different atmospheric problems that students constantly mix up, so keep them separate.

• The stratospheric ozone layer absorbs incoming UV radiation and protected early life enough for it to evolve and survive. Less ozone means more UV at the surface, which causes skin cancer, cataracts, and damage to ecosystems.
• Ozone depletion is mostly anthropogenic, driven by chlorofluorocarbons (CFCs). A natural factor also plays in: ice crystals in the atmosphere at the start of Antarctic spring speed the reactions that break ozone apart.
• The fix is replacing CFCs with substitutes that do not destroy ozone. Hydrofluorocarbons (HFCs) are one common replacement, but the catch is that some HFCs are strong greenhouse gases, so you trade one problem for a smaller one.
• The greenhouse effect is the natural process that keeps Earth warm enough for life. The principal greenhouse gases are carbon dioxide, methane, water vapor, nitrous oxide, and CFCs.
• Water vapor is a greenhouse gas but doesn't drive long-term climate change because it has a short residence time in the atmosphere (it rains out fast). Carbon dioxide and methane stick around.

Greenhouse gases, potency, and global warming

The exam cares a lot about which gases matter most and why.

• Global warming potential (GWP) ranks how much heat a gas traps compared to CO2. Carbon dioxide has a GWP of 1 and is the reference point. CFCs have the highest GWP, then nitrous oxide, then methane.
• A gas can be powerful per molecule (high GWP) but still matter less overall if there isn't much of it. CO2 dominates the warming conversation because of sheer quantity, even with a GWP of 1.
• Rising greenhouse gas levels lead to melting ice sheets and thermal expansion of seawater, both raising sea level.
• Warming pushes disease vectors (like mosquitoes) from the tropics toward the poles, spreading diseases into new regions.
• These pressures change population dynamics and trigger population movements as people and species respond to a shifting climate.

Climate change, ocean warming, and acidification

Most of the warming and most of the absorbed CO2 ends up in the ocean, so the ocean is where many of the biggest effects show up.

• Climate change alters atmospheric circulation. Shifts in Hadley cells and the jet stream redistribute heat and change weather patterns.
• Sea level rise affects marine ecosystems in mixed ways. Flooded continental shelves can create new shallow habitat, while deeper communities may sink below the photic zone and lose the light they need.
• Ocean warming comes straight from rising greenhouse gases. It causes habitat loss plus metabolic and reproductive changes in marine species.
• Coral bleaching happens when warm water makes corals expel the symbiotic algae living inside them, turning the coral white. Some recover, some die.
• Ocean acidification is the drop in ocean pH as seawater absorbs atmospheric CO2. The dissolved CO2 forms carbonic acid, which makes it harder for shelled organisms and corals to build calcium carbonate skeletons.

Biodiversity loss: invasive and endangered species

The back half of the unit zooms in on how all this change pushes species toward extinction.

• HIPPCO is the acronym for the main drivers of biodiversity loss: Habitat destruction, Invasive species, Population growth, Pollution, Climate change, and Overexploitation.
• Habitat fragmentation breaks large habitats into small isolated patches. Roads, pipelines, logging, and clearing for agriculture or development do this, and it leaves species stranded in pieces too small to survive.
• Invasive species thrive outside their normal range and threaten native species. They tend to be generalist, r-selected species (fast reproducers, broad diets) that outcompete natives for resources.
• Species become endangered when they are heavily hunted, have a limited diet, get outcompeted by invasives, or need very specific habitat.
• Not every species is equally at risk. Ones that can adapt or migrate to a new environment are less likely to go extinct than specialists locked into one niche.

Unit 9, Global Change at a glance

Why Unit 9, Global Change matters in APES

This is where the whole course pays off. Earlier units built the pieces (ecosystems, energy flow, pollution, resource use), and Unit 9 shows what happens when human activity scales those pressures up to the entire planet. It ties directly to the course's big ideas about how Earth's systems interact and how human actions disrupt them.

• It demonstrates interconnection. One driver, excess CO2, simultaneously warms the air, raises seas, and acidifies oceans.
• It centers human responsibility. Nearly every problem here is anthropogenic, which means human choices can also be solutions.
• It connects cause to consequence to mitigation, the analytical chain the exam asks you to trace over and over.
• It forces you to compare natural variation (orbital cycles, volcanic activity) against the much faster, human-driven change happening now.

How this unit connects across the course

• Ocean acidification and the carbon cycle build directly on ecosystem cycling and energy flow from The Living World (Units 1-2), and endangered species ties back to biodiversity, speciation, and species diversity covered there.
• Habitat fragmentation, overexploitation, and population growth as HIPPCO drivers connect to carrying capacity and human population dynamics from Populations (Unit 3) and land conversion from Land and Water Use (Unit 5).
• The greenhouse gas story is the payoff of fossil fuel combustion introduced in Energy Resources and Consumption (Unit 6), where you learned which fuels release the most CO2.
• Cross-boundary pollution and CO2 emissions link to Atmospheric Pollution (Unit 7) and Aquatic and Terrestrial Pollution (Unit 8), with Unit 9 scaling those local pollutants up to global consequences.

