What Is the Greenhouse Effect?

The Sun energy reaches Earth as visible light, warming the surface. The warmed surface re-radiates this energy as infrared (heat) radiation. Greenhouse gases — primarily water vapour, carbon dioxide, methane, and nitrous oxide — absorb some of this outgoing infrared radiation and re-emit it in all directions, including back toward the surface. This natural process raises Earth average temperature from about -18 °C (what it would be without an atmosphere) to about +15 °C, making the planet habitable. The problem arises when human activities (burning fossil fuels, deforestation) increase greenhouse gas concentrations, trapping more heat and driving global warming.