UN chief Antonio Guterres said on Monday the world is "sleepwalking to climate catastrophe", with major economies allowing carbon pollution to increase when drastic cuts are needed.
The planet-saving goal of capping global warming at 1.5 degrees Celsius is already "on life support," he told a sustainability conference in London.
Keeping 1.5C in play requires a 45% drop in emissions by 2030 and carbon neutrality by mid-century, according to the UN's Intergovernmental Panel on Climate Change (IPCC).
But even if nations honour newly revised pledges under the Paris Agreement, emissions are still set to rise 14% before the decade ends.
"The problem is getting worse," Mr. Guterres said in a pre-recorded video message. "We are sleepwalking to climate catastrophe."
"If we continue with more of the same, we can kiss 1.5C goodbye," he added. "Even two degrees may be out of reach."
His comments came only hours before the 195-nation IPCC kicks off a two-week meeting to validate a landmark report on options for reducing carbon pollution and extracting CO2 from the air.
The report is expected to conclude that CO2 emissions must peak within a few years if the Paris temperature targets are to be met.
Mr. Guterres described COVID recovery spending as "scandalously uneven" and a missed opportunity to accelerate the turn toward clean energy.
The Russian invasion of Ukraine, he added, could further derail climate action with importers locking in fossil fuel dependence as they scramble to replace Russian oil and gas.
"Countries could become so consumed by the immediate fossil fuel supply gap that they neglect or knee-cap (climate) policies," Mr. Guterres said.
"This is madness. Addiction to fossil fuels is mutually assured destruction."
A bombshell report last year from the intergovernmental International Energy Agency (IEA) concluded that a 1.5C world was incompatible with any new oil or gas developments, or new coal-fired power plants.
Breaking with the usual practice of not singling out countries, Mr. Guterres called out Australia and a "handful of holdouts" for failing to lay out "meaningful" near-term plans to slash emissions.
He also said the development needs and economic structures of China, India, Indonesia and other "emerging economies" prevent them from making similar commitments, especially on coal.
Rich nations should provide money, technology and knowhow to help these emerging economies purge coal from their energy portfolios, he added, pointing to a pathbreaking deal for South Africa unveiled at the COP26 climate summit last November in Glasgow.
"Our planet can't afford a climate blame game," he cautioned. "we can't point fingers while the planet burns."
Wealthy nations in the Organisation for Economic Co-operation and Development (OECD) must phase-out coal by 2030, and all other countries by 2040, Mr. Guterres said.
China and India — both heavily reliant on coal — have resisted a full embrace of the 1.5C goal, along with pressure to set more ambitious short-term emissions reduction targets.
Both nations, however, have set long-term "net-zero" goals for carbon neutrality, 2060 for China and 2070 for India.
G20 countries account for about 80% of global greenhouse gas emissions.
A landmark IPCC report on climate impact and humanity's capacity to adapt, published last month, details an atlas of human suffering and warned that far worse is to come.
Unprecedented floods, heatwaves and wildfires seen across four continents in the last year will all accelerate in coming decades even if the fossil fuel pollution is rapidly brought to heel, the report concluded.
Mr. Guterres was addressing a four-day conference organised by The Economist.