The Los Angeles basin from Mt Wilson from where methane emissions were measured NASA/JPL-Caltech

In a first time demonstration of mapping greenhouse gases from Earth-bound sites, a Nasa study has found methane emissions from Los Angeles to be 18 to 61 percent higher than estimates.

Using two years of observations from a mountaintop instrument, the study was able to provide an accurate estimate of total methane emissions, says Nasa.

Over the sprawling 50 by 110 kilometre LA basin, methane emissions were estimated to be around 400,000 tonnes per year.

The common method to give estimates, adds emissions from all known methane sources.

The study used observations by a spectrometer, which counts the number of molecules in the air by measuring the effect of methane and other gases on the spectrum of sunlight.

The spectrometer measures pollutants in the air between it and each site, as also in the clean air above the mountain. The difference gives the amount of methane in the LA basin.

"The instrument is like a stationary satellite," said Clare Wong, a Nasa postdoctoral fellow at JPL and lead author of the new study published in the journal Atmospheric Chemistry and Physics.

The instrument is part of the California Laboratory for Atmospheric Remote Sensing (CLARS), located about 5,700 feet (1,700 meters) above Los Angeles atop Mt Wilson.

Though not designed to look for sources of methane, the study showed that some areas like large landfills seem to be more significant emitters than others.

The mountaintop instrument is part of the pilot Megacities Carbon Project to monitor emissions from urban areas with populations of more than 10 million.

Cities are the source of about 70 percent of the world's carbon emissions, with the 22 megacities responsible for about half of that.

Methane traps even more heat than carbon dioxide and has a stronger effect on global warming in short timescales. Some of the main urban sources include gas pipeline leaks, landfills, waste water treatment plants and transportation while cattle belch is a major rural contributor.

Thawing of permafrost or perennially frozen ground is also a source of methane emissions that arise from the exposing of organic material below. Permafrost contains around 1,700 gigatonnes of carbon.