Carbon is fundamental to life, being the main component of macromolecules, such as carbohydrates, proteins, lipids and nucleic acids. These are the building blocks of life and make up all living things.

The Earth contains a finite amount of carbon, which is cycled through different reservoirs, such as the atmosphere, the ocean, living organisms and fossil fuels. The carbon cycle is key to many life processes, such as photosynthesis and cellular respiration. Biological reservoirs consist of living organisms, which cycle carbon relatively quickly. On the other hand, geological reservoirs, such as fossil fuels, sediments, rocks, and volcanoes, "lock up" carbon for thousands, if not millions, of years.

The carbon cycle. Carbon dioxide is cycled between the atmosphere and ocean, in what is known as the ocean-atmosphere exchange. Plants uptake carbon dioxide from the atmosphere for photosynthesis. It is returned via the respiration of plants and animals. Carbon locked up beneath the Earth’s surface is returned by volcanic eruptions. Fossil fuels, formed over millions of years from decaying plants and animals, store a lot of carbon. This is released into the atmosphere through human emissions.

Figure 1: The carbon cycle with directional arrows showing the flow of carbon between reservoirs.