AP Syllabus focus:
‘The main greenhouse gases are carbon dioxide, methane, water vapor, nitrous oxide, and chlorofluorocarbons (CFCs).’
Greenhouse gases are a small fraction of the atmosphere, but they strongly influence Earth’s energy balance by absorbing infrared radiation.
This NASA schematic visualizes Earth’s energy budget by tracking what happens to incoming solar radiation—how much is reflected back to space versus absorbed by the atmosphere and surface. It provides a concrete context for why greenhouse gases matter: altering infrared absorption changes the balance between incoming energy and outgoing heat. Source
Knowing the principal greenhouse gases helps you connect human activities and natural processes to atmospheric warming.
What makes a gas a “greenhouse gas”?
Greenhouse gas: An atmospheric gas that absorbs and re-emits outgoing infrared (heat) radiation, contributing to the warming of the lower atmosphere and Earth’s surface.
Greenhouse gases matter because their molecular structures allow them to interact with infrared wavelengths emitted by Earth, unlike nitrogen (N₂) and oxygen (O₂), which make up most of the air but absorb little infrared radiation.
This diagram summarizes the greenhouse effect: most incoming sunlight passes through the atmosphere and warms Earth’s surface, which then emits infrared radiation upward. Greenhouse gases absorb some of that outgoing infrared and re-emit it in all directions, including back toward the surface, increasing warming in the lower atmosphere. Source
Shared traits of the principal greenhouse gases
Present in trace amounts relative to N₂ and O₂
Differ in sources (natural vs. human-caused), removal processes, and atmospheric lifetime
Vary in how strongly they absorb infrared radiation across different wavelengths
Principal greenhouse gases named in the AP syllabus
This NOAA figure shows the contribution of major long-lived greenhouse gases to total radiative forcing over time, with CO₂ as the largest component and CH₄ and N₂O as additional contributors. It also includes a grouped contribution from CFC-related gases, illustrating how even smaller-concentration gases can still add measurable warming influence. Source
Carbon dioxide (CO₂)
Carbon dioxide (CO₂) is a major greenhouse gas because it is abundant enough to strongly affect Earth’s heat retention.
Key sources (human-related): fossil fuel combustion, cement production, and land-use change (especially deforestation)
Key natural exchanges: photosynthesis removes CO₂; respiration and decomposition release CO₂
Important sinks: long-term storage in oceans, biomass, and carbonate rocks (over geologic time)
Why it is “principal”: CO₂ is long-lived enough that emissions can accumulate, altering atmospheric concentration over time
Methane (CH₄)
Methane (CH₄) is a carbon-based gas that can have a strong warming effect per molecule.
Major sources (human-related): livestock digestion, rice paddies, landfills, and leaks during fossil fuel extraction/transport
Natural sources: wetlands and other anaerobic (low-oxygen) environments
Removal: mainly through chemical reactions in the atmosphere
Why it is “principal”: CH₄ concentrations can rise quickly with changes in agriculture and energy systems
Water vapor (H₂O)
Water vapor (H₂O) is the most abundant greenhouse gas in the atmosphere by concentration, but it varies widely by location and time.
Main source: evaporation from oceans, lakes, and soils; also transpiration from plants
Removal: condensation and precipitation (rain/snow)
Why it is “principal”: H₂O strongly absorbs infrared radiation and is tightly linked to temperature and the water cycle
Nitrous oxide (N₂O)
Nitrous oxide (N₂O) is a nitrogen-containing greenhouse gas associated with soils and agricultural practices.
Major sources (human-related): nitrogen fertiliser use and manure management, which enhance microbial processes that release N₂O
Natural sources: soil and ocean microbial activity
Removal: slow breakdown in the atmosphere
Why it is “principal”: it is less abundant than CO₂, but it persists long enough to influence warming
Chlorofluorocarbons (CFCs)
Chlorofluorocarbons (CFCs) are entirely human-made compounds that act as greenhouse gases.
Historical uses: refrigerants, foam-blowing agents, and aerosol propellants
Key feature: very low natural removal rates, so they can remain in the atmosphere for a long time
Why they are “principal”: even at low concentrations, CFCs absorb infrared radiation effectively
Distinguishing the “principal” list from other atmospheric gases
The syllabus list focuses on gases with clear greenhouse behaviour and climate relevance. Many other gases exist in air, but most do not absorb infrared radiation effectively (for example, N₂ and O₂), or they occur at levels too small to be treated as principal in AP Environmental Science.
FAQ
Their molecular structures do not absorb much outgoing infrared radiation.
They can interact with some energy forms, but they are largely transparent to the key infrared wavelengths Earth emits.
Common approaches include:
Continuous monitoring at observatories using infrared spectroscopy and gas analysers
Air sampling with laboratory calibration to international reference scales
Satellite remote sensing for broader spatial coverage (best for some gases)
CFC molecules absorb infrared radiation very efficiently in spectral regions where other gases absorb less.
This means small amounts can still contribute measurable warming.
No. Natural emissions occur, especially from wetlands and other anaerobic environments.
Human activities can add substantial additional methane through agriculture, waste management, and fossil fuel systems.
Water vapour is governed by the water cycle:
Evaporation adds it rapidly when conditions are warm and surfaces are wet
Condensation forms clouds when air cools and reaches saturation
Precipitation removes it, often on timescales of days to weeks
Practice Questions
State two principal greenhouse gases from the AP list. (2 marks)
Any two correct: carbon dioxide (CO₂), methane (CH₄), water vapour (H₂O), nitrous oxide (N₂O), chlorofluorocarbons (CFCs). (1 each)
Explain what makes a gas a greenhouse gas, and describe how sources or removal processes differ for three principal greenhouse gases. (6 marks)
Defines greenhouse gas as absorbing and re-emitting infrared radiation (heat). (1)
Explains that this affects Earth’s energy balance/warming. (1)
Correct description for three gases (1 each, max 3):
CO₂: emitted by fossil fuel burning/deforestation; removed by photosynthesis/ocean uptake.
CH₄: emitted by livestock/rice paddies/landfills/leaks; removed by atmospheric chemical reactions.
H₂O: added by evaporation/transpiration; removed by condensation/precipitation.
N₂O: linked to fertilisers/soils; slow atmospheric removal.
CFCs: synthetic industrial sources; very slow removal.
