Global Carbon Cycle

The gas carbon dioxide (CO2), one atom of carbon (C) combined with two atoms of oxygen (O2), is just one of many chemical forms of carbon on the Earth.

As shown in the diagram, the near-surface environment of the Earth contains approximately 121,000,000 gigatons of carbon (GtC); a gigaton is equivalent to a billion metric tons; the number means "121 million gigatons or 121 million billion metric tons" of carbon).1,2

The carbon cycle diagram shows how carbon is stored in the environment and how a small amount is constantly moving between the land, the ocean, and the atmosphere. When carbon is exposed to the atmosphere, it can combine with oxygen to make CO2. One ton of carbon combining with oxygen makes nearly 4 tons of CO2 gas.3

Carbon in the environment can be divided into three types based on its availability to the atmosphere. The three types are carbon that is locked away in permanent storage and is not available to combine with oxygen and form CO2 in the atmosphere, carbon that is in relatively long-term storage in the land and the ocean, and carbon that is already in the atmosphere, mainly as CO2 gas.

About 78,000,000 GtC or two-thirds of the near-surface carbon on Earth occurs in nearly permanent storage in fossil fuels, limestone rocks, or sediments. Most of this carbon was originally in the atmosphere but has gone into storage underground over millions of years.

Most of the remaining one-third (44,000 GtC or one-third of the total) is in relatively long-term storage in the ocean and at the surface of the land. In the ocean, this carbon occurs as dissolved CO2 gas, as lime in seashells, and in the organic tissues of small marine creatures (i.e., plankton). The deep ocean contains close to 40,000 GtC, while the upper ocean contains only 1000 GtC. About 2000 GtC of carbon is held on the land, where it occurs primarily in plants, animals, and decaying organic matter.

A small part of the carbon, only 750 GtC, less than 1% of all the near-surface carbon on the Earth, occurs in the form of a gas in the atmosphere. Most of this carbon is combined with oxygen as the gas CO2.1

Each year, about 260 GtC (that is, about one-half of 1% of the carbon in relatively long-term storage) moves from the land and ocean to the atmosphere, and a nearly equal amount moves from the atmosphere into temporary storage in the ocean and the land.

This cycle has been relatively constant, but there have been times in the past when CO2 levels in the atmosphere have been relatively high. There have also been periods when the amount of CO2 in the atmosphere has been relatively low. Learn more at Climate Change.

The carbon locked in the Earth’s crust as sedimentary rocks and fossil fuels is basically in permanent storage. The carbon was locked away mainly through natural processes whereby plants and animals convert CO2 to organic and inorganic compounds or through the preservation of plant and animal remains. This volume of carbon is "unavailable" to the atmosphere and is not part of the everyday carbon cycle. However, human actions can release this stored carbon back into the atmosphere and add it to the active carbon cycle. Stored carbon could be added into the active carbon cycle through human actions like plowing farmland, making lime from limestone, or burning fossil fuels like coal, oil, or natural gas.

Since the beginning of the Industrial Revolution, humans have added carbon to the Earth's carbon cycle by their increasing use of fossil fuels, expanding agriculture, and use of other natural resources. Over the past 150 years, the amount of carbon from the human (anthropogenic) CO2 emissions from the use of fossil fuels has grown from a few million tons a year to over 9 GtC a year (learn more at CO2 from Human Action).

Altogether, humans have released over 300 billion tons of carbon once stored in the Earth to the atmosphere as CO2 over the past 150 years. It has been suggested that half of this carbon has been absorbed by the land and the ocean and that the other half has remained in the atmosphere, increasing the level of CO2.4 Currently, the level of carbon as CO2 in the atmosphere is relatively high and is significantly higher than it was 150 years ago (learn more at Climate Change).5

  1. (accessed October 2018).
  2. GtC = gigatons of carbon; 1 gigaton equals 1 billion or 1,000,000,000 metric tons (a metric ton is 1000 kilograms); 1 metric ton = 2204.6 pounds (an English system ton is 2000 pounds).
  3. Based on the ratio of the weights of the atoms of carbon and oxygen, 1 ton of carbon would combine with 2.667 tons of oxygen to form 3.667 tons of CO2.
  4. Accessed from Hadley Centre in fall 2005.
  5. (accessed November 2004)