UN Secretary General Antonio Guterres gave a warning on Saturday that global climate targets agreed in France five years ago were not being met and mankind was heading for a "catastrophic temperature rise".
Co-hosting talks with the UK and France on the anniversary of the 2015 Paris climate deal, Mr Guterres said carbon emissions were still far too high and were causing a planet-wide “dramatic emergency”.
There is hope that recent zero-carbon pledges from big economies, including the incoming Biden administration in the US, but some experts and campaigners say emission cuts are not fast or deep enough.
“Carbon dioxide levels are at record highs,” said Mr Guterres.
“Today, we are 1.2 degrees hotter than before the industrial revolution. If we don’t change course, we may be headed for a catastrophic temperature rise of more than 3 degrees this century.”
The 2015 Paris Agreement has a legally-binding aim of keeping global warming under 2 degrees Celsius over pre-industrial times and to strive to limit the temperature rises to 1.5°C – a target that is now almost beyond reach.
The UN chief said mankind was "not doomed to fail", but urged the world's so-called 20 biggest economies to focus harder on cutting carbon emissions to zero and funding climate mitigation in poorer countries.
“The members of the G20 are spending 50 per cent more in their stimulus and rescue packages [for the coronavirus pandemic] on sectors linked to fossil fuel production and consumption, than on low-carbon energy,” said Mr Guterres.
To meet the 2015 Paris goals, countries must cut emissions of carbon dioxide and other heat-trapping gases. To become carbon neutral, a country must cut emissions to zero, or offset any shortfalls by planting trees or using carbon capture and storage technology.
The UN chief, British Prime Minister Boris Johnson and French President Emmanuel Macron co-hosted the online summit of 75 world leaders to mark the anniversary of Paris and to help tackle rising temperatures ahead of talks in Glasgow, Scotland, in November 2021.
The summit featured video messages from Chinese President Xi Jinping, Indian Prime Minister Narendra Modi, Pope Francis, Tim Cook, CEO of Apple, and others, interspersed with uplifting videos about climate change.
Before the talks on Friday, Britain said it would stop providing money for oil, gas or coal projects overseas.
"Climate change is one of the great global challenges of our age, and it is already costing lives and livelihoods the world over, our actions as leaders must be driven not by timidity or caution, but by ambition on a truly grand scale," said Mr Johnson.
There are other reasons for hope, said Mr Guterres.
Britain, China, Japan, South Korea, the European Union and the US under a Biden Administration are committed to carbon neutrality, the UN says. Together, they account for two-thirds of all carbon dioxide emissions and more than 70 per cent of the world economy.
Ahead of his inauguration of January 20, US President-elect Joe Biden has said he will bring the US back into the Paris deal that the US withdrew from under President Donald Trump, who once called climate change a hoax.
"I'll immediately start working with my counterparts around the world to do all that we possibly can, including by convening the leaders of major economies for a climate summit within my first 100 days in office," Mr Biden said on Saturday.
The President-elect has also appointed John Kerry, a former US Secretary of State and one of the architects of the Paris accord, as his envoy for the climate – the most senior climate change appointment ever made in US politics.
Still, teenage Swedish climate activist Greta Thunberg was not convinced by “big speeches” at the UN, she said.
In a video to mark the event, Ms Thunberg said leaders were “in denial”, setting “distant hypothetical targets” and carving out “new loopholes” while ice caps melt, forests burn and the world gets hotter.
In a report this week, UN experts urged wealthy countries to come good on pledges to raise $100 billion a year to help poor countries mitigate and adapt to climate change. They called for a doubling of grant funds and other measures.