AP Environmental Science Quick Review

Energy flows one way through an ecosystem (sun $arrow$ producers $arrow$ consumers $arrow$ decomposers) and is lost as heat at every transfer, while matter cycles and is reused.

The 10% rule: only about 10% of energy at one trophic level is available to the next (90% is lost as metabolic heat, respiration, or unconsumed/undigested matter). Gross primary productivity (GPP) = total energy captured by producers; net primary productivity (NPP) = $GPP - respiration$, the energy actually available to consumers. Biomass and energy pyramids both shrink going up; a few ecosystems (e.g., some aquatic ones) can have inverted numbers pyramids.

Food chain (single linear energy pathway) vs. food web (realistic network of interconnected feeding relationships across many species).

Forgetting to subtract respiration when asked for NPP, or assuming energy ``recycles'' like matter --- energy flow is unidirectional and dissipates as heat; only matter (C, N, P, H$_2$O) cycles through biogeochemical cycles.

Only 10% of energy passes up each trophic level $arrow$ NPP $=$ GPP $-$ respiration.

Essential elements cycle continuously between living organisms, the atmosphere, oceans, and rocks/soil through specific biological, chemical, and geological pathways.

Carbon cycle: photosynthesis removes atmospheric CO$_2$; respiration, combustion, and decomposition return it. Nitrogen cycle: N$_2$ gas (78% of atmosphere) is biologically unusable until nitrogen-fixing bacteria (e.g., Rhizobium) convert it to ammonia/ammonium; nitrification converts ammonium to nitrites then nitrates; denitrification returns N$_2$ to air. Phosphorus cycle has NO significant atmospheric gas phase --- it cycles through rock weathering, soil, water, and organisms only, making it often the limiting nutrient. The water cycle moves via evaporation, transpiration, condensation, precipitation, and runoff/infiltration.

Nitrogen fixation (N$_2 arrow$ ammonia, done by bacteria) vs. nitrification (ammonium $arrow$ nitrite $arrow$ nitrate, also bacterial) vs. denitrification (nitrate $arrow$ N$_2$ gas, releases nitrogen back to atmosphere).

Treating phosphorus like nitrogen or carbon and assuming it has a major atmospheric gas reservoir --- phosphorus has no biologically significant gas phase, so phosphorus pollution (fertilizer runoff) stays in aquatic systems and drives eutrophication.

P cycle has no gas phase $arrow$ often the limiting nutrient; N needs bacteria to be usable.

Different ecosystems vary enormously in how much biomass they produce per unit area per year, which sets the energy ceiling for everything living there.

Highest NPP per unit area: estuaries, tropical rainforests, swamps/marshes. Lowest NPP: open ocean, tundra, desert. Despite low per-area productivity, the open ocean's huge total area makes it the single largest contributor to total global NPP. Units for productivity are typically $kcal/m^2/yr$ or $g/m^2/yr$.

Productivity per unit area (highest in wetlands/rainforests) vs. total global productivity (highest contribution from the ocean, due to its sheer surface area, despite low per-area productivity).

Assuming the ocean is the most productive ecosystem per unit area because it contributes the most total productivity --- always distinguish ``per area'' rankings from ``total contribution'' rankings on data questions.

Per-area champs: wetlands/rainforest. Total-global champ: open ocean (area, not rate).

Ecosystems change predictably over time after a disturbance, moving through a series of communities toward a relatively stable climax community.

Primary succession begins on lifeless substrate with no soil (bare rock, new volcanic land, retreating glacier) --- pioneer species are typically lichens and mosses that begin soil formation; it takes centuries. Secondary succession begins where soil already exists after a disturbance (fire, farming, logging) and proceeds much faster, starting with grasses/weeds.

Primary succession (no soil present, starts from bare rock) vs. secondary succession (soil already present, disturbance removed vegetation only).

Calling regrowth after a forest fire or abandoned farmland ``primary succession'' --- if soil already existed before the disturbance, it is secondary succession, even after a severe fire.

No soil $arrow$ primary (slow, lichens first); soil present $arrow$ secondary (fast, grasses first).

Higher biodiversity generally increases ecosystem stability and resilience, and the theory of island biogeography predicts species richness from an island's size and isolation.

Ecosystem services (provisioning, regulating, supporting, cultural) are the benefits humans get from healthy ecosystems (pollination, water purification, flood control, climate regulation). Island biogeography: species richness increases with island size and decreases with distance from the mainland (source pool); larger/closer islands reach equilibrium at higher species numbers via higher immigration and lower extinction rates.

Species richness (the number of different species present) vs. species evenness (how equally abundant those species are) --- both combine to determine overall biodiversity/diversity index.

Assuming biodiversity is only about species count --- genetic diversity (within a species) and ecosystem diversity (variety of habitats/communities) are equally tested components of overall biodiversity.

Big & close islands $arrow$ more species (higher immigration, lower extinction).

Every species has a range of environmental conditions it can tolerate and a specific functional role (niche) it plays, and interactions between species shape community structure.

Range of tolerance: optimal range $arrow$ zone of physiological stress $arrow$ zones of intolerance at the extremes. Species interactions: competition ($-/-$), predation and parasitism ($+/-$), mutualism ($+/+$), commensalism ($+/0$). Competitive exclusion principle: two species cannot indefinitely occupy the exact same niche in the same place; resource partitioning lets similar species coexist by dividing resources.

Fundamental niche (the full range of conditions a species COULD occupy) vs. realized niche (the narrower range it actually occupies once competition/predation are factored in).

Mislabeling a $+/-$ interaction --- predation and parasitism are both $+/-$, but predation quickly kills the prey while parasitism harms the host gradually without immediate death; herbivory is also $+/-$ (plant is harmed).

Competitive exclusion $arrow$ resource partitioning lets similar species coexist.

