Global Carbon Cycle Primer

Ecologists have long been interested in carbon. This interest stems from several reasons. First, humans (as well as all of the other plants and animals on Earth) are made primarily of carbon. In fact, nearly 50% of our dry weight is carbon. Ecologists can learn much about ecosystems and what they do by constructing carbon budgets (or energy budgets) from measurements of productivity, food chains, and nutrient cycling.

Second, carbon, in the form of carbon dioxide (CO2), is the major greenhouse gas released to the atmosphere as a result of human activities. The continued release of greenhouse gases is raising the temperature of the earth, disrupting the climates we and our agricultural systems depend on, and raising sea-level. The concentration of CO2 in the atmosphere has already increased by nearly 40% since the start of the industrial revolution sometime around the middle of the 18th century and will continue to increase unless society eliminates the use of fossil fuels.

Most of the increase in atmospheric CO2 concentrations has come from and will continue to come from the use of fossil fuels (coal, oil, and natural gas) for energy, but more than 30% of the increase over the last 150 years came from changes in land use. These include, for example, the clearing of forests and the cultivation of soils for food production [Figure 1].


figure 1

The global carbon cycle involves the carbon in and exchanging between the earth’s atmosphere, fossil fuels, the oceans, and the vegetation and soils of the earth’s terrestrial ecosystems [Figure 2].


Figure 2 (updated 3/2014)

Scientists at the Woods Hole Research Center are involved in determining the role terrestrial ecosystems play in the global carbon cycle. 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 [Figure 3].


Figure 3

If the global totals for photosynthesis and respiration are not equal, carbon either accumulates on land or is released to the atmosphere. Unfortunately, the global rates of photosynthesis and respiration are neither known nor measured well enough to determine annual changes in carbon storage. On the other hand, human use of the land, for example the clearing of forests for croplands, is relatively well documented, both historically and with satellites, and thus can be used to determine changes in the storage of carbon on land.

Research at WHRC has focused on the current and historic releases of carbon that result from changes in land use. The approach used is based on the fact that much of the carbon stored in trees and soils is released to the atmosphere when forests are cleared and cultivated. Some of the release occurs rapidly with burning; some of it occurs slowly as dead plant material decomposes. When forests regrow on cleared land, they withdraw carbon from the atmosphere and store it again in trees and soils. The difference between the total amount of carbon released to the atmosphere and the total amount withdrawn from the atmosphere determines whether the land is a net source or sink for atmospheric carbon. The Center’s approach is thus based on two types of data: rates of land-use change (e.g., annual rates of deforestation) and the changes in carbon that follow changes in land use.

WHRC’s work shows that between 1850 and 2005, about 156 Pg of carbon were released to the atmosphere from changes in land use, worldwide (one Pg [petagram]=1015 grams=one billion metric tonnes). The amount released each year generally increased over the period, and by the 1990s, the rate of release averaged about 1.5 Pg of carbon per year.

When considered with the other terms in the global carbon equation (the atmosphere, fossil fuels, and the oceans), there is an apparent imbalance in the global accounting, and considerable effort has gone into explaining and finding this residual sink of carbon.

Average Annual Fluxes for the period 2000-2008: (Modified from LeQuéré et al., 2009)

Atmospheric increase = Emissions from
fossil fuels
+ Net emissions from
changes in land use
carbon sink
4.1(±0.04) = 7.7 (±0.4) + 1.5 (±0.7) 2.3 (±0.4) 2.8 (±0.9)


Concern about the consequences of a changing climate has led Center researchers to explore how forests might be used to keep carbon out of the atmosphere. Management of forests has a significant potential for reducing the rate at which carbon builds up in the atmosphere, but the major contributor to climatic change, and hence the human activity most in need of change, is use of fossil fuels for energy. Advances in the technology of renewable energy sources, including wood-derived fuels, might reduce reliance on fossil fuels and thus reduce global emissions of carbon dioxide significantly.

More data on CO2 are available from DOE’s Carbon Dioxide Information Center in Oak Ridge and the Global Carbon Project.

Also see The Global Carbon Cycle (pdf).