A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy at thermal infrared wavelengths, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and ozone (O₃). Without greenhouse gases, the average temperature of Earth's surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F). Greenhouse gases exist in many atmospheres, creating greenhouse effects on Mars, Titan and particularly in the thick atmosphere of Venus.

Human activities since the beginning of the Industrial Revolution (around 1750) have increased the atmospheric concentration of methane by over 150% and carbon dioxide by over 50%, up to a level not seen in over 3 million years. Carbon dioxide is causing about 3/4ths of global warming and can take thousands of years to be fully absorbed by the carbon cycle. Methane causes most remaining warming and lasts in the atmosphere for an average of 12 years.

Average global surface temperature has risen by 1.2 °C (2.2 °F) as a result of greenhouse gas emissions. If current emission rates continue then temperatures will surpass 2.0 °C (3.6 °F) sometime between 2040 and 2070, which is the level the United Nations' Intergovernmental Panel on Climate Change (IPCC) says is "dangerous".

The vast majority of anthropogenic carbon dioxide emissions come from combustion of fossil fuels, principally coal, petroleum (including oil) and natural gas. Additional contributions come from cement manufacturing, fertilizer production, and changes in land use like deforestation. Methane emissions originate from agriculture, fossil fuel production, waste, and other sources.