Some species have an ecological importance far out of proportion to their numbers, and species introduced outside their native range can devastate an ecosystem lacking natural predators/controls.

Keystone species have a disproportionately large effect on community structure relative to their abundance (classic example: sea otters controlling sea urchin populations that would otherwise destroy kelp forests). Indicator species signal ecosystem health via their sensitivity to pollution/change (e.g., lichens for air quality, amphibians for water/wetland quality). Invasive species: non-native, spread rapidly, outcompete natives, usually due to lack of natural predators/pathogens in the new range.

Keystone species (removal causes ecosystem collapse, regardless of population size) vs. dominant species (most abundant/highest biomass, but not necessarily critical to structure).

Assuming an invasive species must be intentionally harmful --- most invasives (kudzu, zebra mussels, Burmese pythons) were introduced accidentally or for a different, often well-intentioned, purpose.

Remove the keystone $arrow$ the whole community collapses (sea otter $arrow$ urchin $arrow$ kelp).

Biomes are large-scale terrestrial regions defined mainly by climate (temperature and precipitation), while aquatic biomes/zones are defined by depth, salinity, and light penetration.

Terrestrial biomes (low to high precipitation/varying temp): desert, tundra, grassland/savanna, temperate forest, tropical rainforest, taiga/boreal forest. Aquatic zones: photic zone (light penetrates, photosynthesis occurs) vs. aphotic zone (no light); littoral/limnetic/profundal/benthic zones in lakes; estuaries are highly productive transition zones between fresh and salt water.

Tundra (cold, low precipitation, permafrost, low biodiversity) vs. taiga/boreal forest (cold but with coniferous tree cover, no permafrost at the surface).

Confusing biome (defined by climate, e.g., ``desert'') with ecosystem (a specific functioning community in one location) --- a biome is a category that contains many individual ecosystems sharing similar climate and vegetation type.

Biomes = climate-defined categories; ecosystems = the actual local systems within them.

Populations grow exponentially only when resources are unlimited; in the real world, limited resources cause growth to slow and level off near the environment's carrying capacity.

Exponential growth: $$/dNdt = rN$$ produces a J-shaped curve with no limit. Logistic growth: $$/dNdt = rN≤ft(/K-NK)$$ produces an S-shaped curve that levels off at carrying capacity $K$; growth rate is fastest at $N = K/2$. $r$ = per capita growth rate (birth rate $-$ death rate, ignoring migration for a closed population).

Exponential growth (J-curve, $/dNdt=rN$, unlimited resources) vs. logistic growth (S-curve, $/dNdt=rN/K-NK$, resource-limited, approaches carrying capacity $K$).

Forgetting the $(K-N)/K$ term when writing the logistic equation, or claiming a population grows fastest right at $K$ --- growth rate is actually zero at $K$ (population is stable) and maximum halfway to $K$.

$dN/dt = rN$ (J-curve, unlimited) vs.\ $rN(K-N)/K$ (S-curve, levels off at $K$).

Carrying capacity ($K$) is the maximum population size an environment can sustainably support, set by the scarcest necessary resource and modified by density-dependent and density-independent factors.

Density-dependent limiting factors intensify as population density increases (competition for food/space, predation, disease/parasites spreading faster in crowded populations). Density-independent limiting factors affect a population regardless of its density (natural disasters, extreme weather, habitat destruction). When a population overshoots $K$, a population crash (die-off) typically follows.

Density-dependent factors (effect strengthens as population grows more crowded, e.g., disease, competition) vs. density-independent factors (effect is constant regardless of density, e.g., a hurricane, a wildfire, a cold snap).

Labeling a natural disaster as ``density-dependent'' because it happened to a large population --- classification depends on whether the factor's IMPACT changes with density, not on the population size when it occurred.

Density-dependent $arrow$ worse when crowded; density-independent $arrow$ hits regardless of crowding.

Species evolve different reproductive strategies (r-selected vs.\ K-selected) that trade off offspring quantity against parental investment, reflected in characteristic survivorship curves.

r-selected species: many small offspring, little/no parental care, short lifespan, early reproduction, thrive in unstable/unpredictable environments (insects, weeds, rodents) --- Type III survivorship (high early mortality). K-selected species: few offspring, high parental investment, long lifespan, thrive near carrying capacity in stable environments (elephants, humans, whales) --- Type I survivorship (low early mortality, most death late in life). Type II survivorship = constant mortality risk at all ages (many birds, rodents in some contexts).

Type I survivorship curve (most individuals survive to old age, then die --- K-selected pattern) vs. Type III (massive early die-off, few reach adulthood --- r-selected pattern).

Assuming ``r-selected'' always means a small organism --- classification is about reproductive strategy (many offspring, low investment, opportunistic), not size; some larger organisms can still be relatively r-selected.

r-selected: many, small, Type III. K-selected: few, cared-for, Type I.

As countries industrialize and develop economically, they typically pass through a predictable four-stage shift in birth and death rates called the demographic transition.

Stage 1 (pre-industrial): high birth rate, high death rate, low/stable growth. Stage 2 (developing): death rate falls sharply (sanitation/medicine), birth rate stays high $arrow$ rapid population growth. Stage 3 (industrializing): birth rate falls (urbanization, education, family planning) $arrow$ growth slows. Stage 4 (developed): both rates low $arrow$ stable/slow growth (some countries enter a Stage 5 with declining population). Total fertility rate (TFR) of about 2.1 is ``replacement level'' for a developed country.

Crude birth/death rate (per 1,000 people per year, total population) vs. total fertility rate (average number of children per woman over her lifetime) --- both measure fertility/mortality differently.

Assuming population growth rate is highest in Stage 4 (developed) countries --- the fastest growth actually occurs in Stage 2, where death rates have dropped but birth rates remain high.

