The carbon cycle

Forests are often referred to as the lungs of the world because trees, like other plants, absorb carbon dioxide (CO2) from the air and use it to create food through photosynthesis. The photosynthesis process captures carbon from the carbon dioxide, which is then used by the plants to help them grow, and the whole process releases oxygen (O2) back into the air.Animals, insects, etc. use oxygen, either breathed from the air or absorbed from water, and produce carbon dioxide as a waste product
of the respiration.

Of course, carbon dioxide can also be produced by any process that breaks down carbon-based material such as oil, coal and wood. The most obvious examples are when cars break down petrol (a fuel created out of fossilised material) to covert it into mechanical energy, or when industry also burns fossil fuels such as coal and gas.