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Uncertainty surrounds US livestock methane emission estimates

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November 30, 2017, Pennsylvania State University  read full phys.org article here

A new study of methane emissions from livestock in the United States ….has challenged previous top-down estimates.

The research was conducted because serious discrepancies exist between top-down estimates that suggest the U.S. Environmental Protection Agency is underestimating agricultural emissions by up to 90 percent, and bottom-up estimates accepted by the federal government showing lower emissions.

Top-down emissions estimates involve monitoring atmospheric methane concentrations by satellites or from air samples collected at high altitude by planes, and using models to estimate the sources of emissions. Bottom-up estimates take into account populations and animal emission factors.

In their detailed analysis, researchers used a spatially explicit, bottom-up approach, based on animal inventories and feed-intake-based emission factors, to estimate enteric methane emissions for and methane emissions for cattle, swine and poultry for the contiguous United States.

The researchers estimated methane emissions using a “gridded” approach, dividing the U.S. into 0.1 by 0.1-degree GIS units, which created cells from 31 square miles in the northern United States to 42 square miles in the southern part of the country.

….According to the EPA, the top three sources of anthropogenic methane in the United States are the combined energy sector—natural gas, petroleum systems and coal mining—which makes up 40 percent of the total; livestock, 36 percent of the total; and landfills, 18 percent of the total.

…Methane emissions from livestock operations are the result of microbial fermentation and methanogenesis in the forestomach of ruminants and similar fermentation processes in manure from both ruminant and non-ruminant farm .

Methane is also produced from enteric fermentation in the digestive tract of non-ruminant herbivore species, such as horses, donkeys and mules, as a result of fermentation processes in their hindgut. However, “hindgut fermenters” do not produce nearly as much methane per unit of fermented feed as ruminants, so enteric or manure emissions from equine species were not included in this analysis. Neither were emissions from small ruminants such as sheep and goats, which are negligible in the U.S.

…Overall, the research, which was published this month in Environmental Science and Technology, yielded total U.S. livestock methane emissions of 19.6 billion pounds per year. However, uncertainty surrounding that total is high, researchers acknowledged.

…predicting methane emissions from manure is a more complex process and carries a larger uncertainty in the estimates, the researchers pointed out. Manure composition, type of storage facilities and manure retention time, and environment—particularly temperature—are among the factors that affect methane emissions from manure.

There is great uncertainty in both enteric and manure methane emissions from livestock, Hristov conceded. He said that research around the world has shown that variability in enteric methane emissions largely can be explained with variability in feed dry-matter intake. Nutrient composition of the feed is also important but has a lesser impact on enteric methane production.

Uncertainty surrounds US livestock methane emission estimates The researchers estimated methane emissions using a “gridded” approach, dividing the US into 0.1- by 0.1-degree GIS units, which created cells from 31 square miles in the northern United States to 42 square miles in the southern part of the country. The study pegged total U.S. livestock methane emissions of 19.6 billion pounds per year. This map shows where they are coming from. Credit: Penn State

Read more at: https://phys.org/news/2017-11-methane-emissions-livestock.html#jCp

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