Must-know sheet

Environmental Science must-know sheet

The calculation setups, cycles, pollutants, laws and dates you should know cold for AP Environmental Science. The real exam doesn't give you a formula sheet but does let you use a calculator, so learn every setup here and always show your work with units.

Math setup habits and unit conversions

Units 1, 3, 5, 6, 8, 9

Show the setup, with units, every time: On the calculation free-response question, one point usually goes to the setup and one to the answer. Write each number with its unit, multiply by conversion factors so units cancel, and put the unit on your final answer.
Dimensional analysis: Start with the given value, then multiply by fractions whose top and bottom are equal amounts in different units (like 1,000 g / 1 kg) until only the unit you want is left. Example: 12,000 mi × (1 gal / 30 mi) × (8.9 kg CO₂ / 1 gal) ≈ 3,560 kg CO₂.
Scientific notation: Multiplying: multiply the numbers and add the exponents, so (3 × 10⁴)(2 × 10³) = 6 × 10⁷. Dividing: divide the numbers and subtract the exponents, so (8 × 10⁶) ÷ (2 × 10²) = 4 × 10⁴.
Metric prefixes: kilo (k) = 10³, mega (M) = 10⁶, giga (G) = 10⁹, tera (T) = 10¹². centi (c) = 10⁻², milli (m) = 10⁻³, micro (μ) = 10⁻⁶. So 1 MW = 1,000 kW and 1 kWh = 1,000 Wh.
Percent change = (new − old) ÷ old × 100: Always divide by the starting (old) value. A positive answer is an increase and a negative one is a decrease. Example: CO₂ rising from 317 ppm to 425 ppm is (425 − 317) ÷ 317 × 100 ≈ 34% increase.
Percent of a total = part ÷ whole × 100: Use it for questions like what share of applied fertilizer ran off. To go the other way, part = whole × (percent ÷ 100).
'Times as much' is not the same as percent increase: Going from 1.0 to 2.5 is 2.5 times as much but a 150% increase, because the increase is 1.5 on a base of 1.0.
Per capita = total ÷ number of people: Use it to compare countries fairly, for example energy use or CO₂ emissions per person. A big country can have a large total but a small per-person value.
Energy and power units: A watt (W) is 1 joule per second, a rate. A kilowatt-hour (kWh) is an amount of energy: 1 kWh = 3.6 × 10⁶ J ≈ 3,412 BTU. 1 BTU ≈ 1,055 J; 1 calorie ≈ 4.184 J; 1 kcal (a food Calorie) = 1,000 calories.
Area, volume and mass conversions: 1 hectare (ha) = 10,000 m² ≈ 2.47 acres; 1 km² = 100 ha. 1 m³ = 1,000 L, and 1 L of water has a mass of about 1 kg. 1 metric ton (t) = 1,000 kg ≈ 2,205 lb; 1 kg ≈ 2.2 lb; 1 gallon ≈ 3.79 L; 1 mile ≈ 1.61 km.
Time conversions to have ready: 1 day = 24 h; 1 year = 365 days = 8,760 h. Many energy questions need hours per year, so multiply hours per day by 365.
Parts per million (ppm): 1 ppm means 1 part in 1,000,000. In water, 1 ppm is about 1 mg of pollutant per liter, and 1 ppb (parts per billion) is 1,000 times smaller than 1 ppm.
Check your answer makes sense: Look at the size of the number. A household using millions of kWh a year, or a percent change over 100% for something that fell, means a setup error, often a missing ÷ 1,000 or a divide that should be a multiply.

Population calculations

Unit 3

Crude birth rate (CBR) and crude death rate (CDR): Births or deaths per 1,000 people per year: CBR = births ÷ population × 1,000. Example: 120,000 births in a country of 5,000,000 is a CBR of 24 per 1,000.
Rate of natural increase (%) = (CBR − CDR) ÷ 10: Because CBR and CDR are per 1,000, dividing the difference by 10 turns it into a percent. Example: CBR 32 and CDR 8 give (32 − 8) ÷ 10 = 2.4% per year. This ignores migration.
Growth rate with migration: Growth rate (%) = [(births + immigrants) − (deaths + emigrants)] ÷ population × 100. Example: (120,000 + 15,000 − 40,000 − 5,000) ÷ 5,000,000 × 100 = 1.8% per year.
Rule of 70: doubling time ≈ 70 ÷ growth rate (in percent): Use the percent number, not the decimal: 2% growth doubles in about 35 years, not 3,500. It works for anything growing at a steady percent, like populations, money or energy use.
Exponential growth by doublings: N = N₀ × 2ⁿ: n is the number of doubling times that pass (time ÷ doubling time). Example: 100 bacteria doubling every 20 minutes for 2 hours is 6 doublings, so 100 × 2⁶ = 6,400.
Infant mortality rate: Deaths of babies under 1 year old per 1,000 live births: infant deaths ÷ live births × 1,000. Example: 2,400 ÷ 160,000 × 1,000 = 15 per 1,000.
Total fertility rate (TFR) and replacement level: TFR is the average number of children a woman has in her lifetime. Replacement level is about 2.1 in developed countries and higher where more children die young; a TFR above replacement means long-term growth.
Population density = number of individuals ÷ area: Give it units such as people per km² or deer per hectare. Higher density makes density-dependent factors like disease and competition stronger.
Logistic growth is fastest at half of carrying capacity (K ÷ 2): Below K ÷ 2 there are too few individuals to add many; above it, limited resources slow growth. This is also where maximum sustainable yield is taken.
Percent survival or death from survivorship data: Percent that died in an age interval = (number at start − number at end) ÷ number at start × 100. A large loss in the first interval points to a Type III curve.

