The last two years saw average temperatures exceed a critical warming limit for the first time, Europe's climate monitor said Friday, as the UN demanded "trail-blazing" climate action.

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."

"Blazing temperatures in 2024 require trail-blazing climate action in 2025," UN chief Antonio Guterres said in a statement.

"There's still time to avoid the worst of climate catastrophe. But leaders must act — now."

The UN's World Meteorological Organization (WMO) said six international datasets all confirmed that 2024 was the hottest on record, extending a decade-long "extraordinary streak of record-breaking temperatures."

Last year was also the highest on record across the mainland United States, said the National Oceanic and Atmospheric Administration (NOAA), which provided one of the datasets. 

Another record-breaking year is not anticipated in 2025, as climate sceptic Donald Trump takes office, and a deadline looms for nations to commit to curbing rising levels of greenhouse gases.

But scientists predict that 2025 will likely still rank among the top three warmest years in the history books.

This excess heat supercharges extreme weather, and 2024 saw countries from Spain to Kenya, the United States and Nepal hit by disasters that cost more than $300 billion by some estimates.

Los Angeles is battling deadly wildfires that have destroyed thousands of buildings and forced tens of thousands to flee their homes. US President Joe Biden said the fires were the most "devastating" to hit California and were proof that "climate change is real."  

Stark warning

WMO said its consolidated analysis of the six datasets showed that global average surface temperatures were 1.55 degrees Celsius 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 examined, for its part found that both of the past two years had exceeded the warming limit set out in the 2015 Paris Agreement, as global temperatures soar "beyond what modern humans have ever experienced."

"It is important to emphasise that a single year of more than 1.5°C for a year does NOT mean that we have failed to meet Paris Agreement long-term temperature goals, which are measured over decades rather than an individual year" said WMO chief Celeste Saulo.

Johan Rockstrom of the Potsdam Institute for Climate Impact Research said hitting 1.5C was 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 is nowhere on track to meeting that target.

"We are now teetering on the edge of passing the 1.5C level," said Copernicus climate deputy director Samantha Burgess.

Copernicus records go back to 1940 but other sources of climate data, such as ice cores and tree rings, allow scientists to say the Earth today is likely the warmest its been in tens of thousands of years.

Scientists say every fraction of a degree above 1.5C is consequential, and that beyond a certain point the climate could shift in ways that are difficult to anticipate.

At present levels, human-driven climate change is already making droughts, storms, floods and heatwaves 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, a crucial climate regulator 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 mean higher evaporation and greater moisture in the atmosphere, causing heavier rainfall, feeding energy into cyclones and bringing sometimes unbearable humidity.

Water vapour in the atmosphere hit fresh highs in 2024 and combined with elevated temperatures caused floods, heatwaves and "misery for millions of people," Burgess said.

Scientists say the onset of a warming El Nino phenomenon in 2023 contributed to the record heat that followed.

But El Nino ended in early 2024, and scientists have puzzled over why global temperatures have remained at record or near-record levels ever since.

In December, WMO said if an opposite La Nina event took over in coming months it would be too "weak and short-lived" to have much of a cooling effect.

"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.

Nations agreed to transition away from fossil fuels at a UN summit in 2023 but the latest meeting in November struggled to make any progress around how to make deeper reductions to heat-trapping emissions.