Agriculture and forestry are responsible for between 10% and 32% of the world’s total greenhouse gas emissions from human activities, depending on how much one attributes emissions from land use changes and biomass burning to these sectors.  Depending on this accounting, agriculture and forestry contribute as little as 1% and as much as 24% of carbon dioxide (CO2) emissions, 47% to 52% of methane (CH4) emissions, and 58% to 84% of nitrous oxide (N2O) emissions. The largest source of CO2 emissions is decay of above- and below-ground organic carbon after deforestation. Agricultural CH4 emissions derive from the cultivation of wetland rice, burning of crop residues, and rearing of ruminants such as cows, sheep, goats, buffaloes, and camels. Soil and plant transformations of nitrogen fertilizers account for the majority of the agricultural N2O emissions .
Several external factors influence greenhouse gas emissions from the agricultural sector.
• Agricultural lands, although they have expanded 5% per decade in developing countries and 2.5% per decade in the world as a whole, have not kept pace with population growth, and so agricultural land per person has declined. To meet rising demands for food and fiber, farmers more intensively manage the lands that are already in production. Intensive management often results in a loss of soil carbon particularly from lands recently converted from forests or wetlands to agriculture and primarily through plowing, which accelerates microbial decomposition. Such losses manifest as a net release of CO2 to the atmosphere.
Agricultural land use (109 hectares) in developed and developing countries “Crops” and “Pasture” indicate areas that are in persistent use, whereas “Arable land” indicates areas that receive intermittent use for crops. [After FAOSTAT 2007.]
• People are now wealthier on average and can afford to consume more meat and dairy products. This dietary shift has altered the balance from producing food for direct human consumption to producing feed for domestic animals. The net result is that crop residues, which previously remained in the field and amended soil carbon or slowly decomposed to CO2, have become animal feed, and the animals release a portion of the organic carbon in the feed to the atmosphere as methane (CH4), a greenhouse gas with 23 times the warming potential of CO2.
Worldwide trends Human population (109 people), individual wealth (gross domestic product in constant, year 2000 $U.S. per person per year), and meat and dairy consumption (kilocalories per person per year). “Meat and dairy” includes meat from various animals, seafood, milk, and eggs. [After Development Data Group, The World Bank 2007; FAOSTAT 2007; United Nations Population Division 2008.]
• Policies, intended to nurture the emerging bioethanol industry in the United States and Europe, have shifted some agricultural production from food to fuel. Prices for staple crops such as maize have doubled in 2 years. Whether demand for biofuels will continue to influence the market for agricultural commodities is uncertain. Also uncertain is the potential for biofuels to mitigate greenhouse gas emissions.
Forests cover about 30% of Earth’s land area. Net loss of forested land between 2000 and 2005 averaged 7.3 million hectares, or 0.0002% per year. Certain areas of the world, however, such as equatorial South America, Africa, and Oceania, have suffered losses in excess of 0.5% per year, whereas others, such as the Iberian Peninsula and southeast Asia, have seen gains of more than 0.5% per year. Deforestation (clearing land for agricultural, industrial, commercial, or residential purposes) not only results in the immediate loss of carbon in the lumber removed, but it also leads to decomposition of organic carbon from the leaves, twigs, and roots left behind (from 100 metric tons to 250 metric tons of carbon emitted per hectare).  Several practices, such as slowing deforestation, promoting afforestation (conversion of lands into forests), and managing forests for high carbon intensity could sequester an additional 1.6 × 109 metric tons of carbon per year , or mitigate about 14% of total human CO2 emissions. International agreements including the Kyoto Protocol provide economic incentives to promote such practices , and many of the organizations that supply carbon offsets (credits for projects that sequester greenhouse gases to balance activities that emit them) do so through reforestation.
 Rogner, H.-H., D. Zhou, B. R., P. Crabbé, O. Edenhofer, B. Hare, L. Kuijpers, and M. Yamaguchi (2007) Introduction. In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Metz, B., O. R. Davidson, P. R. Bosch, R. Dave, and L. A. Meyer, eds. Cambridge University Press, New York. pp. 95-116.
Smith, P., D. Martino, Z. Cai, D. Gwary, H. Janzen, P. Kumar, B. McCarl, S. Ogle, F. O’Mara, C. Rice, B. Scholes, and O. Sirotenko (2007) Agriculture. In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Metz, B., O. R. Davidson, P. R. Bosch, R. Dave, and L. A. Meyer, eds. Cambridge University Press, New York. pp. 497-540.
 Nabuurs, G. J., O. Masera, K. Andrasko, P. Benitez-Ponce, R. Boer, M. Dutschke, E. Elsiddig, J. Ford-Robertson, P. Frumhoff, T. Karjalainen, O. Krankina, W. A. Kurz, M. Matsumoto, W. Oyhantcabal, N. H. Ravindranath, M. J. Sanz Sanchez, and X. Zhang (2007) Forestry. In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Metz, B., O. R. Davidson, P. R. Bosch, R. Dave, and L. A. Meyer, eds. Cambridge University Press, New York. pp. 541-584.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents