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No good deed goes unpunished—and that includes trying to slow climate change. By cutting greenhouse gas emissions, humanity will spew out fewer planet-cooling aerosols—small particles of pollution that act like tiny umbrellas to bounce some of the sun’s energy back into space.
“Even more important than this direct reflection effect, they alter the properties of clouds,” says Øivind Hodnebrog, a climate researcher at the Center for International Climate Research in Oslo, Norway. “In essence, they make the clouds brighter, and the clouds reflect sunlight back into space.”
So as governments better regulate air quality and deploy renewable energy and electric vehicles, we’ll get less warming thanks to fewer insulating emissions going into the sky, but some additional warming because we’ve lost some reflective pollution. Hodnebrog's new research suggests that this aerosol effect has already contributed to a significant amount of heating.
The most important component in fossil fuel pollution is gaseous sulfur dioxide, which forms aerosols in the atmosphere that linger for mere days. So slashing pollution has an almost immediate effect, unlike with carbon dioxide, which lasts for centuries in the atmosphere.
It’s a gnarly, unavoidable catch-22, but in no way a reason to keep polluting willy-nilly. Fossil fuel aerosols kill millions of people a year by contributing to respiratory problems, cardiovascular diseases, and other health issues. So by decarbonizing we’ll improve both planetary and human health. The urgency is growing by the day: Last year was by far the hottest on record, and this March was the 10th month in a row to notch all-time highs. Meanwhile, ocean temperatures—boosted by El Niño, the warm band of water that periodically arises in the Pacific, which also added heat to the atmosphere—have soared to and maintained record highs for over a year, stunning scientists.
“The preponderance of those records and the margins by which they were broken was eye-opening,” says Jennifer Francis, senior scientist at the Woodwell Climate Research Center in Massachusetts. “Until society manages to stop increasing the greenhouse blanket, record-smashing events like those in 2023 will become more common, even without the boost from El Niño.”
Slowing down the growth of that insulating blanket is already underway. “We seem to be flattening greenhouse gas emissions, which is a good thing,” says Zeke Hausfather, a research scientist at Berkeley Earth. “But we’re also uncovering some warming that our pollution had historically been masking. And because of that, our models expected—and we seem to be starting to see—some evidence of a speed-up in the rate of surface warming.” This is known in climate science as acceleration. Hausfather points to data showing that since 1970, the warming rate was 0.18 degree Celsius per decade, which has jumped to about 0.3 degree Celsius per decade over the past 15 years.
In his new paper, published in the journal Communications Earth and Environment, Hodnebrog and his colleagues set out to quantify just how much an effect curbing aerosols has had. To start, they gathered measurements between 2001 and 2019 from the Clouds and the Earth’s Radiant Energy System, satellite instruments that detect the difference in the solar energy coming to our planet and the energy reflected back out into space. This is the overall “energy imbalance” of the Earth, with it trending upwards as the world warms.
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The researchers then fed global emissions data into four different state-of-the-art climate models and managed to reproduce those satellite measurements. “When we set the aerosol emissions to constant—so we didn’t include any change over time in the aerosol emissions—then this upward trend in the energy imbalance was much reduced, and we didn’t manage to reproduce the satellite measurements,” says Hodnebrog. “So our main conclusion is that these aerosol emission reductions need to be accounted for in order to explain what we see now, what we measure from space.”
The researchers found that over the past two decades, the reduction in aerosol emissions has accounted for nearly 40 percent of the increase in energy imbalance—that is, the extra warming energy that’s raised global temperatures. “I would be surprised if this will not lead to temporary acceleration in surface temperature warming,” says Hodnebrog of the ongoing tailing off of aerosol emissions.
Projecting forward with aerosols, though, is tricky, because we’re dealing with extraordinarily complex atmospheric processes. For one, modeling cloud formation is notoriously difficult, and it’s hard to tell just how much human-made aerosols contribute to a given cloud versus natural aerosols.
There’s also uncertainty about how strong a cooling effect aerosols have up in the sky. If they have an intense cooling effect, we’ll get more warming in the future as they decrease. It’d be like switching off the planet’s air conditioning. But if they have a milder cooling effect, losing them wouldn’t lead to as much warming. In 2022, a separate team of scientists calculated that if it ends up being the latter case, we’d have a better chance of keeping warming below the 1.5-degree Celsius limit established in the Paris Agreement. (In their new aerosols paper, Hodnebrog and his colleagues accounted for this uncertainty by running those different models, which had different representations of aerosols and their interactions with clouds. Their results were the average of the four models.)
Even in the present day, some scientists are skeptical that we’re seeing acceleration of global warming from reduced aerosols. “Yes, it is responsible for the acceleration in warming during the 1970s to 1980s,” says climate scientist Michael Mann of the University of Pennsylvania. That was when clean-air regulations started requiring “scrubbers” on coal-fired power plants to remove the sulfur dioxide that forms aerosols. “There is no evidence for any acceleration over the past few decades, however.”
Instead, we could be seeing natural variability, Mann says—the rising and falling of global temperatures over the years that Earth would see even in the absence of human-caused warming. Last year was a good illustration of this. Record-smashing temperatures were due to humans failing to stop pumping so much carbon into the atmosphere, but also due in part to the natural emergence of El Niño. “Think of it as a tide on top of a rising sea,” Mann says. “The rising sea—the steady warming—is what we should be concerned about, and that will continue until net emissions reach zero.”
That much is very clear, and very much agreed upon by scientists: Humans need to stop burning fossil fuels, even if losing some aerosols leads to additional warming going forward. “Right now the recent acceleration is borderline significant, which is why there’s some debate,” says Francis. “But aside from all this, the real story is the relentless global warming that we know is caused by the thickening blanket of greenhouse gases owing to human activities.”
