Earth’s warming exceeded 1.5C on an annual basis for the first time in 2024, according to two major climate science agencies. It’s the most potent evidence yet that countries are failing to meet a Paris Agreement goal of limiting global heating to that level as a decades-long average.
The amount of time left to avoid eclipsing the goal “is now wafer thin,” said Colin Morice, a UK Met Office scientist, in a statement.
Scientists sounded the alarm long before last year ended that 2024 would become the hottest year on record and almost certainly the first to surpass the 1.5C limit. Now both of those milestones have been confirmed in official statistical releases from two independent scientific agencies.
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The EU’s Copernicus Climate Service measured the 2024 global average temperature to be 1.6C above the pre-industrial average, and the UK Met Office to be 1.53C above it. (Three other groups are expected to report Friday.)
The clear acceleration in rising temperatures has puzzled scientists, even as the evidence of the fast-warming atmosphere became impossible to miss.
The hottest day ever recorded happened on July 21, 2024 — a record that held until July 22. The planetary heatspike was made 2.5 times more likely by greenhouse gases, according to researchers. Typhoon Gaemi in Asia and Hurricanes Helene and Milton in the US, similarly juiced by climate change, killed hundreds of people and caused colossal damage. There was flooding across Africa’s Sahel and in southeastern Spain; drought in southern Italy and the Amazon River basin; wildfires in central Chile; and landslides in northern India.
Hottest-year status puts 2024 in rarefied company. The warmest year up to now, by a substantial margin? 2023.
But while the heat is clear, scientists are struggling to account for the speed of this recent jump. Something’s pushing up temperatures faster than expected, and the climate detectives have yet to agree on what. After months of research and debate, they have collected suspects — and already let a few go — in what’s become the greatest climate mystery in 15 years.
“The science tells us that we should expect surprises like this,” said Sofia Menemenlis, a doctoral candidate in atmospheric and oceanic sciences at Princeton University. “This is not something that should be completely unexpected in the future, knowing what we know about global warming.”
The landmark status for 2024 can be partly explained by the first five months coinciding with El Niño, a natural warm phase that supercharges global weather. But the planet is heating up so fast that even years with cooling trends, known as La Niña, are counted among the hottest of all time. The Met Office expects 2025 to be the third hottest, behind 2024 and 2023. In fact, the last 10 years are all ranked in the hottest on record, and all but one of the two dozen hottest years happened since 2000.
There’s a simple rule of thumb that greenhouse gases combined with El Niño makes for an exceptionally hot year. But scientists doubt those two factors are enough to account for the recent runup in warming. And they’re debating whether this is a spiky blip in the record or the start of a more lasting acceleration.
The ‘Anti-Hiatus’
For many experts, the mystery calls to mind the hotly debated “hiatus” in global temperatures from about 1998 to 2013, when temperatures seemed to plateau for a time. This prompted a deluge of studies in climate journals as well as public policy debates. But it was ultimately misleading: Postmortems concluded that natural variability, including a string of La Niña years, and incomplete Arctic data buoyed an illusion.
When temperatures began climbing upwards again in the mid-2010s, and once scientists updated their data sets, the hiatus dissolved into thin air.
“That isn’t going to be the case here,” said Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies, who published an influential article in March that articulated his and colleagues’ concerns.
With the current conundrum — call it the ‘anti-hiatus’ — scientists can point to physical reasons that are likely contributing to the fast-rising heat. They just don’t know yet which reasons are most important, or how long the trend will continue.
“We have a lot more physically grounded reasons to think an acceleration is happening than we did to think the slowdown was happening during the hiatus years,” said Zeke Hausfather, a climate researcher affiliated with Berkeley Earth, a non-profit that maintains one of the major temperature datasets.
The global average temperature in 2023 reached 1.48C higher than the preindustrial average, according to the EU’s Copernicus Climate Change Service. Greenhouse gas pollution and El Niño go a long way to explain that heat— about 1.23C of it in 2023, some experts estimate — but there’s more to account for. The sun entered the brighter part of its 11-year cycle, addingless than 0.03C. And a January 2022 volcanic eruption in the southern Pacific shot enough of the ocean skyward to raise the stratosphere’s heat-trapping water-vapor level by a record 10%. Initially considered a warming factor, the volcano gave off heat-reflecting sulfur aerosols that accorded the plume a slight net cooling event. In other words: a red herring.
