Agriculture contributed 9.3% of the U.S. output of greenhouse gases in 2018, mostly nitrous oxide from fertilized fields and methane from cattle, the Environmental Protection Agency reports in an annual inventory of emission sources.
The EPA estimates agriculture emitted the equivalent of 618.5 million metric tons of carbon dioxide, up 2.7% from the year before and 11.5% since 1990, the international baseline for evaluating emission trends. The estimate does not include agriculture’s use of electricity and fossil fuels.
Overall, U.S. carbon emissions increased by 2.9% to 6.67 billion tons, ending a four-year streak of declining emissions. More fossil fuels were consumed as a cold winter and hot summer upped the demand for energy for heating and cooling, according to the EPA.
U.S. emissions have risen by 3.7% since 1990. They have declined, however, by 12.3% since 2005, even with the bump up in 2018.
The report, released last week, is a preliminary draft of an inventory the EPA plans to finalize in April.
Nitrous oxide from soil management accounted for more than half of farm emissions. Nitrous oxide is naturally produced in soils through a process driven by the availability of nitrogen.
Agricultural soils release the majority of nitrous oxide emissions in the U.S. Year-to-year fluctuations depend on the weather, fertilizer use and crop production, according to the EPA.
The second-largest agricultural source was enteric fermentation, the digestive process that leads to livestock belching methane. Emission levels generally follow trends in cattle populations, but dairy cows have been an exception, according to the EPA.
The EPA reported that the dairy cow population has declined 2.6% since 1990, but emissions from enteric fermentation have risen 4.6%. Meanwhile, milk production has increased by 57%, suggesting emissions per unit of milk or beef have decreased, the EPA states.
Other sources of agricultural greenhouse gases include manure management, rice cultivation and field burning, according to the EPA.
Farmland, including pastures, was credited with storing 16.6 million metric tons of carbon. Converting forests to cropland released 55.3 million metric tons of carbon, the EPA estimated.
Greenhouse gases from wildfires were an estimated 141.1 million metric tons in 2017. Fire information for 2018 was not available when EPA compiled the report.
The EPA says it’s continuing to refine methods for estimating farm greenhouse gases.