As the world combat global warming by moving away from fossil fuels, air pollution will also decrease. Less air pollution is a good thing that promises massive benefits for human health. Paradoxically, though, it will also “unmask” fossil fuel warming that until now has been dampened by particles of soot and smog.
A form of geoengineering known as marine cloud brightening could counteract this effect at a global level, a new study suggests. But the devil is in the details: the effects on specific regions of the planet could be quite different, highlighting the need for careful planning to avoid unintended effects.
In the study, researchers used a computer climate model to forecast the effects of seeding four cloudy areas over the eastern Pacific Ocean with microscopic particles of salt known as sea salt aerosols, similar to the salt spray that arises from ocean waves breaking on rocks. Aerosols contribute to the formation of clouds made of especially tiny water droplets, and those especially tiny droplets lead to whiter clouds that reflect more solar radiation back into space, thus cooling the planet.
On its own, shifting to green energy and achieving global net-zero carbon emissions would reduce air pollution enough to unmask nearly 1 °C of fossil fuel warming by the end of the century, the researchers calculated.
It will require billions of kilograms of sea salt aerosols every year, but marine cloud brightening could counteract the unmasking effect of reduced air pollution, holding global average temperature to its 2020 level. The geoengineering strategy would also counteract the increase in global annual precipitation that would otherwise occur as air pollution diminishes, according to the model.
But looking closer, the researchers discovered unintended effects at a regional level. Europe, the U.S., northeastern China, and some areas of the ocean would still see warming. This is partly because the sea salt injections would indirectly cause the ocean conveyor belt known as the Atlantic Meridional Overturning Current to speed up.
And the regional effects of marine cloud brightening on precipitation are even more dramatic than the effects of reduced air pollution itself. Overall, the United States would become hotter and drier by the end of the century, while India, Australia, the Amazon, and the semi-arid Sahel region of Africa would be cooler and wetter than they are now.
The idea that geoengineering has unintended effects shouldn’t be so surprising. After all, injecting massive amounts of carbon dioxide and particulate air pollution into the atmosphere are also forms of geoengineering. But we’re now considering the unintended side effects of geoengineering we might undertake to mitigate the unwanted consequences of our past geoengineering.
Viewed this way, we’ve turned the Earth’s atmosphere into a Rube Goldberg device.
At the very least, as the researchers write, the new study “highlights the need for a more careful implementation strategy to produce both widespread cooling and fewer regional climate risks considering the effects are highly dependent on the strategy and seeding location.”
Source: Yu Y. et al. “Marine cloud brightening mitigates the warming induced by the aerosol reductions toward carbon neutrality.” Communications Earth & Environment 2026.
Image: © Anthropocene Magazine.
