* COP26 aims to secure tougher measures to cut CO2 emissions
* Conference begins with afternoon speeches
* Weekend G20 summit failed to set positive tone
* Thunberg tells leaders: 'It's code red for the Earth' (Adds quotes, background)
By Mark John and Katy Daigle
GLASGOW, Nov 1 (Reuters) - A U.N. conference https://www.reuters.com/business/cop critical to averting the most disastrous effects of climate change opened on Monday, with world leaders, environmental experts and activists pleading for decisive action to halt global warming.
The task of the COP26 conference in the Scottish city of Glasgow was made even more daunting by the failure of the Group of 20 major industrial nations to agree ambitious new commitments at a weekend summit in Rome.
The G20 is responsible for around 80% of emissions of carbon dioxide https://www.reuters.com/world/uk/cop26-what-would-success-look-like-climate-summit-2021-10-31 - the gas produced by burning fossil fuels that is the main cause of the heatwaves, droughts, floods and storms that are growing in intensity worldwide.
"Humanity has long since run down the clock on climate change. It's one minute to midnight on that Doomsday clock and we need to act now," British Prime Minister Boris Johnson told the opening ceremony.
U.N. Secretary-General Antonio Guterres reminded the conference hall that the six hottest years on record have occurred since 2015.
Other speakers, including activists from the poorer countries hardest hit by climate change, delivered a defiant message.
"Pacific youth have rallied behind the cry 'We are not drowning, we are fighting'," said Brianna Fruean from the Polynesian island state of Samoa, at risk from rising sea levels. "This is our warrior cry to the world."
As Johnson took the stage, Swedish activist Greta Thunberg retweeted an appeal for her millions of supporters to sign an open letter accusing leaders of betrayal.
"This is not a drill. It's code red for the Earth," it read. "Millions will suffer as our planet is devastated -- a terrifying future that will be created, or avoided, by the decisions you make. You have the power to decide."
In Rome, the G20 leaders 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 - badly undermining one of COP26's main aims.
Instead, they only recognised "the key relevance" of halting net emissions "by or around mid-century", and set no timetable for phasing out domestic coal power, a major cause of carbon emissions.
Their commitment to phase out fossil fuel subsidies "over the medium term" echoed wording used by the G20 at a summit in Pittsburgh as long ago as 2009.
Discord among some of the world's biggest emitters about how to cut back on coal, oil and gas will not make their task easier.
At the G20, U.S. President Joe Biden singled out China and Russia, neither of which sent its leader to Glasgow, for not bringing proposals to the table.
He told the conference: "Glasgow must be the start of a decade of shared ambition and innovation to preserve our future."
Chinese President Xi Jinping, whose country is by far the biggest emitter of greenhouse gases, ahead of the United States, told the conference in a written statement that developed countries should not only do more but also support developing countries to do better.
President Vladimir Putin of Russia, one of the world's top three oil producers along with the United States and Saudi Arabia, dropped plans to participate in any talks live by video link, the Kremlin said.
Less senior delegates - many of them held up on Sunday by disruptions to trains between London and Glasgow - had more mundane problems.
More than a thousand had to shiver https://www.reuters.com/business/environment/queuing-save-world-delegates-head-into-cop26-summit-venue-2021-11-01 for over an hour in a bottleneck outside the venue to present proof of a negative COVID-19 test and gain access, while being treated by activists to an electronic musical remix of Thunberg's past speeches.
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 (2.7 Fahrenheit) above pre-industrial levels https://www.reuters.com/business/cop/paris-glasgow-cutting-through-climate-jargon-2021-10-27.
To do that, it needs to secure more ambitious pledges https://www.reuters.com/world/uk/cop26-what-would-success-look-like-climate-summit-2021-10-31 to reduce emissions, lock in billions in climate-related financing https://www.reuters.com/business/environment/climate-finance-could-make-or-break-cop26-summit-heres-why-2021-11-01 for developing countries, and finish the rules for implementing the 2015 Paris Agreement, signed by nearly 200 countries.
"Climate financing" could make or break the talks. In 2009, the rich nations most responsible for global warming pledged to provide $100 billion per year by 2020 to help developing countries deal with its consequences.
The commitment has still not been met, generating mistrust and a reluctance among some developing nations to accelerate their emissions reductions.
Barbados Prime Minister Mia Mottley compared the vast sums pumped into the global economy by rich countries' central banks in recent years with the insufficient amounts spent on climate help for poor nations.
"Our people are watching and our people are taking note ... Can there be peace and prosperity if one-third of the world lives in prosperity and two-thirds lives under seas and face calamitous threats to our wellbeing?" she told the conference.
Developed countries confirmed last week they would be three years late in meeting the $100 billion climate finance pledge - which many poor countries and activists say is insufficient anyway.
The pledges made so far 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.
Two days of speeches by world leaders will be followed by technical negotiations. Any deal may not be struck until close to or even after the event's Nov. 12 finish date.
(Reporting by Elizabeth Piper and Jeff Mason; writing by Mark John, Kevin Liffey and Gavin Jones; editing by Barbara Lewis and Alexander Smith)
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