Stage 2 = death rate crashes first (fastest growth); Stage 3 = birth rate catches down.

An age-structure diagram's shape reveals a population's growth trajectory, and a young, wide-based pyramid guarantees continued growth for decades even if fertility rates drop immediately (population momentum).

Expansive pyramid (wide base, narrow top) $arrow$ rapid future growth, many young people entering reproductive age. Stationary pyramid (roughly equal width top to bottom) $arrow$ stable population. Constrictive pyramid (narrower base than middle) $arrow$ declining population. Population momentum: even after fertility drops to replacement level, a young population keeps growing for another generation because so many are entering childbearing age.

Expansive age structure (young/wide base $arrow$ future growth) vs. constrictive age structure (narrow base $arrow$ future decline), read from the shape of the pyramid's base relative to its middle and top.

Assuming population growth stops immediately once fertility rate drops to replacement level (2.1) --- population momentum from a young age structure keeps total population rising for years afterward.

Wide base $arrow$ expansive $arrow$ growing; narrow base $arrow$ constrictive $arrow$ shrinking.

Earth's lithosphere is divided into moving plates, and the type of boundary between them determines the specific geologic hazards (earthquakes, volcanoes) a region faces.

Divergent boundaries: plates move apart, magma rises (mid-ocean ridges, rift valleys). Convergent boundaries: plates collide --- subduction (denser oceanic plate sinks beneath continental/oceanic plate, causing volcanoes and the strongest earthquakes) or continental collision (mountain building, e.g., Himalayas). Transform boundaries: plates slide past each other horizontally (e.g., San Andreas Fault), producing earthquakes but not volcanoes.

Convergent boundary (plates collide, produces volcanoes AND earthquakes via subduction) vs. transform boundary (plates slide past each other, produces earthquakes but NOT volcanoes).

Assuming all plate boundaries produce volcanic activity --- transform boundaries produce only earthquakes; divergent boundaries produce mostly non-explosive volcanism (basaltic, effusive), while subduction zones produce the most explosive, dangerous volcanism.

Divergent $arrow$ pulls apart; convergent $arrow$ collides/subducts (quakes + volcanoes); transform $arrow$ slides (quakes only).

Rocks continuously transform among three types through geologic processes, and soil forms slowly from the weathering of parent rock combined with organic matter accumulation.

Igneous rock forms from cooled magma/lava; sedimentary rock forms from compacted/cemented sediment (often containing fossils); metamorphic rock forms when heat and pressure transform existing rock without melting it. All three can transform into any other type given the right process (melting, weathering/erosion, heat+pressure). Soil horizons top to bottom: O (organic litter), A (topsoil, humus-rich, most fertile), B (subsoil, mineral accumulation), C (weathered parent rock), R (bedrock).

Weathering (physical/chemical breakdown of rock in place) vs. erosion (the transport of weathered material by wind, water, or ice to a new location).

Mixing up soil horizon order --- the fertile, organic-rich A horizon (topsoil) is the layer most vulnerable to erosion and the one agriculture depends on; losing it is far more consequential than losing the C horizon.

O-A-B-C-R top to bottom; A horizon (topsoil) is the fertile layer farming depends on.

Uneven solar heating by latitude drives global atmospheric circulation cells and ocean currents that redistribute heat and determine regional climate and precipitation patterns.

Three atmospheric circulation cells per hemisphere: Hadley (0--30°, rising air at equator = wet tropics, sinking air at 30° = desert belts), Ferrel (30--60°), Polar (60--90°). Coriolis effect deflects moving air/water right in the Northern Hemisphere, left in the Southern Hemisphere. Thermohaline circulation (the ``ocean conveyor belt'') moves heat globally via density differences from temperature and salinity.

Weather (short-term atmospheric conditions, day to day) vs. climate (long-term average weather patterns over decades).

Forgetting that sinking air at 30° latitude (Hadley cell) is WHY most of the world's major deserts sit at that latitude --- sinking air warms and dries, suppressing precipitation, regardless of any nearby ocean.

Rising air at equator = wet; sinking air at 30° = desert belt (Sahara, Arabian, Australian deserts).

ENSO is a periodic, naturally occurring shift in Pacific Ocean surface temperatures and trade wind strength that disrupts weather patterns worldwide for months to over a year.

El Ni\ no: trade winds weaken, warm surface water piles up in the eastern Pacific (instead of the west), suppressing the normal upwelling of cold, nutrient-rich water off South America $arrow$ collapse of fisheries there, altered global rainfall/drought patterns. La Ni\ na is roughly the opposite: stronger trade winds, cooler eastern Pacific, stronger upwelling.

El Ni\ no (weak trade winds, warm eastern Pacific, weak upwelling, fishery collapse off South America) vs. La Ni\ na (strong trade winds, cool eastern Pacific, strong upwelling, typically more Atlantic hurricanes).

Confusing ENSO (a natural, cyclical, short/medium-term ocean-atmosphere oscillation) with long-term anthropogenic climate change --- they are distinct phenomena, though ENSO events are increasingly discussed as being intensified by a warming baseline.

El Ni\ no $arrow$ warm East Pacific, weak upwelling, fisheries crash; La Ni\ na $arrow$ opposite.

Although water covers most of Earth's surface, only a tiny fraction is fresh, liquid, and accessible, making freshwater a scarce and unevenly distributed resource.

About 97% of Earth's water is saltwater (oceans); of the remaining $$3% freshwater, roughly 2/3 is locked in glaciers/ice caps, leaving under 1% of all water on Earth as accessible liquid freshwater (groundwater, lakes, rivers). Aquifers are underground water-bearing rock layers; an unconfined aquifer is recharged directly from the surface, a confined aquifer is trapped between impermeable rock layers and recharges much more slowly.

Unconfined aquifer (recharges relatively quickly from surface infiltration) vs. confined aquifer (isolated by impermeable layers, very slow recharge, more vulnerable to permanent depletion).

