The conversation about climate change largely revolves around greenhouse gases. Agriculture is both a source and sink for greenhouse gases (GHG). A source is a net contribution to the atmosphere, while a sink is a net withdrawal of greenhouse gases. In the United States, agriculture is a relatively small contributor, with approximately 8% of the total greenhouse gas emissions, as seen in Figure 1.
Most agricultural emissions originate from soil management, enteric fermentation (microbial action in the digestive system), energy use, and manure management (Figure 2). The primary greenhouse gases related to agriculture are (in descending order of magnitude) methane, nitrous oxide, and carbon dioxide.
Fact sheet: Contribution of Greenhouse Gases: Animal Agriculture in Perspective (look below the preview box and title for a download link)
|Figure 1: U.S. greenhouse gas inventory with electricity distributed to economic sectors (EPA, 2013)|
Figure 2: U.S. agricultural greenhouse gas sources (Adapted from Archibeque, S. et al., 2012)
Within animal production, the largest emissions are from beef followed by dairy, and largely dominated by the methane produced in during cattle digestion (Figure 3).
Figure 3: Greenhouse gas emissions from livestock in 2008 (USDA, 2011)
Excess nitrogen in agriculture systems can be converted to nitrous oxide through the nitrification-denitrification process. Nitrous oxide is a very potent greenhouse gas, with 310 times greater global warming potential than carbon dioxide. Nitrous oxide can be produced in soils following fertilizer application. This includes both commercial, inorganic fertilizer as well as organic fertilizers like manure or compost.
As crops grow, photosynthesis removes carbon dioxide from the atmosphere and stores it in the plants and soil life. Soil and plant respiration adds carbon dioxide back to the atmosphere when microbes or plants breakdown molecules to produce energy. Respiration is an essential part of growth and maintenance for most life on earth. This repeats with each growth, harvest, and decay cycle, therefore, feedstuffs and foods are generally considered to be carbon “neutral.”
Some carbon dioxide is stored in soils for long periods of time. The processes that result in carbon accumulation are called carbon sinks or carbon sequestration. Crop production and grazing management practices influence the soil’s ability to be a net source or sink for greenhouse gases. Managing soils in ways that increase organic matter levels can increase the accumulation (sink) of soil carbon for many years.
The next largest portion of livestock greenhouse gas emissions is from methane produced during enteric fermentation in ruminants - a natural part of ruminant digestion where microbes in the first chamber of the stomach, the rumen, breaks down feed and produces methane as a by-product. The methane is released primarily through belching.
As with plants, animals respire carbon dioxide, but also store some in their bodies, so they too are considered a neutral source of atmospheric carbon dioxide.
A similar microbial process to enteric fermentation leads to methane production from stored manure. Anytime the manure sits for more than a couple days in an anaerobic (without oxygen) environment, methane will likely be produced. Methane can be generated in the animal housing, manure storage, and during manure application. Additionally, small amounts of methane is produced from manure deposited on grazing lands.
Nitrous oxide is also produced from manure storage surfaces, during land application, and from manure in bedded packs & lots. Related: Archived webinar on GHG Emissions Research in Animal Ag
There are many smaller sources of greenhouse gases on farms. Combustion engines exhaust carbon dioxide from fossil fuel (previously stored carbon) powered vehicles and equipment. Manufacturing of farm inputs, including fuel, electricity, machinery, fertilizer, pesticides, seeds, plastics, and building materials, also results in emissions.
To learn more about how farm emissions are determined and see species specific examples, see the Carbon Footprint resources.
To learn about how to reduce on-farm emissions through mitigation technology and management options, see the Reducing Emissions resources.
Definition: carbon footprint is the total greenhouse gas emissions for a given person, place, event or product.
Carbon footprints are created using a process called life cycle assessment. Life cycle assessment or LCA is a method of resource accounting where quantitative measures of inputs, outputs and impacts of a product are determined.
Life cycle assessment is commonly used to:
boundaries of the system: each higher tier provides a more complete picture of the product’s impacts, however requires more time and resources to complete.
Different researchers may get different results when performing a LCA on the same product. This can happen for many reasons:
Related: Six archived webinars on the sources of animal ag ghg's (some are general and some are species-specific)
If you would like to use the video, slides, or factsheet for educational programs, please visit the curriculum page for download links for this and other climate change topics.
To find more resources on this and related topics, visit the animalagclimatechange.org resource finder.
Author: Crystal A. Powers - University of Nebraska-Lincoln firstname.lastname@example.org
This material was developed through support from the USDA National Institute for Food and Agriculture (NIFA) under award #2011-67003-30206.