Tiny aerosols present a big challenge for global warming

This story was originally published by Wired and appears here as part of the Climate Desk collaboration.

Fossil fuels are rapidly warming the planet, and the aerosols from their combustion kill millions of people each year. So we need to rapidly decarbonize. But in an ironic twist, those aerosols actually have one beneficial side effect: They cool the atmosphere. It creates an odd climate contradiction. If we burn less gas, oil and coal, we’ll stop loading the sky with planet-warming carbon, but we’ll also load it with fewer planet-cooling aerosols.

But exactly how much cooling we get from aerosols, and how strong that effect will be as the world weans off fossil fuels, are huge questions among climate researchers. “It’s taken as read that aerosols are important,” says University of Oxford climate scientist Duncan Watson-Parris. “And this uncertainty in the aerosol effect is a key uncertainty in climate science.”

Last week, Watson-Parris published a paper in the journal Nature Climate Change in which he played out a scenario for how aerosol concentrations will change through the end of the century. It assumes that as we burn less fossil fuel, we’ll produce fewer aerosols. But he was able to tweak how much cooling those aerosols could provide going forward. In one version of the model, which assumed that aerosols have a more intense cooling effect, losing them was a bit like switching off the planet’s air conditioning. The resulting warming would be enough to overshoot the Paris Agreement’s goal of keeping global temperatures from increasing more than 1.5 degrees Celsius.

But if we assume that aerosols actually have a 50 per cent smaller cooling effect, losing them will matter less, and we’ll have a better chance at keeping warming below 1.5 degrees. Pinpointing the size of this effect would be key for policymakers, he points out, who have spent the past two weeks at the COP27 climate conference in Egypt negotiating how much more carbon countries should be allowed to emit.

But nailing down that figure has been difficult, thanks to the dizzying complexity of aerosols and Earth’s atmosphere. Burning fossil fuels produces clouds of microscopic particles, primarily sulfate, which cool the climate in two main ways. “The little particles themselves act like little mirrors, and they reflect some sunlight straight back to space,” says Watson-Parris. “So it’s a little bit like a parasol.” All of these tiny atmospheric parasols shield the surface of the planet from solar radiation.

The second way is more indirect: They influence the formation of clouds, which in turn affect the local climate. “All aerosols act as nuclei on which water vapour in the atmosphere condenses and forms cloud droplets,” says Watson-Parris.

Clouds do this naturally when water condenses around specks of dust. But if you load a given area with extra aerosols, the droplets end up being more numerous, yet smaller: There’s only so much water vapour to go around all the particles. Smaller droplets are brighter than bigger ones, which whitens the cloud, causing it to bounce more of the sun’s energy back into space. “If you make the droplets smaller, they will potentially precipitate less, and the clouds can live longer,” says Watson-Parris. “And this—we call it a lifetime effect—is one of the most uncertain and potentially one of the larger contributions to this overall cooling.”

Interrogating this effect globally remains difficult. For one thing, says Watson-Parris, it’s hard to determine to what extent fossil fuel particles have influenced the formation of a given cloud. (There are a few obvious exceptions, like “ship tracks,” or the sulfur emissions from cargo ships. These provide aerosols that brighten clouds overhead and show up as white streaks on satellite images.) And for another, there’s no historical data to compare modern measurements against. We don’t know the dynamics of clouds before the Industrial Revolution, when fossil fuels were still largely locked underground.

In addition, the atmosphere is an extraordinarily complicated 3D system stretching miles into the sky. Temperatures, humidity and winds are in constant flux. And anthropogenic aerosols are themselves extraordinarily complicated, coming in different sizes and chemical compositions.

Models can simulate how those particles interact with clouds, but any model is necessarily a simplification of reality — there’s just no way for even the burliest supercomputers to account for such complexity. One could more easily model a smaller, isolated chunk of the sky, but that’s not how the atmosphere actually works. It’s a great, big swirling soup of interacting systems. “That’s why there’s so many uncertainties,” says Earth scientist Hailong Wang, who models the influence of aerosols in the atmosphere for the Pacific Northwest National Laboratory. “Different models agree on some aspects, but eventually they give a very large spread in a prediction of how temperature will respond to aerosol changes.”

That’s why scientists can’t yet say that if we burn fewer fossil fuels and reduce aerosols by X amount, we can expect Y amount of warming. There are just too many unknowns. And that’s why researchers like Watson-Parris play around with a range of outcomes. More atmospheric data, they say, and more powerful supercomputers will allow them to run more complicated simulations and get closer to concrete numbers.

In the meantime, if that uncertainty seems rather demoralizing, Watson-Parris says it’s yet another reason to aggressively decarbonize. If we find better ways to take existing particulates out of the air—say, with a new generation of scrubber or filter—but continue to burn fuels that release planet-warming carbon dioxide and methane, we’ll raise temperatures while eliminating the tiny atmospheric parasols that are compensating for some of that heat. And that, he says, would be “a double whammy.”