www.historyoftheuniverse.com Web

Most sunlight energy passes straight through the atmosphere and warms up the Earth's oceans and continents. These get hot and they give out infra red radiation. This radiation cannot travel through some gases in the atmosphere, so it gets trapped. So some gases trap heat in the same way as glass traps heat inside a greenhouse. Hence the name greenhouse effect.
Gases which do this are called greenhouse gases. When there is more greenhouse gas in the atmosphere the Earth heats up. When there is less greenhouse gas the Earth cools. So which gases are these greenhouse gases?

There is much talk of the greenhouse gas carbon dioxide, and evidence from carbon dioxide in air bubbles trapped in ice shows that the amount of CO₂ in the air has increased by about a quarter since the industrial revolution began around 1750.

But you need to consider all greenhouse gases to get the complete picture. Other greenhouse gases include methane and nitrous oxide and water vapor. The most important greenhouse gas is water vapor, and the amount of this varies much more than the amount of carbon dioxide. Also water vapor creates cloud, which stops sunlight coming down to the surface, so the true picture is complex.

The greenhouse effect raises the average temperature of the Earth by more than 30 degrees, so without it the Earth would be a very cold place!