From the BBC
The fossil fuels that we've been burning for more than a century produce two very different kinds of pollution. As is well known, carbon dioxide warms the Earth. But less well-known is sulphur pollution, which forms vast numbers of tiny particles in the air and reflects sunlight back into space, partly counteracting the warming effect of carbon.
Thanks to clean-air policies around the world - like the US's Acid Rain Program in the early 1990s, that reduced gas pollution from power plants - sulphur emissions are falling sharply while carbon dioxide emissions remain near record highs.
Scientists increasingly believe the loss of that cooling effect is slightly [more than slightly — increasing it by 50%!] speeding up global warming.
The past three years were the three warmest ever recorded, and the Met Office says 2027 is "very likely" to replace 2024 as the warmest year on record.
Is it possible that our planet is actually more sensitive to greenhouse gases than we imagined?
And if so, might the climate be changing more rapidly than we feared?
Global temperatures jump around naturally from year to year. El Niño, for example, a natural warming of the Pacific Ocean, can temporarily push them higher. So a run of record hot years does not necessarily mean the underlying rate of global warming has increased.
Reto Knutti and his team at ETH Zurich have tried to strip out the effects of El Niño and other natural fluctuations to reveal the underlying trend.
"It's pretty clear that the rate of warming has accelerated," says Knutti, a professor of climate physics at the Swiss university and a former co-ordinating lead author for the IPCC - the UN-backed body that produces the world's most authoritative assessments of climate science.
A separate study published this year reached a similar conclusion after accounting for several major natural influences.
And it seems aerosols are playing a role.
Burning coal and oil releases sulphur dioxide, which forms tiny particles known as sulphate aerosols. These particles are bright and reflect some sunlight before it can reach and warm the planet's surface.
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| Pollution from ships can create bright trails through clouds, visible from space Source:NASA |
You've probably heard the greenhouse analogy used to explain global warming. Gases like carbon dioxide make it harder for heat to escape from the Earth, rather like the glass windows of a greenhouse.
Well, sulphur pollution acts a bit like dust on the greenhouse windows. The more dust that gathers on those windows, the less sunlight gets inside, and the slower the greenhouse heats up in the first place.
Dr Øivind Hodnebrog, a principal researcher at Cicero, the Norwegian climate research institute, has studied how falling aerosol pollution affects the climate. "Sulphate aerosols, they are really small particles, but they are bright particles," he says.
As well as reflecting sunlight, aerosols also cool the planet by changing the formation of clouds. Aerosols act like tiny seeds around which water vapour can condense, making some clouds brighter or longer-lived. Pollution from ships can even create bright trails through marine clouds visible from space. So more aerosols mean brighter clouds and more reflection, and so less warming.
But sulphur pollution also damages human health (it can get into our lungs and make it harder to breathe), and causes acid rain, which can destroy forests and kill fish - and so countries have spent decades cutting it. Emissions have fallen sharply in Europe, North America and China, while new rules have also reduced sulphur pollution from shipping.
In our greenhouse analogy, it's as if we've wiped some of the dust away from the windows of the greenhouse, allowing in more sunlight.
And while carbon dioxide remains in the climate system for centuries, sulphate aerosols survive in the atmosphere for just days or weeks. So when sulphur emissions fall, their cooling effect disappears almost immediately.
The effect is strongest where sulphur pollution has fallen fastest. One study estimates it 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 another senior author of the IPCC's most recent assessment, estimates that declining aerosol pollution may now be adding around 0.05C to 0.1C of warming per decade [to the existing 0.2C per decade].
But aerosols can only explain part of the recent acceleration in warming. Greenhouse gases remain much more important.
Forster estimates they are currently adding around 0.2C of warming globally per decade. "It is a very significant contribution," he says of the aerosol effect, "but it is not as important as the greenhouse gas contribution by quite a long way."
"The aerosols have masked some of the greenhouse gas warming from earlier," is how Hodnebrog explains it.
"Masked" - that's an interesting word he uses. What he means is cleaner air is not causing the underlying warming; it is removing some of the cooling that had been holding that warming back.