Assuming freshwater scarcity is mainly about total global water quantity --- the real driver on the exam is DISTRIBUTION and accessibility (most freshwater is frozen or deep underground), not the total amount present on Earth.

97% saltwater; of the 3% freshwater, most is frozen $arrow$ under 1% of all water is usable liquid freshwater.

The Green Revolution dramatically increased global food production through high-yield crop varieties, synthetic fertilizer, irrigation, and pesticides, but at significant environmental cost.

Green Revolution technologies: high-yield/hybrid seed varieties, synthetic nitrogen fertilizer, expanded irrigation, mechanization, pesticides. Trade-offs: increased yields fed billions more people, but caused increased water use, fertilizer runoff/eutrophication, pesticide resistance and non-target harm, soil degradation, and reduced crop genetic diversity (monoculture vulnerability).

Monoculture (single crop species planted over a large area --- efficient but vulnerable to pests/disease and reduces biodiversity) vs. polyculture/intercropping (multiple crop species grown together --- more resilient, mimics natural diversity).

Treating the Green Revolution as purely positive or purely negative on an FRQ --- always address BOTH the food-security benefit (fed a rapidly growing population) and the environmental costs (fertilizer/pesticide runoff, monoculture risk, groundwater depletion).

Green Revolution: high-yield seeds + fertilizer + irrigation $arrow$ more food, more runoff/monoculture risk.

Poor land-use practices accelerate soil erosion and nutrient loss far faster than soil naturally forms, but specific conservation techniques can dramatically slow that loss.

Conservation methods: contour plowing (plowing along elevation contours, not up/down slope, to slow water runoff), terracing (steplike flat platforms on steep slopes), no-till/low-till farming (leaves crop residue, reduces erosion and preserves soil structure), crop rotation (alternates crops, e.g., legumes to restore nitrogen, to prevent nutrient depletion), windbreaks/shelterbelts (tree rows that reduce wind erosion), strip cropping.

Contour plowing (follows the land's elevation contours to slow runoff on moderate slopes) vs. terracing (creates flat steps on STEEP slopes, more effective for severe slopes).

Recommending contour plowing on very steep terrain where terracing is required, or vice versa recommending expensive terracing where simple contour plowing would suffice --- match the conservation method to the slope steepness described in the scenario.

Gentle slope $arrow$ contour plowing; steep slope $arrow$ terracing; wind erosion $arrow$ windbreaks.

Irrigation choices trade off water-use efficiency against cost, and converting natural land to agricultural or urban use fundamentally alters local water cycles.

Flood/furrow irrigation: cheap but very inefficient (high evaporation/runoff loss, can cause waterlogging and salinization). Spray/sprinkler irrigation: moderate efficiency. Drip irrigation: delivers water directly to plant roots, most efficient (least water waste, least salinization risk) but most expensive to install. Salinization: irrigation water evaporates and leaves salts behind in the soil, eventually making land unfit for crops.

Salinization (salt buildup in soil from evaporating irrigation water, common in arid-climate flood irrigation) vs. waterlogging (soil becomes saturated, depriving roots of oxygen, from overirrigation/poor drainage).

Assuming drip irrigation is always the ``correct'' recommendation on an FRQ without acknowledging its high installation cost --- a complete answer names both the environmental benefit and the economic trade-off.

Flood = cheap/wasteful; drip = efficient/expensive; evaporation leaves salt behind $arrow$ salinization.

Converting forests and natural land to agriculture, logging, or urban development provides economic benefits but disrupts the ecosystem services those lands once provided.

Deforestation impacts: loss of carbon sink/increased atmospheric CO$_2$, soil erosion (loss of root structure holding soil), reduced local precipitation (less transpiration), habitat/biodiversity loss, disrupted water cycle. Urban sprawl impacts: habitat fragmentation, increased impervious surface $arrow$ increased stormwater runoff and flooding, urban heat island effect (cities are measurably warmer than surrounding rural areas due to pavement/buildings absorbing and re-emitting heat).

Habitat fragmentation (a large habitat is broken into smaller, disconnected patches, harming species needing large ranges) vs. habitat destruction (the habitat is eliminated outright).

Forgetting that impervious surfaces increase runoff VOLUME and speed while decreasing infiltration/groundwater recharge --- this combination is what causes urban flash flooding even from moderate rainfall.

Impervious surfaces $arrow$ less infiltration, more/faster runoff $arrow$ urban flash flooding.

Different mineral extraction methods trade off cost and yield against the scale of land disturbance and pollution they cause.

Surface mining (strip mining, open-pit, mountaintop removal): cheaper, extracts near-surface ore, but destroys large land areas and generates massive waste rock. Subsurface/underground mining: smaller surface footprint but more dangerous and expensive, can cause subsidence. Acid mine drainage: exposed sulfide minerals react with air/water to form sulfuric acid, which leaches toxic heavy metals into waterways --- a major long-term pollution legacy of both surface and subsurface mining.

Surface mining (larger land footprint, cheaper, used for shallow deposits) vs. subsurface mining (smaller footprint, more expensive/dangerous, used for deep deposits).

Assuming mining pollution stops once a mine closes --- acid mine drainage can continue leaching acid and heavy metals into watersheds for decades after a mine is abandoned.

Exposed sulfides + water/air $arrow$ sulfuric acid $arrow$ acid mine drainage leaches heavy metals.

Sustainable yield management aims to harvest a renewable resource at a rate the population can naturally replace, but overharvesting past that point causes collapse.

Maximum sustainable yield (MSY): the largest harvest that can be taken from a renewable resource population indefinitely without depleting it, generally targeted near $N=K/2$ where population growth rate is fastest. Sustainable forestry: selective cutting, shelterwood cutting, and reduced-impact logging preserve more structure than clear-cutting. Overfishing from ignoring MSY, plus bycatch (unintentional catch of non-target species) and habitat-damaging methods (bottom trawling), have collapsed numerous global fisheries.

