"It's one minute to midnight and we need to act now," British Prime Minister Boris Johnson was due to tell them, according to extracts from his speech.
"If we don't get serious about climate change today, it will be too late for our children to do so tomorrow."
Observers had hoped a weekend meeting in Rome of leaders of the G20 nations, which between them emit nearly 80 percent of global carbon emissions, would give a strong impetus to the Glasgow COP26 summit, which was postponed for a year due to the pandemic.
The G20 major economies committed on Sunday to the key goal of 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 this did not convince NGOs, the British prime minister or the United Nations.
"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.
'Going to be very difficult'
"We've inched forward (at the G20). We've put ourselves in a reasonable position for COP in Glasgow but it's going to be very difficult in the next few days," Johnson said Sunday, before warning: "If Glasgow fails, then the whole thing fails."
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 signatories of the Paris agreement -- 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 as he opened the meeting 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 tomorrow," said Namrata Chowdhary from the NGO 350.org.
Eyes on India
While 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, 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 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 cough up $100 billion a year starting in 2020 to help developing nations lower emissions and adapt -- a pledge first made in 2009.
This 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.
"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, also denouncing the refusal of large economies to abandon coal.
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, particularly for those on the front line of the climate crisis who are already suffering the consequences in a world that has heated up by about 1.1 degrees Celsius.
Despite everything, it seems that some are not afraid, or worse, that they are indifferent, she said.
'Not next year. Now'
Her words are likely to find an echo in the speeches from African and Pacific leaders on Monday and Tuesday.
While the Chinese and Russian presidents are not expected in person, dozens of other heads of state and government from US President Joe Biden to EU leaders and Australia's Scott Morrison are making the trip to Glasgow.
Their words and actions will be closely scrutinised, in particular by the young activists who travelled to Scotland despite the obstacles due to the pandemic.
"As citizens across the planet, we urge you to face up to the climate emergency," they said in an open letter from several of them, including Swedish activist Greta Thunberg, who arrived Sunday by train.
"Not next year. Not next month. Now."