DEFINITION OF CARBON CYCLE

The carbon cycle represents a complex series

of processes where the element carbon makes a continuous

and complex exchange between the atmosphere, hydrosphere,

lithosphere and solid Earth, and biosphere. Carbon is

one of the fundamental building blocks of Earth, with most

life-forms consisting of organic carbon and inorganic carbon

dominating the physical environment. The carbon cycle is

driven by energy flux from the Sun and plays a major role in

regulating the planet’s climate.

Several main processes control the flux of carbon on the

Earth, and these processes are presently approximately balanced.

Assimilation and dissimilation of carbon, by photosynthesis

and respiration by life, cycles about 1011 metric

tons of carbon each year. Some carbon is simply exchanged

between systems as carbon dioxide, and other carbon undergoes

dissolution or precipitation as carbonate compounds in

sedimentary rocks.

Atmospheric carbon forms the long-lived compounds

carbon dioxide and methane and the short-lived compound

carbon monoxide that has a very short atmospheric residence

time. Global temperatures and the amount of carbon (chiefly

as CO2) in the atmosphere are closely correlated, with more

CO2 in the atmosphere resulting in higher temperatures.

However, it is yet to be determined if increased carbon flux to

the atmosphere from the carbon cycle forces global warming,

or if global warming causes an increase in the carbon flux.

Since the industrial revolution, humans have increased CO2

emissions to the atmosphere resulting in measurable global

warming, showing that increased carbon flux can control

global temperatures.

The oceans represent the largest carbon reservoir on the

planet, containing more than 60 times as much carbon as the

atmosphere. Dissolved inorganic carbon forms the largest

component, followed by the more mobile dissolved organic

carbon. The oceans are stratified into three main layers. The

well-mixed surface layer is about 246 feet (75 m) thick and

overlies the thermocline, which is a stagnant zone characterized

by decreasing temperature and increasing density to its

base at about 0.6-mile (1-km) depth. Below this lie the deep

cold bottom waters where dissolved CO2 transferred by

descending cold saline waters in polar regions may remain

trapped for thousands of years. Cold polar waters contain

more CO2 because gases are more soluble in colder water.

Some, perhaps large amounts, of this C gets incorporated in

gas hydrates, which are solid, ice-like substances made of

cases of ice molecules enclosing gas molecules like methane,

ethane, butane, propane, carbon dioxide, and hydrogen sulfide.

Gas hydrates have recently been recognized as a huge

global energy resource, with reserves estimated to be at least

twice that of known fossil fuel deposits. However, gas

hydrates form at high pressures and cold temperatures, and

extracting them from the deep ocean without releasing huge

amounts of CO2 to the atmosphere may be difficult.

Carbon is transferred to the deep ocean by its solubility

in seawater, whereas organic activity (photosynthesis) in the

oceanic surface layer accounts for 30–40 percent of the global

vegetation flux of carbon. About 10 percent of the C that

is used in respiration in the upper oceanic layer is precipitated

out and sinks to the lower oceanic reservoir.

The majority of Earth’s carbon is locked up in sedimentary

rocks, primarily limestone and dolostone. This stored

carbon reacts with the other reservoirs at a greatly reduced

rate (millions and even billions of years) compared with the

other mechanisms discussed here. Some cycles of this carbon

reservoir are related to the supercontinent cycle and the

weathering of carbonate platforms when they are exposed by

continental collisions.

The Earth’s living biomass, the decaying remains of this

biomass (litter), and soil all contain significant C reserves that

interact in the global carbon cycle. Huge amounts of carbon

are locked in forests, as well as in arctic tundra. Living vegetation

contains about the same amount of carbon as is in the

atmosphere, whereas the litter or dead biomass contains

about twice the amount in the living biomass. It is estimated

that land plants absorb 100 gigatons of carbon a year and

return about half of this to the atmosphere by respiration.

The remainder is transformed to organic carbon and incorporated

into plant tissue and soil organic carbon.

Understanding the global carbon cycle is of great importance

for predicting and mitigating climate change. Climatologists,

geologists, and biologists are just beginning to

understand and model the consequences of changes to parts

of the system induced by changes in other parts of the system.

For instance, a current debate centers on how plants respond

to greater atmospheric CO2. Some models indicate that they

may grow faster under enhanced CO2, tending to pull more

C out of the atmosphere in a planetary self-regulating effect.

This is known as the fertilization effect. Many observations

and computer models are being performed to investigate the

effects of natural and human-induced changes (anthropogenic)

to the global carbon cycle, and to better understand

what the future may hold for global climates.

See also GAS HYDRATES; GREENHOUSE EFFECT.

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