A new study on methane has challenged previous research that suggests the US Environmental Protection Agency (EPA) has been underestimating emissions from livestock.
Right now, the EPA predicts livestock make up 36 percent of the country's total methane emissions - just behind the combined energy sector, which makes up 40 percent of the total.
Their results are based on a bottom-up estimate, which calculates methane emissions by considering livestock populations and animal emission factors, like feed.
But that's not the only way to estimate methane emissions. A top-down approach monitors atmospheric methane concentrations from satellites or air samples collected by high altitude planes.
Here's where things start to get worrying.
When previous studies have employed a top-down method, there are serious discrepancies in the data. The findings suggest the EPA could be underestimating agricultural methane emissions by up to 90 percent.
Compared to CO2, methane emissions are roughly 30 times more potent as a heat-trapping gas. So finding out how much methane the U.S. actually produces is exceedingly important in our fight against climate change.
For the study, researchers used a bottom-up approach, based on animal inventories and emissions from feed intake. Using this information, they estimated the enteric methane emissions for cattle and then the manure methane emissions for swine, poultry and cattle in the contiguous US.
To be as spatially exact as possible, they split the U.S. into a grid of cells that ranged from 31 square miles to 42 square miles.
"This level of detail enabled us to more accurately assess agricultural methane emissions based on activities involving livestock," said lead researcher Alex Hristov.
"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."
Overall, the study found US livestock produce 19.6 billion pounds of methane emissions per year.
"Our analysis showed that the EPA's estimates are close to reality," said Hristov.
But not quite so fast. While the study appears to largely agree with EPA emission estimates (apart from a few discrepancies in the spatial distribution of the emissions), the researchers admit there is a high level of uncertainty surrounding the results.
That's because predicting methane emissions from manure is complex. There are a ton of little factors that can affect how much methane manure produces, like the composition of the manure, the type of storage, the environment and temperature. Unfortunately, complexity creates breeds uncertainty.
"If methane emissions from livestock in this country really are twice as high as what is estimated now—and we don't believe they are—that would put a big target on agriculture to take measures to cut these emissions," said Hristov.
"Having an accurate and spatially explicit assessment of methane emissions from livestock is critical for reconciliation of top-down and bottom-up approaches, and it's the starting point in any mitigation effort."
The research was published in Environmental Science and Technology.