While this does not mean the internationally-agreed 1.5C warming threshold has been permanently breached, the United Nations warned it was in "grave danger".

"Today's assessment from the World Meteorological Organization (WMO) is clear," UN chief Antonio Guterres said. "Global heating is a cold, hard fact."

He added: "Blazing temperatures in 2024 require trail-blazing climate action in 2025. There's still time to avoid the worst of climate catastrophe. But leaders must act -- now."

The WMO said six international datasets all confirmed that 2024 was the hottest year on record, extending a decade-long "extraordinary streak of record-breaking temperatures".

The United States became the latest country to report its heat record had been shattered, capping a year marked by devastating tornadoes and hurricanes.

The announcement came just days before President-elect Donald Trump, who has pledged to double down on fossil fuel production, was set to take office.

Excess heat is supercharging extreme weather, and 2024 saw countries from Spain to Kenya, the United States and Nepal suffer disasters that cost more than $300 billion by some estimates.

Los Angeles is currently battling deadly wildfires that have destroyed thousands of buildings and forced tens of thousands to flee their homes.

- 'Stark warning' -

Another record-breaking year is not anticipated in 2025, as a UN deadline looms for nations to commit to curbing greenhouse gas emissions.

"My prediction is it will be the third-warmest year," said NASA's top climate scientist Gavin Schmidt, citing the US determination that the year has begun with a weak La Nina, a global weather pattern that is expected to bring slight cooling.

The WMO's analysis of the six datasets showed global average surface temperatures were 1.55C above pre-industrial levels.

"This means that we have likely just experienced the first calendar year with a global mean temperature of more than 1.5C above the 1850-1900 average," it said.

Europe's climate monitor Copernicus, which provided one of the datasets, found that both of the past two years had exceeded the warming limit set out in the 2015 Paris Agreement.

Global temperatures had soared "beyond what modern humans have ever experienced", it said.

Scientists stressed that the 1.5C threshold in the Paris Agreement refers to a sustained rise over decades, offering a glimmer of hope.

Still, Johan Rockstrom of the Potsdam Institute for Climate Impact Research called the milestone a "stark warning sign."

"We have now experienced the first taste of a 1.5C world, which has cost people and the global economy unprecedented suffering and economic costs," he told AFP.

- On the edge -

Nearly 200 nations agreed in Paris in 2015 that meeting 1.5C offered the best chance of preventing the most catastrophic repercussions of climate change.

But the world remains far off track.

While Copernicus records date back to 1940, other climate data from ice cores and tree rings suggest Earth is now likely the warmest it has been in tens of thousands of years.

Scientists say every fraction of a degree above 1.5C matters -- and that beyond a certain point the climate could shift in unpredictable ways.

Human-driven climate change is already making droughts, storms, floods and heat waves more frequent and intense.

The death of 1,300 pilgrims in Saudi Arabia during extreme heat, a barrage of powerful tropical storms in Asia and North America, and historic flooding in Europe and Africa marked grim milestones in 2024.

- 'Stark warning' -

The oceans, which absorb 90 percent of excess heat from greenhouse gases, warmed to record levels in 2024, straining coral reefs and marine life and stirring violent weather.

Warmer seas drive higher evaporation and atmospheric moisture, leading to heavier rainfall and energizing cyclones.

Water vapour in the atmosphere hit fresh highs in 2024, combining with elevated temperatures to trigger floods, heatwaves and "misery for millions of people", Copernicus climate deputy director Samantha Burgess said.

Scientists attribute some of the record heat to the onset of a warming El Nino in 2023.

But El Nino ended in early 2024, leaving them puzzled by persistently high global temperatures.

"The future is in our hands -- swift and decisive action can still alter the trajectory of our future climate," said Copernicus climate director Carlo Buontempo.

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