Clear-cutting (removes all trees in an area at once --- cheap, high erosion risk, resets to early succession) vs. selective cutting (removes only mature/marked trees --- preserves canopy and soil structure, more sustainable).

Assuming MSY should target the population's maximum size ($K$) rather than $K/2$ --- population growth rate is actually zero at $K$, so sustainable harvest is maximized near the midpoint of the logistic curve, not at carrying capacity itself.

MSY targets $≈ K/2$ (fastest regrowth), not $K$ itself (growth rate $=0$ there).

Coal, oil, and natural gas formed over millions of years from buried organic matter under heat and pressure, making them nonrenewable on any human timescale.

Coal forms from ancient swamp/plant material (highest carbon, most polluting when burned, especially sulfur dioxide). Oil and natural gas form from marine organisms buried under sediment. Extraction methods include conventional drilling and hydraulic fracturing (``fracking,'' injects high-pressure water/sand/chemicals to release gas/oil from shale, raising concerns about groundwater contamination and induced seismicity). Fossil fuel combustion is the dominant source of anthropogenic CO$_2$ and a major source of SO$_2$, NO$_x$, and particulate pollution.

Nonrenewable resource (fossil fuels, forms over geologic timescales far slower than human consumption, effectively finite) vs. renewable resource (replenished on a human timescale, e.g., solar, wind, biomass, if used sustainably).

Treating natural gas as ``clean'' with zero environmental concern --- while it burns cleaner than coal (less CO$_2$, SO$_2$, and particulates per unit energy), methane leakage during extraction/transport is itself a potent greenhouse gas concern.

Coal = dirtiest (most C, most SO$_2$); natural gas = cleanest-burning fossil fuel but leaks methane.

Nuclear power generates large amounts of low-carbon electricity through controlled fission chain reactions, but raises distinct concerns about radioactive waste and catastrophic accident risk.

Fission: splitting heavy atoms (typically U-235) releases energy used to boil water, produce steam, and turn a turbine --- same basic electricity-generation principle as fossil fuel plants, but no direct CO$_2$ emissions from the reaction itself. Concerns: long-lived radioactive waste requiring secure storage for thousands of years, high construction cost, and the risk (though rare) of severe accidents (e.g., Chernobyl, Fukushima).

Nuclear fission (splitting heavy atoms, the technology used in all current commercial nuclear power plants) vs. nuclear fusion (combining light atoms, powers the sun, not yet commercially viable for power generation).

Claiming nuclear power directly emits greenhouse gases like a fossil fuel plant --- the fission reaction itself is carbon-free; nuclear's main environmental drawbacks tested on the exam are waste disposal and accident risk, not routine CO$_2$ emissions.

Fission (splitting U-235) $arrow$ low-carbon electricity, but long-lived radioactive waste.

Renewable energy sources are naturally replenished on human timescales, but each has distinct trade-offs in reliability, land use, and environmental side effects.

Solar (photovoltaic or thermal): intermittent (needs sun), no emissions during operation, land-use footprint. Wind: intermittent, bird/bat mortality concern, no emissions during operation. Hydroelectric: reliable/on-demand, but dams disrupt river ecosystems, block fish migration (e.g., salmon), and flood upstream land. Geothermal: reliable base-load power but geographically limited to tectonically active regions. Biomass: burning organic matter is considered ``carbon neutral'' if replanted at the rate it's harvested, but still emits pollutants when burned.

Intermittent renewables (solar, wind --- output varies with weather/time of day, need storage or backup) vs. reliable/base-load renewables (hydro, geothermal --- can generate consistently on demand).

Assuming all renewables are impact-free --- dams (fish migration blockage, habitat flooding), wind turbines (bird/bat strikes), and even solar/wind farms (land use, mining for materials) all carry real environmental trade-offs the exam expects you to name.

Solar/wind = intermittent, low-impact; hydro/geothermal = reliable, but dams disrupt rivers.

Reducing overall energy demand through conservation and efficiency is often the cheapest and fastest way to cut both costs and emissions, without needing new power sources.

Energy conservation = reducing energy USE through behavior change (turning off lights, driving less). Energy efficiency = getting the same output/service using LESS energy input (LED bulbs, better insulation, hybrid/electric vehicles, cogeneration/combined heat-and-power systems that capture waste heat). Cogeneration can push overall fuel-to-useful-energy efficiency well above 50%, far higher than a standard power plant that discards waste heat.

Energy conservation (reducing consumption through behavior/lifestyle) vs. energy efficiency (reducing the amount of energy needed to accomplish the same task through better technology).

Treating conservation and efficiency as identical on an FRQ --- an efficiency upgrade (e.g., LED bulbs) can happen with ZERO behavior change, while conservation requires actually using less.

Efficiency $=$ better tech, same task, less energy; conservation $=$ using less, period.

Global energy consumption is dominated by fossil fuels but is shifting, and the electrical grid must balance supply and demand in real time, creating unique challenges for intermittent renewables.

Fossil fuels (coal, oil, natural gas) still supply the large majority of global primary energy, though renewable share is rising fastest of any source category. The grid requires constant matching of generation to demand; intermittent sources (solar, wind) require either backup generation, energy storage (batteries, pumped hydro), or grid-scale demand management to remain reliable. Developed nations consume disproportionately more energy per capita than developing nations.

Primary energy consumption (total energy used across all sectors: electricity, transportation, heating, industry) vs. electricity generation alone (only one subset/use of total primary energy).

Assuming a country's total energy consumption and its electricity generation mix are the same thing --- a country can have a very ``clean'' electricity grid while still consuming huge amounts of fossil fuel for transportation and heating.

