AP Environmental Science: Global Change — AP Environmental Science
1. Overview of Global Change ★★☆☆☆ ⏱ 3 min
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. It is Unit 9 in the latest AP Environmental Science CED, accounting for 10–15% of your total exam score, making it a high-weight, frequently tested unit that integrates content from all earlier syllabus topics.
2. Stratospheric Ozone Depletion ★★☆☆☆ ⏱ 4 min
The stratosphere, located 10–50 km above Earth's surface, contains an ozone layer ($O_3$) 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 chlorofluorocarbons (CFCs) — once widely used in refrigerants, aerosol propellants, and foam-blowing agents — drift up to the stratosphere. Inert in the troposphere, CFCs break down under high-energy UV radiation, releasing reactive chlorine atoms that act as catalysts to destroy ozone:
Cl + O_3 \rightarrow ClO + O_2 \\ ClO + O \rightarrow Cl + O_2
A single Cl atom can destroy up to 100,000 $O_3$ molecules before being removed, and CFCs have an atmospheric lifetime of 50–100 years, so impacts persist for decades after emissions stop. Depletion is worst over Antarctica: cold winter temperatures form polar stratospheric clouds that activate stored chlorine, which is released when spring UV levels rise, creating the annual ozone hole.
Impacts include increased skin cancer rates (a 1% ozone drop correlates to a 2% skin cancer increase), reduced crop yields, and disrupted aquatic food webs. The 1987 Montreal Protocol phased out 99% of CFC production; the 2016 Kigali Amendment extended this to phase out HFCs (potent GHG CFC replacements), with ozone projected to recover to 1980 levels by 2050–2070.
3. Greenhouse Effect, Climate Change and Ocean Acidification ★★★☆☆ ⏱ 6 min
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 radiation. Greenhouse gases (GHGs) absorb this IR radiation, trapping heat and keeping Earth's average surface temperature at ~15°C, rather than -18°C without GHGs.
The enhanced (anthropogenic) greenhouse effect occurs when human activities emit excess GHGs, increasing trapped heat and driving anthropogenic climate change. A key metric is Global Warming Potential (GWP), the heat 1 ton of a gas traps over 100 years relative to 1 ton of $CO_2$. Total emissions are measured in $CO_2$ equivalent ($CO_2e$):
\text{Total CO}_2e = \sum (\text{mass of gas} \times \text{GWP of gas})
Oceans absorb ~30% of all anthropogenic $CO_2$ emissions, reducing atmospheric warming but driving ocean acidification. When $CO_2$ dissolves in seawater, it reacts to form carbonic acid that releases hydrogen ions:
CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^- \rightarrow 2H^+ + CO_3^{2-}
Increased $H^+$ lowers ocean pH: pre-industrial pH was 8.2, today it is 8.1, a 30% increase in $H^+$ concentration. Since pH is a negative logarithmic scale, small pH drops correspond to large increases in $H^+$. The primary impact is reduced carbonate availability, which calcifying organisms (coral, shellfish, pteropods) need to build calcium carbonate shells. Coral bleaching is caused by elevated ocean temperatures, while acidification reduces coral growth and slows recovery after bleaching.
4. Biodiversity Loss ★★☆☆☆ ⏱ 3 min
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 extinction rate is 100–1000 times the natural background rate, leading scientists to call this the 6th mass extinction (Holocene extinction).
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.
5. Mitigation, Adaptation and Global Policy ★★★☆☆ ⏱ 4 min
AP examiners test the distinction between two categories of global change responses in nearly every administration:
- **Mitigation**: Actions that reduce GHG emissions or remove GHGs from the atmosphere to reduce the magnitude of future climate change. Examples: transitioning to renewable energy, improving energy efficiency, reforestation, carbon capture and storage.
- **Adaptation**: Actions that adjust human or natural systems to reduce harm from current or expected climate change impacts. Examples: building sea walls, developing drought-resistant crops, managed retreat from high-risk flood zones.
The landmark 2015 Paris Agreement, signed by 196 countries, sets a global goal to limit warming to well below 2°C, preferably 1.5°C, above pre-industrial levels. Countries submit updated emissions reduction targets (nationally determined contributions, NDCs) every 5 years.
Common Pitfalls
Why: The same chemical is present in both layers, so students mix up their functions
Why: Most media coverage focuses on the negative impacts of anthropogenic warming
Why: Many forget that pH is a negative logarithmic scale
Why: Students mix up the core purpose of each strategy type
Why: Both impacts are driven by rising $CO_2$ emissions, so students conflate them