Such intensification of agricultural activities is linked to global climate change, particularly through increased atmospheric emissions of greenhouse gases such as methane and nitrous oxide. Three agricultural activities contribute directly to greenhouse gas emissions: cultivation of flooded rice fields, enteric fermentation in livestock, and use of nitrogenous fertilizers. The United Nations Environment Programme's (UNEP) Information Unit for Climate Change (IUCC) Fact Sheet 1 (1990a) provides an overview of human activities that contribute to climate change.
The decomposition of fertilizers and crop residues results in Methane Emissions from Flooded Rice Cultivation. This source of emissions is becoming more significant, because the acreage under rice cultivation increased by 40 percent worldwide between 1950 and 1984. Of the total acreage, more than 90 percent is located in Asia. In dryland rice production no methane is produced.
Another substantial source of methane is Methane Emissions from Livestock. Livestock--such as sheep, goats, camels, cattle, and buffalo--provide food and supply manure and power for agriculture. The global livestock population has increased considerably since the 1960s. As a by-product of digestion, however, livestock release significant amounts of methane to the atmosphere.
Global climate change may also result from Nitrous Oxide Emissions from Application of Nitrogenous Fertilizer. Since the introduction of high-yielding crop varieties, nitrogenous fertilizers have played an important role in increasing yields. But as a result of microbial action in the soil, these chemicals release nitrous oxide. The nitrogen cycle illustrates the mechanism for nitrous oxide emissions (U.S. Office of Technology Assessment 1991).
Chapter 4 of the U.S. Environmental Protection Agency report Policy Options for Stabilizing Global Climate offers an overview of agricultural activities that contribute to global climate change (Lashof and Tirpak 1990). This report also provides data on current methane emissions by source, regional contributions to greenhouse warming, trends in domestic animal populations, rice paddy production, nitrous oxide emissions by fertilizer type, and nitrogenous fertilizer consumption. The Intergovernmental Panel on Climate Change (1991) also estimates agricultural contributions to methane and nitrous oxide emissions in Climate Change: The IPCC Response Strategies.