What it covers: How humans generate and consume energy — fossil fuels, nuclear power, and the major renewable sources.
Exam weight: About 10–15% of the AP Environmental Science exam — one of the most topic-dense units alongside Unit 5 (13 topics).
The big question: How does each energy source work, and what environmental tradeoffs come with it?
Big Idea covered: Energy Transfer (ENG).
Key topics at a glance
Fossil Fuels
Coal, oil, natural gas. Extracted via drilling/fracking/mining; combustion releases CO2 and pollutants. Unevenly distributed globally.
Nuclear Power
Fission of uranium-235. Low-emission baseload power, but produces long-lived radioactive waste and carries meltdown risk.
Solar & Wind
Intermittent (depend on sun/wind availability). Solar = photovoltaic cells; wind = turbines. Both need storage/backup for reliability.
Hydroelectric
Reliable baseload renewable, but dams disrupt fish migration and flood upstream habitat.
Geothermal
Taps Earth's internal heat; most viable in geologically active regions (e.g., near plate boundaries — links to Unit 4).
Biomass & Hydrogen
Biomass: carbon-neutral over growth cycle. Hydrogen fuel cells: clean output (water), but most H2 production still relies on fossil fuels.
Energy Conservation
Reducing demand via efficiency + behavior change — often the cheapest way to cut energy-related emissions ("the first fuel").
Global Consumption Patterns
Developed nations use more energy per capita than developing nations due to industrialization and infrastructure.
The key terms you must know
Nonrenewable resource — regenerates far slower than consumed (fossil fuels).
Renewable resource — replenished on a human timescale (sunlight, wind, water).
Fracking — high-pressure fluid injection to extract oil/gas from rock.
Nuclear fission — splitting atomic nuclei to release energy.
Baseload power — the minimum continuous electricity demand that must be reliably met.
Intermittent energy — power sources (solar, wind) that don't produce consistently.
Photovoltaic (PV) cell — converts sunlight directly into electricity.
Hydrogen fuel cell — generates electricity via H2 + O2 reaction, producing only water.
Energy conservation — reducing energy demand through efficiency and behavior.
Carbon-neutral — releasing no net additional carbon (e.g., biomass over its growth cycle).
Key themes to remember
Every energy source has a tradeoff. Even "clean" renewables have land use, intermittency, or wildlife impacts.
Reliability vs. emissions is a recurring tension. Fossil fuels and nuclear provide reliable baseload power; solar/wind are clean but intermittent.
Geography determines feasibility. Geothermal needs tectonic activity; hydro needs rivers; this unit connects directly to Unit 4's Earth systems.
Conservation is the cheapest "energy source." Reducing demand avoids the cost and impact of generating new supply.
Global energy use is unequal. Developed nations consume disproportionately more energy per capita than developing nations.
Common exam traps
Nuclear power is low-emission, not zero-risk. Don't conflate "no combustion emissions" with "no environmental concerns" — waste storage is the key risk.
Biomass isn't automatically "clean." It's carbon-neutral over its growth cycle, but burning it still releases local air pollutants.
Most hydrogen today comes from fossil fuels. Don't describe hydrogen fuel cells as inherently emission-free without acknowledging production source.
Solar and wind are intermittent, not unreliable in a bad way — distinguish "doesn't always produce" from "doesn't work."
Hydroelectric is renewable but not impact-free. Dam construction has major ecological costs despite being a clean energy source.
Don't confuse energy efficiency with energy conservation. Efficiency = using less energy for the same output; conservation = broader demand reduction, including behavior change.