Elevated levels of carbon dioxide and other “greenhouse” gases may be contributing to higher global average temperatures in recent years, a trend frequently referred to as global warming. Some scientists predict that such warming could increase sea levels, stimulate weather that is more violent, alter patterns of disease, and have other potentially damaging effects. Carbon, a constituent of all the main greenhouse gases, is believed to play a pivotal role in the regulation of the earth’s temperature.
The major human activities that cause greenhouse gases to build up are the burning of wood and fossil fuels, and changes in land use that result in fewer plants capturing less carbon dioxide from the atmosphere. Since the beginning of the industrial revolution (around 1750), carbon dioxide in the air has risen more than 30 percent, from 278 to 368 parts per million, and other carbon-containing greenhouse gas concentrations have increased even more. Methane concentrations, for example, have risen by nearly 120 percent since the beginning of the 19th century, from approximately 790 parts per billion in 1850, to the present level of over 1,725 parts per billion.
Concerns over potentially negative effects of prolonged global warming have stimulated interest in restraining the buildup of greenhouse gases in the atmosphere. Proponents of immediate action favor measures aimed at slowing or stopping the buildup of greenhouse gases by using less fossil fuel, or using it more efficiently. An alternate approach that is gaining more attention shifts the focus away from slowing generation of greenhouse gases to removing greenhouse gases from the atmosphere and storing, or “sequestering,” them in a variety of ways. This study focuses on those efforts.
Advocates of sequestration point to numerous benefits of this approach, including lower costs of carbon dioxide control, reduced impact on lifestyles of people in developed countries, and greater ability to achieve domestic greenhouse gas reductions without transferring wealth or technology abroad.
There are three major ways to store additional carbon in forests (including soil):
- Preventing Long-term Deforestation. In some cases, forested areas are harvested but not replanted, or land is converted to non-forestry or non-agricultural use. This limits the Earth’s natural ability to regulate greenhouse gases, including carbon dioxide. This can be offset by reducing long-term or permanent deforestation.
- Increasing Tree-planting. More trees can be planted on marginal land, such as less-productive agricultural land. This directly removes additional carbon dioxide from the atmosphere, and locks it away in the woody tissue of the trees.
- Improving Forest Management. Advanced forest management practices can be adopted more quickly and uniformly, particularly in developing countries where they have not yet taken hold. With such techniques, forests can be managed for long-term health and carbon storage as well as for wood production.
Carbon can also be stored in agricultural soils, which can retain as much carbon as forest soil. Practices that maximize both plant growth and carbon-retention in soils (called conservation tillage) can be used for cotton and most grain crops. Specific conservation-tillage practices that can increase carbon-retention by agricultural soils include:
- No-till Cultivation. Tilling agricultural soils liberates trapped carbon, and is not always necessary for crop cultivation.
- Maximizing Carbon Retention Through Fertilizer and Herbicide Use. Changing the way that fertilizers and herbicides are used can alter the way that agricultural soils store carbon-bearing matter underground, maximizing the retention of carbon.
- Planting of “Cover Crops.” Fields with fewer cover crops are more likely to undergo degradation of subsurface carbon-bearing. Cover crops can be planted on “fallow” agricultural lands to preserve soil nutrients and carbon content between primary crop periods.
Many farmers are already adopting these practices to achieve higher production (and usually lower costs), allowing additional marginal land to be retired and reforested.
Finally, some forest and agricultural land can be used to grow trees and crops for “biomass.” Biomass is a term used to encompass the many different types of carbon-bearing plant matter that are not ultimately used for food production. Biomass includes unusable portions of agricultural crops, such as crop stubble, wheat stalks, and corn stalks. Biomass can be burned directly, or can be converted to fuels such as methanol, and has the potential to replace a significant amount of petroleum-derived fossil fuels.
While burning fossil fuels such as oil and coal adds carbon to the atmosphere, biomass avoids this by recycling the same carbon over and over again. Although carbon is released to the atmosphere when biomass fuel is burned, the carbon is pulled out of the atmosphere when the crops are re-planted.
Global carbon dioxide emissions contained about 8.2 gigatons of carbon in 2000 and, with “business as usual,” could reach 14.5 gigatons in 2050.1 Based on evaluation of published studies, potential amounts of emissions that could be stored in plants and soil or avoided by using biomass fuel, and their approximate costs, can be summarized as follows:
- Changes in Forest Management. This could result in about 2 gigatons of carbon stored per year, at an average cost around $4 per ton of carbon stored.
- Changes in Agricultural Management. This could result in about 1 gigaton per year in soil, with operational savings offsetting most of the costs.
- Use of Biomass Fuels. This could substitute for 2 gigatons of annual fossil fuel emissions at a cost similar to the replaced fossil fuels.
Summing up these three areas, a projected potential annual savings of 5 gigatons would offset or avoid 35 percent of the “business as usual” emissions in 2050. Savings would grow toward this annual rate as sequestration efforts are established, be maintained for 30 to 50 years, and then slowly taper off as forests and soils approach their carbon capacity.