World leaders began arriving on Monday at a UN conference critical to averting the most disastrous effects of climate change.
The COP26 conference in the Scottish city of Glasgow opens a day after the G20 economies failed to commit to a 2050 target to halt net carbon emissions - a deadline widely cited as necessary to prevent the most extreme global warming.
Instead, their talks in Rome only recognised "the key relevance" of halting net emissions "by or around mid-century", set no timetable for phasing out coal at home and watered down promises to cut emissions of methane, a greenhouse gas many times more powerful than carbon dioxide.
Swedish activist Greta Thunberg asked her millions of supporters to sign an open letter accusing leaders of betrayal.
"As citizens across the planet, we urge you to face up to the climate emergency," she tweeted. "Not next year. Not next month. Now."
Many of those leaders take to the stage in Glasgow on Monday to defend their records and in some cases make new pledges at the start of two weeks of negotiations that conference host Britain is billing as make-or-break.
"Humanity has long since run down the clock on climate change. It's one minute to midnight and we need to act now," British Prime Minister Boris Johnson will tell the opening ceremony, according to advance excerpts of his speech.
"If we don't get serious about climate change today, it will be too late for our children to do so tomorrow."
Discord among some of the world's biggest emitters about how to cut back on coal, oil and gas, and help poorer countries to adapt to global warming, will not make the task easier.
At the G20, US President Joe Biden singled out China and Russia, neither of which is sending its leader to Glasgow, for not bringing proposals to the table.
US National Security Adviser Jake Sullivan, on board Air Force One with Biden, said Glasgow could put pressure on those who had not yet stepped up, but that it would not end the global effort.
"It is also critical for us to recognise that the work is going to have to continue after everyone goes home," he told reporters.
Chinese President Xi Jinping, whose country is by far the biggest emitter of greenhouse gases and ahead of the United States, will address the conference on Monday in a written statement, according to an official schedule.
President Vladimir Putin of Russia, one of the world's top three oil producers along with the United States and Saudi Arabia, has dropped plans to participate in any talks live by video link, the Kremlin said.
Turkish President Tayyip Erdogan will also stay away. Two Turkish officials said Britain had failed to meet Ankara's demands on security arrangements and protocol.
Delayed by a year because of the COVID-19 pandemic, COP26 aims to keep alive a target of capping global warming at 1.5 degrees Celsius above pre-industrial levels - a level scientists say would avoid its most destructive consequences.
To do that, it needs to secure more ambitious pledges to reduce emissions, lock in billions in climate-related financing for developing countries, and finish the rules for implementing the 2015 Paris Agreement, signed by nearly 200 countries.
Existing pledges to cut emissions would allow the planet's average surface temperature to rise 2.7C this century, which the United Nations says would supercharge the destruction that climate change is already causing by intensifying storms, exposing more people to deadly heat and floods, raising sea levels and destroying natural habitats.
Developed countries confirmed last week that they would be three years late in meeting a promise made in 2009 to provide $100 billion a year in climate finance to developing countries by 2020.
"Africa is responsible for only 3 per cent of global emissions, but Africans are suffering the most violent consequences of the climate crisis," Ugandan activist Evelyn Acham told the Italian newspaper La Stampa.
"They are not responsible for the crisis, but they are still paying the price of colonialism, which exploited Africa's wealth for centuries," she said. "We have to share responsibilities fairly."
Two days of speeches by world leaders starting Monday will be followed by technical negotiations. Any deal may not be struck until close to or even after the event's November 12 finish date.