Could We Slow Global Warming By Making Clouds Brighter?


Dr. Katie Spalding

Katie has a PhD in maths, specializing in the intersection of dynamical systems and number theory.

Freelance Writer


Get outta here sun, we don't want you. Image credit: OlegRi/

As the consequences of climate change get more and more extreme, it makes sense that the solutions to the crisis might do the same. One idea that’s been turning heads recently fits that profile perfectly: it’s big, it’s bold, and at first glance, it seems more the purview of a supervillain than a scientist. Put simply: we BLOCK OUT THE SUN.

Okay, maybe that was putting it too simply. The suggestion is actually to populate the atmosphere with clouds of seawater particles – not to block out the Sun exactly, but to reflect it.


“Clouds reflect solar radiation (sunlight) back to space, producing cooling effects locally, and across the planet,” the Marine Cloud Brightening Project (MCBP), a research group comprising scientists and engineers from the University of Washington, the Palo Alto Research Center, and the Pacific Northwest National Library, explains on its website. If we could just figure out how to create and manipulate clouds, we might be able to reflect enough sunlight to counteract some of the human-made warming of the planet.

Mind-boggling though it may sound, they’re not the first to suggest an idea like this. This is the branch of science known as geoengineering, and it is not without controversy: some experts believe it could cause more problems than it solves, while others say it’s the only way to combat the climate crisis.

“The reflectivity of clouds increases as the number of water droplets inside the cloud increases and their size decreases,” the project explains. “[This makes] the clouds brighter and longer-lasting, reflecting sunlight and increasing cooling.”

Now, you might be thinking that “just create a bunch of shiny clouds” is not much less of a tall order than “stop global warming”, but it’s not as baffling as it sounds. For decades, scientists have had an idea of how it could work, mostly because it’s already been happening. Thousands of ships crisscross our oceans every day, expelling emissions full of greenhouse gases, toxic pollutants – and tiny particles that mix with the clouds overhead and reflect sunlight away from the Earth.


“As these particles mix into low clouds, particularly clouds over the ocean, they alter their properties by adding droplet nuclei, catalyzing more small droplets to form and brightening the clouds,” explains the MCBP website. “The large quantities of man-made particles currently produced by industrial and other human activities are likely cooling the planet enough to be significantly off-set warming caused by greenhouse gases, but this effect is not well quantified.”

Of course, reflecting a bit of sunlight probably won’t be much use in the fight against climate change if it’s achieved by sending fleets of oil-fueled ships across the world – which is why the MCBP is turning to more eco-friendly ways to shine up their clouds. Taking their lead from Mother Nature herself, they’re using one of the most abundant substances on Earth for their clouds: seawater.

“Over remote parts of the ocean, most [cloud-forming particles] are of natural origin and include sea salt from crashing ocean waves,” wrote MCBP scientists Kate Murphy, Gary Cooper, Sarah Doherty and Rob Wood in an explainer for IEEE Spectrum. “The aim of the MCB Project is to consider whether deliberately adding more sea salt … to low marine clouds would cool the planet.”

There are many advantages to the idea, should it prove successful. Seawater is free and environmentally harmless, and the proposed testing methodology – spraying it from ships – wouldn’t require unnecessary cost or emissions. But the team has a way to go before then: for the moment, the idea is firmly in the initial stages.


“First, we need to find out if we can reliably and predictably increase reflectivity,” explain the team. “Second, we need more modeling to understand how MCB would affect weather and climate both locally and globally. It will be crucial to study any negative unintended consequences using accurate simulations before anyone considers implementation.”

Only then – and only after they develop a suitable spray system to deliver the particles – will the team be able to test the idea in a real-world setting, they say – and only as small-scale experiments. But should the experiments prove successful, they may give the world a vital tool to slow climate change – and maybe give humanity enough time to solve the problem properly.


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  • global warming,

  • Geoengineering