Any gas that absorbs infrared radiation and re-emits it can act as a greenhouse gas. The most important ones in Earth’s atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Together they keep the surface warmer than it would otherwise be.

Water vapor is the strongest overall contributor, yet its concentration is largely controlled by temperature. Carbon dioxide and methane have longer atmospheric lifetimes and are directly increased by human activities. Methane is especially potent on a per-molecule basis over short time scales.

Concentrations are measured continuously at sites such as Mauna Loa. Ice cores show that current levels of carbon dioxide and methane are higher than at any time in at least 800,000 years. Reducing emissions of these gases is the central strategy for limiting further warming.

• Absorbs and re-emits infrared radiation
• Includes CO2, methane, water vapor
• Concentrations rising from human activity
• Drives the enhanced greenhouse effect