Fossil fuels still dominate global primary energy; renewables need storage/backup for grid reliability.

Every energy source, including renewables, has an environmental footprint from extraction/construction through operation and disposal --- comparing them requires a full life-cycle view.

Comparing energy sources on the exam requires weighing: land-use footprint, greenhouse gas emissions across the full life cycle (not just during operation), water use, wildlife impact, waste products, and reliability. Mountaintop removal coal mining and oil sands extraction are especially destructive nonrenewable extraction methods; large-scale hydro and biomass are the renewables with the largest ecological trade-offs.

Operational emissions (released while actually generating power, near-zero for wind/solar/nuclear/hydro) vs. life-cycle emissions (includes manufacturing, mining raw materials, construction, and decommissioning --- nonzero even for renewables, but still far lower than fossil fuels).

Claiming any renewable source has literally zero environmental impact --- FRQ scoring rewards naming the SPECIFIC trade-off (bird strikes for wind, habitat flooding for hydro, land use for solar) rather than a blanket ``renewables are impact-free'' claim.

Full life-cycle comparison, not just operating emissions --- every source has SOME trade-off.

Primary pollutants are emitted directly into the air; secondary pollutants form through chemical reactions in the atmosphere, and both concentrate dangerously under certain weather conditions.

Primary pollutants: CO, SO$_2$, NO$_x$, particulate matter, VOCs (emitted directly). Secondary pollutants: ground-level (tropospheric) ozone and photochemical smog, formed when VOCs and NO$_x$ react in sunlight; also acid rain precursors combining in the atmosphere. Industrial/gray smog (coal-burning, sulfur-based) vs. photochemical/brown smog (vehicle exhaust + sunlight, ozone-based). Temperature inversion: a layer of warm air traps cooler air (with pollutants) near the surface, preventing normal vertical mixing and letting pollution accumulate --- common in valley cities and worsened by geography.

Primary pollutant (emitted directly from a source, e.g., NO$_x$ from a tailpipe) vs. secondary pollutant (formed by chemical reaction of primary pollutants in the atmosphere, e.g., ground-level ozone).

Confusing ground-level (tropospheric) ozone (a harmful secondary pollutant/smog component near the surface) with stratospheric ozone (the beneficial UV-blocking ozone layer high in the atmosphere) --- ``ozone is good up high, bad nearby'' is the key exam distinction.

Tropospheric ozone = bad (smog, near ground); stratospheric ozone = good (blocks UV, high up).

Sulfur dioxide and nitrogen oxides from fossil fuel combustion react with water vapor in the atmosphere to form acids that fall as precipitation, damaging ecosystems far from the pollution source.

$SO_2 + H_2O arrow H_2SO_3$ (sulfurous acid, further oxidizes to sulfuric acid); $NO_x$ reacts similarly to form nitric acid. Normal rainwater is slightly acidic (pH $≈ 5.6$, from dissolved CO$_2$); acid rain is well below this. Effects: leaches nutrients (Ca, Mg) and mobilizes toxic aluminum from soil, acidifies lakes/streams (harming fish, especially at early life stages), damages forests (especially at high elevation) and stone/metal structures (e.g., statues, buildings). Acid rain can travel far via prevailing winds, making it a transboundary/regional pollution issue, not just a local one.

Acid rain's ecosystem damage mechanism (leaching essential nutrients out of soil AND mobilizing toxic aluminum into soil/water) vs. simply ``making water acidic'' --- both nutrient loss and aluminum toxicity independently harm plants and aquatic life.

Assuming acid rain only affects the area immediately around the polluting power plant --- prevailing winds can carry SO$_2$/NO$_x$ hundreds of miles before they convert to acids and fall, making this a classic interstate/international pollution problem on FRQs.

$SO_2/NO_x$ + atmospheric water $arrow$ acids; travels far downwind $arrow$ leaches soil nutrients, mobilizes Al.

Human-made chlorofluorocarbons (CFCs) catalytically destroy stratospheric ozone, but a coordinated global treaty phasing them out has allowed the ozone layer to begin recovering.

CFCs (once used in refrigerants, aerosols) release chlorine atoms in the stratosphere that catalytically break down ozone (O$_3$) --- one chlorine atom can destroy thousands of ozone molecules without being consumed. This thinning is most severe over Antarctica (the ``ozone hole''), due to unique polar stratospheric cloud chemistry. The Montreal Protocol (1987) is the international treaty that phased out CFC production and is widely cited as the most successful international environmental agreement, with the ozone layer on track to recover by mid-century.

Ozone depletion (a stratospheric chemistry problem caused by CFCs, addressed by the Montreal Protocol) vs. global climate change (a greenhouse gas/heat-trapping problem, addressed by different treaties like the Paris Agreement) --- these are separate problems, though some ozone-depleting substances are ALSO potent greenhouse gases.

Confusing ozone depletion with global warming as the ``same problem'' --- they have different causes (CFCs vs. CO$_2$/CH$_4$), different mechanisms, and were addressed by different treaties (Montreal Protocol vs. Kyoto/Paris).

CFCs catalytically destroy stratospheric O$_3$ $arrow$ Montreal Protocol (1987) phased them out $arrow$ recovery underway.

Indoor air pollution can be as harmful as outdoor pollution and disproportionately affects developing nations, while U.S. federal law sets enforceable outdoor air quality standards.

Major indoor pollutants: radon (naturally occurring radioactive gas seeping from soil/rock, the second-leading cause of lung cancer in the U.S. after smoking), carbon monoxide (odorless, binds hemoglobin more strongly than O$_2$), asbestos, formaldehyde, and especially indoor smoke from burning solid fuels (wood, dung, coal) for cooking/heating in developing regions --- a major global respiratory health burden. The Clean Air Act (1970, U.S.) established National Ambient Air Quality Standards (NAAQS) for six criteria pollutants (CO, Pb, NO$_2$, O$_3$, SO$_2$, PM) and is enforced by the EPA.

