Why isn't Earth a frozen ball at −18 °C?

Earth isn't frozen because a few gases in the air act like a one-way blanket. Sunlight passes straight through them and warms the ground. The warm ground then gives off its own heat as invisible infrared radiation, and that is what greenhouse gases such as water vapour and carbon dioxide absorb. They hold the heat back instead of letting it escape to space. Without this greenhouse effect, Earth's surface would average about −18 °C. With it, the average is around 15 °C: a 33 °C bonus that makes oceans liquid and life possible.

The surprising part is how little gas does the job. Nitrogen and oxygen make up 99 percent of dry air, and heat passes right through them. The physicist John Tyndall, who from 1859 was the first to measure how gases absorb infrared, found that the effect came from a tiny fraction of the atmosphere: mostly water vapour, with carbon dioxide also playing a significant part.

The name is a bit of a fib. A real greenhouse stays warm mainly because its glass stops warm air from drifting away. The atmosphere does something else: it slows down the radiation of heat into space. The word "greenhouse" only stuck in 1901, decades after Joseph Fourier first described the idea in 1824.

It isn't good or bad in itself. It's the reason Earth is habitable. The trouble is that burning fuels adds more of these gases, thickening the blanket: scientists call that the enhanced greenhouse effect.