Environmental Issues

Biofuels and the Carbon Balance

by Carol E. Whitman, Ecologist, and Gary R. Evans, Special Assistant for Global Change Issues, Office of the Assistant Secretary for Science and Education. USDA, Washington, DC.

The burning of fossil fuels for energy has contributed to the buildup of carbon dioxide (CO₂) in the atmosphere. As the demand for energy continues to expand with population and economic growth, so will the need for alternative fuels that have no net effect on atmospheric carbon concentrations. One alternative is fuel made from crop biomass (biofuel).

This approach to balancing the carbon cycle has so far received scant attention, compared with proposals to plant more trees to sequester carbon from the atmosphere. But fuels made from plant biomass can reduce our dependence on imported, nonrenewable fossil fuels, slow the net increase of atmospheric CO₂ and help bring the Earth's carbon cycle back into balance.

The Carbon Cycle

Carbon is one of the basic elements of all living organisms. It exists in two principal forms: organic carbon derived from living things, and inorganic carbon, that is, gaseous and mineral compounds. Since carbon is central to life, its cycling between inorganic and organic phases is an important part of the Earth's ecology (see fig. 1).

CO₂, the predominant inorganic form of carbon, is found both in the air and dissolved in water. It is the principal source of carbon for living organisms. Aquatic and land plants convert CO₂ water (H₂O), and light energy into organic carbon during photosynthesis:

light energy + CO₂ + H₂O (CH₂O) + O₂.

In this way, light energy is stored in organic compounds as chemical energy, and energy and inorganic carbon (CO₂) are made available to living organisms.

In order to meet their energy needs, plants break down some of these organic compounds through the process of respiration:

(CH₂O) + O₂ CO₂ + H₂O + energy.

But much of the carbon remains in the plant's structure until it dies or is eaten by animals. On land, plant residues then become incorporated into the soil as soil organic matter. This constitutes a large pool of the Earth's organic carbon.

Animals also release CO₂ during respiration, eliminate carbon in waste material, and retain carbon in their bodies until they die. As the dead plants, dead animals, and animal waste material decay, CO₂ is returned to the air or water.

This cycle of carbon moving between an inorganic store and organic systems occurs on a relatively short time scale of several decades or less. Within this time frame, the carbon is balanced; that is, there is no net gain or loss in either carbon pool.

Other pathways occur over a much longer time scale. Sometimes the dead remains of plants and animals fail to decompose and under certain conditions are converted into fossil fuels (such as coal, oil, and natural gas), rock, or diamonds. This process takes place over millions of years, forming an almost permanent carbon reservoir.

Carbon Out of Balance

Over the past several hundred years, human beings have begun to alter many of the flows in the carbon cycle, and with them, the carbon balance of the Earth. The burning of coal, oil, and gas has released some of the permanent store of organic carbon to form CO₂. In addition, native forests and grasslands have been cleared for agriculture and development. Often forests are burned, prematurely returning carbon to the air. The reduction in number and size of forests has decreased the overall ability to assimilate CO₂ from the atmosphere. Without a steady source of plant residues, the carbon tied up in soil organic matter also declines, resulting in a net release of CO₂ into the air.

Human activities have caused a net increase in atmospheric CO₂ from a preindustrial level of 280 parts per million volume (ppmv) to 350 ppmv today. At current levels of energy consumption, CO₂ is expected to double over the next century. This increase in atmospheric CO₂ is significant because of its role in regulating the Earth's temperature.

Like a blanket around the Earth, atmospheric CO₂ absorbs heat radiation emitted by the sun-warmed planet, and radiates it back to the surface. In effect, CO₂ helps to keep the Earth warm. This natural "greenhouse effect" provides human beings, animals, and plants with a comfortable living environment. But increases in atmospheric CO₂ like adding another blanket, may cause the Earth's temperature to rise.