Energy calculations

Unit 6

Energy used (kWh) = power (W) × hours ÷ 1,000: Dividing by 1,000 turns watt-hours into kilowatt-hours. Example: a 60 W bulb on 5 h a day for a year uses 60 × 5 × 365 ÷ 1,000 = 109.5 kWh.
Cost = kWh × price per kWh: Example: 109.5 kWh × 0.15 dollars per kWh ≈ 16.43 dollars a year. To find savings from a switch, use the difference in watts: (60 W − 9 W) × 5 h × 365 ÷ 1,000 ≈ 93 kWh saved per bulb per year.
Efficiency = useful energy out ÷ energy in × 100: To find the fuel energy needed, divide the output by the efficiency (as a decimal): 100 units of electricity from a 35%-efficient plant needs 100 ÷ 0.35 ≈ 286 units of fuel. Multiplying here is the classic mistake.
Efficiency of a chain = multiply the steps: Overall efficiency is the product of each step's efficiency. Example: a 35% power plant, 90% transmission and a 5% incandescent bulb give 0.35 × 0.90 × 0.05 ≈ 1.6% of the fuel energy ending up as light.
Yearly output = rated power × 8,760 h × capacity factor: Capacity factor is the share of its rated output a plant actually produces on average. Example: a 2 MW wind turbine at 35% makes 2 × 8,760 × 0.35 = 6,132 MWh a year.
Number of homes supplied = yearly output ÷ yearly use per home: Convert to the same unit first (1 MWh = 1,000 kWh). Example: 6,132 MWh = 6,132,000 kWh; ÷ 10,500 kWh per home ≈ 584 homes.
Fuel needed for a power plant: Electric energy out ÷ efficiency = fuel energy in; fuel energy ÷ energy per kg = kg of fuel. Watch that power in MW is a rate (10⁶ J each second), so multiply by seconds to get joules.
Payback period = up-front cost ÷ yearly savings: Subtract any rebate or tax credit from the cost first. Example: solar panels costing 18,000 dollars with a 30% tax credit cost 12,600 dollars; at 1,500 dollars saved a year, payback is 12,600 ÷ 1,500 = 8.4 years.
Net energy and energy return: Net energy = energy out − energy in; energy return ratio = energy out ÷ energy in. Corn ethanol's ratio is only a little above 1, which is why it is a weak fuel choice.
Carbon to CO₂: multiply by 44 ÷ 12: Each carbon atom (mass 12) becomes one CO₂ molecule (mass 44), so 1 t of carbon makes about 3.67 t of CO₂. Burning 1 kg of methane (CH₄, mass 16) makes 44 ÷ 16 = 2.75 kg of CO₂; the extra mass comes from oxygen in the air.
Half-life: amount left = starting amount × (½)ⁿ: n = time passed ÷ half-life. Example: 800 g with a 30-year half-life is 800 × (½)³ = 100 g after 90 years. Decay is never linear: after two half-lives a quarter remains, not zero.
Time to reach a fraction: Count halvings: 1/2, 1/4, 1/8, 1/16 is four half-lives. Example: strontium-90 (half-life about 29 years) falls to 1/16 in about 4 × 29 = 116 years.

Ecology, land, water and pollution calculations

Units 1, 5, 7, 8, 9

NPP = GPP − respiration: Gross primary productivity is all the energy producers capture; net primary productivity is what is left after their own respiration and is what consumers can eat. Units are usually kcal/m²/yr or g/m²/yr.
Light and dark bottle method: Measure dissolved oxygen (DO) at the start, then in a bottle kept in light and one kept dark. Respiration = initial − dark; NPP = light − initial; GPP = NPP + respiration = light − dark.
10% rule for energy between trophic levels: Energy at the next level ≈ energy at this level × 0.10. Example: 10,000 kcal of producers supports about 1,000 kcal of primary consumers, 100 kcal of secondary and 10 kcal of tertiary consumers.
pH is a log scale: each unit is 10 times: A drop of n pH units means 10ⁿ times more acidic. pH 4.0 rain is 10² = 100 times more acidic than a pH 6.0 lake. Normal rain is about 5.6.
Decibels are a log scale too: Every +10 dB is 10 times the sound intensity, and sounds roughly twice as loud. 90 dB is 100 times as intense as 70 dB.
Biomagnification factor = concentration higher up ÷ concentration lower down: Example: a pesticide at 2.0 ppm in large fish and 25 ppm in fish-eating birds has increased 25 ÷ 2.0 = 12.5 times.
LD50 dose = LD50 (mg/kg) × body mass (kg): LD50 is given per kilogram of body weight. A substance with an LD50 of 50 mg/kg would kill half of a group of 20 kg animals at about 1,000 mg each. A lower LD50 means a more toxic substance.
Water volume = area × depth: Convert to meters first, then use 1 m³ = 1,000 L. Example: 2 cm of rain on a 10,000 m² lot is 10,000 × 0.02 = 200 m³ = 200,000 L.
Water to apply = water crops need ÷ irrigation efficiency: Example: crops need 400,000 L; flood irrigation that delivers 80% needs 400,000 ÷ 0.80 = 500,000 L, while drip at 95% needs about 421,000 L.
Ore needed = metal wanted ÷ ore grade: Example: 1 t of copper from 0.5% ore needs 1 ÷ 0.005 = 200 t of ore (199 t of waste). Halving the grade doubles the ore and the waste.
Sustainable yield = how much the resource grows each year: A forest of 10,000 m³ growing 3% a year has a sustainable yield of about 300 m³/yr; harvesting more shrinks it. For populations, the maximum sustainable yield comes at about half of carrying capacity.
Ecological footprint: number of Earths = footprint ÷ biocapacity: Both are in global hectares (gha) per person. A 6.4 gha footprint against 1.6 gha of biocapacity per person would need 6.4 ÷ 1.6 = 4 Earths.
Reading graphs and tables: Read values off the axes with units, describe the trend (increase, decrease, peak, leveling off) and calculate slope or rate as change in y ÷ change in x. Identify the independent variable (x-axis), dependent variable (y-axis) and control.

