A carbon flux is the amount of carbon exchanged between Earth’s carbon pools, the oceans, atmosphere, land, and living things and is typically measured in units of gigatonnes of carbon per year (GtC/yr). A gigatonne is a tremendous amount of mass, roughly twice the mass of all humans on Earth combined, or the mass of about 200 million elephants. These carbon pools contain enormous quantities of carbon and exchange this matter in various ways. The Earth’s carbon is exchanged globally in what is known as the carbon cycle. This cycle exchanges immense quantities of carbon each year, with values shown in Figure below. (Remember, each value in this figure represents 1 gigatonne, the mass of 200 million elephants.) The carbon cycle balances almost perfectly naturally, however when humans introduce carbon that was originally buried underground, this introduces an imbalance, as shown in red text in Figure. The carbon cycle of the Earth. Numbers represent the mass of carbon in gigatonnes (not the molecules, just carbon alone) that is cycled in a year. Yellow text is the natural carbon cycle, with red text showing human effects. Notice that the 9 gigatonnes of carbon that humans are emitting (~35 gigatonnes of carbon dioxide) becomes an extra 4 gigatonnes in the atmosphere, an extra 3 gigatonnes of photosynthesis and an extra 2 gigatonnes in the ocean every year. This is how humans are changing the natural carbon cycle.