British Prime Minister Boris Johnson welcomed more than 120 world leaders to historic climate talks in Glasgow Monday with the stark warning: "It's one minute to midnight, and we need to act now."

Global heads of state and government were convening for a two-day summit at the start of the UN's COP26 climate conference, with US President Joe Biden set to jet in from separate G20 talks in Rome.

The G20 members including China, India and Western nations collectively emit nearly 80 percent of global carbon emissions, but campaigners' hopes for more decisive action heading in to COP26 were dashed.

The G20 economies committed on Sunday to limiting global warming to 1.5 degrees Celsius above pre-industrial levels -- the most ambitious target of the landmark 2015 Paris Agreement.

They also agreed to end funding for new "unabated" coal plants abroad -- those whose emissions have not gone through any filtering process -- by the end of 2021.

But how to get to 1.5 was left largely undefined -- and the COP26 negotiators were left with yawning blanks to fill in over the next fortnight.

"While I welcome the G20's recommitment to global solutions, I leave Rome with my hopes unfulfilled -- but at least they are not buried," UN Secretary General Antonio Guterres said on Twitter.

Richer countries have failed to meet a $100 billion annual target for climate finance by 2020. Poorer nations like India want more cash to move past their reliance on coal and other fossil fuels.

Johnson kicks off the Glasgow summit from 1200 GMT, having admitted to a "road to Damascus" conversion to the threat of climate change.

"It's one minute to midnight and we need to act now," Johnson was due to tell them in his keynote speech, according to Downing Street.

"If we don't get serious about climate change today, it will be too late for our children to do so tomorrow."

'Need to do better'

The Glasgow gathering, which runs until November 12, comes as an accelerating onslaught of extreme weather events across the world underscores the devastating impacts of climate change from 150 years of burning fossil fuels.

The current commitments of the Paris signatories -- if they were followed -- would still lead to a "catastrophic" warming of 2.7 Celsius, according to the UN.

COP26 marks the "last, best hope to keep 1.5C in reach", summit president Alok Sharma said on Sunday.

"If we act now and we act together, we can protect our precious planet," he said.

Climate advocacy groups expressed disappointment at the statement released at the end of the G20 summit.

"These so-called leaders need to do better. They have another shot at this: starting (Monday)," said Namrata Chowdhary from the NGO 350.org.

Eyes on India

China, by far the world's biggest carbon polluter, has just submitted to the UN its revised climate plan, which repeats a long-standing goal of peaking emissions by 2030.

But President Xi Jinping is absent from Glasgow, and India is now at the centre of expectations.

India has yet to submit a revised "nationally determined contribution".

But if Prime Minister Narendra Modi announces new efforts to curb emissions in his own speech Monday, it could put more pressure on China and others, said Alden Meyer, a senior associate at climate and energy think tank E3G.

"If he feels confident enough that there's going to be financing and technology assistance from Europe, the US, Japan and others, he might signal that India is willing to update its NDC," Meyer said.

Another pressing issue is the failure of rich countries to deliver on their $100 billion pledge, which was first made in 2009.

The goal has been postponed to 2023, exacerbating the crisis of confidence between the North, responsible for global warming, and the South, which is the victim of its effects.

'Not next year. Now'

"Climate finance is not charity. It is a question of justice," stressed Lia Nicholson, on behalf of the Alliance of Small Island States vulnerable to climate change.

Forecasts by the UN climate experts panel (IPCC) that the threshold of a 1.5 Celsius increase could be reached 10 years earlier than expected, around 2030, are "terrifying", she said.

"Climate finance is not charity. It is a question of justice," stressed Lia Nicholson, on behalf of the Alliance of Small Island States vulnerable to climate change.

Forecasts by the UN climate experts panel (IPCC) that the threshold of a 1.5 Celsius increase could be reached 10 years earlier than expected, around 2030, are "terrifying", she said.

The leaders' words will be closely scrutinized in particular by a group of young activists who travelled to Scotland by train despite obstacles due to the coronavirus pandemic, which has entailed strict health protocols at COP26.

"As citizens across the planet, we urge you to face up to the climate emergency," they said in an open letter signed by Swedish activist Greta Thunberg among others.

"Not next year. Not next month. Now."