The balanced atmosphere

The Carbon Cycle

Carbon is a constituent of all organic compounds, many of which are essential to life on Earth.

The source of the carbon found in living matter is carbon dioxide (CO₂) in the air or dissolved in water.

Carbon dioxide (CO₂), a colorless gas having a faint, sharp odor and a sour taste; it is a minor component of the Earth's atmosphere (about 3 volumes in 10,000).

Algae and terrestrial green plants (producers) are the chief agents of carbon dioxide fixation through the process of photosynthesis, through which carbon dioxide and water are converted into simple carbohydrates.

Carbohydrates are used by the producers to carry on metabolism, the excess being stored as fats and polysaccharides. The stored products are then eaten by consumer animals, from protozoan to man, which convert them into other forms.

All animals return CO₂ directly to the atmosphere as a by-product of their respiration.

The carbon present in animal wastes and in the bodies of all organisms is released as CO2 by decay, or decomposer, organisms (chiefly bacteria and fungi) in a series of microbial transformations.

Part of the organic carbon--the remains of organisms--has accumulated in the Earth's crust as fossil fuels (e.g., coal, gas, and petroleum), limestone, and coral. The carbon of fossil fuels, removed from the cycle in prehistoric time, is now being released in vast amounts as CO₂ through industrial and agricultural processes, much of it quickly passing into the oceans and there being "fixed" as carbonates. If oxygen is scarce (as in sewage, marshes, and swamps) some carbon is released as methane gas.

US Emissions of Carbon Dioxide are detailed in the EPA web page.

The presence of the gas in the atmosphere keeps some of the radiant energy received by the Earth from being returned to space, thus producing the so-called greenhouse effect.

The global carbon cycle

The global carbon cycle involves the earth's atmosphere, fossil fuels, the oceans, and the vegetation and soils of the earth's terrestrial ecosystems.

A simplified annual average atmospheric carbon dioxide budget for the 1980s, with fluxes given in units of petagrams C per year (Pg C /yr).

Each year the world's terrestrial ecosystems withdraw carbon from the atmosphere through photosynthesis and add it again through respiration and decay. The withdrawals and additions of carbon can be seen in the regular seasonal oscillation of CO₂ concentrations in the atmosphere.

As the figures below show, CO2 concentrations have varied over the past 150 thousand years but the increases in CO2 over the past 150 years since the beginning of the industrial revolution are much more rapid than the natural variations of CO2.

Atmospheric CO2 concentration from Mauna Loa

NASA mission Orbiting Carbon Observatory (OCO) will measure Carbon Dioxide and fluxes and exchange with a global perspective

For the decade of the 1980s, the global carbon cycle can be summarized as follows (units are PgC. One Pg [petagram] = one billion metric tonnes = 1000 x one billion kg):

Atmospheric increase	=	Emissions from Fossil fuels	+	Net emissions from changes in land use	-	Oceanic uptake	-	"Missing" carbon sink
3.3 (±0.2)	=	5.5 (±0.5)	+	1.6 (±0.7)	-	2.0 (±0.8)	-	1.8 (±1.2)

Attention on the global carbon cycle over more than 25 years has focused on the apparent imbalance in the carbon budget in the above equation -- the so-called "missing sink," missing because the accumulation of carbon has not been observed.

The terms in the global carbon equation above can be found graphically over the period 1850-1990 in the figure below:

Additional information can be found at the Woods Hole Research Center web pages for the Carbon Cycle

Graphic: Manipulating the carbon cycle (NYT). Scientist believe than more than half the carbon dioxide put in the atmosphere by the burning of fossil fuels is absorbed by the Earth's oceans, plants and soils.