Sources of Greenhouse Gas Emissions
Agriculture Sector Emissions
Total U.S. Greenhouse Gas Emissions by Economic Sector in 2013
Agricultural activities - the cultivation of crops and livestock for food - contribute to emissions in a variety of ways:
- Various management practices for agricultural soils can lead to production and emission of nitrous oxide (N2O). The large number of different activities that can contribute to N2O emissions from agricultural lands range from fertilizer application to methods of irrigation and tillage. Management of agricultural soils accounts for over half of the emissions from the Agriculture sector.*
- Livestock, especially cattle, produce methane (CH4) as part of their digestion. This process is called enteric fermentation, and it represents almost one third of the emissions from the Agriculture sector.
- The way in which manure from livestock is managed also contributes to CH4 and N2O emissions. Manure storage methods and the amount of exposure to oxygen and moisture can affect how these greenhouse gases are produced. Manure management accounts for about 12% of the total greenhouse gas emissions from the Agriculture sector in the United States.
- Smaller sources of emissions include rice cultivation, which produces CH4, and burning crop residues, which produce CH4 and N2O.
More national-level information about emissions from agriculture can be found in the agriculture chapter in the Inventory of U.S. Greenhouse Gas Emissions and Sinks.
*Management of agricultural soils can also lead to emissions of carbon dioxide (CO2). However, these emissions are included under the Land Use, Land-Use Change, and Forestry sector.
Emissions and Trends
In 2013, greenhouse gas emissions from agriculture accounted for approximately 9% of total U.S. greenhouse gas emissions. Greenhouse gas emissions from agriculture have increased by approximately 17% since 1990. One driver for this increase has been the 54% growth in combined CH4 and N2O emissions from livestock manure management systems, reflecting the increased use of emission-intensive liquid systems over this time period. Emissions from agricultural soil management have also increased by about 17% since 1990. Emissions from other agricultural sources have either remained flat or changed by a relatively small amount since 1990.
To learn about projected greenhouse gas emissions to 2020, visit the U.S. Climate Action Report 2014 (PDF) (310 pp., 23.1 MB).
Greenhouse Gas Emissions from Agriculture
Reducing Emissions from Agriculture
The table shown below provides some examples of opportunities to reduce emissions from agriculture. For a more comprehensive list of options and a detailed assessment of how each option affects different gases, see Chapter 8 of the Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change .
|Type||How Emissions are Reduced||Examples|
|Land and Crop Management||Adjusting the methods for managing land and growing crops.||
|Livestock Management||Adjusting feeding practices and other management methods to reduce the amount of CH4 resulting from enteric fermentation.||Improving pasture quality to increase animal productivity, which can reduce the amount of CH4 emitted per unit of animal product. Also, increased productivity can be accomplished through breeding.|
6,822 million metric tons of CO2 equivalent--what does that mean?
An Explanation of Units
A million metric tons is equal to about 2.2 billion pounds, or 1 trillion grams. For comparison, a small car is likely to weigh a little more than 1 metric ton. Thus, a million metric tons is roughly the same mass as 1 million small cars!
The U.S. Inventory uses metric units for consistency and comparability with other countries. For reference, a metric ton is a little bit larger (about 10%) than a U.S. "short" ton.
GHG emissions are often measured in carbon dioxide (CO2) equivalent. To convert emissions of a gas into CO2 equivalent, its emissions are multiplied by the gas's Global Warming Potential (GWP). The GWP takes into account the fact that many gases are more effective at warming Earth than CO2, per unit mass.
The GWP values appearing in the Emissions webpages reflect the values used in the U.S. Inventory, which are drawn from the IPCC's Second Assessment Report (SAR). For further discussion of GWPs and an estimate of GHG emissions using updated GWPs, see Annex 6 of the U.S. Inventory and the IPCC's discussion on GWPs.