Greenhouse effect
Gases such as carbon dioxide, water vapour and methane trap some of the heat Earth gives off, keeping the planet warm.
Sunlight warms Earth's surface, and the warm surface gives off energy as invisible infrared radiation. Some gases in the atmosphere absorb that infrared and send part of it back down, so heat escapes to space more slowly. This is the greenhouse effect, and the gases responsible, chiefly water vapour, carbon dioxide and methane, are greenhouse gases.
The effect itself is natural and essential; without it Earth would be far too cold for most life. The concern is that burning fossil fuels and other human activities have added large amounts of carbon dioxide and methane, strengthening the effect and warming the climate.
Not all greenhouse gases are equal. Scientists compare them using the global warming potential: how much heat a tonne of a gas traps over a set period, usually 100 years, compared with a tonne of carbon dioxide. Methane's value over 100 years is about 27 to 30. It lasts only about a decade in the air on average, far less than carbon dioxide, but while it is there it traps much more heat per tonne. That is why letting methane escape unburned is worse for the climate than burning it to carbon dioxide and water.
A common mix-up is between the greenhouse effect and the ozone hole. They are separate problems: the ozone layer blocks ultraviolet light, and its thinning is not the main cause of global warming.