The Carbon Cycle
Carbon exists in all living things and has been called the building block of life. Carbon also exists in nonliving things, such as carbon dioxide (an invisible gas) and rocks such as limestone. Our understanding of how carbon moves between the atmosphere, ocean, and land is central to quantifying how increases in greenhouse gases and changes in land surface (i.e., deforestation and soil erosion) will affect climate in the future. The carbon cycle is a complex series of processes that describes how carbon is exchanged between the atmosphere, ocean, and land (i.e., plants, soil, and the Earth's crust). One component of the carbon cycle is the exchange of carbon between the atmosphere and plants (i.e., carbon dioxide absorbed by plants via photosynthesis), while volcanic eruptions that inject carbon dioxide into the atmosphere represent an exchange of carbon between the Earth's crust and the atmosphere. Although carbon is constantly moving between different parts of the Earth, the total amount of carbon on the planet is constant. In this way, if the amount of carbon in the ocean were to go down, then the amount of carbon in the atmosphere or land would have to go up. This property of carbon conservation allows us in theory to keep track of all carbon movements throughout the Earth.
Today, the burning of coal and petroleum for energy is just an acceleration of one component of the carbon cycle. For example, the carbon in coal would normally remain in the ground for thousands of years until erosion or plate tectonics eventually released this carbon back into the atmosphere. So when humans burn fossil fuels they are really just moving carbon from the land into the atmosphere.1 Well-defined estimates exist for how much carbon goes into the atmosphere each year due to the burning of fossil fuels and changes in the land surface. There are also various organizations that are working to take carbon out of the atmosphere and put it back into the land. For example, the planting of a tree over a period of time will store atmospheric carbon in the trunk, branches, and roots of the tree. However, true to the carbon cycle, if that tree were to burn in a fire, the carbon would be liberated back into the atmosphere. So careful monitoring of the carbon cycle is necessary to understand how human factors affect atmospheric carbon levels and climate.
1. Fossil fuels are a nonrenewable resource, and limited reserves exist on our planet. Estimates vary as to how many years each energy source will be readily available at an acceptable economic cost, and range from ten to fifty years for oil, thirty-five to eighty years for natural gas, and one hundred to two hundred years for coal.