Matter cycles and energy flow

Unit 1

Energy flows one way; matter cycles: Energy enters as sunlight and leaves as heat, so ecosystems need a constant energy input. Atoms like carbon, nitrogen and phosphorus are reused over and over by moving between reservoirs.
First and second laws of thermodynamics: First law: energy is not created or destroyed, only changed in form. Second law: every transfer loses some energy as heat, which is why only about 10% passes up each trophic level and food chains are short.
Carbon cycle: the key processes: Photosynthesis takes CO₂ out of the air; respiration, decomposition and burning put it back. The ocean absorbs and releases CO₂ at its surface, and carbon is stored for millions of years in sedimentary rock (like limestone) and fossil fuels.
Carbon cycle: fast and slow: Exchanges between air, plants, animals and the surface ocean are fast. Burying carbon in sediments, rock and fossil fuels is very slow, so burning fossil fuels moves carbon to the air far faster than it can be stored again. Sedimentary rock is the largest carbon reservoir.
Nitrogen fixation: N₂ → NH₃ (then NH₄⁺): Bacteria, either free in the soil or in root nodules of legumes like beans and clover, turn nitrogen gas into ammonia, which becomes ammonium. Lightning and factory-made (synthetic) fertilizer also fix nitrogen.
Nitrification: NH₄⁺ → NO₂⁻ → NO₃⁻: Soil bacteria turn ammonium into nitrite and then nitrate, which plants absorb easily.
Assimilation and ammonification: Assimilation: plants take up ammonium or nitrate and build it into proteins and DNA, and animals get nitrogen by eating them. Ammonification: decomposers turn nitrogen in wastes and dead bodies back into ammonia and ammonium.
Denitrification: NO₃⁻ → N₂O → N₂: Bacteria in low-oxygen (waterlogged) soil turn nitrate back into nitrogen gas, returning it to the atmosphere, the largest nitrogen reservoir. N₂O released along the way is a greenhouse gas.
Phosphorus cycle: Phosphate is released as rocks weather, taken up by plants, passed through food webs and returned by decomposers; much ends up in ocean sediment and returns only when geologic uplift raises the rock. It has no major gas phase, so it cycles slowly and often limits plant and algae growth, especially in fresh water.
Water (hydrologic) cycle: Driven by the sun: evaporation and transpiration (water vapor from plant leaves) move water up, condensation forms clouds, precipitation brings it down, and it either infiltrates into groundwater or flows as runoff. The ocean holds about 97% of Earth's water; only about 2.5% is fresh, and most of that is ice.
Limiting nutrient: The nutrient in shortest supply caps growth even when others are plentiful. Nitrogen often limits land plants and marine algae; phosphorus often limits freshwater algae. Adding the limiting nutrient causes the biggest growth jump.
Trophic levels: Producers (autotrophs) → primary consumers (herbivores) → secondary consumers → tertiary consumers. Decomposers and detritivores break down dead matter at every level and return nutrients to the soil.

