Brightening clouds with salt could reduce global warming

Marine cloud brightening (MCB) is a form of geoengineering in which salt water is fired into the clouds in order to increase their brightness and reflectivity. The method shows promise in helping to curb warming temperatures due to climate change, however there may be unforeseen ecological consequences.

Cloud cover

Injecting sea salt aerosol into the clouds can restrict the “future global-mean surface air temperature and precipitation change,” said a study published in Communications Earth & Environment. In a computer simulation, the scientists salted “four cloudy regions in the eastern Pacific Ocean every year from 2020 to 2100” and found that the injection “compensates well for the global warming induced by anthropogenic aerosol reductions over both land and ocean.” Scientists, in a separate research project in the U.K., are testing the geoengineering process in a three-story “cloud chamber,” with the potential for a real world test in 2028.

MCB enhances the “natural process of cloud formation” similar to the “natural effects of sea spray on cloud properties over the ocean,” said a release about the U.K. project. The sea salt aerosol particles “act as sites for the formation of cloud droplets when the air becomes humid enough, the more particles present, the more cloud droplets form and the more reflective clouds become.” The sea salt then “scatters more sunlight back to space and prevents some solar radiation from reaching the Earth’s surface in that area.” With less light reaching the planet, the temperatures cool.

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The process is not perfect. In the simulation, MCB was found to “not fully mitigate the warming in some regions, including Europe, the U.S., northeastern China, central and eastern Siberia and the Arctic,” while it did help in other regions, said the study. This discrepancy is likely because the sea salt injections “would indirectly cause the ocean conveyor belt known as the Atlantic Meridional Overturning Circulation to speed up,” said Anthropocene magazine.

MCB also affected rainfall. Though the total amount of rain globally remained the same as in 2020 when the simulation began, the distribution varied. The U.S. “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.”

Silver lining

Real world testing of MCB is still needed despite the study’s findings. “One model cannot settle whether marine cloud brightening could work safely in the real atmosphere over decades,” said Earth.com. Cloud behavior remains difficult to simulate because “droplets, particles, winds and ocean currents interact at many scales.” There is “very limited understanding of whether such approaches are scientifically sound, so it is essential that we understand whether spraying sea water can be performed effectively and what the effects might be,” Hugh Coe, a professor of atmospheric composition at The University of Manchester and the lead of the U.K. project, said in the release.

The focus of the “cloud lab” tests is “to find the ideal ‘Goldilocks’ size for the salt particles, said The Times. “Too large, and they risk soaking up all the moisture before smaller droplets can form. Too small, and they won’t ‘activate’ properly, meaning the cloud won’t brighten enough.” If the tests are successful, MCB could be tested in the real atmosphere as soon as two years from now. Geoengineering is a controversial measure that is “opposed by environmentalists who fear it is an excuse for not cutting the carbon emissions driving climate change,” said The Times. Other experts argue that reducing emissions is not enough.

“Decarbonization is the only sustainable route out of the climate crisis,” Mark Symes, the program director of the U.K.’s Advanced Research and Invention Agency, which is funding the project, said to The Independent. “However, decarbonization is not happening quickly enough to protect many parts of the world from the worst effects of global heating.”