AP Environmental Science Study Guide: Earth Systems, Human Impact, and Calculation Questions

AP Environmental Science is the most interdisciplinary AP science course — it combines ecology, geology, chemistry, physics, economics, and policy to study human interactions with the natural world. Students who engage with the course's environmental urgency typically find the material more compelling than the typical AP science, but the breadth of topics requires systematic coverage.

The exam has a distinctive feature shared by few other AP courses: the free-response section explicitly asks for solutions and evaluations of real environmental problems. This tests not just content knowledge but the ability to apply environmental science to propose and critique interventions.

Earth systems and biogeochemical cycles

Energy flow through ecosystems: Producers (photosynthesis) → primary consumers → secondary consumers → tertiary consumers. Each trophic level loses approximately 90% of energy as heat — only about 10% is passed upward. This 10% rule explains why food chains are short and why energy pyramids are narrow at the top.

Biogeochemical cycles: Know each cycle's reservoirs, fluxes, and human perturbations:

Carbon cycle: Atmospheric CO₂ ↔ photosynthesis (removes) and respiration/combustion (adds) ↔ ocean dissolution ↔ long-term storage in fossil fuels and carbonate rock. Human perturbation: burning fossil fuels adds approximately 10 Gt C/year to the atmosphere, more than natural flux pathways can absorb.

Nitrogen cycle: Atmospheric N₂ (unusable) → nitrogen fixation (lightning, bacteria — converts to NH₃/NO₃⁻) → plant uptake → decomposition → denitrification (back to N₂). Human perturbation: synthetic fertilisers (Haber-Bosch process — produces 150 Mt N/year) cause eutrophication and dead zones when runoff reaches water bodies.

Phosphorus cycle: No atmospheric component — cycles through rock weathering → soil → organisms → sedimentation. Slow cycle, easily disrupted by agricultural runoff (phosphorus → eutrophication → algal blooms → hypoxia).

Water (hydrological) cycle: Evaporation → condensation → precipitation → runoff/infiltration → groundwater → transpiration. Human perturbations: irrigation depletes aquifers faster than recharge (Ogallala Aquifer), dams alter river hydrology, impervious surfaces increase runoff.

Biodiversity and ecosystem function

Why biodiversity matters: Ecosystem stability (more species → more functional redundancy → greater resilience to disturbance), ecosystem services (provisioning, regulating, cultural, supporting services — CICES framework), genetic diversity for future adaptation.

Threats to biodiversity (HIPPO acronym): Habitat destruction (most significant threat — especially deforestation), Invasive species (no natural predators, outcompete native species), Pollution (direct toxicity plus indirect effects like endocrine disruption), Population (human population growth drives all other threats), Overharvesting (fishing, poaching, bushmeat).

Conservation strategies: Protected areas (national parks, biosphere reserves), wildlife corridors (connecting habitat fragments), sustainable fisheries management (MSC certification, catch limits, marine protected areas), CITES (Convention on International Trade in Endangered Species), captive breeding programs (genetic rescue, reintroduction).

Energy resources and EROI

Energy Return on Investment (EROI): EROI = energy returned / energy invested. High EROI = efficient energy source. Traditional coal: EROI ~ 80. Early oil and gas: EROI ~ 100 (now declining as easy reserves are depleted, ~ 10–20 for recent extraction). Nuclear: ~ 14. Solar PV: ~ 8–12. Wind: ~ 18–20. Corn ethanol: ~ 1.3–1.6 (barely breaks even). The trend toward lower-EROI fossil fuels makes the energy transition to renewables more economically compelling over time.

Fossil fuel extraction and impacts: Conventional oil (vertical wells), hydraulic fracturing/fracking (horizontal wells, high-pressure injection to crack shale — concerns: water contamination, methane leakage, induced seismic activity), mountaintop removal coal mining (valley fill, acid mine drainage), surface mining vs underground mining trade-offs.

Renewable energy: Solar (photovoltaic vs concentrated solar), wind (onshore vs offshore), hydroelectric (run-of-river vs reservoir — trade-offs: salmon migration, methane from reservoirs), geothermal, tidal/wave. Know the land use, intermittency, and environmental impact trade-offs for each.

Pollution: sources, mechanisms, and remediation

Air pollution:

• Primary pollutants (emitted directly): CO (incomplete combustion), SO₂ (coal burning), NO_x (high-temperature combustion), particulate matter (PM₂.₅ and PM₁₀), VOCs (volatile organic compounds from solvents and fuels).
• Secondary pollutants (formed in atmosphere): ozone (photochemical smog — NO_x + VOCs + sunlight → O₃), sulfuric acid (SO₂ + H₂O → H₂SO₄ — acid deposition).
• Thermal inversion: warm air layer traps cold air below → pollutants cannot disperse → acute pollution events (London 1952, Donora 1948).

Water pollution: Point source (identifiable discharge point — factory pipe, sewage outlet) vs non-point source (diffuse — agricultural runoff, urban stormwater). Eutrophication sequence: nutrient input (N and P) → algal bloom → algae decompose → bacteria consume O₂ → hypoxia → dead zone. Examples: Gulf of Mexico dead zone from Mississippi agricultural runoff.

Solid waste and waste management hierarchy: Refuse (don't create the waste) → Reduce → Reuse → Recycle → Recover energy → Dispose. Landfill issues: leachate, methane generation (can be captured for energy), liner failure.

Global change: the most tested area in FRQ

Climate change mechanism: CO₂, CH₄, N₂O, and water vapour absorb outgoing longwave radiation and re-emit it in all directions, including back toward Earth's surface (enhanced greenhouse effect). Evidence: temperature records (each of the last ten years has been among the ten warmest on record), ice cores (CO₂ and temperature correlation over 800,000 years), sea level rise (thermal expansion + ice melt), ocean acidification (CO₂ + H₂O → H₂CO₃ → reduced pH → affects calcium carbonate shells).

Ozone depletion: CFCs photolytically decompose in the stratosphere, releasing Cl atoms that catalytically destroy ozone (one Cl can destroy 100,000 O₃ molecules). The Antarctic ozone hole forms in southern spring — polar stratospheric clouds provide surfaces for reactions. Recovery is occurring following the Montreal Protocol's phase-out of CFCs.

Ocean acidification: As atmospheric CO₂ increases, more dissolves in the ocean → forms carbonic acid → lowers pH (0.1 units since pre-industrial, representing a 26% increase in H⁺ concentration). Effects: reduces calcification in corals, molluscs, echinoderms; affects fish olfaction and behaviour.

Use the Spaced Repetition Flashcard Tool for key facts, rates, and legislation. The Pomodoro Timer works well for APES calculation practice — 25 minutes of calculation drills followed by review. For the chemistry concepts that underpin APES, see the AP Chemistry study guide.