Sunlight warms the ground, and the warm ground gives its heat back out as light you cannot see: infrared.
Infrared light lies just beyond red, at the long-wave end of the spectrum. Its waves are longer than red light's and carry less energy. It is the warmth you feel on your face from a fire. Everything gives off infrared, and the warmer it is, the more it gives off. Ground at Earth's average temperature, about 15 °C, gives off infrared with waves around 10,000 nm long, some 14 times the length of red light's.
About half of the sunlight reaching Earth passes through the air and warms the land and the oceans. Their infrared then heads back up. Nitrogen and oxygen let it pass. A few other gases absorb some of it, and each molecule that does passes the energy on: it bumps into its neighbours and warms the air, or it gives off infrared of its own, in any direction. Some of that infrared heads back down and warms the ground again.
This warming is the greenhouse effect, and the gases that cause it are greenhouse gases. Earth's surface averages about 15 °C. Scientists calculate that a planet with the same sunlight, reflecting the same share of it, and no greenhouse gases would average about −18 °C, so the greenhouse effect keeps Earth's surface about 33 °C warmer.
Which gases do the work
| What absorbs the infrared | Share |
|---|---|
| water vapour | about 50% |
| clouds | about 25% |
| carbon dioxide | about 20% |
| methane, ozone and other gases | about 5% |
Clouds are tiny drops of water and bits of ice, and they absorb infrared too. Water vapour does about half the work. The amount in the air follows the temperature: warmer air holds about 7% more water vapour for each degree Celsius, and the vapour rains out within about ten days. Carbon dioxide and methane stay up for years, so they set the level. As they warm the air, it holds more water vapour, which warms it further.