A greenhouse gas (GHG) is a gas that absorbs and emits radiant energy within the thermal infra-red range, causing the greenhouse effect which is critical to supporting life, and initially was a precursor to life moving out of the ocean onto land.
In a nutshell, Earth receives energy from the Sun, which has a temperature of circa 5,500°C, in the form of ultraviolet, visible, and near-infrared radiation. Part of the incoming solar energy is reflected to space by the atmosphere and clouds or is absorbed by the atmosphere and clouds. Most of the remaining energy is absorbed at the surface of Earth. Because the Earth’s surface is colder than the Sun, it radiates at wavelengths that are much longer than the wavelengths that were absorbed, thus allowing this thermal radiation that was going back towards the Sun to be captured by the atmosphere and warming it. The atmosphere also gains heat by sensible and latent heat fluxes from the surface and radiates energy both upwards and downwards. The intensity of downward radiation (i.e. the strength of the greenhouse effect) depends on the amount of GHG that the atmosphere contains. This leads to a higher equilibrium temperature than what it would be without the atmosphere (i.e. -15°C or less rather than 15°C).
The primary greenhouse gases in Earth’s atmosphere are water vapor (H2O), which remains in atmosphere for only a few days, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3), chlorofluorocarbons (CFCs) and hydrofluorocarbons (e.g. HCFCs and HFCs). Other GHGs are sulphur hexafluoride (SF6), nitrogen trifluoride (NF3) and perfluorocarbons (PFCs).
Human activities since the beginning of the Industrial Revolution in about 1750 have produced a 40% increase in the atmospheric concentration of carbon dioxide. This enhanced greenhouse effect from anthropogenic activities (“anthropos”, ἄνθρωπος, is Greek for human), is informally referred to as global warming or, much better, climate change. To put things in perspective, the last time the atmospheric concentration of carbon dioxide was this high was circa 1 million years ago, the past few years have been the warmest on record and across the globe the average sea levels increase by 3 to 4 mm each year because of melting ice and water increases in volume as overall the oceans’ water heats up.
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 cutting down carbon-absorbing forests, or leaving them to rot, and other changes in land use. To put it in numbers, if the GHG emissions associated to eating 1g protein of legumes are x, they become 5x in case of wheat, 40x in case of poultry and 250x in case of beef. The leading source of methane emissions is agriculture, closely followed by gas venting and fugitive emissions from the fossil-fuel industry. 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.