How do gases in the air keep the planet warm, and which gases matter most?
Topic 9.3 The Greenhouse Effect: explain the greenhouse effect, identify the major greenhouse gases, and distinguish the natural effect from the enhanced effect.
A focused answer to APES Topic 9.3, covering how the greenhouse effect works, the major greenhouse gases (carbon dioxide, methane, nitrous oxide, water vapor, CFCs), the difference between the natural and enhanced greenhouse effect, global warming potential and residence time, with a worked global warming potential calculation.
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What this topic is asking
The College Board (Topic 9.3) wants you to explain the greenhouse effect, identify the major greenhouse gases, and distinguish the natural effect from the enhanced effect.
How the greenhouse effect works
The major greenhouse gases
Natural versus enhanced greenhouse effect
Why this matters
The greenhouse effect is the mechanism behind all of Unit 9's climate topics, building on the atmosphere of Unit 4 and the carbon cycle of Unit 1. The AP exam constantly tests the distinction between the greenhouse effect (trapping outgoing infrared, warming) and ozone depletion (CFCs, UV), and between the natural and the enhanced effect.
Try this
Q1. Identify three major greenhouse gases. [1 point]
- Cue. Any three of carbon dioxide, methane, nitrous oxide, water vapor, CFCs/HFCs.
Q2. Distinguish the natural greenhouse effect from the enhanced greenhouse effect. [2 points]
- Cue. The natural greenhouse effect keeps the Earth warm enough for life; the enhanced greenhouse effect is the additional warming caused by the greenhouse gases humans add, mainly from burning fossil fuels and agriculture, which drives climate change.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ). (a) Explain how the greenhouse effect warms the Earth. (b) Identify three major greenhouse gases. (c) Distinguish the natural greenhouse effect from the enhanced greenhouse effect. (d) Explain why methane is a more potent greenhouse gas than carbon dioxide per molecule.Show worked answer →
A 4-point FRQ on the greenhouse effect.
(a) Explain (1 point): greenhouse gases absorb outgoing infrared (longwave) radiation from the Earth and re-emit it, trapping heat in the lower atmosphere and warming the surface.
(b) Identify (1 point): any three of carbon dioxide, methane, nitrous oxide, water vapor, and CFCs/HFCs.
(c) Distinguish (1 point): the natural greenhouse effect keeps the planet warm enough for life; the enhanced effect is the extra warming from human-added greenhouse gases.
(d) Explain (1 point): methane absorbs infrared radiation far more effectively per molecule than carbon dioxide, giving it a higher global warming potential, though it is less abundant and shorter-lived.
Markers reward the absorb-and-re-emit infrared mechanism, three valid greenhouse gases, the natural-versus-enhanced distinction, and the higher per-molecule potency of methane.
AP 2018 (style)1 marksSection I (multiple choice). Greenhouse gases warm the Earth by: (A) reflecting incoming sunlight back to space (B) absorbing and re-emitting outgoing infrared radiation (C) depleting stratospheric ozone (D) blocking ultraviolet radiation. Justify your choice.Show worked answer →
A 1-point MCQ on the greenhouse effect. The answer is (B).
Greenhouse gases absorb the outgoing infrared (longwave) radiation that the warmed Earth emits and re-emit some of it back downward, trapping heat in the lower atmosphere. They do not mainly reflect sunlight (A), deplete ozone (C, that is CFCs) or block UV (D, that is the ozone layer). The trap is confusing the greenhouse effect (trapping outgoing infrared) with ozone's UV-blocking role; they are different.
Related dot points
- Topic 9.4 Increases in the Greenhouse Gases: identify the human activities that increase greenhouse gases and explain why their concentrations are rising.
A focused answer to APES Topic 9.4, covering the human activities that raise greenhouse gases (fossil fuel combustion, deforestation, agriculture, landfills, industry), the specific sources of carbon dioxide, methane and nitrous oxide, the role of the carbon cycle, and the Keeling Curve evidence, with a worked emissions calculation.
- Topic 9.5 Global Climate Change: describe the evidence and effects of global climate change and explain the role of positive feedback loops.
A focused answer to APES Topic 9.5, covering the evidence for global climate change, its effects (rising temperatures, melting ice, sea-level rise, extreme weather, shifting species), positive feedback loops (ice-albedo, permafrost methane, water vapor), the difference between weather and climate, and mitigation and adaptation, with a worked sea-level reasoning example.
- Topic 9.1 Stratospheric Ozone Depletion: explain how CFCs deplete stratospheric ozone and describe the consequences of a thinner ozone layer.
A focused answer to APES Topic 9.1, covering the protective role of stratospheric ozone, how CFCs release chlorine that catalytically destroys ozone, the Antarctic ozone hole, the consequences of increased UV (skin cancer, cataracts, harm to ecosystems), and the difference from ground-level ozone, with a worked catalytic-destruction reasoning example.
- Topic 1.4 The Carbon Cycle: describe the major reservoirs and fluxes of the carbon cycle and explain how natural processes and human activities move carbon between them.
A focused answer to APES Topic 1.4, covering carbon reservoirs and fluxes, photosynthesis and respiration, decomposition, combustion, the ocean as a carbon sink, and how fossil fuel burning alters the cycle, with a worked carbon-flux calculation.
- Topic 7.4 Atmospheric CO2 and Particulates: describe the natural and human sources of atmospheric carbon dioxide and particulate matter and their effects.
A focused answer to APES Topic 7.4, covering the natural and human sources of atmospheric carbon dioxide and particulate matter, the difference between PM10 and PM2.5, why fine particles are most dangerous, the health and environmental effects, with a worked particulate exposure calculation.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)