Greenhouse gas

About half the light energy reaching Earth's atmosphere passes through the air and clouds to the surface, where it is absorbed and re-emitted as infrared heat. About 90% of this heat is absorbed by greenhouse gases and re-radiated, slowing its loss to space. This greenhouse effect is essential to life on Earth; but human-made emissions are trapping more heat, and scientists attribute the global warming trend observed since the mid-20th century to the human expansion of the greenhouse effect.

Carbon dioxide enters the atmosphere through natural processes such as volcanic eruptions and through human activities such as burning fossil fuels and deforestation; plants remove it as part of the carbon cycle. Methane comes from both natural sources, such as plant matter breaking down in wetlands, and human sources, such as producing and transporting coal, gas and oil, livestock, and decaying organic waste. Fluorinated gases are synthetic and are typically emitted in smaller quantities than other greenhouse gases, but for a given mass they trap substantially more heat than carbon dioxide. Chlorofluorocarbons (CFCs) — compounds entirely of industrial origin, once used as refrigerants, solvents and spray-can propellants — are also among the key greenhouse gases; some fluorinated gases are sometimes used as substitutes for CFCs and other ozone-depleting substances.

Water vapour is the most abundant greenhouse gas. But because a warming ocean increases the amount of it in the atmosphere, it is not a direct cause of climate change but a feedback: as Earth warms, water vapour increases, which leads to further warming.