Andrew G. Jordan, Ph.D.
July 2, 2007
There is growing public concern over the threat of rising global temperatures. Though reasons are clouded by scientific uncertainties, many experts predict that the earth is on the verge of non-reversible climate change.
While scientists disagree on the extent humans are responsible, they acknowledge greenhouse gases can trap increasing amounts of the sun's energy keeping heat near the earth's surface. They also concur atmospheric carbon dioxide (CO2) levels have increased from about 280 parts per million from the beginning of the industrial age to more than 375 ppm today. Other greenhouse gases such as methane and nitrous oxide are increasing at proportionate rates. If managed sensibly, reducing greenhouse gases can contribute to more pleasing surroundings while balancing economic, environmental and social benefits.
The most dramatic means to reduce greenhouse gases could come about by reducing emissions from burning fossil fuels. Proposals include mandates to increase vehicle fuel efficiency standards, scrubbing CO2 from coal burning power plants and funding research and development for low emission fuels. Encouraging adoption of renewable energy resources such as wind, solar, bio-diesel and ethanol also can lessen the burden of greenhouse gases.
While media attention is on gas emissions, many overlook the fact that healthy agriculture and forestry practices can extract millions of tons of CO2 from the atmosphere through photosynthesis. Growing plants help offset CO2 produced from burning fuels. Globally, photosynthesis and respiration have kept CO2 in check in the past,1 and photosynthesis will continue to play a part in CO2 removal long before scientists and politicians agree on how to cut emissions from fossil fuels.
All living plants extract CO2 from the air. Plant products often are used as animal food or consumed by soil microorganisms, both of which complete the natural carbon cycle putting CO2 back into the atmosphere. Conversely cellulose, a major constituent of wood and cotton, can be thought of as a giant carbon sink because carbon included in cellulose and formed into textile and wood products remains for many years.2
The broad leaves of the cotton plant operate as factories using the energy of the sun to make useful products. In fact, a cotton leaf may be viewed as a solar collector crammed full of tiny photosynthetic cells. The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the products, sugar and oxygen, exit. The sugars go to various parts of the plant providing building blocks for roots, stems, and fruit while oxygen is emitted back to the atmosphere. Cotton's fruit, called a cotton boll, is harvested and separated into various parts such as fibers for textiles, seeds for oil, protein and carbohydrates, and waste products for compost, soil residue and soil organic carbon.
 Facts to Ponder
In one year, cotton plants in a typical world cotton field will extract about 10,0003,4 pounds of CO2 per acre to make the fiber, oil, protein and other plant parts.
Of those 10,000 pounds, nearly 1,0005 pounds of CO2 taken from the air is used to produce cotton fibers and about 450 6 pounds is extracted to produce some 170 pounds of vegetable oil. Cottonseed oil, as all vegetable oils are, is a rich source of energy and can be used for food, bio-fuel or industrial products providing a positive energy balance for cotton production.7
On one acre, 7,000 pounds of oxygen8 is released back into the atmosphere, enough to supply the respiratory needs of a family of 5 for two years.9
Other parts of the plant including protein and carbohydrates are fed to dairy cattle and other livestock and used for bedding. The remaining crop residues are left on the soil surface protecting soil particles from wind and water erosion and adding organic matter to the soil. Increasing soil carbon makes nutrients readily available for plants, conserving moisture and allowing roots to reach easily into the soil.
In 2005 the world cotton textile fiber production reached 55 billion pounds10 with most going to apparel, home furnishings and industrial products.
More than 80 billion pounds of CO2 were removed from the atmosphere to form the cellulose in the fiber. (While more than 1 trillion pounds of CO2 were removed by cotton plants during the growing season, most was deposited back as an amendment to the soil. The remaining 80 billion pounds is bound in fibers for a considerable period of time.)
The U.S. Environmental Protection Agency reports that a typical U.S. passenger vehicle emits 11,09011 pounds of CO2 per year. Carbon sequestered in the world cotton fiber supply is the oil equivalent of taking 7.25 million passenger vehicles from the highways.
