(AFP) - G20 leaders on Sunday committed to the key goal of limiting global warming to 1.5 degrees Celsius, but campaigners slammed a "lack of ambition" as make-or-break UN climate talks opened in Glasgow.

Following a two-day meeting in Rome, the Group of 20 major economies agreed that keeping that goal -- which had initially been raised in the landmark 2015 Paris Agreement -- would require "meaningful and effective actions", according to the final summit statement.

The leaders, whose nations between them emit nearly 80 percent of carbon emissions, also promised action on coal, but failed to set a clear target on another key goal, to reach "net zero" emissions.

"We're proud of these results but we must remember that it's only the start," said Italian Prime Minister Mario Draghi, host of the talks.

Earlier, the COP26 climate talks opened in Scotland with a warning by summit president Alok Sharma that they were the "last, best hope" to keep the goal of limiting global warming to 1.5 Celsius above pre-industrial levels.

As an accelerating onslaught of extreme weather events underscores the devastating impacts of burning fossil fuels, he said: "If we act now and we act together we can protect our precious planet."

- Keep 1.5 within reach -

In the statement, G20 leaders reaffirmed their support for the Paris agreement goals of keeping "the global average temperature increase well below 2 degrees and to pursue efforts to limit it to 1.5 degrees above pre-industrial levels".

They added that "keeping 1.5 degrees within reach will require meaningful and effective actions and commitment by all countries, taking into account different approaches", through the development of national plans that "align long-term ambition with short- and medium-term goals".

But experts say meeting the 1.5 degree target means slashing global emissions nearly in half by 2030 and to "net-zero" by 2050 -- and the G20 set no firm date, speaking only of reaching the goal of net zero "by or around mid century".

The leaders did agree to end funding for new unabated coal plants abroad -- those whose emissions have not gone through any filtering process -- by the end of 2021.

- Lacking ambition -

Environmental campaign group Greenpeace slammed the final statement as "weak, lacking both ambition and vision", saying G20 leaders "failed to meet the moment".

"If the G20 was a dress rehearsal for COP26, then world leaders fluffed their lines," said Executive Director Jennifer Morgan.

Friederike Roder, senior director at anti-poverty group Global Citizen, told AFP the summit had produced "half-measures rather than concrete actions".

However, French President Emmanuel Macron had earlier said it was too early to write off the success of the talks in Glasgow.

More than 120 heads of state and government, including US President Joe Biden, India's Narendra Modi and Australia's Scott Morrison, are heading to the Scottish city.

"Let's not forget that in Paris, in 2015, nothing was decided in advance," Macron told the weekly Journal du Dimanche.

China -- by far the world's biggest carbon polluter -- plans to make its economy carbon neutral before 2060, but has resisted pressure to offer nearer-term goals.

India, meanwhile, argues that if net-zero by 2050 is the global goal, then rich countries should be carbon neutral ten years earlier to allow poorer, emerging nations a larger carbon allowance and more time to develop.

- 'Dream big' -

Earlier Sunday, Draghi, Prince Charles and Pope Francis had all called on G20 leaders -- and by extension, the wider group of world leaders meeting in Glasgow -- to think big.

Calling climate change "the defining challenge of our times", Draghi warned: "Either we act now... or we delay acting, pay a much higher price later, and risk failing."

Prince Charles, a committed environmentalist and summit guest, had also reminded leaders of their "overwhelming responsibility" to keep climate change in check.

Pope Francis, who is outspoken on the issue and received several G20 leaders at the Vatican this weekend, said: "This is a moment to dream big, to rethink our priorities... The time to act, and to act together, is now!"

© Agence France-Presse