Carbon dioxide

Colourless gas
At normal temperatures carbon dioxide is a colourless gas that exists naturally in the Earth’s atmosphere. The air that we breathe has about 0.03% of carbon dioxide in it. 
Carbon dioxide is an important part of the carbon cycle and essential for life on Earth. Without it temperatures on our planet would be very low.

Photosynthesis
Plants use the sunlight and take up the CO2 in the atmosphere to produce energy and oxygen. This process is called photosynthesis.

Production
The gas is also produced when fossil fuels are burned. Carbon dioxide that has been stored for millions of years in coal, oil and natural gas is set free. Living creatures produce carbon dioxide when they breathe out. It is emitted from volcanoes and hot springs. Deforestation sets carbon that is stored in trees free.

Greenhouse gas
Carbon dioxide is an important greenhouse gas. Light that enters the atmosphere it is converted to heat. Higher amounts of CO2 make the atmosphere denser and keep it from getting out. CO2 is a gas that is highly responsible for global warming. Since the Industrial Revolution of the 1800s the amount of CO2 in our atmosphere has risen by 40%.

Absorption by nature
Throughout the centuries carbon dioxide has been in balance. Now more carbon dioxide is being produced than can be absorbed by nature.

Enhanced greenhouse effect

There are two meanings of the term greenhouse effect.

1. There is a natural greenhouse effect that keeps the Earth's climate warm and habitable.
2. There is also the man-made enhanced greenhouse effect, which is the enhancement of Earth's natural greenhouse effect by the addition of greenhouse gases from the burning of fossil fuels (mainly petroleum, coal, and natural gas).

In order to understand how the greenhouse effect operates, we need to first understand infrared radiation. Greenhouse gases trap some of the infrared radiation that escapes from the Earth, making the Earth warmer that it would otherwise be. You can think of greenhouse gases as sort of a blanket for infrared radiation. It keeps the lower layers of the atmosphere warmer, and the upper layers colder, than if the greenhouse gases were not there.

About 80-90% of the Earth's natural greenhouse effect is due to water vapour, a strong greenhouse gas. The remainder is due to carbon dioxide, methane, and a few other minor gases.

It is the carbon dioxide concentration that is increasing, due to the burning of fossil fuels (as well as from some rainforest burning). This is the man-made portion of the greenhouse effect, and it is believed by many scientists to be responsible for the global warming of the last 150 years.

Greenhouse gases

By their percentage contribution to the greenhouse effect on Earth the four major gases are:

• water vapour, 36–70%
• carbon dioxide, 9–26%
• methane, 4–9%
• ozone, 3–7%