Climate Change and Geoengineering: Artificially Cooling Planet Earth by Thinning Cirrus Clouds

cirrus clouds
Scientists propose a way to cool the planet by targeting cirrus clouds. Greg Younger/Flickr

As part of the Paris Agreement in 2015, nearly 200 world leaders agreed to curb greenhouse gas emissions and strive to keep temperatures at 1.5 degrees Celsius above pre-industrial levels in order to avoid dangerous and irreversible climate change by the end of the century.

At present, climate scientists regard warming of two degrees above pre-industrial levels as the threshold for global warming. After this point, extreme weather will become more likely—increasing the risks of storms, droughts and a rise in sea levels. Consequences include food and water scarcity, and increased migration as parts of the planet become uninhabitable.

If global emissions continue on their current trajectory, some scientists estimate we will surpass the two-degree limit by 2050. And with Donald Trump poised to pull the U.S. out of the Paris Agreement, the chance of achieving the set target looks even less likely.

Over recent decades, scientists from across the globe have been discussing the potential of geoengineering—the deliberate manipulation of the environment that could, in theory, cool the planet and help stabilize the climate.

There are main two types of geoengineering. The first involves removing carbon dioxide from the atmosphere and storing it. This is already being done on an industrial scale, but it is not effective enough at the moment to cope with the huge levels of emissions. The other type, solar radiation management, is more radical—an attempt to reduce the amount of sunlight absorbed by the planet by reflecting it away.

Many ways of doing this have been proposed. One of the most widely discussed (and riskiest) involves the injection of reflective aerosols into the upper atmosphere. This plan is based on the cooling effect of volcanoes: Sulfur dioxide emitted in an eruption causes the formation of droplets of sulfuric acid. These reflect the sunlight away, creating a cooling effect. But this plan could also go very wrong. The sulfuric acid could strip away the ozone layer, leaving Earth completely exposed to the sun's radiation.

In an article published in the journal Science, Ulrike Lohmann and Blaž Gasparini, from the ETH Zurich, in Switzerland, discuss a variation of this idea: the thinning of cirrus clouds to target the long-wave radiation coming from Earth.

Cirrus clouds are thin and wispy clouds that form at high altitudes and do not reflect much solar radiation back into space, creating a greenhouse effect. The higher the altitude at which they form, the larger the warming effect on the climate. And in a warmer climate, cirrus clouds form at higher altitudes.

So what if we got rid of them? These clouds could be thinned out—leading to a reduction in their warming effect—by seeding them with aerosol particles like sulfuric or nitric acid, which act as "ice nucleating particles" or INPs. If these are injected into the level of the atmosphere where cirrus clouds form, the way they form would be altered, resulting in thinner clouds that have less of a warming effect.

"The maximum cirrus seeding potential would be achieved by removing all cirrus clouds," they write. "If cirrus thinning works, it should be preferred over methods that target changes in solar radiation, such as stratospheric aerosol injections, because cirrus thinning would counteract greenhouse gas warming more directly."

But Lohmann and Gasparini warn that the plan comes with major drawbacks. It could, they say, lead to even more cirrus clouds being formed, exacerbating global warming in the process.

climate change
The sun rises over an oil field in California’s Monterey Formation. David McNew/Getty Images

"Unintended cirrus formation is especially pronounced if the seeded INPs start to nucleate ice at very low relative humidities.... If cirrus seeding is not done carefully, the effect could be additional warming rather than the intended cooling. If done carefully, the negative radiative effect from cirrus seeding should be stronger in a warmer climate, in which the overall radiative effect of cirrus clouds will be larger."

Because of the dangers, the scientists say any plan to thin cirrus clouds should be limited to specific times and places, where it would be most effective. "Contrary to solar radiation management methods, cirrus seeding is more effective at high than at low latitudes. A small-scale deployment of cirrus seeding could therefore be envisioned—for instance, in the Arctic to avoid further melting of Arctic sea ice," they say, but the scientists add that there are many questions that need to be answered before cirrus thinning can be further explored.

"It is also important to remember that, like solar radiation management, cirrus thinning cannot prevent the CO2 increase in the atmosphere and the resulting ocean acidification," they conclude. "For the time being, cirrus cloud thinning should be viewed as a thought experiment that is helping to understand cirrus cloud–formation mechanisms."