Richard A. Houghton

Richard Houghton, Senior Scientist, Carbon Research

Understanding the Global Carbon Cycle

We (ecologists) have been interested in carbon for a long time, first, because carbon is what we (as well as all of the other plants and animals on earth) are made of (50% of our dry weight). Ecologists can learn a lot about ecosystems and what they do for us by constructing carbon budgets (or energy budgets) from measurements of productivity, food chains, and nutrient cycling.

The second reason that carbon is of interest is because 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 about 30% since the start of the industrial revolution sometime around the middle of the 19th century and will continue to increase unless societies choose to change their ways.

Most of the increase in atmospheric CO2 concentrations came from and will continue to come from the use of fossil fuels (coal, oil, and natural gas) for energy, but about 25% of the increase over the last 150 years came from changes in land use, for example, the clearing of forests and the cultivation of soils for food production [Figure 1].

Graph - CO2 release

Figure 1 (select image for larger version - JPG, 109KB)

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

Global Carbon Cycle
Figure 2 (select image for larger version - GIF, 65KB)

We at the Center are involved with 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].

Atmospheric CO2 at Mauna Loa
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.

Research at the Center has focused on the current and historic releases of carbon that result from changes in land use. The approach we use 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. Our 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.

Our work shows that between 1850 and 2000 about 155 Pg of carbon were released to the atmosphere from changes in land use, worldwide (one Pg [petagram]=one billion metric tonnes=1000 x one billion kg). The amount released each year generally increased over the period, and by the 1990s the rate of release averaged about 2 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, or missing sink, of carbon.

Atmospheric increase = Emissions from fossil fuels + Net emissions from changes in land use - Oceanic uptake - Missing carbon sink
3.2 (±0.2) = 6.3 (±0.4) + 2.2 (±0.8) - 2.4 (±0.7) - 2.9 (±1.1)

Concern about the consequences of a changing climate has led us to explore how forests might be used to withdraw carbon from the atmosphere. They have a significant potential for reducing the rate at which carbon builds up in the atmosphere (see "Using Forests to Sequester Carbon"), 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 our 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.