Greenhouse gas Greenhouse gas A greenhouse gas is a gas that absorbs and emits radiant energy within the thermal infrared range. Greenhouse gases cause the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide and ozone. Without greenhouse gases, the average temperature of Earth's surface would be about , rather than the present average of . The atmospheres of Venus, Mars and Titan also contain greenhouse gases. Human activities since the beginning of the Industrial Revolution (around 1750) have produced a 40% increase in the atmospheric concentration of carbon dioxide (), from 280 ppm
Idealized greenhouse model for the required amount of heat energy finally to be radiated out into space from the top of the atmosphere. The greenhouse effect can be illustrated with an idealized planet. This is a common "textbook model": the planet will have a constant surface temperature T and an atmosphere with constant temperature T. For diagrammatic clarity, a gap can be depicted between the atmosphere and the surface. Alternatively, T could be interpreted as a temperature representative of the surface and the lower atmosphere, and T could be interpreted as the temperature of the upper atmosphere. In order to justify that T