Global Change

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.

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
• Ozone Depletion and Climate Change: Two Problems, Not One17 min · 3 objectivesDistinguish stratospheric ozone depletion from the greenhouse effect · Explain the chemistry of CFC-driven ozone destruction · Compare greenhouse gases by lifetime and warming potential
• Feedback Loops, Ocean Acidification, and Invasive Species17 min · 3 objectivesDistinguish positive from negative feedback and identify climate examples · Explain ocean acidification chemistry and its effect on calcifying organisms · Explain why invasive species succeed and evaluate control options

Formulas in Unit 9

Every term in Unit 9

All 43 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.

The CFC catalytic cycle

UV frees a chlorine atom from a CFC; Cl reacts with O₃ to give ClO and O₂; ClO then reacts with a free oxygen atom, releasing Cl unchanged. Because the chlorine is regenerated, one atom destroys thousands of ozone molecules before it is removed.

Why the ozone hole forms over Antarctica

The polar vortex isolates cold air through the winter, polar stratospheric clouds form and convert chlorine into reactive forms, and returning spring sunlight sets the destruction going all at once. The hole is seasonal and geographic for those reasons, not because CFCs were emitted there.

Polar stratospheric clouds

Ice particles forming only below about −78 °C. Their surfaces convert stable chlorine reservoirs into forms that release chlorine the moment sunlight returns, which is the step that turns a global pollutant into a polar hole.

HCFCs and HFCs as replacements

HCFCs contain chlorine but break down in the troposphere, so far less reaches the stratosphere; HFCs contain none and do not deplete ozone. HFCs are, however, potent greenhouse gases, which the Kigali Amendment now phases down.

UV-A, UV-B and UV-C

UV-C is the most energetic and is absorbed entirely by ozone and oxygen. UV-B is partly absorbed and is what increases with ozone depletion, causing sunburn, cataracts and skin cancer. UV-A passes through largely unaffected either way.

Keeping ozone depletion and climate change apart

Different gases, different altitude, different mechanism. Ozone depletion is chlorine and bromine destroying stratospheric ozone and admitting UV; climate change is CO₂ and methane trapping outgoing infrared in the troposphere. Some gases do both, but conflating them is a standard lost point.

Radiative forcing

The change in the planet's energy balance in watts per square meter caused by a given agent, positive for warming and negative for cooling. It lets CO₂, methane, aerosols and solar variation be compared on one scale.

Why CO₂ matters most despite a low GWP

Methane traps far more heat per molecule, but CO₂ is emitted in vastly greater quantity and persists for centuries rather than about a decade. Total contribution is potency times quantity times lifetime, and CO₂ wins on the last two.

Atmospheric lifetime of greenhouse gases

Methane about 12 years, nitrous oxide about 115, CFCs 50–100, and a large fraction of CO₂ for centuries to millennia. Short-lived gases respond quickly to cuts; CO₂ commits the planet to warming long after emissions stop.

The Keeling curve

The continuous CO₂ record from Mauna Loa since 1958. It rises steadily and saws up and down each year as Northern Hemisphere plants take up carbon in summer and release it in winter — direct evidence that the increase is real and that the biosphere breathes.

Ice cores as a climate record

Trapped air bubbles give past atmospheric composition directly, and oxygen isotope ratios in the ice give past temperature. Together they show CO₂ and temperature moving together for 800,000 years, with current CO₂ far outside that range.

Ice-albedo feedback

Warming melts reflective ice, exposing darker ocean or land that absorbs more sunlight, which warms further and melts more ice. It amplifies the original change, making it a positive feedback and part of why the Arctic warms fastest.

Permafrost and methane

Thawing permafrost lets microbes decompose organic carbon frozen for millennia, releasing CO₂ and methane that cause further warming and further thaw. The stored carbon is roughly twice what is currently in the atmosphere.

A negative feedback in the climate system

Warming increases plant growth where water and nutrients allow, removing some CO₂; warming also increases infrared emission to space as T⁴. Both oppose the change — but neither is remotely large enough to offset emissions.

Thermal expansion vs ice melt

Roughly a third to a half of observed sea level rise is water expanding as it warms, the rest is land ice entering the ocean. The expansion term is why sea level keeps rising for centuries even after temperatures stabilize.

Why melting sea ice does not raise sea level

Floating ice already displaces its own weight of water, so melting it changes volume negligibly — the Archimedes result. Sea level rise comes from land ice: Greenland, Antarctica and mountain glaciers.

Carbonate chemistry and shell building

Dissolved CO₂ forms carbonic acid, which releases hydrogen ions that combine with carbonate. Less free carbonate means calcifiers must spend more energy to build shells, and existing shells begin to dissolve where the water is corrosive enough.

Why an invasive species succeeds

It arrives without the predators, parasites and competitors that limited it at home, often with high reproductive output and broad tolerance. Disturbed habitats give it the opening, which is why invasion and habitat damage travel together.

Controlling an invasive species

Prevention and early detection are by far the cheapest. Once established, options are mechanical removal, chemical control, and biological control — the last effective but risky, since the introduced control agent can itself become invasive.

Which HIPPCO factor is largest

Habitat destruction, including fragmentation, is the leading cause of extinction worldwide, with invasive species generally second. Climate change is rising fast and interacts with the others by moving suitable habitat away from protected areas.

CITES

The treaty regulating international trade in endangered species, listing them in appendices by how threatened they are. It works on trade rather than habitat, so it addresses ivory and exotic pets effectively and deforestation not at all.

Mitigation vs adaptation

Mitigation reduces the forcing — emissions cuts, efficiency, renewables, sequestration. Adaptation reduces the harm from warming already committed — sea walls, drought-tolerant crops, moved infrastructure. Both are needed, and a question naming one is not asking about the 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.
• If an answer says the ozone hole causes global warming, or that carbon dioxide destroys ozone, it has merged two separate problems and will lose the point regardless of what else it says. Name the **layer** — stratosphere or troposphere — early in any answer to keep them apart.

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

All 9 units of AP Environmental Science

1. Unit 1 · The Living World: Ecosystems
2. Unit 2 · The Living World: Biodiversity
3. Unit 3 · Populations
4. Unit 4 · Earth Systems & Resources
5. Unit 5 · Land & Water Use
6. Unit 6 · Energy Resources & Consumption
7. Unit 7 · Atmospheric Pollution
8. Unit 8 · Aquatic & Terrestrial Pollution
9. Unit 9 · Global Change