Key equations and processes

• Ocean acidification chemistry: CO2 + H2O forms H2CO3 (carbonic acid), which dissociates into H+ and HCO3-. The extra H+ ions lower ocean pH. Know how to write and read this when a prompt asks why oceans acidify.
• Greenhouse effect process: incoming shortwave solar radiation is absorbed by Earth's surface, re-emitted as longwave (infrared) heat, and trapped by greenhouse gases. Use this to explain why more gas means more warming.
• Global warming potential (GWP) comparison: CO2 = 1 as the baseline; CFCs greater than N2O greater than CH4 in potency. Use it to rank gases by per-molecule heat-trapping strength.
• Coral bleaching process: warm water stress causes corals to expel symbiotic algae, removing their color and food source. Trace the cause (ocean warming) to the visible effect (white coral).
• CFC ozone destruction: UV light frees chlorine from CFCs, and that chlorine catalytically breaks ozone molecules apart. Explain why one chlorine atom can destroy many ozone molecules.

Unit 9, Global Change on the AP exam

Unit 9 is 15-20% of the exam, one of the heaviest weights in the course, and its content shows up across multiple-choice and free-response questions. You'll explain causes and effects (why oceans acidify, why corals bleach, how rising greenhouse gases raise sea level), and you'll connect a human activity to an environmental consequence. Expect to analyze data and stimulus material like graphs of CO2 concentration over time, temperature trends, or ice extent, then describe what the data shows and propose a solution. FRQs commonly ask you to identify a problem, explain its mechanism, describe a mitigation strategy, and justify your reasoning, so practice writing in those verbs. Because this unit pulls from everywhere else, a Unit 9 question can easily require you to bring in fossil fuels (Unit 6), pollution (Units 7-8), or biodiversity (Unit 2), so prep it as a synthesis unit, not an isolated one.

Essential questions

• How do human activities turn natural processes like the greenhouse effect into global-scale problems?
• Why does the same root cause, excess atmospheric CO2, produce so many different effects across the atmosphere and oceans?
• What makes some species adapt or survive global change while others go extinct?
• How can the chemicals and behaviors that cause environmental problems also become the basis for solutions?

Key terms to know

• Stratospheric ozone: the ozone layer that absorbs incoming UV radiation and shields life at Earth's surface.
• Chlorofluorocarbons (CFCs): human-made chemicals that destroy stratospheric ozone and also act as potent greenhouse gases.
• Hydrofluorocarbons (HFCs): CFC substitutes that don't deplete ozone but some of which are strong greenhouse gases.
• Greenhouse gas: a gas that traps outgoing heat, including CO2, methane, water vapor, nitrous oxide, and CFCs.
• Global warming potential (GWP): a measure of how much heat a gas traps relative to CO2, which is set at 1.
• Residence time: how long a gas stays in the atmosphere, which is why water vapor doesn't drive long-term warming.
• Thermal expansion: the rise in sea level caused by warmer ocean water taking up more volume.
• Coral bleaching: the whitening of coral when heat stress forces it to expel its symbiotic algae.
• Ocean acidification: the drop in ocean pH as seawater absorbs atmospheric CO2 and forms carbonic acid.
• HIPPCO: the acronym for the main drivers of biodiversity loss (habitat destruction, invasives, population growth, pollution, climate change, overexploitation).
• Habitat fragmentation: the breaking of large habitats into small, isolated patches by roads, logging, or development.
• Invasive species: a generalist, often r-selected species that thrives outside its range and outcompetes native species.
• Endangered species: a species at risk of extinction due to factors like hunting, limited habitat, or competition.
• Photic zone: the sunlit upper layer of ocean where photosynthesis happens, which deeper communities can lose access to as sea level shifts.

Common mix-ups

• Ozone depletion is not the greenhouse effect. Ozone depletion lets in more UV radiation and happens in the stratosphere; the greenhouse effect traps outgoing heat and drives warming. They're separate problems that happen to share some chemicals like CFCs.
• Ocean warming and ocean acidification are different effects of CO2. Warming is about trapped heat raising water temperature; acidification is about CO2 dissolving into seawater and lowering pH. CO2 causes both, but through different pathways.
• High GWP doesn't mean a gas causes the most warming overall. CFCs have a far higher GWP than CO2, but CO2 still drives the most warming because there is so much more of it in the atmosphere.
• Not every non-native species is invasive. A species only counts as invasive when it threatens native species; some non-native species are harmless or even beneficial.