UNIVERSITY PARK, Pa. — A new study of methane emissions from livestock in the United States — led by a researcher in Penn State's College of Agricultural Sciences — 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 methane 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 livestock 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 cattle and manure 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.
"This level of detail enabled us to more accurately assess agricultural methane emissions based on activities involving livestock," explained lead researcher Alex Hristov, professor of dairy nutrition, who is a member of the current National Academy of Sciences Anthropogenic Methane Committee.
"We must have more specific information about methane emissions that combines local livestock populations and characteristics with distribution of landscape features — and a gridded inventory approach provides that," he said.
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.