AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/9/9-3)
Unit 9 · Topic 9.3
9.3 The Greenhouse Effect
The greenhouse effect is the natural process by which certain gases trap heat radiated from Earth's surface, keeping the planet warm enough for life. Gases differ in how strongly they trap heat, which is compared using global warming potential (GWP), with carbon dioxide set at 1.
Key terms
- greenhouse effect
- infrared radiation
- greenhouse gas
- global warming potential (GWP)
- methane
- nitrous oxide
How the greenhouse effect works
Sunlight reaches Earth mostly as visible light, with some ultraviolet and near-infrared. About 30 percent is reflected back to space by clouds, ice and bright surfaces. The rest is absorbed by the land, oceans and air, warming them.
The warm surface gives off energy as infrared radiation (heat), which has longer wavelengths than sunlight. Greenhouse gases in the atmosphere absorb some of that infrared radiation and then re-emit it in all directions, including back toward the surface. That extra energy keeps the lower atmosphere and surface warmer than they would otherwise be.
Without any greenhouse effect, Earth's average surface temperature would be about −18 °C instead of about 15 °C, and most of the planet would be frozen. The problem isn't the greenhouse effect itself; it is the enhanced greenhouse effect caused by humans adding more greenhouse gases (topic 9.4).
The main greenhouse gases
| Gas | Main sources | How long it stays | Approximate GWP (100 years) |
|---|---|---|---|
| Carbon dioxide (CO₂) | Burning fossil fuels, deforestation, cement making; natural respiration and decomposition | Centuries or longer for part of it | 1 (the reference) |
| Methane (CH₄) | Livestock digestion, rice paddies, landfills, natural gas leaks, coal mining; natural wetlands | About 12 years | About 25–30 |
| Nitrous oxide (N₂O) | Fertilizer on farm soils, manure, fuel burning | More than 100 years | About 265–300 |
| CFCs | Old refrigerants and aerosol propellants (now banned) | Decades to a century or more | Thousands; over 10,000 for some |
| Water vapor (H₂O) | Evaporation, controlled mostly by temperature | Days | Not given a GWP; acts as a feedback |
Global warming potential
Global warming potential (GWP) compares how much heat a given mass of a gas traps over a set time, usually 100 years, compared with the same mass of CO₂. CO₂ is defined as 1. A gas's GWP depends on how strongly each molecule absorbs infrared radiation and how long it stays in the atmosphere.
The order you need to know from highest to lowest GWP is CFCs, then nitrous oxide, then methane, then CO₂. Exact values differ between scientific reports, and a question will give you the numbers to use if a calculation needs them.
High GWP doesn't make a gas the biggest cause of warming. CO₂ has the lowest GWP of the four, but people release so much of it that it causes the largest share of human-caused warming. Water vapor is the most abundant greenhouse gas, but it stays in the air for only days before falling as rain or snow, so it doesn't drive climate change on its own. Its amount follows temperature instead: warmer air holds more water vapor, which traps more heat, a positive feedback.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
Converting to CO₂-equivalents
A dairy farm emits 10 metric tons of methane and 2 metric tons of nitrous oxide per year. Using GWPs of 28 for methane and 265 for nitrous oxide, calculate the farm's total emissions of these two gases in metric tons of CO₂-equivalent (CO₂e).
Show the solutionHide the solution
- Step 1: CO₂-equivalent = mass of gas × its GWP.
- Step 2: Methane: 10 t × 28 = 280 t CO₂e.
- Step 3: Nitrous oxide: 2 t × 265 = 530 t CO₂e.
- Step 4: Total: 280 + 530 = 810 t CO₂e.
Answer: 810 metric tons of CO₂-equivalent per year.
- Example 2
Trap: the gas with the highest GWP isn't the main driver
A student argues that because nitrous oxide has a GWP about 265 times that of CO₂, cutting N₂O should be the top priority for slowing climate change. What is missing from this reasoning?
Show the solutionHide the solution
- Step 1: GWP compares warming per unit of mass, not total warming.
- Step 2: Total impact = amount emitted × GWP. Humans emit vastly more CO₂ each year than N₂O, by mass.
- Step 3: Because of the huge amount, CO₂ is responsible for the largest share of human-caused warming even though each ton is weaker.
- Step 4: Cutting N₂O is still worthwhile, but cutting CO₂ addresses the largest share.
Answer: The argument ignores the amount emitted. CO₂'s much larger emissions make it the biggest contributor despite its low GWP.
Common mistakes
- Saying the greenhouse effect is bad in itself. The natural greenhouse effect makes Earth habitable; the problem is the human-enhanced effect.
- Saying greenhouse gases trap incoming sunlight. They mainly absorb outgoing infrared radiation from the surface.
- Mixing up the greenhouse effect with ozone depletion. They involve different radiation (infrared versus UV) and different layers.
On the exam
- Be ready to explain the greenhouse effect in terms of wavelengths: shortwave sunlight in, longwave infrared out, absorbed and re-emitted by greenhouse gases.
- Know the GWP order (CFCs > N₂O > CH₄ > CO₂) and how to compute CO₂-equivalents when values are given.
Connected topics
Videos
Check yourself
4 questions on 9.3 The Greenhouse Effect. Pick an answer to see if you got it, and why.
Which statement best describes how greenhouse gases warm Earth's surface?
A landfill releases 5 metric tons of methane in a year. Using a global warming potential of 28 for methane, what is this release equal to in metric tons of CO₂?
Which list orders these greenhouse gases from highest to lowest global warming potential?
Water vapor is the most abundant greenhouse gas, but it is not considered a main driver of long-term climate change. Which statement best explains why?
0 of 4 answered