A debate over the causes of recent record temperatures has highlighted a complex climate science issue: the role of sulphur pollution in masking greenhouse gas warming. While fossil fuel combustion produces carbon dioxide, it also generates sulphur dioxide, which forms particles that reflect sunlight and cool the planet. Decades of clean-air policies in Europe, North America, and China have dramatically reduced sulphur emissions to protect human health and prevent acid rain, but this has an unexpected side effect.
As sulphate aerosol pollution has declined, the cooling effect these particles provided has diminished, allowing more solar radiation to reach the Earth’s surface. Scientists including Reto Knutti at ETH Zurich and Piers Forster at the University of Leeds have found evidence that the rate of underlying global warming has accelerated in recent years. While natural climate variations like El Niño can cause year-to-year temperature fluctuations, research accounting for these factors suggests an actual increase in the warming trend.
Forster estimates that declining aerosol pollution is currently adding approximately 0.05 to 0.1 degrees Celsius of warming per decade, though this contribution remains substantially smaller than the warming from greenhouse gases, which add roughly 0.2 degrees per decade. In regions where sulphur emissions have fallen most sharply, such as western-central Europe, the effect is more pronounced. Aerosols act similarly to dust on greenhouse windows, with sulphate particles creating brighter clouds and reflecting sunlight, while carbon dioxide traps heat for centuries.
The Met Office projects that 2027 is very likely to surpass 2024 as the warmest year on record. While cleaner air is removing some of the cooling effect that had previously counteracted greenhouse gas warming, it accounts for only part of the recent acceleration in global warming, with carbon dioxide remaining the dominant driver of climate change.
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