Antonio Guterres' figure shatters temperature targets of Paris climate deal, which aimed for heating well below 2C and preferably capped at 1.5C above pre-industrial levels.
A failure to slash global emissions is setting the world on a "catastrophic" path to 2.7 degrees Celsius heating, UN chief Antonio Guterres has warned, just weeks before crunch climate talks.
His comments on Friday come as a United Nations report on global emissions pledges found instead of the reductions needed to avoid the worst effects of the climate crisis, they would see "a considerable increase".
This shows "the world is on a catastrophic pathway to 2.7-degrees of heating," Guterres said in a statement.
The figure shatters the temperature targets of the Paris climate agreement, which aimed for warming well below 2C and preferably capped at 1.5C above pre-industrial levels.
"Failure to meet this goal will be measured in the massive loss of lives and livelihoods," Guterres said.
Nations slow-walk their responses
Under the landmark 2015 Paris deal, nations committed to slashing emissions, as well as to provide assistance to the most climate-vulnerable countries.
But the window for action is narrowing as nations slow-walk their responses.
Last month a bombshell "code red" for humanity from the Intergovernmental Panel on Climate Change (IPCC) warned that Earth's average temperature will be 1.5C higher around 2030, a decade earlier than projected only three years ago.
"We have to act, all of us, we have to act now," said US President Joe Biden on Friday, urging the world to bring its "highest" ambition to the UN climate conference in Glasgow in November.
"Those who have not yet done so, time is running out," Biden said in the White House at the start of a virtual summit with nine foreign leaders.
With only 1.1C of warming so far, the world has seen a torrent of deadly weather disasters intensified by the climate crisis in recent months, from asphalt-melting heatwaves to flash floods and untameable wildfires.
The IPCC says emissions should be around 45 percent lower by 2030 compared with 2010 levels to meet the 1.5C goal.
But current pledges by 191 countries would see emissions 16 percent higher at end of the decade than in 2010 – a level that would eventually cause the world to warm 2.7C.
"Overall greenhouse gas emission numbers are moving in the wrong direction," said UN climate chief Patricia Espinosa in a press conference.
But she said there was a "glimmer of hope" from 113 countries that had updated their pledges, including the United States and European Union.
These new pledges, known as Nationally Determined Contributions (NDCs), would see their emissions reduced 12 percent by 2030 compared to 2010.
The Paris deal included a "ratchet" mechanism for countries to review and toughen up their climate pledges every five years.
Despite an end of the 2020 deadline, many major emitters have yet to issue new targets.
That includes China – the world's biggest emitter – which has said it will reach net zero emissions by 2060 but has not yet delivered its NDC to spell out emissions reductions by 2030.
Meanwhile, updates from Brazil and Mexico were actually weaker than pledges they submitted five years ago, according to an analysis by the World Resources Institute.
The UN report was a "damning indictment" of global progress on climate, particularly by G20 nations, responsible for around 80 percent of emissions, said Mohamed Adow, who leads the think tank Power Shift Africa.
"They are the countries which have caused this crisis and yet are failing to show the leadership required to lead us out of this mess," he said.
Time to 'deliver'
Another unfulfilled pledge will be a flashpoint at the Glasgow summit – the promise by wealthy nations to provide annual climate funding of $100 billion from 2020 to poorer countries, who bear the greatest impact of heating.
The Organisation for Economic Co-operation and Development on Friday said progress was "disappointing", with developing countries receiving $79.6 billion in 2019.
It warned that the 2020 target would be missed.
"The fight against climate change will only succeed if everyone comes together to promote more ambition, more cooperation, and more credibility," said Guterres.
"It is time for leaders to stand and deliver or people in all countries will pay a tragic price."