The Earth's atmosphere
The level of carbon dioxide in the atmosphere is maintained by several processes, including photosynthesis [photosynthesis: The chemical change that occurs in the leaves of green plants. It uses light energy to convert carbon dioxide and water into glucose. Oxygen is produced as a by-product of photosynthesis. ], respiration [respiration: Chemical change that takes place inside living cells, which uses glucose and oxygen to produce the energy organisms need to live. Carbon dioxide is a by-product of respiration ] and combustioncombustion: Combustion is the process of burning by fire..
Green plants remove carbon dioxide from the atmosphere by photosynthesis. Living organisms - including all plants and animals - release energy from their food using respiration. Respiration and combustion - burning - both release carbon dioxide into the atmosphere.
These processes form a carbon cycle in which the proportion of carbon dioxide in the atmosphere remains about the same. The animation should help you to understand how the cycle works. Note that you do not need to know about decomposition and fossilisation.
Carbon dioxide is produced by burning fossil fuels. Increased energy consumption is leading to a rise in the use of fossil fuels, which in turn increases the release of carbon dioxide into the atmosphere.
The rising human population is adding to atmospheric carbon dioxide in other ways, too. When land is cleared for timber and farms, there are fewer trees to remove carbon dioxide from the atmosphere by photosynthesis. If the fallen trees are burned or left to rot, additional carbon dioxide is released into the atmosphere. This is particularly important when forests are cleared to make way for farms. Not only are there then fewer trees to absorb carbon dioxide, but the burning of the trees releases carbon dioxide.