Climate & Environment
Cleaner Air May Be Unmasking Faster Global Warming, Scientists Warn
A social media dispute in July, during Britain's record-breaking hot summer, has drawn attention to an emerging and unsettling area of climate research. Reform UK's deputy leader Richard Tice shared a Telegraph article headlined "Heatwaves caused by fall in pollution," arguing that cleaner air rather than carbon dioxide explained rising temperatures and claiming the public had been "gaslit." While Tice was incorrect to suggest carbon dioxide is not the main driver of global warming, the underlying science he referenced is genuine and more nuanced than his claim suggested.
Burning fossil fuels for more than a century has released two very different types of pollution. Carbon dioxide warms the planet, a well-established fact. Less widely known is sulphur pollution, which creates tiny particles in the atmosphere that reflect sunlight back into space, partially offsetting carbon's warming effect. Clean-air policies worldwide, such as the United States' Acid Rain Program introduced in the early 1990s, have sharply cut sulphur emissions even as carbon dioxide emissions remain near record levels. Many scientists now believe the loss of this cooling effect is contributing to a slight acceleration in global warming.
The past three years have been the three warmest on record, and the Met Office says 2027 is "very likely" to overtake 2024 as the hottest year yet. This has prompted scientists to examine whether the planet might be more sensitive to greenhouse gases than previously believed, and whether warming could be happening faster than expected.
Global temperatures fluctuate naturally year to year, partly due to El Niño, a periodic warming of the Pacific Ocean. Reto Knutti, a professor of climate physics at ETH Zurich and former IPCC co-ordinating lead author, has worked with colleagues to filter out such natural variability. "It's pretty clear that the rate of warming has accelerated," he said. A separate study published this year reached similar conclusions after accounting for major natural influences.
Aerosols appear to play a significant role. Burning coal and oil produces sulphur dioxide, which forms sulphate aerosols—small, bright particles that reflect sunlight before it warms the Earth's surface. Dr Øivind Hodnebrog, a principal researcher at the Cicero climate institute in Norway, compares this to dust settling on the windows of a greenhouse, reducing the amount of sunlight that enters and slowing warming. Aerosols also affect cloud formation, acting as seeds for water vapor condensation, which can brighten or extend the life of clouds, further reflecting sunlight. Ship pollution has even been shown to leave visible bright trails in marine clouds, observable from space.
However, sulphur pollution harms human health and causes acid rain, prompting decades of emission reductions across Europe, North America, China, and international shipping. Unlike carbon dioxide, which persists in the atmosphere for centuries, sulphate aerosols dissipate within days or weeks, meaning their cooling effect vanishes almost immediately once emissions drop.
The impact is most pronounced where sulphur pollution has declined fastest. One study estimates it has added roughly half a degree Celsius to summer warming in western-central Europe since 1980. Prof Piers Forster of the University of Leeds, also a senior IPCC author, estimates that reduced aerosol pollution is adding approximately 0.05C to 0.1C of warming per decade. Greenhouse gases remain the dominant factor, contributing around 0.2C of warming per decade, according to Forster. "It is a very significant contribution," he said of aerosols, "but it is not as important as the greenhouse gas contribution by quite a long way." Hodnebrog explains that aerosols have been masking earlier greenhouse gas warming rather than causing new warming themselves.
To understand the fuller picture, scientists examine Earth's energy imbalance—the difference between the energy the planet absorbs from the sun and the energy it radiates back into space. Knutti notes this imbalance has roughly tripled over the past two decades, though the exact increase varies depending on the comparison period. While greenhouse gases trap heat trying to escape, the energy imbalance shows that more sunlight is also entering the system, which points to changes in cloud cover as another contributing factor.
As oceans warm, low cloud cover in some regions appears to diminish, allowing more sunlight to reach and heat the dark ocean surface below—creating a feedback loop where warming reduces clouds, which then allows further warming. The World Meteorological Organization reports that oceans have warmed more than twice as fast in the past two decades compared with the period between 1960 and 2005. Dr Paulo Ceppi of Imperial College London says his research indicates this cloud feedback effect is more significant than sulphur pollution in explaining the recent decline in low cloud cover.
Considerable uncertainty remains. Knutti acknowledges that scientists cannot yet fully separate the effects of cleaner air, natural variation, and warming-induced changes on clouds. This uncertainty is significant because clouds are notoriously difficult to model accurately in climate simulations, leading to varying predictions of "climate sensitivity"—how strongly the climate responds to greenhouse gases.
Knutti's team found that climate models which most accurately predicted recent warming and the rise in Earth's energy imbalance also tend to be the ones projecting greater climate sensitivity. This suggests that models forecasting more warming may be more accurate than previously assumed. His analysis indicates the same level of emissions could result in about 25% more warming than earlier estimates suggested. A study published last year in the journal Science reached similar conclusions, finding that models predicting less warming struggled to match observed increases in Earth's energy imbalance.
Under current government policies, the world is on track for approximately 2.8C of warming by the end of the century. Forster suggests that if this new evidence holds, the same emissions trajectory could instead lead to warming closer to 3.5C. However, he urges caution, noting that the observational data underpinning these estimates spans only a relatively short period. "We have to be very cautious of how we go about interpreting observations that last for one or two decades," he said, adding that the satellite record remains short and natural variability large.
Ultimately, cleaner air has not caused climate change—but removing sulphate pollution has eliminated some of the cooling that had been offsetting greenhouse gas warming, contributing to the recent acceleration. More broadly, evidence suggests the climate may be more sensitive to greenhouse gases than previously assumed, meaning the same level of emissions could produce greater warming than expected. Every fraction of a degree of additional warming raises the risk of more severe heatwaves, floods, droughts, and wildfires. While uncertainties remain, scientists agree on one central point: greater greenhouse gas emissions will continue to drive higher global temperatures, making rapid emissions cuts increasingly urgent if the climate proves more sensitive than previously calculated.