Speaking at a university in Brazil's capital, Simon Stiell said global heating was "dangerously high," but that real progress had been made since the landmark Paris Agreement.
He conceded many countries would miss a February 10 deadline to submit their next round of climate plans -- giving them until September to deliver "first-rate" emissions roadmaps.
Brazil is set to host the next global climate conference, COP30, in November.
"We are already headed in the right direction. We just have to implement, and implement more and faster," said the former Grenadan environment minister.
Quickly after his White House return, President Donald Trump announced the US withdrawal from the Paris deal for the second time.
"A country may step back -- but others are already stepping into their place to seize the opportunity, and to reap the massive rewards: stronger economic growth, more jobs, less pollution and far lower health costs, more secure and affordable energy," said Stiell.
He said economic reality would drive action, with climate investment now at $2 trillion.
Self-interest, he said, "above all other factors, is why the clean energy shift is now unstoppable: because of the colossal scale of economic opportunity it presents."
- Climate costs -
Only a handful of countries have so far submitted their climate plans, including Brazil and Britain, with big emitters China and the European Union expected to follow later in the year.
A UN official said that over 170 countries had indicated they were working on their new emissions goals and planned to submit them this year, most of them before COP30.
When the Paris deal was signed ten years ago, the world was heading for 5 degrees Celsius of warming above pre-industrial levels.
That was "a death sentence for humanity as we know it," said Stiell, noting that the current trajectory of 3C was still catastrophic.
The safer limit under the Paris deal is 1.5C, but scientists say that is slipping out of reach.
Last year was the hottest on record, and the combined average temperature of 2023 and 2024 exceeded the 1.5C threshold for the first time.
On Thursday, Europe's climate monitor said last month was the hottest January on record.
Last year's contentious COP29 meeting in Baku ended with richer countries agreeing to provide at least $300 billion annually by 2035 to help poorer nations progress their green transition and build resilience.
The actual need has been estimated at $1.3 trillion in developing countries -- many of whom are facing crushing debts.
Stiell said the focus this year would be to find other sources of money to plug the gap.
He stressed the funding was "not charity" but a way to curb inflation caused by climate disasters.
"Just take rising food prices, which have the fingerprints of climate-driven droughts, floods, and wildfires all over them," he said.
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