References:
1
www.esig.ucar.edu/nacp
2
R. Malcolm Brown, Jr. Microbial Cellulose: A New Resource for Wood, Paper, Textile, Food and Specialty Products. The University of Texas Austin, Texas, www.botany.utexas.edu/facpages/mbrown/position1.htm
3
Pinter, P.J., B.A. Kimball, J.R. Mauney, G.R. Hendrey, K.F. Lewin and J. Nagy. 1994. Effects of free-air carbon dioxide enrichment on PAR absorption and conversion efficiency by cotton. Ag. And Forest Met. 70:209-230.
4
Arizona cotton research showed 1500 grams above ground biomass per square meter or about 13,000 pounds per acre. Given that world average cotton yields are about 50% of Arizona yields, biomass estimates were adjusted accordingly to 6500 pounds biomass per typical acre. On average more than 9,500 pounds CO2 per acre were removed from the air.
5
Cotton fiber, nature's purest form of cellulose, consists of 41% carbon. CO2 is 27% carbon which for each pound cellulose produced, 0.41/0.27 (1.48 pounds) CO2 is taken from the air and 1.07 pounds of O2 are produced.
6
Approximately 170 pounds vegetable oil is produced on one acre of cotton. Cotton seed oil is 72% carbon and CO2 is 27%, a ratio of 0.72/0.27 or for each pound of oil produced, 2.67 pounds CO2 are utilized.
7
Cotton production like all agriculture requires energy inputs. Cottonseed oil, a product of cotton production, is packed with energy equivalent to about 25 to 30 gallons of petro-diesel per acre. Energy represented in cottonseed oil is from 4 to 10 times the energy content as the tractor and implement fuel used at farm level. Energy consumed in the production of fertilizers, plant health products and for operating ginning varies depending on the local geographic area. The ability of individual farmers to employ energy saving conservation tillage practices, opportunity to rotate cotton with nitrogen-fixing legume crops, access to plant and animal compost and utilization of energy friendly biotech crops substantially reduce the need for energy inputs. The net energy content of cottonseed oil is estimated to be approximately a 1 to 1 offset for all production and ginning inputs. While cottonseed oil may or may not be used in producing bio-diesel used on the farm the influx of oil into the vegetable oil market makes more vegetable oils available for bio-diesel. Though not included in these comparisons, energy of cottonseed protein and carbohydrates fed to animals represents a significant biological energy offset Technology and equipment for converting gin by-products to direct combustion fuels in the form of dry pellet fuels with energy equivalent to 21 gallons of petro-diesel per acre of cotton is becoming increasingly feasible (USDA Ginning Laboratory Research report (http://journal.cotton.org) The Journal of Cotton Science, 7:205-216(2003)). Gasifier technologies have been demonstrated and are being promoted to produce liquid pyrolosis products (bio-oil) and synthetic gases from fermentation to make bio-based fuels and chemicals. New technologies for cellulose ethanol production from field residues show that ethanol yields from 21 to 113 gallons per metric ton dry matter are possible (http://www1.eere.energy.gov/biomass/ethanol_yield_calculator.html). From 1 to 1.5 tons dry matter from cotton stalks and burrs (21 to 170 gallons ethanol) could be available from cotton fields while leaving sufficient surface residue for soil erosion protection. By far, when all energy products and by products are considered cotton is a net producer of energy in addition to its net carbon sequestration in fiber.
8
Oxygen production estimates are based on the ratio that 32 pounds oxygen are produced for each 30 pounds of biomass, the same ratio as the total sugar to total oxygen in the following basic formula for photosynthesis: 6 H2O + 6 CO2 —> C6H12O6 + 6 O2 . There is no claim that the world is likely to run out of oxygen before it runs out of fossil fuels, however it is an interesting fact that oxygen is a positive contribution of growing plants.
9
NASA calculates for moderate exercise, a grown person consumes 0.84 kg oxygen per day, or 670 pounds per person per year.
10
Foreign Agricultural Service, USDA, Office of Global Analysis, Table 1, 2, p 7.
11
USEPA, Average Annual Emission and Fuel Consumption for Passenger Cars and Light Trucks. Office of Transportation and Air Quality. EPA420-F-00-013, April 2000
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