The milestone was confirmed by the European Union's Copernicus Climate Change Service (C3S), which said climate change is pushing the planet's temperature to levels never before experienced by modern humans.
"The trajectory is just incredible," C3S director Carlo Buontempo told Reuters, describing how every month in 2024 was the warmest or second-warmest for that month since records began.
The planet's average temperature in 2024 was 1.6C higher than in 1850-1900, the "pre-industrial period" before humans began burning CO2-emitting fossil fuels on a large scale, C3S said. Last year was the world's hottest since records began, and each of the past ten years was among the ten warmest on record.
Britain's Met Office confirmed 2024's likely breach of 1.5C, while estimating a slightly lower average temperature of 1.53C for the year. US scientists will also publish their 2024 climate data on Friday.
Governments promised under the 2015 Paris Agreement to try to prevent average temperatures exceeding 1.5C, to avoid more severe and costly climate disasters. The first year above 1.5C does not breach that target, which measures the longer-term average temperature.Â
Buontempo said rising greenhouse gas emissions meant the world was on track to soon also blow past the Paris goal - but that it was not too late for countries to rapidly cut emissions to avoid warming rising further to disastrous levels.
"It's not a done deal. We have the power to change the trajectory from now on," he said.
The impacts of climate change are now visible on every continent, affecting people from the richest to the poorest countries on earth.
Wildfires raging in California this week have killed several people and destroyed hundreds of homes. In 2024, Bolivia and Venezuela also suffered disastrous fires, while torrential floods hit Nepal, Sudan and Spain, and heatwaves in Mexico and Saudi Arabia killed thousands.
Climate change is worsening storms and torrential rainfall, because a hotter atmosphere can hold more water, leading to intense downpours. The amount of water vapour in the planet's atmosphere reached a record high in 2024.
But even as the costs of these disasters spiral, political will to invest in curbing emissions has waned in some countries.
US President-elect Donald Trump, who takes office on January 20, has called climate change a hoax, despite the global scientific consensus that it is human-caused and will have severe consequences if not addressed.
The US experienced 24 climate and weather disasters in 2024 in which the cost of damages exceeded $US1 billion ($A1.6 billion), including Hurricanes Milton and Helene, according to the National Oceanic and Atmospheric Administration.
Chukwumerije Okereke, a professor of global climate governance at Britain's University of Bristol, said the 1.5C milestone should serve as "a rude awakening to key political actors to get their act together".
"Despite all the warnings that scientists have given, nations... are continuing to fail to live up to their responsibilities," he told Reuters.
Concentrations in the atmosphere of carbon dioxide, the main greenhouse gas, reached a fresh high of 422 parts per million in 2024, C3S said.
Zeke Hausfather, a research scientist at US non-profit Berkeley Earth, said he expected 2025 to be among the hottest years on record, but likely not top the rankings.
"It's still going to be in the top three warmest years," he said.
That's because while the biggest factor warming the climate is human-caused emissions, temperatures in early 2024 got an extra boost from El Nino, a warming weather pattern which is now trending towards its cooler La Nina counterpart.
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