Radon (a naturally occurring radioactive gas from soil/rock, an indoor hazard with no outdoor-source pollution mandate) vs. CFCs/industrial pollutants (human-manufactured, regulated as outdoor emission sources).

Assuming indoor air is automatically cleaner/safer than outdoor air --- indoor concentrations of radon, CO, and cooking-fire particulates can be far higher than typical outdoor pollution levels.

Radon = \#2 cause of U.S. lung cancer; Clean Air Act (1970) sets NAAQS for 6 criteria pollutants, EPA-enforced.

Excess nutrients (especially nitrogen and phosphorus) entering water bodies trigger algal blooms that ultimately deplete dissolved oxygen and can kill aquatic life.

Cultural eutrophication: human-caused nutrient pollution (fertilizer runoff, sewage, detergents) accelerates the natural eutrophication process. Sequence: excess N/P $arrow$ algal bloom $arrow$ algae die and are decomposed by aerobic bacteria $arrow$ decomposition consumes dissolved oxygen $arrow$ hypoxia/anoxia (``dead zone'') $arrow$ fish kills. Key water quality indicators: dissolved oxygen (DO, should be high for healthy water), biochemical oxygen demand (BOD, high BOD signals heavy organic pollution/oxygen depletion risk), turbidity, and indicator species like mayfly larvae (need clean, oxygen-rich water) vs. sludge worms (tolerate pollution).

Dissolved oxygen (DO, a direct water quality measurement --- higher is healthier) vs. biochemical oxygen demand (BOD, measures how much oxygen DECOMPOSERS would consume breaking down organic matter present --- higher BOD means MORE pollution and predicts future oxygen depletion).

Assuming algae themselves directly kill fish --- the fish kill is actually caused by oxygen depletion from bacterial decomposition of dead algae AFTER the bloom, not the algae's presence itself.

Excess N/P $arrow$ algal bloom $arrow$ decomposers consume O$_2$ $arrow$ hypoxia $arrow$ fish kill.

Water pollution sources are classified by whether they come from a single identifiable location or from diffuse runoff across a wide area, which fundamentally shapes how each is regulated.

Point source pollution: comes from a single, identifiable location (a factory discharge pipe, a sewage outfall) --- easier to regulate and monitor. Nonpoint source pollution: comes from diffuse, widespread sources (agricultural runoff, urban stormwater, atmospheric deposition) --- much harder to regulate/trace and is now the LARGER contributor to U.S. water pollution overall. The Clean Water Act (1972, U.S.) regulates point-source discharges into navigable waters via the NPDES permit system, enforced by the EPA; it addresses nonpoint sources far less directly, through best-management-practice incentives rather than permits.

Point source (single identifiable pipe/outfall, directly regulated under Clean Water Act permits) vs. nonpoint source (diffuse runoff from farms/streets/lawns, much harder to regulate, now the dominant pollution source in the U.S.).

Assuming the Clean Water Act effectively controls agricultural fertilizer runoff --- nonpoint source pollution like farm runoff is largely UNregulated by direct permit under the Act, which is why it remains the leading cause of U.S. water quality impairment today.

Point source = one pipe, regulated (NPDES permits); nonpoint = diffuse runoff, the bigger, harder-to-regulate problem.

Greenhouse gases trap outgoing infrared radiation in the atmosphere, warming Earth's surface --- a natural process essential to life that human emissions have dangerously intensified.

Key greenhouse gases (by warming contribution/potency): water vapor (most abundant, but a feedback not a direct human driver), carbon dioxide (largest human-driven contributor, from fossil fuel combustion and deforestation), methane (CH$_4$, far more potent per molecule than CO$_2$ but shorter atmospheric lifetime --- from livestock, landfills, rice paddies, fossil fuel extraction), nitrous oxide (N$_2$O, from fertilizer/agriculture), and CFCs. Global warming potential (GWP) compares a gas's heat-trapping ability relative to CO$_2$ over a set time period (usually 100 years).

The greenhouse effect (a natural, necessary process without which Earth would be far too cold for life) vs. anthropogenic (enhanced) global warming (the human-caused INTENSIFICATION of that natural effect from added greenhouse gases).

Treating the greenhouse effect itself as inherently bad --- the natural greenhouse effect keeps Earth's average temperature roughly 33°C warmer than it would otherwise be; the exam-tested problem is the human-caused enhancement of it, not its mere existence.

CO$_2$ = most CO$_2$-equivalent total impact; CH$_4$ = most potent per molecule, shorter-lived.

Multiple independent lines of physical evidence confirm recent rapid warming, and its downstream consequences cascade across ice, oceans, weather, and ecosystems worldwide.

Evidence: ice core CO$_2$/temperature records, rising global average surface temperature, shrinking glaciers/sea ice/ice sheets, rising sea level (from thermal expansion of water AND melting land ice), ocean acidification (oceans absorbing excess atmospheric CO$_2$, lowering pH via $CO_2 + H_2O arrow H_2CO_3$), and phenological shifts (earlier springs, shifting species ranges). Consequences: more frequent/intense extreme weather, coral bleaching, coastal flooding, disrupted agriculture, and biodiversity loss from habitats shifting or disappearing faster than species can migrate/adapt.

Sea level rise from thermal expansion (warmer water physically takes up more volume) vs. sea level rise from land-ice melt (glaciers/ice sheets on land add NEW water to the ocean) --- melting floating sea ice, by contrast, does NOT significantly raise sea level (it already displaces its mass in water).

Claiming melting Arctic sea ice directly raises sea level --- floating ice already displaces its own weight in water, so its melting has minimal direct effect; it's melting LAND ice (Greenland, Antarctica, mountain glaciers) and thermal expansion that actually raise sea level.

