Global Change
Unit 9 of AP Environmental Science, worth 15–20% of the exam. 13 questions below, each with the working. Every answer was checked by a second pass before it was published.
Stratospheric ozone, the greenhouse effect, global climate change, ocean warming and acidification, invasive species, endangered species, human impacts on biodiversity.
How this unit is tested
Start by keeping the three big atmospheric/ocean processes separate in your head, because the exam loves to test whether you confuse them: ozone depletion is a stratospheric UV-shielding problem caused mainly by CFCs, the greenhouse effect is a tropospheric heat-trapping process caused by CO₂, CH₄, N₂O and water vapor, and ocean acidification is a chemical consequence of CO₂ dissolving into seawater. For any question, first identify which of these three systems is being described, then apply the specific mechanism (catalytic chlorine cycle, radiative absorption/re-emission, or carbonic acid formation).
For climate change questions, practice tracing cause-and-effect chains: an emission source leads to an atmospheric concentration change, which leads to a physical change (temperature, ice extent, sea level), which leads to a biological or feedback consequence. AP free-response questions frequently ask you to explain one link in this chain, so be ready to state a mechanism in a full sentence rather than just naming a term.
For biodiversity topics (invasive species, endangered species, human impacts), organize your knowledge around causes using a mnemonic like HIPPCO (Habitat loss, Invasive species, Pollution, Population growth, Climate change, Overharvesting). When given a scenario, identify which of these forces is at work and describe the mechanism of harm, not just the outcome.
Finally, practice distinguishing correlation from mechanism: the exam rewards answers that explain *why* something happens (e.g., why lower pH reduces carbonate ion availability) over answers that just restate the observation (e.g., 'the ocean is more acidic').
What you have to know
Catalytic ozone destruction
Chlorine atoms released from CFCs react with ozone (Cl + O₃ → ClO + O₂) and are then regenerated (ClO + O → Cl + O₂), so a single Cl atom can destroy thousands of O₃ molecules before being removed from the stratosphere.
Greenhouse effect
Greenhouse gases (CO₂, CH₄, N₂O, water vapor) are transparent to incoming shortwave solar radiation but absorb and re-emit outgoing longwave infrared radiation, trapping heat in the troposphere and warming Earth's surface.
Ocean acidification reaction
CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻; increased atmospheric CO₂ dissolves into seawater, raises H⁺ concentration (lowering pH), and consumes carbonate ions (CO₃²⁻) that calcifying organisms need.
Radiative forcing
A measure of the change in energy balance in the Earth-atmosphere system caused by a given factor (e.g., a greenhouse gas); positive forcing warms the surface, and forcing depends on both a gas's heat-trapping ability per molecule and its atmospheric concentration.
HIPPCO framework
The main human-caused drivers of biodiversity loss: Habitat destruction, Invasive species, Pollution, Population growth (human), Climate change, and Overharvesting.
13 practice questions
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Which chemical class is primarily responsible for catalytic destruction of stratospheric ozone?
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Answer. Chlorofluorocarbons (CFCs)
CFCs are broken down by UV radiation in the stratosphere, releasing chlorine atoms that catalytically destroy ozone molecules through a repeating reaction cycle. Because the cycle regenerates Cl, one atom can destroy thousands of O₃ molecules. -
Explain why a single chlorine atom can destroy thousands of ozone molecules rather than just one.
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Answer. Because the chlorine reaction with ozone is catalytic — chlorine is regenerated after the reaction, not consumed.
Cl reacts with O₃ to form ClO and O₂; ClO then reacts with a free O atom to reform Cl and O₂. Since Cl is regenerated at the end of the cycle, it can repeat the destructive reaction many times before being removed from the stratosphere. -
Describe the basic mechanism by which greenhouse gases warm the troposphere.
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Answer. They allow incoming shortwave solar radiation to pass through but absorb outgoing longwave infrared radiation and re-emit it, trapping heat near Earth's surface.
Solar energy reaches Earth mostly as visible light, which greenhouse gases don't absorb well. Earth re-radiates this energy as infrared, which greenhouse gases do absorb and re-emit in all directions, including back toward the surface, raising surface and lower-atmosphere temperatures. -
Why does methane (CH₄) have a much higher global warming potential per molecule than CO₂, yet contribute less total radiative forcing historically?
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Answer. CH₄ absorbs infrared radiation more strongly per molecule, but it is far less abundant in the atmosphere and has a shorter atmospheric lifetime than CO₂.
Global warming potential accounts for both the heat-trapping strength per molecule and how long a gas persists. CH₄ traps more heat per molecule over short time frames but breaks down in roughly a decade, while CO₂ is weaker per molecule but persists for centuries and is far more abundant, giving it greater cumulative forcing. -
Write the chemical process by which dissolved atmospheric CO₂ lowers ocean pH.
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Answer. CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
Dissolved CO₂ reacts with water to form carbonic acid, which dissociates into hydrogen ions and bicarbonate. The added H⁺ ions lower the water's pH, making it more acidic. -
Explain why ocean acidification harms calcifying organisms like corals and mollusks, beyond simply 'the water is more acidic.'