Ecology, biodiversity and population models

Units 1, 2, 3

Species interactions: Predation (+/−), parasitism (+/−), competition (−/−), mutualism (+/+) and commensalism (+/0). Resource partitioning lets competing species share a resource by using it at different times, places or in different ways.
Biomes are set by temperature and precipitation: Read a climatograph: tropical rainforest is hot and very wet, desert is very dry, temperate grassland has moderate rain with cold winters, taiga is cold with conifers, and tundra is coldest with permafrost and no trees.
Aquatic biomes are set by salinity, depth, temperature and light: Freshwater biomes (lakes, rivers, streams, freshwater wetlands) supply drinking water; marine biomes include the open ocean, coral reefs, salt marshes and estuaries. Shallow, sunlit, nutrient-rich water near coasts (estuaries, salt marshes, reefs) is the most productive, and ocean algae make a large share of Earth's oxygen.
Three levels of biodiversity: Genetic diversity (variety within a species), species diversity (richness = number of species; evenness = how balanced their numbers are) and habitat or ecosystem diversity. More genetic diversity helps a population survive change; a bottleneck removes it.
Four types of ecosystem services: Provisioning (goods like food, wood, medicine), regulating (climate control, flood protection, water purification), cultural (recreation, spiritual value) and supporting (soil formation, nutrient cycling, photosynthesis).
Island biogeography: Larger islands hold more species (more habitats, lower extinction rates), and islands closer to the mainland hold more species (higher immigration rates). The same applies to habitat islands like park fragments.
Range of tolerance: Optimal range (thrives) → zones of physiological stress (survives poorly) → zones of intolerance (dies). Pollution or warming can push conditions out of a species' optimal range.
Succession: Primary succession starts on bare rock with no soil (pioneers: lichens, mosses); secondary succession follows a disturbance that leaves soil (fire, abandoned farm) and is faster. Over time soil depth, biomass and species diversity usually increase.
Natural disruptions and adaptation: Disruptions can be periodic (regular, like seasons or tides), episodic (occasional, like droughts or hurricanes) or random (unpredictable, like earthquakes and eruptions). Over long times climate and sea level change too, so species must migrate, adapt or go extinct; adaptation happens by natural selection acting on inherited traits over generations.
Keystone and indicator species: A keystone species has an outsized effect on its ecosystem relative to its numbers (sea otters, beavers, wolves). An indicator species shows ecosystem health by being sensitive to change (lichens for air quality, mayfly larvae and trout for clean water).
Specialist vs. generalist species: Specialists have narrow niches and do well only in stable conditions, so they are prone to extinction. Generalists have broad niches, adapt to change and often become invasive.
r-selected vs. K-selected species: r-selected: many offspring, little parental care, early maturity, boom-and-bust populations (insects, many fish). K-selected: few offspring, lots of care, late maturity, populations near carrying capacity, slow recovery (elephants, whales).
Survivorship curves: Type I: most survive to old age (humans, K-selected). Type II: a steady death rate at all ages (some birds, rodents). Type III: most die young (many fish, insects, plants; r-selected).
Exponential vs. logistic growth: Exponential growth makes a J-curve when resources are unlimited. Logistic growth makes an S-curve that levels off at carrying capacity (K). A population that overshoots K usually suffers a dieback.
Density-dependent vs. density-independent factors: Density-dependent factors get stronger as the population gets crowded (disease, competition, predation, food supply). Density-independent factors hit regardless of crowding (drought, fire, floods, storms).
Age structure diagrams: Wide base (pyramid): rapid growth. Nearly straight sides: slow or no growth. Narrow base, wider top: declining population.
Demographic transition model: Stage 1 pre-industrial: high birth and death rates, slow growth. Stage 2 transitional: death rate falls (better food, medicine, sanitation), fast growth. Stage 3 industrial: birth rate falls, growth slows. Stage 4 post-industrial: both low, stable or shrinking population.
What lowers fertility: More education and jobs for women, access to family planning, later marriage, lower infant mortality, urbanization (children cost more) and government pension systems.

Earth systems

Unit 4

Plate boundaries: Convergent (plates collide): mountains, volcanoes, island arcs, trenches and earthquakes; subduction happens when ocean crust sinks under another plate. Divergent (plates pull apart): seafloor spreading, mid-ocean ridges, rift valleys. Transform (plates slide past): earthquakes, like the San Andreas Fault.
Soil horizons, top to bottom: O (organic litter and humus), A (topsoil: minerals mixed with humus), B (subsoil: minerals leached from above build up here), C (weathered parent material) and R (bedrock). Some profiles also show an E horizon of leached material between A and B.
Soil texture: sand, silt, clay: Particle size: sand is largest (0.05–2 mm), then silt, then clay (under 0.002 mm). Sandy soil drains fast (high permeability) but holds little water; clay holds water and nutrients but drains slowly. Loam, about 40% sand, 40% silt and 20% clay, is best for most crops; use the soil texture triangle to name a soil.
Atmosphere composition: About 78% nitrogen (N₂), 21% oxygen (O₂), 0.9% argon, about 0.04% (around 420 ppm) carbon dioxide, plus variable water vapor.
Atmosphere layers: Troposphere (0 to about 12 km; weather; temperature falls with height), stratosphere (to about 50 km; the ozone layer absorbs UV, so temperature rises with height), mesosphere (temperature falls), thermosphere (temperature rises), exosphere.
Global wind circulation: Strong sunlight at the equator heats air, which rises, expands, cools and drops its rain; the air flows poleward and sinks around 30° N and S, making deserts there (Hadley cells). The Coriolis effect bends moving air right in the Northern Hemisphere and left in the Southern, giving trade winds (0–30°, from the east), westerlies (30–60°) and polar easterlies (60–90°).
Seasons come from Earth's 23.5° tilt: Tilt changes the angle of sunlight and the hours of daylight through the year; distance from the sun is not the cause. Insolation is strongest at the equator because sunlight strikes most directly.
Rain shadow: Moist air rises up the windward side of a mountain, cools and drops rain; the dry air sinking down the leeward side warms, making a dry rain shadow.
Watershed: All the land that drains into one river, lake or other body of water. Anything done upstream in the watershed, such as farming, paving or logging, affects water quality downstream.
El Niño (ENSO): Trade winds weaken, warm water spreads east toward South America and cold, nutrient-rich upwelling off Peru shuts down, hurting fisheries; weather shifts worldwide (often wetter in the southern US and drier in Australia and Indonesia). La Niña is the opposite: stronger trade winds, more upwelling, cooler eastern Pacific.