So if cleaner air is only responsible for part of the recent acceleration in global warming, what else is at work?
To see what else might be going on, scientists zoom out and look at the Earth as a whole: how much energy it absorbs from the Sun, and how much heat it loses back into space. Scientists call the difference Earth's energy imbalance.
Cleaner air is part of that picture. Fewer sulphate particles mean less sunlight is reflected away - in our greenhouse analogy, the windows are cleaner.
For a long time, the Earth has been absorbing more energy than it loses. That surplus is what warms the planet. And Knutti says that imbalance has roughly tripled over the past two decades - although the precise increase depends on which years are compared.
Greenhouse gases principally work by making it harder for heat to escape out into space. That's what we're all familiar with. But when you look at Earth's energy imbalance, you find that there's also more sunlight getting in (and therefore more heat that can be absorbed).
So what else might be letting more sunlight in?
Much of that change appears to involve something we have already encountered in this story: clouds. We saw how sulphur pollution can make some clouds brighter and longer-lived, reflecting more sunlight away. But global warming itself can also change clouds. As some parts of the ocean warm, low cloud cover can diminish. That matters because clouds are bright, while the ocean beneath them is dark. Remove some cloud and more sunlight reaches the sea, where much of it is absorbed as heat.
That can create a feedback: warming reduces some cloud cover, which allows more sunlight in, which produces still more warming.
And the oceans are certainly heating up. The World Meteorological Organization says the oceans have warmed more than twice as fast over the past two decades than they did between 1960 and 2005.
Dr Paulo Ceppi, a climate physicist at Imperial College London, says his research suggests this feedback loop between climate change and clouds is more important than sulphur pollution in explaining the recent decline in low clouds.
But there's still lots of uncertainty. Scientists say the impacts of cleaner air, natural variations and the effects of warming itself are all tangled together. And, when I asked Knutti how much of the cloud change came from each, his answer was simple: "We don't know."
And this is where the story takes an even more worrying turn.
That uncertainty matters because clouds are one of the hardest things to represent in the computer simulations scientists use to project future warming. Some models show more warming than others for the same amount of greenhouse gas emissions.
Scientists call this variability "climate sensitivity": how much the climate responds to gases like carbon dioxide.
And the recent observations give Knutti and his colleagues a new way to test those models. They found that models which best predicted the recent warming, as well as the rise in the Earth's energy imbalance, are also the ones with a more sensitive climate.
In other words, the models that predict more warming are looking like the most accurate. His team's analysis suggests that the same emissions could produce about 25% more warming than we previously thought.
And a study published in the journal Science last year points in a similar direction. Models which projected relatively little warming struggled most to reproduce the observed rise in Earth's energy imbalance.
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 this new evidence proves correct, the same broad emissions path could instead produce warming closer to 3.5C.
It sounds serious - but there are important reasons for caution. [For climate scientists, yes. For governments and the general population, no. Because prudence suggests we assume the worst. If that is wrong, we've reduced emissions faster than we were going to do anyway. If it's right, we instead face climate and civilisational catastrophe.]
Warming appears to have accelerated, but Forster says that does not mean it will keep doing so.
He points out that the recent observations that underpin these 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 these new studies do not overturn previous estimates.
But still, some scientists are alarmed.
So - is cleaning up the air making global warming worse?
In one narrow sense, yes. Removing sulphate pollution takes away some of the cooling that had been counteracting greenhouse warming. That really has contributed to the recent acceleration. But cleaner air has not caused climate change.
The aerosol story may help explain part of that acceleration. But the wider evidence raises a more worrying possibility: that the climate responds more strongly to greenhouse gases than central estimates have assumed.
If that's correct, the same emissions could produce more warming than we expected. Climate risks increase with every fraction of a degree of warming, with the potential for more severe heatwaves, floods, droughts, and wildfires as temperatures rise.
Some of the science we have explored here is complex and important questions remain unresolved. But on the central point, the science is clear: the more greenhouse gases we emit, the warmer the planet will become.
And, if the climate really is more sensitive than previously thought, cutting emissions quickly becomes even more important.
See also:
Significant acceleration of global heating since 2015
