Topic 20.3 · Core
Air Pollution and the Enhanced Greenhouse Effect
The greenhouse effect itself is not the problem — it's necessary for life on Earth. What Cambridge actually wants you to explain is the enhanced version: what happens when human activity pushes methane and carbon dioxide concentrations higher than they would otherwise be.
Three terms, three separate meanings
The greenhouse effectis the natural warming process in which certain atmospheric gases absorb outgoing infrared radiation emitted from Earth’s surface and re-radiate some of it back down, keeping the lower atmosphere and surface warmer than they would otherwise be. Without it, Earth’s surface would be too cold to support life as we know it — so never present the greenhouse effect itself as something harmful. The enhanced greenhouse effect is the strengthening of that same natural process because human activity has raised the concentration of greenhouse gases — chiefly methane and carbon dioxide for this syllabus point — so more outgoing infrared radiation is absorbed and re-radiated back down than before. Climate changeis the resulting long-term shift in climate patterns, such as average temperature and rainfall, that follows from the enhanced greenhouse effect; it is Cambridge’s own required term, even though “global warming” is the everyday phrase for the temperature-rise part of it.
How the mechanism actually works
Earth’s surface emits infrared radiation outward. Greenhouse gas molecules in the atmosphere absorb some of that outgoing radiation and re-radiate it in different directions, including back down toward the surface. That re-radiation reduces the net energy the Earth system loses to space, which is what warms the lower atmosphere and the surface. Raise the concentration of greenhouse gases, and more outgoing radiation gets absorbed and sent back — more greenhouse gas leads to more radiation absorbed, which leads to greater warming.
Carbon dioxide: important sources
Human activities that directly release carbon dioxide include the combustion of fossil fuels and the burning of vegetation during land clearance. Deforestation raises atmospheric carbon dioxide too, but through two distinct mechanisms rather than one: carbon dioxide can be released when the cleared vegetation is burned or decomposes, and fewer trees remain afterwards to remove carbon dioxide from the atmosphere by photosynthesis. The second of those is a lost carbon sink, not a new source — a distinction covered in full on deforestation.
Methane: important sources
Important human-linked sources of methane include ruminant livestock such as cattle, the anaerobic decomposition of organic waste in landfill, and waterlogged agriculture such as rice cultivation.
What climate change can mean for organisms
A long-term shift in temperature and rainfall patterns changes the environmental conditions organisms depend on. Species unable to tolerate the new conditions, or unable to move to somewhere more suitable, can decline — which is exactly why climate change appears again as one of the named reasons a species becomes endangered on sustainable resources and endangered species.
What this topic does not require
This objective is deliberately narrow: it covers methane and carbon dioxide, and the enhanced greenhouse effect and climate change that follow from them. Acid rain, ozone depletion and general particulate air pollution are real environmental issues, but they are not part of this specific Cambridge requirement, and shouldn’t be added into an answer that’s really being asked about greenhouse gases.