While humans have been enthusiastically overheating our planet for the last couple of centuries, other forms of our pollution have been quietly offsetting a bit of the damage, helping to cool the climate by affecting cloud formation.

But the extent of the cooling provided by aerosols isn't thoroughly understood, and getting an accurate picture of it is crucial for accurate models of climate change. A paper in Nature this week uses a vast database of cloud data to show that aerosol particles may not lead to as much cooling as we thought.

Aerosols cool the planet, but how much?

Aerosols are tiny atmospheric particles of sea spray, smoke, dust, and other substances. They can come from natural sources like sandstorms, as well as from human activities like fossil fuel combustion.

These particles are one of the many forces affecting the Earth's climate, because they scatter, reflect, and absorb light. They also play an essential role in cloud formation, because cloud droplets are formed by water vapor condensing onto aerosol particles. So, an increase in aerosols created by human activity means changes for cloud formation, which in turn tends to cool the planet.

But the extent of this cooling isn't understood. It counterbalances an "unknown fraction" of the global heating caused by greenhouse gases, write meteorologist Velle Toll and his colleagues in their paper.

This creates a hidden complication for any measures to reduce fossil fuel reliance, as it would simultaneously reduce aerosol emissions. If aerosols have been playing a large role in mitigating rising temperatures, reduced emissions would also reduce this cooling effect. Figuring out how these factors balance out is crucial if we're going to accurately estimate the carbon emissions targets that would limit warming to the 1.5ºC set by the Paris Agreement.

More aerosols, more cloud droplets

The light-reflecting properties of aerosols are reasonably well-understood, and they play a limited role in their cooling capabilities. Much of the uncertainty lingers instead around their role in cloud formation. Aerosols increase the number of water droplets in low-level clouds but decrease the size of those droplets. This creates clouds with a greater surface area for reflecting sunlight away from the Earth and a greater cooling effect than unpolluted clouds.

What's not clear is whether aerosols lead to clouds that hold more or less water than unpolluted clouds. The size and concentration of water droplets has sometimes been found to lead to higher water content and sometimes to lower water content. Because the water content of clouds also affects how much cooling they create, this uncertainty has meant that it's impossible to pin down the degree of cooling caused by aerosols.

Toll and his colleagues isolated the effect of aerosols in cloud water content by comparing polluted clouds with nearby unpolluted clouds.

"Distinguishing polluted from unpolluted clouds is generally not possible with the naked eye," writes Nicolas Bellouin, one of the authors of the paper, in a blog on the subject. But by using satellite imaging technology that can measure in the near-infrared spectrum, it's possible to see that "clouds made up of smaller droplets appear brighter than clouds made up of larger droplets."

The near-infrared images loop and swirl like impressionist paintings, showing distinct bright streaks in the clouds that trace the movement of aerosols emitted by wildfires, human emissions, and volcanoes. The researchers gathered data from 2,400 of these pollution tracks from around the world, covering a period of 15 years.

The results did not tell a simple story. Sometimes the water content of the polluted clouds was higher, and sometimes lower. Overall, the average change was minimal, but there was a small average decrease in water content in the polluted clouds compared to unpolluted ones.

More accurate climate models

Lower water content in polluted clouds means that aerosols are creating opposite effects simultaneously: smaller, more abundant water droplets make more reflective clouds, but those same clouds have less water, which undercuts their reflectiveness. The researchers estimate that this offsets about 23% of the cooling caused by the smaller droplets, although this estimate could change with future research.

The result means that there's a "constraint on the overall cooling effect of aerosol emissions," writes Anna Possner in an accompanying article in Nature. Having a better handle on the complex effects of aerosols, she adds, "reduces one of the key uncertainties in climate science."

Nature, 2018. DOI: 10.1038/s41586-019-1423-9 (About DOIs).