The greenhouse effect of solar radiation on the Earth's surface caused by greenhouse gases
Radiative forcing of different contributors to climate change in 2011, as reported in the fifth IPCC assessment report.

A greenhouse gas (sometimes abbreviated GHG) is a gas that absorbs and emits radiant energy within the thermal infrared range. Greenhouse gases cause the greenhouse effect[1] on planets. The primary greenhouse gases in Earth's atmosphere are water vapor (H
), carbon dioxide (CO
), methane (CH
), nitrous oxide (N
), 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] The last time the atmospheric concentration of carbon dioxide was this high was over 3 million years ago.[7] This increase has occurred despite the uptake of more than half of the emissions by various natural "sinks" involved in the carbon cycle.[8][9]

The vast majority of anthropogenic carbon dioxide emissions come from combustion of fossil fuels, principally coal, petroleum (including oil) and natural gas, with additional contributions coming from deforestation and other changes in land use.[10][11] The leading source of anthropogenic methane emissions is agriculture, closely followed by gas venting and fugitive emissions from the fossil-fuel industry.[12][13] Traditional rice cultivation is the second biggest agricultural methane source after livestock, with a near-term warming impact equivalent to the carbon-dioxide emissions from all aviation.[14]

At current emission rates, temperatures could increase by 2 °C (3.6°F), which the United Nations' Intergovernmental Panel on Climate Change (IPCC) designated as the upper limit to avoid "dangerous" levels, by 2036.[15]