Cleaner air is speeding up global warming, scientists say

Sulphur pollution from burning coal and oil reflects sunlight and has partly offset greenhouse warming for decades. As clean-air laws cut those emissions in Europe, North America and China, that cooling is disappearing — and climate scientists estimate the loss may now be adding roughly 0.05C to 0.1C of warming per decade.

What is sulphur pollution doing to the climate?

Burning fossil fuels releases both carbon dioxide, which traps heat, and sulphur dioxide, which forms tiny bright particles called sulphate aerosols. These particles bounce some sunlight back into space before it reaches the Earth’s surface, and they also make clouds brighter and longer-lived by acting as seeds around which water vapour condenses. Dr Øivind Hodnebrog, a principal researcher at the Norwegian climate research institute Cicero, describes them this way: “Sulphate aerosols, they are really small particles, but they are bright particles.”

How much warming does cleaner air add?

Sulphate aerosols survive in the atmosphere for only days or weeks, while carbon dioxide persists for centuries, so their cooling effect fades almost as soon as emissions fall. One study estimates that cleaner air has added around half a degree to summer warming in western-central Europe since 1980. Prof Piers Forster, a climate scientist at the University of Leeds and a senior author of the IPCC’s most recent assessment, estimates that declining aerosol pollution is adding around 0.05C to 0.1C per decade, compared with roughly 0.2C per decade from greenhouse gases. “It is a very significant contribution,” Forster says of the aerosol effect, “but it is not as important as the greenhouse gas contribution by quite a long way.”

Why do scientists say warming has accelerated?

Global temperatures swing naturally from year to year, so a run of hot years does not by itself show that the underlying rate has risen. Such swings include El Niño, which has been linked to a near-record Atlantic hurricane season. Reto Knutti, a professor of climate physics at ETH Zurich and a former co-ordinating lead author for the IPCC, and his team tried to strip out El Niño and other natural fluctuations. “It’s pretty clear that the rate of warming has accelerated,” Knutti says. The past three years were the three warmest on record, after individual monthly records including a month that tied July 2023 as the hottest on record, and the Met Office says 2027 is “very likely” to replace 2024 as the warmest year.

Is the climate more sensitive than thought?

Knutti’s team used recent observations to test the computer models used to project future warming. The models that best reproduced the recent warming, and the rise in Earth’s energy imbalance — the gap between the energy the planet absorbs from the Sun and the heat it radiates back to space — were also those with a more sensitive climate. Their analysis suggests the same emissions could produce about 25% more warming than previously thought, and a study published in the journal Science last year points in a similar direction. Under policies governments currently have in place, the world is estimated to be heading for around 2.8C of warming by the end of the century; Forster says that if the new evidence proves correct, the same broad emissions path could instead produce warming closer to 3.5C.

How confident are scientists in this?

Forster urges caution. He says accelerated warming does not mean the rate will keep rising, and notes that the observations underpinning the estimates cover 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 says. The satellite record is short, natural variations are large, and the new studies do not overturn previous estimates.

Cleaner air has not caused climate change; it has removed some of the cooling that was holding warming back. The harder question is whether the climate responds more strongly to greenhouse gases than central estimates assume, which would mean more severe heatwaves, floods, droughts and wildfires for the same emissions.

Frequently asked questions

Does cleaning up air pollution cause global warming?

No. Sulphate aerosols reflect sunlight and masked part of the warming from greenhouse gases, so cutting them removes some cooling and adds to warming. Carbon dioxide remains the primary driver of climate change.

How much extra warming does falling sulphur pollution add?

Prof Piers Forster of the University of Leeds estimates declining aerosol pollution is adding around 0.05C to 0.1C of warming per decade, while one study estimates cleaner air has added about half a degree to summer warming in western-central Europe since 1980.

Is the climate more sensitive to greenhouse gases than scientists thought?

Not established. Reto Knutti of ETH Zurich and his team say models that best matched recent warming are the more sensitive ones, implying about 25% more warming for the same emissions, but Forster warns the observation period is short and the studies do not overturn previous estimates.

Which years were the warmest on record?

The past three years were the three warmest recorded, and the Met Office says 2027 is “very likely” to replace 2024 as the warmest year.

With reporting from BBC News.

Stock image, not from the events described. Brand Brewery Smokestack (7808293518) — Nitram242 / Wikimedia Commons (CC BY 2.0)