While efforts to avert disastrous climate change zoom in on cars, industry and power plants belching harmful gases into the air, a potent source of global warming is stealthily stewing underground: rotting trash.

Landfills packed with decomposing dinner leftovers and grass clippings are among the world's top sources of methane -- one of the most powerful heat-trapping gases contributing to the Earth's warming.

The worst dumping grounds "are places where climate change is being caused on a very great scale," David Newman, who heads the International Solid Waste Association, said on the sidelines of the UN conference in Paris where a climate rescue pact is edging towards possible agreement.

Curbing emissions, mainly from burning oil, coal and gas, is a key thrust of the 195-nation talks billed as the last chance to avert worst-case-scenario global warming.

But scientists estimate that dumps produce at least 10 percent of man-made methane, making it the world's third biggest source of the gas after energy production and agriculture.

Methane has a much shorter lifespan in the atmosphere than carbon dioxide (CO2) -- the most abundant greenhouse gas -- but traps about 20 times more per unit of the heat radiated from Earth's surface.

Rotting garbage in landfills emits the gas because it is buried and therefore decomposes without oxygen -- anaerobically. A backyard compost pile exposed to air would generally not produce large quantities of the gas.
Experts warn the methane is seeping out of landfills around the world despite efforts to boost recycling and cut waste.

In the European Union, more than 100 million tonnes of trash are dumped every year, though the 28-nation bloc has issued a directive to limit the volumes sent to landfills by 2025.

The United States, China, India and many other nations send millions more tonnes of their waste to the dump annually for burial.

Veerabhadran Ramanathan, a climate science expert at University of California, San Diego, said tackling methane is key to limiting global warming to the UN ceiling of two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels.

"You can't do it with (cutting) CO2 alone, we have lost that luxury," he said. "It's too late -- you need to bring in these other pollutants."

- 'Uncontrolled methane' -

How to deal with landfill gas -- up to 60 percent methane -- has long been a problem for dump operators.

Gas has been known to escape from dumps, leak into homes and explode, notably in Britain and Denmark.

Many landfills now collect the gas, burn it off or use it to make electricity.    
For example, gas from Shanghai's Laogang landfill, one of China's largest, will provide enough electricity per year for 100,000 families, said Gary Crawford, a vice president of international affairs at water, waste and energy giant Veolia.   

"The project contributes to significant greenhouse gas emission reductions, over 700,000 tonnes of CO2 equivalent in 2014," he said.

Those emissions are on par with the pollution emitted by 147,300 cars in a year.

However, environmentalists have been critical of using landfill gas as an energy source. American advocacy group Sierra Club issued a report in 2010 saying burning methane results in a net increase in pollution.

Sierra Club said it is better to keep organic waste out of landfills "so that uncontrolled methane is not generated in the first instance".

The benefits of generating electricity in this way, it said, are outweighed by methane escaping into the air.

Ramanathan stressed that methane emissions can be largely averted.  

"CO2 is an inescapable consequence of burning fossil fuels," he said, but for methane, "there are ways to avoid" it.