The ability to absorb and re-emit infrared energy is what makes CO2 an effective heat-trapping greenhouse gas. Not all gas molecules are able to absorb IR radiation. For example, nitrogen (N2) and oxygen (O2), which make up more than 90% of Earth's atmosphere, do not absorb infrared photons. CO2 molecules can vibrate in ways that simpler nitrogen and oxygen molecules cannot, which allows CO2 molecules to capture the IR photons.

Greenhouse gases and the greenhouse effect play an important role in Earth's climate. Without greenhouse gases, our planet would be a frozen ball of ice. In recent years, however, excess emissions of carbon dioxide and other greenhouse gases from human activities (mostly burning fossil fuels) have begun to warm Earth's climate at a problematic rate. Other significant greenhouse gases include water vapor (H2O), methane (CH4), nitrous oxide (N2O) and ozone (O3).