A greenhouse gas (sometimes abbreviated GHG) is a gas that absorbs andemits radiant energy within the thermal infrared range. Greenhouse gases cause the greenhouse effect[1] on planets. The primary greenhouse gases inEarth'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),[2] rather than the present average of 15 °C (59 °F).[3][4][5] 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 45% increase in the atmospheric concentration of carbon dioxide, from 280 ppm in 1750 to 415 ppm in 2019.[6] This increase has occurred despite the uptake of more than half of the emissions by various natural "sinks" involved in the carbon cycle.[7][8] The vast majority ofanthropogenic carbon dioxide emissions come from combustion of fossil fuels, principally coal, oil, and natural gas, with additional contributions coming from deforestation, changes in land use, soil erosion and agriculture (including livestock).[9][10] The leading source of anthropogenic methane emissions is animal agriculture, followed by fugitive emissions from gas, oil, coal and other industry, solid waste, wastewater and rice production.[11]
Should greenhouse gas emissions continue at their rate in 2019, global warmingcould cause Earth's surface temperature to exceed historical values as early as 2047, with potentially harmful effects on ecosystems, biodiversity and human livelihoods.[12] At current emission rates, temperatures could increase by 2 °C, which the United Nations' Intergovernmental Panel on Climate Change(IPCC) designated as the upper limit to avoid "dangerous" levels, by 2036.[13]
