A greenhouse gas is a gas in the atmosphere that sustains life by allowing the sun's rays to pass through it and warm the planet but prevents this warmth from escaping back into space. Without such gases, the average temperature on the earth's surface would drop to about minus 18 degrees centigrade, in contrast to the current average of roughly 15 degrees.
The main greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, ozone and fluorinated gases. They are released into the atmosphere naturally from various sources, including evaporation of water from the oceans and elsewhere, from volcanos, from the respiration of animals, and from the decomposition of organic matter.
The level of greenhouse gases was relatively stable for thousands of years, with natural processes removing excess quantities of them as they were released. However, this situation began to change several hundred years ago due to the large-scale burning of fossil fuels, massive deforestation, and increasingly widespread agriculture, with the level of carbon dioxide in the atmosphere now about 50 percent higher than at the start of the industrial era (around 1750). The result has been a gradual rise in average temperatures, accompanied by an increased severity and frequency of storms, heat waves and wildfires, as well as by melting of glaciers and the permafrost, rising sea levels, acidification of the oceans, and an accelerating extinction of plant and animal species.