Land use, farming and sustainability

Unit 5

Tragedy of the commons: A shared, unowned resource gets overused because each user gains from taking more while the cost is spread over everyone. Examples: overfishing, overgrazing, air pollution. Fixes include regulation, quotas, permits and ownership.
Clearcutting effects: Cheap and efficient for logging, but it causes erosion, warmer soil and streams, more flooding and habitat loss, and it releases stored carbon.
Green Revolution (mid-1900s): High-yield crop varieties, synthetic fertilizer, pesticides, irrigation and machinery greatly increased food output, but also increased fossil fuel use, pollution, monocultures and loss of crop genetic diversity.
Harmful farming practices: Tilling loosens topsoil so wind and water erode it; slash-and-burn clears forest and releases carbon and nutrients; extra fertilizer runs off and causes eutrophication; monocultures are vulnerable to pests.
Irrigation methods, least to most efficient: Flood and furrow (cheap, lose much water to evaporation and runoff), spray (less waste, costs more and needs energy), drip (most efficient, most expensive). About 70% of the fresh water people use goes to farming. Overwatering causes waterlogging; evaporating irrigation water leaves salts behind (salinization).
Pesticide resistance: Spraying kills the susceptible pests, so the few with inherited resistance survive and reproduce, and the population becomes resistant over generations (natural selection).
Integrated pest management (IPM): Combines biocontrol (natural predators), crop rotation, intercropping, traps and monitoring, using chemicals only when pests pass a threshold. It cuts pesticide harm but takes more time, knowledge and cost.
Meat production: CAFOs (feedlots) are cheap and efficient but concentrate manure that pollutes water, and routine antibiotics speed antibiotic resistance. Grazing systems need more land; rotational grazing lets pasture regrow, while overgrazing causes erosion, compaction and desertification. Meat needs far more land, water and energy per gram of protein than plants.
Overfishing and bycatch: Catching fish faster than they reproduce leads to fishery collapse. Bycatch is unwanted animals caught along with the target; bottom trawling destroys seafloor habitat.
Mining terms: Ore is rock rich enough in a metal to mine; overburden is the soil and rock removed to reach it; tailings are the leftover waste after extraction and slag is the waste from smelting. Surface mining strips land; subsurface mining is more dangerous and costly. Acid mine drainage lowers stream pH and releases metals.
Urbanization and runoff: Impervious surfaces (roads, roofs, parking lots) block infiltration, so more runoff reaches streams faster and floods are bigger. Solutions: permeable pavement, rain gardens, green roofs, more trees, building up instead of out, public transit. Pumping groundwater near coasts can cause saltwater intrusion.
Soil conservation practices: Contour plowing (rows run across the slope, along its contour lines), terracing (steps on hillsides), strip cropping, windbreaks (rows of trees), no-till farming, crop rotation, green manure, and limestone to raise the pH of acidic soil.
Aquaculture: Farming fish in controlled water needs little space and fuel, but wastes pollute water, disease spreads to wild fish, escaped fish compete with wild ones, and feed may come from wild-caught fish.
Sustainable forestry: Reforestation, selective cutting instead of clearcutting, buying certified wood, reusing wood, IPM or removing diseased trees, and prescribed burns that clear built-up fuel to prevent huge wildfires.
Sustainability and indicators: Using resources so future generations can meet their needs. Indicators to watch: biodiversity, food production, atmospheric CO₂, global temperature, human population and resource depletion.

Energy sources at a glance

Unit 6

Renewable vs. nonrenewable: Nonrenewable sources (coal, oil, natural gas, uranium) form far more slowly than we use them. Renewables (sun, wind, flowing water, geothermal heat) are replenished; biomass is renewable only if not used faster than it regrows.
Who uses the most energy: Fossil fuels supply most of the world's energy. Developed countries use far more energy per person than developing ones, and use rises as a country industrializes; which source a place uses depends on cost, availability and the local geography (sun, wind, rivers, fossil fuel deposits).
How a thermal power plant works: Fuel (coal, gas, oil, biomass or nuclear fission) heats water into steam → steam spins a turbine → the turbine turns a generator → electricity. Steam is condensed with cooling water, which can cause thermal pollution. Coal plants turn only about a third of the fuel's energy into electricity.
Cogeneration (combined heat and power): One fuel source makes both electricity and useful heat: the waste heat from generating power is used to heat buildings or water or run industrial processes, so far more of the fuel's energy is put to use.
Coal ranks: Peat → lignite → bituminous → anthracite, with heat, pressure and time. Anthracite has the most carbon and energy per kilogram. Burning coal releases the most CO₂ per unit of energy of the fossil fuels, plus SO₂, NOₓ, particulates and mercury.
Natural gas and oil: Natural gas is mostly methane and releases about half the CO₂ of coal for the same energy, but leaked methane is a strong greenhouse gas. Crude oil is refined into gasoline, diesel and other products. Fracking can contaminate groundwater and release VOCs; tar sands take lots of water and energy to process.
Nuclear fission: Neutrons split uranium-235 atoms, releasing heat; control rods absorb neutrons to control the reaction. No CO₂ while running, but spent fuel stays radioactive for thousands of years, mining has impacts, and accidents (Three Mile Island 1979, Chernobyl 1986, Fukushima 2011) are possible.
Biomass and biofuels: Wood, charcoal, crop waste and dried manure are cheap and widely available but release CO₂, CO, NOₓ and particulates, and overharvesting causes deforestation. Ethanol (from corn or sugarcane) and biodiesel replace some gasoline and diesel; corn ethanol has a low net energy.
Solar: Photovoltaic (PV) cells turn sunlight directly into electricity; active solar uses pumps and collectors to heat water, and passive solar uses building design (south-facing windows in the Northern Hemisphere, thermal mass). No pollution while running, but panels need mined metals and large farms use land.
Hydroelectric and tidal: Falling water spins turbines; renewable with no air pollution once built. Dams flood land and displace people, block fish migration (fish ladders help), trap sediment, and change downstream temperature and flow; reservoirs can release methane. Tidal power uses the rise and fall of the tides to spin turbines in the same way, but suits only a few coastlines.
Geothermal: Heat from inside the earth (partly from radioactive decay) makes steam for turbines or heats buildings directly; ground-source heat pumps work almost anywhere. Large plants need hot rock near the surface and can release hydrogen sulfide (H₂S).
Hydrogen fuel cell: 2H₂ + O₂ → 2H₂O + electricity: The only exhaust is water, but hydrogen must first be made (often from natural gas, or by electrolysis using electricity) and is hard to store and transport, so it carries energy rather than being a source.
Wind: Moving air spins turbines; renewable, cheap to run and pollution-free while operating. Drawbacks: it is intermittent, turbines kill birds and bats, and some people object to noise and appearance.
Energy conservation: Efficient appliances and LED bulbs, insulation, programmable thermostats, passive solar design, CAFE (corporate average fuel economy) standards, hybrid and electric vehicles, carpooling and public transit.

