Escalating global temperatures are an increasing concern among climatologists, economists and government officials. Even private sector companies are assessing their vulnerability to increasing temperatures, ocean levels and erratic weather. Two strategies for slowing the rising temperatures involve either reducing the production of green house gases, primarily CO2 produced by burning fossil fuels (oil, coal, natural gas) or converting the CO2 now in the air to solid or liquid carbon compounds.
The process of converting atmospheric CO2 to solid or liquid compounds is called "carbon sequestration.” There are a variety of high tech solutions being explored for sequestering atmospheric CO2, such as liquefying it and pumping it underground under high pressure. But probably the simplest, most effective and most efficient way to sequester atmospheric CO2 is to let plants do that work for us.
Plants as Carbon Sequestration Machines
Plants are carbon sequestration machines. They take in carbon dioxide from the atmosphere and with energy gained from sunlight through photosynthesis convert the CO2 to complex sugar compounds and pure oxygen. The complex sugar compounds are converted further by the plant into cellulose, energy and other molecules required by the plant. The oxygen is expelled as waste.
But some plants do carbon sequestration better than others. Annuals, for example do little carbon sequestration over their life cycle. Once an annual plant dies at the end of the season, its carbon is slowly released back into the atmosphere as natural decomposition processes take over. Perennial grasses, forbs and herbaceous plants do a better job, since their root systems persist in the ground and thereby sequester some carbon year to year, although their tops die back and annually decompose into simpler molecules, including carbon dioxide. In research being conducted in Kansas, deep-rooted prairie grasses, forbs and herbaceous perennials have been found to sequester as much as 1/3 of a ton of carbon per acre per year (Rice, 2002).
Young Trees Most Effective at Sequestering Carbon
One of the most efficient plant systems for sequestering carbon is a young tree. Young trees sequester carbon at a comparatively high rate, converting CO2 into wood as they grow. A tree will be a net user of CO2 for most of its life as it continues to grow. As a tree matures its will continue to sequester the carbon held in its wood, but its ability to convert CO2 slows with its growth. Eventually, when a tree dies, it will release its carbon dioxide back into the atmosphere slowly, if it decays naturally, quickly, if it is burned as fuel, or for a much longer period if it is converted to lumber or furniture.
Highways as Carbon Producers
Highways produce carbon, first by the construction process, but more significantly over time with the carbon dioxide produced by combustion of petroleum-based fuels by the vehicles using the highways.
Table 1. Carbon dioxide emissions from building one lane-mile of urban highway over 50 years
Construction, building materials, and maintenance: 3,500 tons
Net congestion relief: -7,000 tons
Additional vehicle travel on the facility: 90,000 tons
Induced vehicle travel off the facility: 30,000–100,000 tons
Total: 116,500-186,500 tons
A mile of highway produces from 2,330 - 3,730 tons per year of CO2. Conversely, a healthy tree stores about 13 pounds of carbon annually -- or 2.6 tons per acre each year. An acre of trees absorbs enough CO2 over one year to equal the amount produced by driving a car 26,000 miles (Nowak, 1993).
By including trees and meadows of deep-rooted perennial grasses and herbaceous plants as a part of highway design, either within or adjacent to the right-of-way or in areas removed from the project, at least a part of the carbon produced by a highway project can be offset.