Fast Carbon, Slow Carbon:

Most of Earth's carbon is stored in rocks. The rest is in the ocean, atmosphere, plants, soil and fossil fuels. Carbon flows between each system in an exchange called the carbon cycle, which has fast and slow components. Any change that shifts carbon out of one system, shifts it into another. The carbon cycle is balanced if the amount of carbon cycling from the land and ocean into the atmosphere equals the amount cycling back into the land and ocean systems.

The fast carbon cycle happens on annual and interannual time scales as plants on land and in the ocean take carbon dioxide out of the air during photosynthesis. Using energy from the sun, plants combine carbon dioxide (CO2) and water (H2O) to form sugar (CH2O) and oxygen (O2). The carbon is returned to the air through respiration by the plants and animals who have eaten the plants, decay and decomposition after the plants die, or through burning. The fast carbon cycle has a strong annual cycle: carbon dioxide concentrations in the air decrease during spring and summer when plants are growing and increase during winter when plants die or go dormant.

carbon cycle

Fast carbon cycle diagram shows the movement between land, atmosphere and ocean. Numbers are in gigatons of carbon per year for natural (yellow) and human (red) components.White numbers indicate stored carbon. (from adapted from U.S. DOE, Biological and Environmental Research Information System).

The slow carbon cycle happens over millions of years. Through chemical reactions and tectonic activity, carbon takes between 100-200 million years to move between rocks, soil, ocean, and atmosphere. The slow cycle returns carbon to the atmosphere through volcanoes. When volcanoes erupt, they release carbon dioxide to the atmosphere. At present, volcanoes emit between 100-400 million metric tons of carbon dioxide per year. By comparison, human industry releases about 30 billion tons of carbon dioxide per year by burning fossil fuels - that's about 300 times more than volcanoes.


The Carbon Crisis in 90 seconds

Earth Observatory feature: The Carbon Cycle

NASA Carbon Monitoring System


Educational Resources:

Teaching Bundle: Carbon Cycle and Climate Change

NASA Know Your Earth Carbon Quiz


Satellite Information:

NASA Aqua's Atmospheric Infrared Sounder (AIRS)

Orbiting Carbon Observatory 2 (OCO-2)


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