Air pollutants and their effects

Unit 7

Primary vs. secondary pollutants: Primary pollutants are released directly (CO, SO₂, NOₓ, particulates, VOCs). Secondary pollutants form when primary ones react in the air (ground-level ozone, sulfuric and nitric acid).
The six criteria air pollutants: Under the Clean Air Act, the EPA sets national limits on carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO₂), ground-level ozone (O₃), particulate matter (PM₁₀ and PM₂.₅) and sulfur dioxide (SO₂).
Sulfur dioxide (SO₂): Mainly from burning coal. Irritates the lungs and forms sulfuric acid, a main cause of acid rain, and sulfate particles.
Nitrogen oxides (NOₓ): From high-temperature combustion in vehicles and power plants. Irritate the lungs, form nitric acid in acid rain and are a key ingredient of photochemical smog.
Carbon monoxide (CO): From incomplete combustion (car exhaust, faulty heaters, indoor fires). It binds to hemoglobin and blocks oxygen delivery, which can kill in closed spaces.
Particulate matter (PM₁₀, PM₂.₅): Tiny solid or liquid particles from burning fuel, wildfires, dust and volcanoes. The number is the particle size in micrometers; PM₂.₅ is more dangerous because it reaches deep into the lungs. Causes haze and blocks sunlight.
Lead (Pb): Once mainly from leaded gasoline, now from old paint, old pipes and smelting. A neurotoxin that is especially harmful to children's brain development.
Mercury (Hg): Released mainly by burning coal. It settles into water, bacteria convert it to methylmercury, and it biomagnifies in fish; it damages the nervous system.
Photochemical smog and ground-level ozone: NO₂ + sunlight → NO + O; O + O₂ → O₃. VOCs react with NO so ozone builds up instead of being broken down. Worst in hot, sunny, traffic-heavy cities; on a typical day NO peaks during morning rush hour, NO₂ a few hours later and O₃ in the afternoon. Ozone irritates lungs and damages plants.
Thermal inversion: Normally air cools with height so polluted air rises and disperses. In an inversion a warm layer sits over cooler surface air, trapping pollution near the ground, as in Donora, Pennsylvania (1948).
Acid deposition: SO₂ and NOₓ react with water to form sulfuric and nitric acids that fall as acid rain or dry deposition. It lowers the pH of lakes and soils, leaches nutrients and releases toxic aluminum, harms fish and trees, and dissolves limestone and marble. Limestone (calcium carbonate) bedrock buffers lakes against it.
Indoor air pollutants: In developing countries, smoke from burning wood, charcoal or dung for cooking (particulates, CO). Elsewhere: CO from faulty heaters, radon-222 from uranium decay in rock (a leading cause of lung cancer), asbestos (mesothelioma, asbestosis), lead dust from old paint, mold, and VOCs such as formaldehyde from glues, carpets and particleboard.
Air pollution control devices: Catalytic converters change CO, NOₓ and hydrocarbons into CO₂, N₂ and H₂O. Wet scrubbers spray a lime slurry to remove SO₂ and particles. Electrostatic precipitators charge particles so they stick to plates; baghouse filters trap particles. Vapor recovery nozzles capture gasoline VOCs; fluidized bed combustion burns coal with limestone to cut SO₂.
Prevention beats control: Burning less fuel (conservation, transit), switching to low-sulfur coal or natural gas, and using renewables prevent pollutants from forming in the first place.
Noise pollution: Traffic, construction, airports, ships and industry. Long exposure above about 85 dB can damage hearing, and noise interferes with animals' communication, hunting and migration (ship noise and whales).

