A decade-long surge of the potent greenhouse gas methane threatens to make the fight against global warming even harder, top researchers warned Monday.

"Additional attention is urgently needed to quantify and reduce methane emissions," they wrote in the Environmental Research Letters journal, summarising the findings of a consortium of 81 scientists.

After rising slowly from 2000 to 2006, the concentration of methane in the air climbed 10 times more quickly the following decade, according to that study, which was published in the peer-reviewed Earth System Science Data.

The unexpected -- and largely unexplained -- increase was especially sharp in 2014 and 2015.

"Keeping global warming below two degrees Celsius (3.6 degrees Fahrenheit) is already a challenging target," they said, referring to the goal set in the 196-nation Paris climate pact, which entered into force last month.

"Such a target will become increasingly difficult if reductions in methane emissions are not also addressed strongly and rapidly."

With only 1 C (1.8 F) of warming above pre-industrial era levels so far, the world has seen an uptick in extreme weather, including droughts, superstorms, heat waves and coastal flooding boosted by rising seas.

On current trends, average global temperatures are on track to jump by more than 3 C (5.4 F) by 2100, even if national carbon-cutting pledges annexed to the Paris Agreement are honoured.

Without those pledges, the increase would be much higher.

- Trapping the sun's heat -

To date, efforts to keep the planet from overheating have focused mostly on the main greenhouse gas carbon dioxide, a by-product of burning fossil fuels that accounts for at least 70 percent of warming.

But even as CO2 output has started to plateau, methane (CH4) -- responsible for about 20 percent of the increase in global temperatures -- is soaring.

Indeed, the pace of recent emissions aligns with the most pessimistic scenarios laid out by the UN's top science authority, the Intergovernmental Panel on Climate Change.

CH4 is 28 times more efficient at trapping the sun's heat. As with carbon dioxide, Earth naturally absorbs and releases methane.

But industrialisation and a surging human population have upset a long-standing natural balance, leaving an excess of both heat-trapping gases in the atmosphere.

Even if scientists agree that total emissions of methane are rising sharply, they remain uncertain as to why.

Today, some 60 percent of methane originates from human activity, the rest coming from wetlands and other natural sources.

About a third of human-generated methane is a byproduct of the fossil fuel industry.

- Fracking boom, coal power -

Researchers point to a surge in coal-generated power in China, along with leakage from the natural gas fracking boom in the United States.

"Both these regions are thought to play a role" in the sudden hike, said Marielle Saunois, lead author of the editorial as well as the review, and an assistant professor at the University of Versailles Saint Quentin.

But coal-fired plants and leaks from gas production are not sufficient, and do not jibe with the dramatic increase in the last two years, she told AFP.

A more likely culprit, the study concluded, is livestock production and agriculture (especially rice paddies), which together account for nearly two-thirds of manmade methane emissions.

A third possibility is a slow-down in the natural chemical reaction in the atmosphere that breaks down CH4.

A more frightening prospect -- that climate change has started to unlock massive natural stores of the gas in sub-Arctic permafrost -- has been set aside, said Saunois.

"Right now, it is a very minor factor," she said by phone. "But there's still a high degree of uncertainty, and not necessarily a consensus among scientists."

When it comes to climate change, methane's saving grace is that it is much more short-lived in the atmosphere that CO2.

That means that actions taken to reduce emissions will show rapid results, the researchers said.

"Trends in methane emissions should be taken seriously," said Stefan Schwietzke, an expert at the US National Oceanic and Atmospheric Administration whose own estimates of CH4 output, recently published in Nature, are even higher.

"But we should not forget that we also need to reduce CO2 emissions, no matter what."