AP Environmental Science Cheat Sheet 2026
Key concepts, formulas, and shortcuts for every APES unit — earth systems, biodiversity, populations, energy resources, pollution, and global change. Printable one-page reference.
🌍 Earth Systems & Resources
| Topic | Key Facts |
|---|---|
| Earth's layers | Crust → mantle (convection drives plate tectonics) → outer core (liquid iron, magnetic field) → inner core (solid iron). Lithosphere = crust + upper mantle. |
| Plate tectonics | Divergent (spreading, rifts) → Convergent (subduction, mountains, volcanoes) → Transform (earthquakes, no new crust). Subduction = oceanic under continental. |
| Rock cycle | Igneous (cooled magma) ↔ Sedimentary (deposited layers) ↔ Metamorphic (heat & pressure). Weathering → erosion → deposition → lithification. |
| Soil layers (horizons) | O (organic litter) → A (topsoil, humus, most fertile) → E (leached) → B (subsoil, minerals accumulate) → C (parent material) → R (bedrock) |
| Atmosphere layers | Troposphere (weather, CO₂) → Stratosphere (ozone, UV absorption) → Mesosphere → Thermosphere → Exosphere. Temp decreases in troposphere, increases in stratosphere. |
| Solar angle & seasons | Direct sunlight at equator year-round. Axial tilt causes seasons. Hadley cells: equatorial air rises, moves poleward, sinks ~30°N/S (deserts). Coriolis deflects winds right in N. hemisphere. |
🌿 Biodiversity & Ecosystems
| Concept | Key Rule |
|---|---|
| Biomes | Temperature & precipitation determine biome type. Tropical rainforest = hot & wet, highest diversity. Desert = hot & dry. Tundra = cold, permafrost, low diversity. Temperate deciduous = moderate, seasonal. |
| Ecosystem services | Provisioning (food, water), Regulating (climate, floods), Cultural (recreation), Supporting (nutrient cycling, photosynthesis). Services often undervalued economically. |
| Trophic levels & energy | 10% rule: only ~10% of energy transfers up. Producers → Primary consumers → Secondary → Tertiary. Biomass pyramids can be inverted in aquatic systems. |
| Biodiversity hotspots | High species richness + high endemism + threatened. Tropical forests and coral reefs have highest biodiversity. HIPPCO = main threats: Habitat loss, Invasive species, Population growth, Pollution, Climate change, Overconsumption. |
| Keystone species | Disproportionate effect relative to biomass. Remove them → trophic cascade (sea otters → urchins → kelp). Wolves → elk → riverbank vegetation (Yellowstone). |
| Island biogeography | Larger islands + closer to mainland = more species. Immigration rate decreases as island fills; extinction rate increases. Equilibrium species number where curves intersect. |
👥 Populations & Human Demographics
| Concept | Formula / Rule |
|---|---|
| Population growth rate | r = (births − deaths) + (immigration − emigration) per 1000 pop. Expressed as % per year. |
| Rule of 70 | Doubling time (years) = 70 / growth rate (%). Growth rate 2% → doubles in 35 years. |
| Total fertility rate (TFR) | Average children per woman. Replacement level TFR ≈ 2.1 in developed countries. TFR > 2.1 → population grows; < 2.1 → shrinks over time. |
| Demographic transition model | Stage 1: high birth + high death (pre-industrial). Stage 2: death rate drops (pop. grows fast). Stage 3: birth rate drops. Stage 4: low birth + low death (stable). Stage 5: declining (some developed nations). |
| Age structure diagrams | Wide base (Stage 2) = rapid growth. Uniform (Stage 4) = stable. Narrow base = declining. Many elderly + few young = pension strain. |
| IPAT equation | I = P × A × T (Environmental Impact = Population × Affluence × Technology). Technology can reduce or increase T factor. |
⚡ Energy Resources
| Source | Pro | Con | Key fact |
|---|---|---|---|