Water and land pollution, toxicology and health

Unit 8

Point vs. nonpoint sources: Point source: one identifiable spot, like a factory pipe or sewage outfall, easier to regulate. Nonpoint source: spread out, like runoff from farms, lawns and streets, much harder to control.
Eutrophication, step by step: Extra nitrogen and phosphorus (fertilizer, manure, sewage) → algal bloom → algae block light and plants below die → algae die → bacteria decompose them and use up dissolved oxygen → hypoxia, fish kills and a dead zone. Oligotrophic water is low in nutrients; eutrophic water is high.
Dissolved oxygen (DO): Most fish need several mg/L of DO; water below about 2 mg/L is hypoxic, and the Gulf of Mexico dead zone below the Mississippi River is a classic example. Cold, fast-moving water holds more oxygen than warm, still water.
Thermal pollution: Power plants and factories return cooling water warmer. Warm water holds less oxygen and raises organisms' metabolism, causing thermal shock and fish deaths.
Persistent organic pollutants (POPs): Human-made, carbon-based chemicals like DDT, PCBs and dioxins that break down very slowly, dissolve in fat rather than water and travel long distances on wind and water.
Bioaccumulation vs. biomagnification: Bioaccumulation: a chemical builds up in one organism over its lifetime. Biomagnification: the concentration rises at each higher trophic level, so top predators carry the most. Examples: DDT thinning bald eagle and pelican eggshells, mercury in tuna and swordfish.
Endocrine disruptors: Chemicals that mimic or block hormones, linked to birth defects, developmental and reproductive problems, and male fish showing female traits. Examples: DDT, BPA, phthalates; the herbicide atrazine is suspected.
Oil spills and plastics: Oil coats feathers and fur and poisons marine life; cleanup uses booms, skimmers and dispersants. Ocean plastics entangle animals or are swallowed, and they break into microplastics that enter food webs.
Coral reefs, wetlands and mangroves: Reefs are harmed by warming, sediment runoff, acidification and destructive fishing. Wetlands and mangroves filter water, absorb floods, buffer storm surges, store carbon and shelter young fish, but are drained and filled for development.
Sanitary landfill design: A clay and plastic liner, a leachate collection system, methane collection pipes, daily soil cover and groundwater monitoring wells. Trash breaks down slowly without oxygen, producing methane, which can be captured for energy.
Other waste disposal: Incineration cuts volume by about 90% and can make electricity, but releases air pollution and leaves toxic ash. Open dumps pollute water. E-waste contains lead, mercury and cadmium and needs special recycling. Ocean dumping harms marine life.
Waste reduction order: Reduce first, then reuse, then recycle. Composting turns food and yard waste into soil. Recycling saves resources and energy but still costs money and energy.
Sewage treatment stages: Primary: screens and settling tanks remove solids. Secondary: bacteria break down organic matter in aerated tanks. Tertiary: removes leftover nutrients (nitrogen, phosphorus) and chemicals. Then disinfection with chlorine, UV light or ozone; leftover sludge is landfilled, burned or treated for use as fertilizer.
LD50, ED50 and threshold dose: LD50: the dose that kills 50% of a test population, in mg per kg of body weight; lower LD50 means more toxic. ED50: the dose that causes a set effect in 50%. Threshold dose: the point where harm first appears; below it no effect is detected. All are read from a dose-response curve, usually from animal studies.
Acute vs. chronic exposure: Acute: a single high exposure with quick effects. Chronic: low exposure over a long time with effects that can take years to show, which makes it hard to prove one pollutant caused a disease.
Pollution-linked diseases: Dysentery and cholera from sewage-contaminated water, mesothelioma from asbestos, respiratory damage from ground-level ozone and particulates.
Pathogens and vectors: Malaria (parasite, mosquitoes), West Nile virus and Zika (viruses, mosquitoes), plague (bacterium, fleas on rodents), tuberculosis (bacterium, airborne), cholera (bacterium, contaminated water), SARS and MERS (coronaviruses, respiratory). Warming lets vectors spread toward the poles; crowding and poor sanitation help disease spread.

Global change

Unit 9

Stratospheric ozone depletion: UV light breaks chlorine off CFCs in the stratosphere; Cl + O₃ → ClO + O₂, then ClO + O → Cl + O₂, so one chlorine atom destroys many ozone molecules. Thinner ozone lets more UV-B reach the ground, raising rates of skin cancer and cataracts and harming plants and plankton. The Antarctic ozone hole is worst in the Southern Hemisphere spring.
Good ozone vs. bad ozone: Stratospheric ozone is good: it absorbs harmful UV. Ground-level (tropospheric) ozone is bad: it's a secondary pollutant in smog that damages lungs and plants.
CFC replacements: HCFCs harm ozone less and were a temporary fix; HFCs don't harm ozone, but many are powerful greenhouse gases, so they are now being phased down too (Kigali Amendment, 2016).
Greenhouse effect: Sunlight warms the surface, which gives off infrared radiation; greenhouse gases absorb it and send some back toward the surface. The natural effect keeps Earth livable; extra gases from human activity cause enhanced warming.
Main greenhouse gases and sources: CO₂ (burning fossil fuels, deforestation), methane (livestock, rice paddies, landfills, natural gas leaks, thawing permafrost), nitrous oxide (fertilizer, manure, burning fuel), CFCs and HFCs (refrigerants), water vapor (rises as air warms, amplifying other gases).
Global warming potential (GWP): Heat-trapping power over 100 years compared with CO₂ = 1: methane about 25–30, nitrous oxide about 265–300, CFCs in the thousands. CO₂ still causes the most warming because there is so much of it.
Evidence of warming: The Keeling Curve (direct CO₂ measurements at Mauna Loa since 1958) shows a steady rise with a yearly zigzag from Northern Hemisphere plant growth. Ice cores trap ancient air bubbles that record past CO₂ and temperature, showing they rise and fall together.
Effects of climate change: Melting glaciers, sea ice and permafrost; sea level rise; more extreme weather; shifting ranges for species and disease vectors; coastal flooding that displaces people; possible changes to ocean currents.
Sea level rise has two causes: Thermal expansion of warming seawater and melting land ice (glaciers and ice sheets). Melting floating sea ice does not raise sea level.
Positive feedback loops: Melting ice exposes darker land and water that absorb more sunlight (lower albedo), causing more warming and melting, which is why the poles warm fastest. Thawing permafrost releases CO₂ and methane, causing more warming.
Ocean warming: The ocean has absorbed most of the extra heat. Warm water causes coral bleaching, where corals expel their symbiotic algae (zooxanthellae) and can starve; species shift toward the poles or deeper water.
Ocean acidification: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻: The ocean absorbs CO₂, forming carbonic acid that lowers pH. The extra H⁺ ties up carbonate ions (CO₃²⁻), so corals, shellfish and some plankton struggle to build calcium carbonate (CaCO₃) shells and skeletons. Surface pH has fallen from about 8.2 to about 8.1, roughly 25–30% more H⁺.
Invasive species: Non-native species that spread fast because they lack natural predators and competitors; usually generalists and r-selected. Examples: zebra mussels, kudzu, cane toads, Burmese pythons, Asian carp. Control by inspections, removal and biocontrol.
Traits of endangered species: Small population or range, specialized diet or habitat, low reproductive rate (K-selected), large territory needs, or high value to poachers.
HIPPCO: threats to biodiversity: Habitat destruction, Invasive species, Population growth, Pollution, Climate change and Overexploitation. Habitat loss and fragmentation are the biggest threats.
Conservation tools: Protected areas, wildlife corridors that link fragmented habitat, habitat restoration, captive breeding, sustainable land use, and laws like the Endangered Species Act and CITES.

