Global Change — AP Environmental Science APES Study Guide

For: AP Environmental Science candidates sitting AP Environmental Science.

Covers: Stratospheric ozone depletion, the greenhouse effect and climate change, ocean acidification, biodiversity loss, and global change mitigation and adaptation policies aligned to the latest AP ES CED.

You should already know: Algebra 1, basic biology and chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Environmental Science style for educational use. They are not reproductions of past College Board papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official College Board mark schemes for grading conventions.

1. What Is Global Change?

Global change refers to large-scale, long-term shifts in Earth’s natural systems, driven by a combination of natural processes (e.g., volcanic eruptions, solar variability) and anthropogenic (human-caused) activities, that have cascading impacts across all abiotic and biotic spheres of the planet. This topic is Unit 9 in the latest AP Environmental Science CED, and accounts for 10–15% of your total exam score, making it a high-weight, frequently tested unit that integrates content from all earlier parts of the syllabus. Common synonyms for related concepts include anthropogenic climate disruption, global environmental change, and planetary boundary disruption.

2. Stratospheric ozone depletion

The stratosphere, located 10–50 km above Earth’s surface, contains a layer of ozone (O₃) that absorbs 97–99% of harmful incoming UV-B and UV-C radiation, protecting living organisms from DNA damage, skin cancer, cataracts, and reduced photosynthetic activity.

Ozone depletion occurs when synthetic chemicals called chlorofluorocarbons (CFCs) — once widely used in refrigerants, aerosol propellants, and foam-blowing agents — drift up to the stratosphere. Inert in the lower troposphere, CFCs break down when exposed to high-energy UV radiation in the stratosphere, releasing reactive chlorine (Cl) atoms that act as catalysts to destroy ozone: $$Cl+O_3\to ClO+O_2$$ $$ClO+O\to Cl+O_2$$ A single Cl atom can destroy up to 100,000 O₃ molecules before being removed from the stratosphere, and CFCs have an atmospheric lifetime of 50–100 years, so impacts persist for decades after emissions stop. Examiners frequently test why depletion is worst over Antarctica: cold winter temperatures form polar stratospheric clouds that provide surfaces for chemical reactions that activate stored Cl atoms, which are released when spring UV levels rise, creating the annual Antarctic ozone hole.

Impacts of depletion include increased skin cancer rates (a 1% drop in ozone correlates to a 2% increase in skin cancer), reduced crop yields, and disrupted aquatic food webs from phytoplankton damage. The 1987 Montreal Protocol, the most successful global environmental agreement in history, phased out 99% of CFC production globally, with ozone layers projected to recover to 1980 levels by 2050–2070. The 2016 Kigali Amendment extended the protocol to phase out hydrofluorocarbons (HFCs), CFC replacements that are potent greenhouse gases.

Worked Example: If 1 kg of CFC-12 destroys 100,000 kg of ozone over its atmospheric lifetime, how much ozone is destroyed by the 12 million kg of CFC-12 emitted globally in 1987? $$1.2\times 10^7\text{ kg CFC}\times 10^5{\text{ kg O}}_3/\text{kg CFC}=1.2\times 10^{12}{\text{ kg O}}_3\text{ destroyed}$$

3. Greenhouse effect and climate change

The natural greenhouse effect is an essential process that supports life on Earth: incoming shortwave solar radiation passes through the atmosphere, is absorbed by Earth’s surface, and is re-radiated as longwave infrared (IR) radiation. Greenhouse gases (GHGs) including CO₂, CH₄, N₂O, water vapor, and CFCs absorb this IR radiation, trapping heat and keeping Earth’s average surface temperature at ~15°C, rather than the -18°C it would be without GHGs.

The enhanced (anthropogenic) greenhouse effect occurs when human activities emit excess GHGs into the atmosphere, increasing the amount of heat trapped and driving anthropogenic climate change. A key metric for comparing GHG impact is Global Warming Potential (GWP), the amount of heat 1 ton of a gas traps over a 100-year period relative to 1 ton of CO₂: CO₂ = 1, CH₄ = 28–36, N₂O = 265–298, CFCs = 1,000–10,000. Total emissions are measured in CO₂ equivalent (CO₂e): $${\text{Total CO}}_{2}e=\sum (\text{mass of gas}\times \text{GWP of gas})$$

Evidence for anthropogenic climate change includes ice core data showing atmospheric CO₂ levels never exceeded 300 ppm in 800,000 years, but now exceed 420 ppm, and global average temperatures have risen 1.1°C since the pre-industrial era. Impacts include sea level rise from thermal expansion and land ice melt, more frequent and intense extreme weather (heatwaves, hurricanes, droughts, floods), shifting species ranges, and disrupted precipitation patterns that reduce global food security.

