The emissions estimate for nitrous oxide in this year's report is three times larger than the estimate last year, as a result of the implementation of the new IPCC methods for estimating nitrous oxide emissions. Estimated nitrous oxide emissions have been roughly constant in the 1990s, with no clear trend. Agriculture is the major source, and agricultural emissions are dominated by emissions from nitrogen fertilization of agricultural soils. The emissions factor for this source has been raised somewhat, but the bulk of the increase has been caused by the counting of nitrogen in manure used as fertilizer and nitrogen in crop residues, in addition to nitrogen from chemical fertilizers. Also, following the revised IPCC guidelines, secondary nitrous oxide emissions from nitrogen in agricultural runoff into streams and rivers have been incorporated.
Motor vehicles equipped with catalytic converters also emit significant amounts of nitrous oxide.
Certain chemical processes, fuel combustion, and wastewater treatment plants are smaller sources of nitrous oxide emissions. Emissions of these three classes of gases account for about 2 percent of U.S. GWP-weighted emissions. There are several other categories of chemicals that also qualify as greenhouse gases but are excluded from the Framework Convention on Climate Change and the Kyoto Protocol because they are already controlled under the Montreal Protocol on Ozone-Depleting Substances. On the other hand, many of the gases have atmospheric lifetimes measured in the hundreds or thousands of years, and consequently they are potent greenhouse gases with global warming potentials hundreds or thousands of times higher than that of carbon dioxide per unit of molecular weight. Land Use and Forestry Forest lands in the United States are net absorbers of carbon dioxide from the atmosphere.