Floating sea ice melting $≈$ no sea-level change; land ice melting + thermal expansion $arrow$ real sea-level rise.

As oceans absorb roughly a quarter of human CO$_2$ emissions, seawater chemistry shifts toward acidity, threatening organisms that build calcium carbonate shells and skeletons.

$$CO_2 + H_2O arrow H_2CO_3 arrow H^+ + HCO_3^-$$ The added hydrogen ions lower ocean pH and also reduce the availability of carbonate ions ($CO_3^2-$) needed by shell/skeleton-building organisms (corals, mollusks, some plankton) to form calcium carbonate (CaCO$_3$). This threatens coral reef structure, shellfish fisheries/aquaculture, and the base of many marine food webs.

Ocean acidification (a direct chemical consequence of CO$_2$ dissolving in seawater, lowering pH) vs. coral bleaching (corals expelling their symbiotic algae, primarily triggered by elevated water TEMPERATURE, though acidification compounds coral stress).

Merging ocean acidification and coral bleaching into a single cause on an FRQ --- acidification is a pH/carbonate-chemistry problem from dissolved CO$_2$, while bleaching is primarily a heat-stress problem; both harm coral reefs but through different mechanisms and both should be named separately for full credit.

Ocean absorbs CO$_2$ $arrow$ carbonic acid $arrow$ pH drops & carbonate ions drop $arrow$ shells/reefs harder to build.

Total human environmental impact is a function of three multiplying factors: how many people there are, how much each consumes, and how damaging the technology used to produce that consumption is.

$$I = P × A × T$$ where $I$ = environmental Impact, $P$ = Population, $A$ = Affluence (consumption per person), $T$ = Technology (impact per unit of consumption). Reducing any one factor reduces total impact; developed nations often have lower $P$ but far higher $A$ and sometimes higher $T$-driven impact per capita than developing nations with larger $P$ but lower $A$.

Affluence ($A$, consumption/resource use per person --- driven by wealth and lifestyle) vs. Technology ($T$, the environmental impact/efficiency of the methods used to produce and deliver that consumption --- can raise OR lower total impact depending on whether it's cleaner or dirtier tech).

Assuming population ($P$) is always the dominant driver of a country's total environmental impact --- for many developed, low-population-growth countries, high affluence ($A$) and technology choices ($T$) contribute far more to total impact ($I$) than population size alone.

$I = P × A × T$ --- impact isn't just about population; consumption and technology multiply it.

Global environmental problems that cross borders require international cooperation, while domestic policy uses a mix of regulatory, market-based, and voluntary tools to change behavior.

Key agreements: Montreal Protocol (1987, phased out CFCs, ozone layer); Kyoto Protocol (1997, set binding emissions targets for developed nations); Paris Agreement (2015, nearly all nations set voluntary nationally determined contributions to limit warming). Domestic policy tools: command-and-control regulation (sets legal limits/standards, e.g., Clean Air/Water Acts), market-based approaches like cap-and-trade (sets a total emissions cap, allows trading of permits, creating a financial incentive to cut emissions cheaply) and carbon taxes (directly prices emissions), and subsidies/tax incentives for cleaner technology.

Cap-and-trade (government sets a firm total emissions CAP, market sets the price of permits through trading) vs. a carbon tax (government sets a firm PRICE per ton of emissions, the market determines the resulting quantity emitted).

Confusing the Kyoto Protocol (binding targets, only developed/``Annex I'' nations) with the Paris Agreement (voluntary nationally determined contributions, nearly all nations including developing ones) --- know which treaty is legally binding and which nations it covers.

Cap-and-trade fixes the QUANTITY (cap), lets price float; carbon tax fixes the PRICE, lets quantity float.

Shared, unregulated resources tend to be overexploited unless managed collectively, and the burdens of environmental degradation fall disproportionately on marginalized communities.

Tragedy of the commons (Garrett Hardin, 1968): individuals acting in their own rational self-interest overuse a shared, unregulated resource (open pasture, ocean fisheries, atmosphere) because the benefit of use is private but the cost of depletion is shared --- solutions include privatization, government regulation, or community-based management agreements. Environmental justice: the principle that no group of people should bear a disproportionate share of environmental harm (pollution, hazardous waste siting) or be excluded from environmental decision-making, regardless of race or income --- low-income and minority communities are statistically more likely to live near major pollution sources.

Tragedy of the commons (overuse of a SHARED, open-access resource due to individual incentives) vs. simple pollution/overharvesting by a single private actor on their own property (no shared-resource dynamic involved).

Proposing only ``more regulation'' as the sole solution to a tragedy-of-the-commons scenario on an FRQ --- full credit typically also accepts privatization or community-based co-management as valid alternative solutions, matched to the specific resource described.

Commons overused because benefit is private, cost is shared $arrow$ fix via regulation, privatization, or community management.

Know $dN/dt$, IPAT, NPP, the 10% rule, and Rule of 70 cold --- these appear across nearly every unit.

When two terms sound alike, ask: is the SOURCE direct or indirect? Is the resource finite or renewable?

EPA enforces; Clean Air/Water Acts regulate emissions/discharges; Superfund cleans up; ESA protects species.

When a question names a cycle, curve, or classification, first identify which of these lists it belongs to.

Units first $arrow$ state trend in words $arrow$ show all math $arrow$ explain the mechanism $arrow$ give a specific fix.

80 MCQ (60%, 90 min) + 3 FRQ --- design, data-analysis, calculation-based (40%, 70 min). Calculator allowed throughout.

Cause $arrow$ mechanism $arrow$ direct effect $arrow$ downstream impact $arrow$ targeted solution --- trace every link.

Convert units first, divide by the ORIGINAL value for % change, and always attach units to the final answer.