Worked Example: A small rural community emits 120 tons of CO₂, 3 tons of CH₄ (GWP = 30), and 0.2 tons of N₂O (GWP = 270) annually. What is its total annual CO₂e emissions? $${\text{CH}}_4{\text{ CO}}_{2}e=3\text{ t}\times 30=90\text{ t}$$ $${\text{N}}_2{\text{O CO}}_{2}e=0.2\text{ t}\times 270=54\text{ t}$$ $${\text{Total CO}}_{2}e=120+90+54=264{\text{ t CO}}_{2}e$$

4. Ocean acidification

Oceans absorb ~30% of all anthropogenic CO₂ emissions, reducing atmospheric warming but driving a chemical change called ocean acidification. When CO₂ dissolves in seawater, it reacts to form carbonic acid, which dissociates to release hydrogen ions: $$C{O}_2+H_{2}O\to H_{2}C{O}_3\to H^{+}+HC{O}_3^{-}\to 2H^{+}+C{O}_3^{2-}$$ Increased H⁺ concentration lowers ocean pH: pre-industrial ocean pH was 8.2, it has dropped to 8.1 today, representing a 30% increase in H⁺ concentration (since pH is a negative logarithmic scale, a 0.1 pH drop = ~1.26x higher H⁺ concentration). Under high-emissions scenarios, ocean pH could drop to 7.9 by 2100, a 150% increase in H⁺ concentration.

The primary impact of ocean acidification is reduced availability of carbonate ions (CO₃²⁻), which calcifying organisms including coral, shellfish, pteropods, and coccolithophores use to build calcium carbonate (CaCO₃) shells and skeletons. Low carbonate levels slow growth, weaken shells, and can even dissolve existing shells in severely acidified waters. Examiners often test the difference between ocean acidification and coral bleaching: bleaching is caused by elevated ocean temperatures (1–2°C above average for 4+ weeks) that cause corals to expel their symbiotic zooxanthellae algae, while acidification reduces coral growth rates and slows recovery after bleaching events.

Worked Example: If ocean pH drops from 8.1 to 7.9 by 2100 under a high-emissions scenario, what is the percentage increase in hydrogen ion concentration? $$[H^{+}]\text{ at pH 8.1}=10^{-8.1}=7.94\times 10^{-9}\text{ M}$$ $$[H^{+}]\text{ at pH 7.9}=10^{-7.9}=1.26\times 10^{-8}\text{ M}$$ $$\text{Percentage increase}=\frac{1.26\times 10^{-8}-7.94\times 10^{-9}}{7.94\times 10^{-9}}\times 100=58.7\%\text{ increase}$$

5. Biodiversity loss

Biodiversity refers to the variety of life on Earth across three interconnected levels: genetic diversity (variation within a species), species diversity (number of distinct species in an ecosystem), and ecosystem diversity (variety of habitats and ecological processes across the planet). The current global rate of extinction is 100–1000x the natural background rate of ~1 extinction per million species per year, leading scientists to call the current event the 6th mass extinction, or Holocene extinction.

The primary drivers of biodiversity loss are summarized by the acronym HIPPCO, tested frequently on the AP exam: Habitat destruction (the #1 cause, e.g., tropical deforestation for palm oil agriculture), Invasive species (non-native species that outcompete native organisms), Pollution (including plastic, chemical, and nutrient pollution), Population growth (human population growth increasing resource demand), Climate change (shifting species ranges and disrupting ecosystems faster than many organisms can adapt), and Overexploitation (overharvesting of species for food, medicine, or the pet trade).

Impacts of biodiversity loss include reduced ecosystem resilience (ability to recover from disturbance), loss of ecosystem services (pollination, water purification, carbon sequestration, natural medicine), and reduced global food security.

Worked Example: A 2023 IUCN assessment found that 27% of 150,000 assessed species are at risk of extinction. How many species does this represent? $$0.27\times 150,000=40,500\text{ species at risk of extinction}$$

6. Mitigation and adaptation policies

Global change responses are divided into two categories, a distinction examiners test in every APES administration:

• Mitigation: Actions that reduce GHG emissions or remove GHGs from the atmosphere to reduce the magnitude of future climate change. Examples include transitioning to renewable energy (solar, wind, geothermal), improving energy efficiency, reforestation and afforestation, carbon capture and storage (CCS), and reducing deforestation through programs like REDD+.
• Adaptation: Actions that adjust human or natural systems to reduce harm from current or expected climate change impacts. Examples include building sea walls and flood barriers for coastal communities, developing drought-resistant crop varieties, implementing urban heat action plans, and managed retreat of communities from high-risk flood zones.

Key global policies include the 2015 Paris Agreement, signed by 196 countries, which sets a global goal to limit warming to well below 2°C, preferably 1.5°C, above pre-industrial levels. Countries submit nationally determined contributions (NDCs) every 5 years outlining their emissions reduction targets.

