The greenhouse effect is the process by which certain gases in the atmosphere trap heat from the sun, keeping Earth warm enough to support life — but an excess of these gases causes additional, harmful warming.
Sunlight passes through the atmosphere and warms Earth's surface. The surface then radiates some of that heat back out as infrared radiation. Greenhouse gases — mainly carbon dioxide, methane, and water vapor — absorb part of that outgoing heat and re-radiate it back toward the surface, rather than letting it escape directly into space.
The natural greenhouse effect is essential — without it, Earth's average temperature would be far below freezing. The problem is the "enhanced" greenhouse effect: burning fossil fuels, deforestation, and industrial activity have sharply increased atmospheric CO₂ and methane levels since the industrial era, trapping more heat than the climate system is adapted to and driving global warming.
Last reviewed: September 2026