<P> A greenhouse gas (abbrev . GHG) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range . This process is the fundamental cause of 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 − 18 ° C (0 ° F), rather than the present average of 15 ° C (59 ° F). In the Solar System, the atmospheres of Venus, Mars and Titan also contain gases that cause a greenhouse effect . </P> <P> Human activities since the beginning of the Industrial Revolution (taken as sometime between the years 1740 and 1754) have produced a 40% increase in the atmospheric concentration of carbon dioxide, from 280 ppm in 1750 to 406 ppm in early 2017 . This increase has occurred despite the uptake of a large portion of the emissions by various natural "sinks" involved in the carbon cycle . The vast majority of Anthropogenic carbon dioxide (CO) emissions (i.e., emissions produced by human activities) come from combustion of fossil fuels, principally coal, oil, and natural gas, with comparatively modest additional contributions coming from deforestation, changes in land use, soil erosion, and agriculture (including animal agriculture), though some of the emissions of this sector are offset by carbon sequestration . </P> <P> It has been estimated that if greenhouse gas emissions continue at the present rate, Earth's surface temperature could exceed historical values as early as 2047, with potentially harmful effects on ecosystems, biodiversity and the livelihoods of people worldwide . Recent estimates suggest that on the current emissions trajectory the Earth could pass a threshold of 2 ° C global warming, which the United Nations' IPCC designated as the upper limit to avoid "dangerous" global warming, by 2036 . </P>

What is the most important gas in the greenhouse effect