WWF - For a living planet

The science

The Earth’s climate is driven by a continuous flow of energy from the sun. Heat energy from the sun passes through the Earth’s atmosphere and warms the Earth’s surface. As the temperature increases, the Earth sends heat energy (infrared radiation) back into the atmosphere. Some of this heat is absorbed and reflected back to Earth by gases in the atmosphere, such as carbon dioxide (CO₂), water vapour, methane, nitrous oxide, ozone and halocarbons.

The greenhouse effect

These gases, which are all naturally occurring, act as a blanket, trapping in the heat and preventing it from being reflected too far from the Earth. They keep the Earth's average temperature at about 15°C: warm enough to sustain life for humans, plants and animals. Without these gases, the average temperature would be about minus 18°C - too cold for higher life. This natural warming effect is also sometimes called the ‘greenhouse effect’.

Carbon dioxide (CO₂)

CO₂ is the most significant of the gases in our atmosphere which keep the Earth warm. Four billion years ago its concentration in the atmosphere was much higher than today (80% compared to today's 0.03%), but most of it was removed through photosynthesis over time. All this carbon dioxide became locked in organisms and then minerals such as oil, coal and petroleum inside the Earth's crust.

The natural carbon dioxide cycle

A natural carbon dioxide cycle keeps the amount of CO₂ in our atmosphere in balance. Decaying plants, volcanic eruptions and the respiration of animals release natural CO₂ into the atmosphere, where it stays for about 100 years. It is removed again from the atmosphere by photosynthesis in plants and by dissolution in water (for instance in the oceans).