Key laws and treaties with dates

Units 7, 8, 9

National Environmental Policy Act (NEPA), 1969: Signed January 1, 1970. Requires federal agencies to prepare an environmental impact statement (EIS) before major projects. The EPA was created later in 1970, the same year as the first Earth Day.
Clean Air Act, 1970 (major amendments 1977 and 1990): Lets the EPA set national air quality standards for the six criteria pollutants and limit emissions from vehicles, factories and power plants. The 1990 amendments created a cap-and-trade program for SO₂ that sharply cut acid rain.
Clean Water Act, 1972: Regulates discharges of pollutants into surface waters, requiring permits for point sources, with the goal of making waters fishable and swimmable. It also protects wetlands from being filled without a permit.
US ban on DDT, 1972: The EPA banned most uses of DDT after Rachel Carson's Silent Spring (1962) drew attention to its harm to wildlife; bald eagle and pelican populations later recovered.
Endangered Species Act (ESA), 1973: Protects species listed as endangered or threatened and their critical habitat in the US, and bans harming, harassing or trading them.
CITES, 1973 (in force 1975): The Convention on International Trade in Endangered Species: an international treaty that controls or bans trade in endangered plants and animals and products made from them, like ivory.
Safe Drinking Water Act, 1974: Sets maximum contaminant levels for public drinking water supplies in the US.
Resource Conservation and Recovery Act (RCRA), 1976: Controls hazardous waste 'from cradle to grave' (from creation to disposal) and sets standards for landfills.
CERCLA (Superfund), 1980: Pays to clean up abandoned hazardous waste sites and makes polluters pay where they can be found. Passed after the Love Canal disaster.
Montreal Protocol, 1987: International treaty that phased out CFCs and other ozone-destroying chemicals; the ozone layer is slowly recovering. Often called the most successful environmental treaty. The Kigali Amendment (2016) added a phase-down of HFCs.
Kyoto Protocol, 1997 (in force 2005): Set binding greenhouse gas cuts for developed countries only. The US signed but never ratified it.
Stockholm Convention, 2001 (in force 2004): International treaty to eliminate or restrict persistent organic pollutants (POPs) such as PCBs and most uses of DDT.
Paris Agreement, 2015: Nearly every country set its own emissions target (nationally determined contributions) to keep warming well below 2 °C, aiming for 1.5 °C. The US left in 2020, rejoined in 2021 and left again in January 2026.

Case studies and disasters with dates

Units 5, 6, 7, 8

Dust Bowl, 1930s: Drought plus plowing that removed native grasses on the Great Plains let wind strip away topsoil. It led to soil conservation practices like windbreaks and contour plowing.
Donora, Pennsylvania smog, 1948: A thermal inversion trapped pollution from zinc and steel works in a river valley for days, killing about 20 people and sickening thousands.
London Great Smog, 1952: Coal smoke trapped by an inversion killed about 4,000 people within days (later estimates are much higher), leading to Britain's Clean Air Act of 1956.
Minamata, Japan, 1950s: A factory released mercury into a bay; it biomagnified in fish and caused severe nerve damage and deaths in people who ate them (Minamata disease, identified in 1956).
Cuyahoga River fire, 1969: Oily industrial waste on this Ohio river caught fire, one of several such fires, and helped build support for the Clean Water Act.
Love Canal, New York, late 1970s: Homes and a school were built on a buried chemical waste dump; leaking chemicals led to evacuations in 1978 and to the Superfund law (CERCLA) in 1980.
Three Mile Island, Pennsylvania, 1979: A partial meltdown at a US nuclear plant released only a small amount of radiation, but it slowed nuclear power growth in the US.
Bhopal, India, 1984: A leak of toxic methyl isocyanate gas from a pesticide plant killed thousands of people, one of the worst industrial disasters ever.
Chernobyl, Soviet Union (now Ukraine), 1986: A reactor explosion and fire spread radioactive material across Europe; the area around the plant is still an exclusion zone.
Exxon Valdez, Alaska, 1989: A tanker spilled about 11 million gallons of crude oil into Prince William Sound, killing huge numbers of seabirds and marine mammals; it led to the Oil Pollution Act of 1990.
Deepwater Horizon, Gulf of Mexico, 2010: An offshore drilling rig exploded, killing 11 workers, and spilled oil for 87 days, the largest marine oil spill in US history.
Fukushima, Japan, 2011: An earthquake and tsunami knocked out cooling at a nuclear plant, causing meltdowns and radioactive releases and evacuations.
Flint, Michigan, 2014: Switching the city's water source without proper corrosion control let lead leach from old pipes into drinking water.
Aral Sea: Rivers feeding this Central Asian lake were diverted from the 1960s on to irrigate cotton; the lake shrank dramatically, leaving salty, polluted dust and a collapsed fishery.