New research from the United States has found unexpectedly large methane emissions from cities.
Agriculture - particularly livestock farming - has been previously identified as the major single source of human-influenced sources of methane.
Some studies have identified livestock farming (mostly beef and dairy farming) as contributing about 40 per cent of annual methane production.
Rice production, gas and oil drilling sites, and waste in landfills and sewage treatment plants have been identified as other major human-influenced sources of methane.
But this latest research suggests cities may be a larger source of methane than previously identified.
Measurements taken by aircraft flying over five major US cities have found unexpectedly large methane emissions.
Recorded methane emissions were twice as high as the United States Environment Protection Authority estimates.
Researchers said they suspected most of the extra methane came from leakage from old cast-iron pipes and losses from inefficient appliances.
A second study looking at Los Angeles found methane emissions peaked in winter, which appeared to be related to residential and commercial natural gas use rather than from gas-fired power plants.
In the first study, reported in the American Geophysical Union journal Geophysical Research Letters, an instrumented airplane took 20 research flights between April 8, 2018, and May 12, 2018, totalling 120 flight hours.
The flights covered Washington, Baltimore, Philadelphia, New York, Providence and Boston
It found unexpectedly large emissions - twice the total amount of methane and almost 10 times the amount estimated from natural gas.
"The amount of natural gas that flows into these older cities is very large," lead author Genevieve Plant, of the University of Michigan, said.
"That means leakage from old cast-iron pipes and 'end-use' losses from inefficient appliances could be potentially be significant.
"We found methane emissions in the five largest cities that we sampled are significant - about double what the EPA estimates for the total emissions."
National Oceanic and Atmospheric Administration scientist Colm Sweeney said a recent study estimated emissions from natural gas fields to the edge of the cities were 60pc higher than EPA estimates.
But that study was not focused on examining the loss during distribution and end-use in homes and businesses.
The new study directly measured downwind plumes of emissions coming from these five cities, which included leaks from pipes and appliances.
This allowed scientists to quantify the total amount of methane coming from the city.
Scientists analysed the ratio of ethane to methane to calculate how much methane was due to natural gas leakage.
"There's a real lack of data on urban emissions, including end-use losses," Dr Sweeney said.
"In our study, we found leaks and end-use losses in just five cities was larger than what EPA estimates for leaks in the entire country.
"That was kind of stunning.
"The problem is we still do not exactly where it is coming from. Is it the pipes? Is it the appliances?
"What we do know is that this provides a huge opportunity for emissions reduction if we can determine the source of these leaks"
While there have been a handful of other studies of urban areas scattered around the country, this study sampled one of the largest metropolitan megaplexes, home to about 12pc of the US population.
This story first appeared on Australian Dairyfarmer
