Image copyright EPA Image caption Fires globally are in sharp decline

Scientists think they can now better explain the recent surge in methane levels seen in the Earth's atmosphere.

Although only a trace component in the air, CH4 is a greenhouse gas and has been rising rapidly since about 2006.

Tropical wetlands and fossil fuels are suspected as major sources - but the sums do not add up.

Only if methane reductions stemming from fewer global fires are considered can the CH4 budget be made to balance, says a new Nasa-led study.

John Worden from the US space agency's Jet Propulsion Laboratory in Pasadena, California, and colleagues report their work in the journal Nature Communications.

Methane concentration in the atmosphere currently stands just above 1,850 parts per billion (1,850 molecules of CH4 for every billion molecules of air). But in the early 2000s, it hovered around 1,770ppb.

Emissions from oil and gas production, such as losses from fracking operations; and microbial production in wet tropical environments, such as marshes and rice paddies, have been put forward as explanations for the subsequent rise.

However, when their estimated contributions are added together, they exceed the observed changes in the atmosphere.

Image copyright THINKSTOCK Image caption Rice paddies are likely a significant source of the methane increase

Dr Worden's team took another look at the problem by considering the impact of global fires. These are in rapid decline.

Wildfires in particular have been going down as the amount of land under agricultural control has expanded.

The area of the planet burned each year decreased by about 12% between the early 2000s and the more recent period of 2007 to 2014, according to satellite observations. And because fires will emit methane (among a range of combustion products) - if there are fewer fire events, this CH4 source must also be in reverse.

Dr Worden's team calculates that 17 teragrams per year of the methane increase in the atmosphere is due to fossil fuels, another 12 is coming from wetlands or rice farming, while fires are decreasing by about 4 teragrams per year.

The three numbers combine to 25 teragrams a year - the same as the observed increase.

Carbon dioxide is regarded as the main driver behind human-induced warming on Earth, and its rise in concentration continues unabated.

Nonetheless, despite its trace amount in the air, methane is also viewed as a greenhouse gas of concern.

CH4 is about 30 times better than CO2, over a century timescale, at trapping heat in the atmosphere.

Complex computer models are used to try to project how Earth will warm given a certain mix of gases, and right now methane's growth rate is close to a path that would take the world into a very challenging future, scientists say.