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Answer. The extra H⁺ ions bind with carbonate ions (CO₃²⁻), reducing the carbonate available for organisms to build calcium carbonate shells and skeletons.
Calcifying organisms need free carbonate ions to precipitate CaCO₃. As H⁺ concentration rises, more carbonate is tied up in bicarbonate, leaving less available for shell and skeleton formation, which slows growth and can cause shell dissolution. -
What causes coral bleaching, and why is it dangerous for the coral?
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Answer. Heat stress causes corals to expel their symbiotic zooxanthellae algae, cutting off the coral's main food and pigment source.
Zooxanthellae provide corals with the majority of their energy through photosynthesis and give them their color. When water temperatures rise, this symbiosis breaks down, the coral turns white (bleaches), and it becomes nutrient-starved and highly vulnerable to disease and death. -
Which international treaty specifically addressed stratospheric ozone depletion, as opposed to greenhouse gas emissions?
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Answer. The Montreal Protocol
The Montreal Protocol (1987) phased out production of CFCs and other ozone-depleting substances and is credited with the ongoing recovery of the ozone layer, distinct from climate treaties like the Kyoto Protocol or Paris Agreement, which target greenhouse gases. -
Describe the ice-albedo feedback loop and classify it as positive or negative.
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Answer. It is a positive feedback loop: warming melts ice and snow, exposing darker land or ocean surface that absorbs more solar radiation than reflective ice, causing further warming and more melting.
Ice and snow have high albedo (reflectivity), bouncing sunlight back to space. As they melt, the darker surfaces underneath absorb more heat, amplifying the original warming, which is the defining feature of a positive feedback loop. -
A lake ecosystem loses its native fish after a non-native predator species is introduced and rapidly outcompetes them for food with no natural predators of its own. Which biodiversity threat does this scenario primarily illustrate?
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Answer. Invasive species
An introduced species that lacks natural checks (predators, competitors, diseases) in its new environment and causes ecological harm, such as outcompeting native species, is by definition an invasive species, one of the HIPPCO drivers of biodiversity loss. -
Habitat fragmentation reduces biodiversity even when total remaining habitat area seems adequate. Explain one mechanism for why this happens.
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Answer. Fragmentation increases the proportion of 'edge habitat' relative to core habitat, exposing more of the ecosystem to disturbance, and it isolates populations, reducing gene flow and increasing extinction risk from inbreeding or local disasters.
When continuous habitat is broken into smaller patches, edge effects (changes in microclimate, increased predation, invasive species access) penetrate a larger share of each patch. Isolated populations also can't interbreed, lowering genetic diversity and resilience, an effect not captured by area alone. -
Using the HIPPCO framework, identify the primary driver of biodiversity loss in a scenario where a fish population collapses because commercial fleets removed adults faster than the population could reproduce.
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Answer. Overharvesting
HIPPCO's 'O' stands for overharvesting — removing individuals from a population faster than it can naturally replace them, which is distinct from habitat loss, pollution, or invasive species pressures described in this scenario. -
Explain why rising global temperatures and ocean acidification are considered two separate consequences of the same underlying cause.
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Answer. Both result from increased atmospheric CO₂ from fossil fuel combustion, but temperature rise occurs through the greenhouse effect trapping heat, while acidification occurs through direct chemical dissolution of CO₂ into seawater forming carbonic acid.
The shared cause is elevated atmospheric CO₂. One pathway is physical/radiative (heat trapping via the greenhouse effect), and the other is chemical (CO₂ dissolving into water and altering its acid-base chemistry) — students should be able to trace both pathways from the single CO₂ source.
What people get wrong
- Confusing stratospheric ozone depletion with the greenhouse effect/climate change — they are different problems (UV shielding vs. heat trapping) with different primary chemicals (CFCs vs. CO₂/CH₄); state which system a question is asking about before answering.
- Assuming the Montreal Protocol addressed climate change — it specifically phased out ozone-depleting substances (CFCs); the treaties addressing greenhouse gas emissions are the Kyoto Protocol and Paris Agreement.
- Thinking one chlorine atom destroys only one ozone molecule — the cycle is catalytic, so a single Cl atom is regenerated and can destroy many thousands of O₃ molecules before it leaves the stratosphere.
- Describing ocean acidification as 'CO₂ making water acidic' without the mechanism — always mention that CO₂ forms carbonic acid, which releases H⁺ and lowers carbonate ion availability, since that carbonate loss is what actually harms calcifying organisms.
- Labeling every non-native species as 'invasive' — a species is only invasive if it causes ecological or economic harm; many introduced species are harmless, so don't equate 'non-native' with 'invasive' on the exam.
- Misidentifying feedback loops — a positive feedback amplifies the original change (e.g., melting ice lowers albedo, increasing absorbed heat, causing more melting), while a negative feedback dampens it; check which direction the loop pushes the system before naming it.
Drill this unit until it sticks
These questions come back on a schedule built from what you get wrong, alongside the rest of AP Environmental Science. Free, and no account needed to start.