| Coal | Cheap, abundant, energy-dense | Most CO₂ per unit energy, SOₓ/NOₓ, mining damage | Used mostly for electricity generation. Surface mining destroys mountaintops. |
| Natural gas | Cleaner than coal, versatile | Methane leaks during extraction (potent GHG) | Fracking increased US production dramatically; linked to earthquakes |
| Nuclear | No CO₂ during operation, high energy density | Radioactive waste (lasts 10,000+ years), high upfront cost, accident risk | Fission of U-235. Half-life of waste isotopes is the key issue. |
| Solar | No GHG during operation, abundant | Intermittent (no sun at night), land use, panel manufacturing impact | Photovoltaic (PV) cells convert sunlight to electricity. Fastest growing sector. |
| Wind | No GHG during operation, cheapest new electricity in many regions | Intermittent, bird/bat mortality, visual impact, noise | Offshore wind more consistent than onshore. Turbines need 5–25 m/s winds. |
| Hydroelectric | Reliable, no GHG during operation, long lifespan | Habitat destruction, dam floods land, blocks fish migration | Dams trap sediment (delta starvation downstream). 17% of global electricity. |
🏭 Pollution & Remediation
| Pollutant | Source | Effect | Solution |
|---|---|---|---|
| CO₂, CH₄, N₂O | Fossil fuels, agriculture, deforestation | Greenhouse effect → climate change | Renewable energy, reduced meat consumption, reforestation |
| SO₂ / NOₓ | Coal combustion, vehicle exhaust | Acid rain (pH < 5.6), respiratory disease | Scrubbers, catalytic converters, cap-and-trade |
| Ground-level ozone (O₃) | NOₓ + VOCs + sunlight → photochemical smog | Respiratory damage, crop damage, reduces visibility | Reduce VOC and NOₓ emissions; catalytic converters |
| Particulate matter (PM₂.₅) | Combustion, dust, industry | Lung disease, heart disease, premature death | Filters, reduced combustion |
| Eutrophication | Nitrogen/phosphorus runoff (fertilizers, sewage) | Algal bloom → decomposers use O₂ → dead zone (hypoxia) | Riparian buffers, reduced fertilizer, constructed wetlands |
| Biomagnification | Fat-soluble toxins (DDT, mercury, PCBs) | Concentration increases at higher trophic levels; top predators most affected | Bans on persistent toxins (DDT banned 1972 in US) |
🌡️ Global Change & Climate
| Topic | Key Facts |
|---|---|
| Greenhouse gases (ranked by potency) | Water vapor (#1 by volume but not human-caused), CO₂ (#1 anthropogenic driver), CH₄ (80× CO₂ over 20 years), N₂O (265× CO₂), fluorinated gases (thousands× CO₂) |
| Ozone depletion | CFCs (from refrigerants, aerosols) release Cl in stratosphere → Cl catalytically destroys O₃. Antarctic ozone hole (spring). Montreal Protocol (1987) banned CFCs — ozone layer recovering. |
| Ocean acidification | CO₂ + H₂O → carbonic acid → lowers pH → dissolves calcium carbonate shells (corals, mollusks, pteropods). pH dropped ~0.1 since industrialization (30% more acidic). |
| Sea level rise | Causes: (1) thermal expansion of seawater (~50%), (2) melting glaciers and ice sheets (~50%). ~3.7 mm/year currently; accelerating. Threatens coastal populations. |
| Climate feedback loops | Positive (amplifying): ice-albedo feedback (less ice → less reflection → more warming → less ice), methane from permafrost thaw. Negative (dampening): increased evaporation → more clouds → more reflection. |
| Policy responses | Kyoto Protocol (1997, binding targets for developed nations), Paris Agreement (2015, voluntary NDCs, keep warming < 2°C). Carbon tax, cap-and-trade, renewable portfolio standards. |
APES calculation shortcuts: Energy efficiency = useful output / total input × 100%. Doubling time = 70 / % growth rate. EROI = energy out / energy in to produce it (higher = better; coal ~30:1, tar sands ~3:1, solar PV rising from ~4:1 to ~20:1).