Worked Example: A reforestation project sequesters 22 tons of CO₂ per hectare per year. How many hectares of forest are needed to sequester the 264 t CO₂e annual emissions of the community from Section 3? $$\frac{264{\text{ t CO}}_{2}e}{22{\text{ t ha}}^{-1}{\text{yr}}^{-1}}=12\text{ hectares of forest}$$

7. Common Pitfalls (and how to avoid them)

• Wrong move: Confusing stratospheric "good ozone" with tropospheric "bad ozone", and blaming tropospheric ozone pollution for ozone depletion. Why students do it: The same chemical is present in both layers, so students mix up their functions. Correct move: Explicitly state that ozone depletion only impacts the stratospheric ozone layer, while tropospheric ozone is a toxic air pollutant and GHG that does not repair the ozone layer.
• Wrong move: Claiming the greenhouse effect is entirely harmful. Why students do it: Most media coverage focuses on the negative impacts of anthropogenic warming. Correct move: Always specify the "enhanced" or "anthropogenic" greenhouse effect when referring to harmful human-driven warming, and note that the natural greenhouse effect is essential to support life on Earth.
• Wrong move: Treating pH changes as linear, e.g., assuming a 0.2 pH drop equals a 20% increase in H⁺ concentration. Why students do it: Many forget that pH is a negative logarithmic scale. Correct move: Always calculate H⁺ concentration with the formula $[H^{+}]=10^{-pH}$ before computing percentage changes.
• Wrong move: Classifying mitigation actions as adaptation, e.g., calling solar panel installation an adaptation strategy. Why students do it: Students mix up the core purpose of each strategy type. Correct move: Ask two questions: Does this action reduce GHG emissions or remove CO₂? If yes, it is mitigation. Does it reduce harm from existing or expected warming? If yes, it is adaptation.
• Wrong move: Attributing all coral bleaching to ocean acidification. Why students do it: Both impacts are driven by rising CO₂ emissions, so students conflate them. Correct move: State clearly that coral bleaching is primarily caused by elevated ocean temperatures, while ocean acidification reduces coral growth rates and slows recovery after bleaching events.

8. Practice Questions (AP Environmental Science Style)

Question 1 (Multiple Choice)

Which of the following correctly pairs the driver of a global change impact with the global policy designed to address it? A) CO₂ emissions / Montreal Protocol B) CFC emissions / Kigali Amendment C) HFC emissions / Paris Agreement D) CFC emissions / Montreal Protocol

Worked Solution: Correct answer is D. CFCs are the primary driver of stratospheric ozone depletion, and the 1987 Montreal Protocol phased out global CFC production. A is wrong because the Montreal Protocol addresses ozone depletion, not CO₂-driven climate change. B is wrong because the Kigali Amendment addresses HFCs, not CFCs. C is wrong because the Paris Agreement addresses overall GHG emissions, not HFCs specifically.

Question 2 (Free Response Part A)

A dairy farm emits 75 tons of CO₂, 8 tons of CH₄ (GWP = 30), and 0.3 tons of N₂O (GWP = 270) annually. Calculate the farm’s total annual CO₂e emissions. Show all work.

Worked Solution:

1. Calculate CH₄ CO₂e: $8\text{ t}\times 30=240{\text{ t CO}}_{2}e$
2. Calculate N₂O CO₂e: $0.3\text{ t}\times 270=81{\text{ t CO}}_{2}e$
3. Sum all emissions: $75+240+81=396{\text{ t CO}}_{2}e$ total annual emissions.

Question 3 (Free Response Part B)

Identify one mitigation strategy and one adaptation strategy that the farm could implement to address its contributions and vulnerability to climate change, and justify each choice.

Worked Solution:

• Mitigation: Install solar panels on barn roofs to power farm operations, reducing reliance on grid electricity generated from fossil fuels, which cuts the farm’s annual CO₂ emissions.
• Adaptation: Plant drought-resistant forage crops for cattle, which reduces crop failure risk during periods of prolonged drought driven by climate change, stabilizing the farm’s food supply for its herd.

9. Quick Reference Cheatsheet

10. What's Next

This Global Change unit is the capstone of the AP Environmental Science curriculum, integrating content from all preceding units: atmospheric science (Unit 4), pollution (Unit 7), land and water use (Unit 5), and energy resources (Unit 6). Mastery of this unit will also help you connect cross-cutting themes like sustainability, resource use, and human-environment interactions that are tested across 100% of the AP exam, including the multiple-choice section and both free-response questions (the experimental design FRQ and problem-solving FRQ). Questions on this unit are also frequently paired with data interpretation prompts, so practicing CO₂e and pH calculations will help you earn easy math points on test day.

If you need help working through additional practice questions, clarifying tricky concepts like GWP calculations or ozone chemistry, or want to test your mastery with full-length timed practice exams, reach out to Ollie, your 24/7 APES tutor on the OwlsPrep platform. You can also access additional study materials for all APES units on the homepage to build a comprehensive study plan tailored to your exam timeline, whether you are studying 3 months or 3 weeks before your test date.