Carbon Cycle

The Earth's Carbon Cycle
The Earth maintains a natural carbon balance. When concentrations of carbon dioxide (CO²) are upset, the system gradually returns to its natural state. This natural readjustment works slowly, compared to the rapid rate at which humans are moving carbon into the atmosphere by burning fossil fuels. Natural carbon removal can't keep pace, so the concentration of CO2 in the atmosphere increases.

In the following sections, we will examine the Earth's natural carbon balance and how humans are affecting this balance.

1. The Natural Carbon Balance
Carbon continually exchanges within a closed system consisting of the atmosphere, oceans, biosphere, and landmass. There are short- and long-term cycles at work.

Short-Term Cycles:
Carbon is exchanged rapidly between plants and animals through respiration and photosynthesis, and through gas exchange between the oceans and the atmosphere.

Long-Term Cycle:
Over millions of years, carbon in the air is combined with water to form weak acids that very slowly dissolve rocks. This carbon is carried to the oceans where some forms coral reefs and shells. These sediments may be moved deep into the Earth by drifting continents and eventually released into the atmosphere by volcanoes.

2. Upsetting The Balance :. Human Impact
Like all other animals, humans participate in the natural carbon cycle, but there are also important differences. By burning coal, oil, and natural gas, humans are adding carbon dioxide (CO²) to the atmosphere much faster than the carbon in rocks is released through natural processes. And clearing and burning forests to create agricultural land converts organic carbon to carbon dioxide gas. The oceans and land plants are absorbing a portion, but not nearly all of the CO2 added to the atmosphere by human activities.

Human Impact On The Carbon Cycle
The red arrow, representing rapid fossil fuel burning, indicates the main way in which humans affect the natural carbon cycle. Carbon dioxide (CO²) levels are increasing because the natural system cannot keep pace with this new emission source. The natural processes that permanently remove this additional carbon – ocean uptake and sedimentation – work extremely slowly.

Time
Natural changes to the carbon cycle have been very slow compared to the rate at which humans are adding CO² to the atmosphere. The redistribution of the added CO2 between the atmosphere, oceans, and biosphere takes hundreds of years, and the removal of the added carbon from the short-term cycle by the long-term cycle takes thousands of years.

© National Academy of Sciences