(Adds detail on top 10 emitters)

By Kate Abnett, Valerie Volcovici, Ilze Filks and Jeff Mason

GLASGOW, Nov 2 (Reuters) - More than 100 countries have joined an effort led by the United States and European Union to slash emissions of the potent greenhouse gas methane 30% by 2030 from 2020 levels, an initiative aimed at tackling one of the main causes of climate change.

Methane https://www.reuters.com/business/environment/global-watchdog-track-promised-cuts-potent-greenhouse-gas-methane-2021-10-31 is the main greenhouse gas after carbon dioxide. It has a higher heat-trapping potential than CO2 but breaks down in the atmosphere faster - meaning that cutting methane emissions can have a rapid impact in reining in global warming.

The Global Methane Pledge, launched at the COP26 summit in Glasgow on Tuesday after being announced in September, now covers countries representing nearly half of global methane emissions and 70% of global GDP, U.S. President Joe Biden said.

"Together, we're committing to collectively reduce our methane by 30% by 2030. And I think we can probably go beyond that," Biden said at the COP26 summit in Glasgow, Scotland.

"It's going to boost our economies, saving companies money, reducing methane leaks, capturing methane to turn it into new revenue streams, as well as creating good paying union jobs for our workers."

Among the new signatories was Brazil - one of the world's biggest emitters of methane. The pledge now includes six of the world's 10 biggest methane emitters: the United States, Brazil, Indonesia, Nigeria, Pakistan and Mexico.

China, Russia, India and Iran, also top-10 methane emitters, have not signed up. Those countries were all included on a list identified as targets to join the pledge, first reported by Reuters.

"Methane is one of the gases we can cut fastest. Doing that will immediately slow down climate change," European Commission President Ursula von der Leyen said.

WIDENING THE PACT

Since it was first announced in September with a handful of signatories, the United States and European Union have worked to get the world's biggest methane emitters to join the partnership.

There were roughly 60 countries signed up only last week, after a final diplomatic push from the United States and EU ahead of the COP26 summit https://www.reuters.com/business/cop.

While it is not part of the formal U.N. negotiations, the methane pledge could rank among the most significant outcomes from the COP26 conference, given its potential impact in holding off disastrous climate change.

A U.N. report in May said steep cuts in methane emissions this decade could avoid nearly 0.3 degree Celsius of global warming by the 2040s. Failing to tackle methane, however, would push out of reach the 2015 Paris Agreement's objective to limit the global rise in temperature to 1.5C above pre-industrial levels and avoid the worst impacts of climate change.

The 30% methane cut, which is not legally binding, would be jointly achieved by the signatories, and cover all sectors. Key sources of methane emissions include leaky oil and gas infrastructure, old coal mines, agriculture and landfill sites.

If fulfilled, the pledge is likely to have the biggest impact on the energy sector, since analysts say fixing leaky oil and gas infrastructure is the fastest and cheapest way to curb methane emissions.

The United States is the world's biggest oil and natural gas producer, while the EU is the biggest importer of gas.

The United States on Tuesday unveiled its own sweeping proposal https://www.reuters.com/business/environment/us-unveils-crackdown-methane-starting-with-oil-gas-rules-2021-11-02 to crack down on methane emissions with a focus on the oil and gas sector. The main regulation could take effect as soon as 2023 and slash methane from oil and gas operations by 74% from 2005 levels by 2035, according to the U.S. Environmental Protection Agency.

The EU and Canada both plan to unveil methane legislation addressing the energy sector later this year.

(Reporting by Kate Abnett in Brussels, Valerie Volcovici in Washington, Ilze Filks and Jeff Mason in Glasgow Editing by Matthew Lewis and Alistair Bell)