That leaves 0.2C still unexplained.
Sulfur’s Cooling Effect Wanes
Sulfur aerosols released by power plants and vehicles have a cooling effect on the atmosphere, canceling out as much as a third of humanity’s heat-trapping emissions historically. When environmental rules cut sulfur — as acid-rain restrictions have done since the early 1990s — it comes with the perverse tradeoff of letting more heat break through to the planet’s surface.
Since international shipping regulations requiring low-sulfur fuels went into effect in 2020, scientists have seen a 74% drop in related sulfur aerosol emissions. That benefits human health, even at the short-term cost of temporarily higher temperatures. Similarly, 70% cuts to China’s sulfur pollution since its 2006 peak are reducing the overall atmospheric load — and for a time putting upward pressure on the temperature.
Scant Clouds, More Heat
Long-sought declines in sulfur aerosols may be contributing to more heat in an indirect way, too. These tiny specks encourage water vapor to condense into clouds. Since there are fewer of them in the air, that could be worsening conditions for cloud formation. That means less cloud cover, and that’s a real problem.
Low-lying clouds reflect light back out to space, the way white polar ice caps do. They’re a part of the Earth’s albedo, or surface brightness. Sparser low clouds mean more heat hitting us where we live, and that’s what’s been happening in the last 20 years, especially the last several.
Helge Goessling, a climate scientist at the Alfred Wegener Institute in Germany, started looking at changes in the North Atlantic, when ocean temperatures spiked there in early 2023, and noticed an unusual increase in the amount of solar energy reaching the surface. “We thought, ‘Oh, this is really quite something’,” Goessling said.
The planet’s albedo declined to a record low in 2023, Goessling and other scientists concluded in a paper published in December. It’s a possible consequence of the falling aerosols. The satellite temperature record is only several decades old, which means experts technically can’t rule out similar naturally occurring patterns in prior eras.
But the amount of warming the paper attributed to the lowered albedo is very close to the unattributed heat: 0.2C.
“Usually we do our small pieces of puzzles here and there, having small contributions to the big conversation,” Goessling said. “This one is only a small piece of a big puzzle, but still, it is one that fitted so neatly.”
Those same researchers expressed concern that “the 2023 extra heat may be here to stay.” The atmosphere may be more sensitive to greenhouse gases than previously thought, and “we may thus be closer to the temperature targets defined in the Paris Agreement” than was imagined, they wrote.
The warming beyond 1.5C seen last year doesn’t mean the Paris Agreement is breached. Diplomats and scientists would not consider the 1.5C limit exceeded until temperatures had topped it for 20 years or more.
If global warming itself is melting reflective clouds from the sky, “that would sort of be the worst of the options,” Hausfather said.
Another potential factor could be the 2023-2024 El Niño in particular. It comes after three cooler La Niña phases in a row. Some climate models suggest that when an El Niño follows multiple La Niñas, there’s a small chance that unanticipated heat can pour out of the oceans.
El Niño’s influence on the global temperature usually peaks months after it starts. And that is likely enough to explain 2024’s margin over the preceding year. This year is almost certain to be slightly cooler simply because the Pacific Ocean has returned to a neutral state—and may slip into a La Niña phase soon. But the extent and duration of the marginal effects are murkier. How much heat will continue to come from sulfur particles falling from the sky? How much from ground heat rising and melting away low-lying clouds? Ultimately, scientists are back to asking the core question of their profession: Just how fast is the world warming?
Karsten Haustein, a climate scientist at Leipzig University, said the recent temperature records are significant but he’s mindful to avoid “hiatus” levels of frenzy and isn’t convinced the albedo paper is a breakthrough. “Playing into that panic is not something I’m going to do,” he said.
Still, breaching 1.5C of warming even for a year is “a big deal,” he acknowledged: “Look at the fricking trend.”
Photograph: Destroyed vehicles and flood debris in a residential area of Alfafar, Spain in November 2024. Photo credit: Angel Garcia/Bloomberg
Topics Climate Change