Abstract

Carbon is an element that plays an essential role in energy and chemical exchanges inside and on the surface of the planet Earth. Because it can easily combine with other elements or with itself, carbon is the fundamental element for life on Earth. In its combined forms, it is stored in the Earth’s surface subsystems, which are the lithosphere, the ocean, the atmosphere and the biosphere. The carbon cycle exchanges carbon atoms between these reservoirs, with characteristic times ranging from short (day or year) to long (millennia or more). Living beings contribute significantly to these exchanges, which result in a global state of a dynamic carbon equilibrium. Human activity since the beginning of the industrial era (1850) has disrupted this balance. Combustions massively inject CO2 gas into the atmosphere, at the expense of fossil carbon which was stored in a durable way. The consequence is the increase of the Earth’s average global temperature and climate change. The carbon balance and carbon neutrality measures have thus become central elements for a climate policy aimed at restoring the balance of the Earth system.