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Temperatures over the past three years have been even higher than expected, provoking a debate among scientists. Almost everyone agrees that global warming has accelerated. But some researchers say it is speeding up even more than climate models show, while others argue that the surge in temperatures is due to natural fluctuations that will soon go away.

Depending on who is right, we could have even less time than we thought to avoid or adapt to catastrophic impacts.

“Ultimately, this is a question of how bad climate change is going to be,” says at non-profit organisation Berkeley Earth in California.

Earth was warming at a steady rate of about 0.18°C per decade until the 2010s, when observed temperatures seemed to begin rising slightly faster.

Then, 2023 became the hottest year on record by a margin of 0.17°C, more than expected even with a slight acceleration in warming in the 2010s. Deadly floods struck Libya, unprecedented cyclones pummelled Mozambique and Mexico, and record wildfires torched towns in Canada, Chile, Greece and Hawaii.

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That year, at Columbia University, New York – who famously told the US Congress in 1988 that humans, not natural fluctuations, were heating the planet – published a widely discussed with colleagues arguing that the rate of warming had accelerated to about 0.32°C per decade after 2010.

They attributed this mainly to the “Faustian bargain” humanity made with aerosol pollution from fossil fuel burning. Sulphur aerosols reflect sunlight back into space, and they also help form reflective clouds. For decades, these aerosols and clouds have masked some of the warming from carbon dioxide emissions.

Now that the world is reducing aerosol pollution, which kills millions of people each year, this hidden warming is being unmasked, and climate change is accelerating, the paper argued.

China, the world’s biggest fossil fuel emitter, began a “war on pollution” when it hosted the 2008 Beijing Olympics. Since then, it has reduced its sulphur aerosol emissions by at least 75 per cent.

At the same time, the International Maritime Organization has been cracking down on sulphur emissions from ships. The air is less dirty over the oceans than over land, so this reduction in aerosols from ships can result in very few clouds, and ship tracks – lines of cloud that typically trail vessels – have .

As a result of both actions, global sulphur dioxide emissions have fallen 40 per cent since the mid-2000s. “The atmosphere is cleaner, so more solar radiation is coming in,” says at the European Union’s Copernicus Climate Change Service.

That seemed to be underscored when 2024 was even hotter than 2023, exceeding 1.5°C above pre-industrial levels for the first time – a step towards missing the more ambitious Paris Agreement target. Temperatures remained almost as high in 2025, when heatwaves killed thousands in Europe and cyclones wreaked havoc on South-East Asia and Jamaica.

But while most scientists agree that reductions in our aerosol emissions have accelerated global warming, they disagree on how much. The 0.32°C per decade rate found by Hansen and his colleagues was than the 0.24°C rate estimated by the United Nations Intergovernmental Panel on Climate Change and the 0.29°C average given by the latest generation of climate models.

A major complication is that natural fluctuations have also been affecting the planet’s temperature. The first was the arrival in 2020 of an especially strong maximum in the roughly 11-year solar cycle, when sunspots and magnetic activity began boosting the amount of sunlight reaching Earth.

Then, in 2022, the eruption of a massive underwater volcano near Tonga in the South Pacific shot 146 million tonnes of water vapour, a greenhouse gas, into the stratosphere. At the same time, it ejected sulphur aerosols that cooled the atmosphere somewhat.

Finally, 2023 and 2024 saw a strong El Niño, a natural climate pattern in which weakening trade winds allow a mass of warm water to slosh back across the central and eastern Pacific, inflating the global temperature.

To find out how much global warming is accelerating, scientists have to estimate and exclude the impact of this natural variability, then try to fit a curve to a trend in observed temperatures that is only beginning to emerge. Less natural variability would mean more acceleration, and vice versa.

Earlier this month, a statistical analysis by  at the University of Potsdam, Germany, and US statistician Grant Foster found that global warming has jumped to about 0.36°C per decade since 2o14.

But scientists like at the University of Pennsylvania have argued that Rahmstorf, Hansen and others have overestimated the impacts of aerosols and underestimated the impacts of natural fluctuations. He says that he and his colleagues are working on a study that finds only a modest quickening since the 1990s.

“Recent warmth, which is greatly influenced by the 2023-2024 El Niño event, is entirely consistent with standard climate model simulations,” says Mann. “There is no need to invoke any ad hoc mechanisms, including a supposed acceleration within the past decade.”

But it is possible that unexpected climate feedback loops have also been contributing to the recent heat. The biggest wild card is clouds, which are too small and scattered to be represented well by climate models.

A last year by at the Alfred Wegener Institute in Germany and his colleagues attributed about 0.2°C of the almost 1.5°C of warming in 2023 to a decline in low-lying clouds. While some of this cloud loss was due to the reduction in sulphur pollution, some of it may be because of an “emerging low-cloud feedback”, the researchers said.

Traditionally, cold, moist air over the subtropical ocean and the warm, dry air above have formed what is known as a temperature inversion, keeping the layers separate. But if climate change is increasingly warming that cold air and breaking down the inversion, the dry air could be descending and reducing moisture and thus clouds, says Goessling.

“The more you warm up, the more you would be able to dissolve your low-level clouds,” he says. “It’s really most likely that there is a low-cloud feedback taking place.”

If the acceleration can be blamed mostly on sulphur reductions, then climate change will probably slow down again in future decades, once there is no more sulphur pollution to cut. But if climate feedback loops have been unleashed, then it could potentially continue to speed up.

That would mean we have underestimated climate sensitivity, or the amount of warming that will occur from a given rise in atmospheric CO2.

“The worst-case scenario would be that this is coming from a cloud feedback that the models aren’t predicting and would mean that we actually have a much more sensitive climate than any of the models are projecting,” says at the University of Miami, Florida.

The world is on track for 2.7°C of warming this century under current policies. Those projections, however, have an uncertainty of roughly plus or minus 1°C. More acceleration could mean Earth is headed for something closer to 3.7°C and humanity would have to cut carbon emissions even more to avoid devastating impacts.

“3.7°C… makes some regions uninhabitable,” says Hausfather. “2.7°C would still be bad, but many more regions could potentially adapt to that.”

Either way, fossil fuel emissions are also accelerating, a trend that governments must reverse to have any hope of limiting impacts, says Burgess.

“Global warming is a bit faster, but we’ve also lost time because we’ve not seen the ambitious measures to decarbonise our society,” she says.