If we ever get desperate enough to try artificially cooling the planet to slow global warming, we might want to think about modifying cirrus clouds. Thinning out these feathery, high-altitude clouds could cool the climate while having a relatively small effect on rainfall.
As the effects of climate change become more apparent, scientists are increasingly researching geoengineering: methods to artificially cool the climate. Many of these techniques involve reflecting some of the Sun’s radiation back into space, offsetting the warming effect of greenhouse gases. For example, we could inject aerosol particles into the stratosphere, mimicking the cooling effect of a volcanic eruption.
However, tinkering with the climate could have nasty side-effects, triggering droughts in some regions, for instance. So climatologists are trying to figure out how the various methods will play out in practice.
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Long Cao of Zhejiang University in Hangzhou, China and his colleagues modelled three geoengineering methods: pumping aerosols in the stratosphere; “marine cloud brightening”, in which low-level clouds over the sea are made whiter and therefore more reflective; and “cirrus cloud thinning”, which reduces the coverage and thickness of high-level cirrus clouds.
Pop those clouds
Cirrus cloud thinning is a relatively new idea, . Cirrus clouds are made of tiny ice crystals, so the idea is to spray powder into the air from planes. Ice crystals will form around each grain of powder and become so heavy that they fall, reducing the amount of cirrus.
In each run of their climate model, the team first doubled the concentration of carbon dioxide in the air, then simulated one of the geoengineering methods and observed what happened. Each method was done with sufficient force to offset all the warming.
In line with previous studies, average global precipitation – the amount of rain, hail and snow – never returned to normal after geoengineering. However, the disruption to precipitation was significantly smaller when cirrus cloud thinning was used.
“Broadly speaking, I think cirrus cloud thinning, if it works in reality, would be preferable to stratospheric aerosol injections,” says Cao. “It seems the known side-effects of cirrus cloud thinning are less than stratospheric aerosol injection and marine cloud brightening.”
He says cirrus cloud thinning also has an intuitive appeal, because the method allows excess heat trapped by carbon dioxide to escape into space, rather than stopping sunlight reaching the Earth.
Stubborn clouds
“The key question is whether you could actually get it to work,” says Helene Muri of the Norwegian University of Science and Technology in Trondheim, who previously showed that in theory . We don’t fully understand how the ice crystals form, so manipulating the process is tricky.
“Cirrus clouds have been overlooked a little bit,” says Muri. Worse, the powder injection must be done just right, because too much makes cirrus clouds last longer, causing more warming.
Cao adds that we also know less about the side-effects of cirrus cloud thinning.
However, Muri’s team found that cirrus cloud thinning should affect precipitation less than other forms of geoengineering – in line with Cao’s findings.
If we do try geoengineering, the best approach might be a mix of techniques. Last year Cao’s team showed that could get both global average temperature and precipitation back to preindustrial levels, although there would still be regional changes. This mix-and-match approach was dubbed “cocktail geoengineering”.
“It’s a bit like when you’re taking medication for one illness and then you have to take another one to treat the symptoms,” says Muri. She would prefer to cut greenhouse gas emissions and if necessary artificially remove some of the gases from the air, and “see how we go from there”.
Journal of Geophysical Research: Atmospheres
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