9.3 The Greenhouse Effect

How the greenhouse effect works

The Sun emits short-wave radiation that passes through the atmosphere and warms Earth's surface. The surface re-emits long-wave (infrared) radiation, which greenhouse gases (GHGs) absorb and re-radiate in all directions, trapping heat. This natural greenhouse effect keeps Earth warm enough to support life.

Key greenhouse gases

Major GHGs include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases (CFCs, HFCs). Each gas has a different global warming potential (GWP), measuring how much heat it traps relative to CO2 over time. Methane has a much higher GWP than CO2 but shorter atmospheric lifetime.

Human enhancement of the effect

Burning fossil fuels, deforestation, and agriculture increase GHG concentrations, intensifying the natural greenhouse effect — causing enhanced (anthropogenic) global warming. Ice cores and atmospheric measurements (like the Keeling Curve) show CO2 levels rising sharply since industrialization, correlating with rising global temperatures.

Key terms

Greenhouse effect

Natural process where greenhouse gases absorb and re-radiate long-wave radiation, warming Earth's atmosphere.

Global warming potential (GWP)

A measure of how much heat a gas traps relative to CO2 over a specific time period.

Keeling Curve

A graph showing the continuous rise in atmospheric CO2 concentrations since measurements began in 1958.

Major greenhouse gases

Water vapor, CO2, methane, nitrous oxide, and fluorinated gases (CFCs, HFCs).

Anthropogenic global warming

Enhanced warming caused by human activities like fossil fuel combustion and deforestation increasing GHG concentrations.

Try an AP-style question

How do greenhouse gases warm Earth?

1. They turn oxygen into carbon dioxide
2. They reflect sunlight back to space
3. They absorb and re-emit infrared radiation
4. They block all incoming sunlight