It’s a sweltering summer in Australia, and the corals on the Great Barrier Reef are showing early signs of stress. The authority that manages the largest coral reef system in the world is expecting another bleaching event in the coming weeks—if that happens, it will be the sixth time since 1998 that spikes in water temperatures wipe out swathes of corals that are home to countless marine animals. Three of these bleaching events, which make corals more susceptible to disease and death, have happened in the last six years alone. When corals experience extreme and prolonged heat stress, they expel the algae living in their tissues and turn completely white. This can have devastating impacts on the thousands of fish, crabs and other marine species that rely on the reefs for refuge and food. To slow the rate at which ocean warming is bleaching the coral, some scientists are looking to the skies for a solution. Specifically, they’re looking at clouds.
Clouds bring more than just rain or snow. During the day, like massive parasols, clouds reflect some of the sunlight away from the Earth and back into space. Marine stratocumulus clouds are particularly important: they lie at low altitudes, are thick and cover about 20 percent of the tropical ocean area, cooling the water beneath. This is why scientists are exploring whether their physical properties could be altered to block even more sunlight. On the Great Barrier Reef, the hope is to provide some much-needed relief to coral colonies during increasingly frequent heat waves. But there are also projects aimed at global cooling that are more controversial.
The idea behind the concept is simple: Large amounts of aerosols would be sprayed into the clouds above the ocean in order to increase their reflectivity. Scientists have known for decades that the particles in the polluting tracks left by ships, which look much like the contrails seen behind planes, can brighten existing clouds. That’s because these particles create a seed for cloud droplets; and the more numerous and smaller the cloud droplets, the whiter and better the clouds are at reflecting sunlight before it hits—and heats—the Earth.
Of course, shooting pollutant aerosols into clouds is not a suitable technological fix to global warming. The late British physicist John Latham had already proposed in 1990 to use salt crystals from evaporated seawater instead. Seawater is plentiful, benign and above all free. His colleague Stephen Salter, emeritus professor of engineering design at the University of Edinburgh, then suggested deploying a fleet of some 1,500 remote-controlled ships that would sail the ocean, suck up water and spray a fine mist into clouds to make them brighter.
As greenhouse gas emissions continued to rise, so did interest in Latham and Salter’s unusual proposal. Since 2006, the pair have been collaborating with around 20 experts from the University of Washington, the Palo Alto Research Center and other institutions as part of the Marine Cloud Brightening Project (MCBP). The project group is now researching whether the deliberate addition of sea salt to the low, puffy stratocumulus clouds over the ocean would have a cooling effect on the planet.
Clouds off the west coasts of North America, South America and central to southern Africa appear to be particularly amenable to brightening, according to Sarah Doherty, an atmospheric scientist at the University of Washington in Seattle, who has been managing the MCBP since 2018. Cloud droplets do form naturally over the ocean when moisture gathers around salt particles, but adding just a little more salt to them could increase the clouds’ reflecting power. Brighten the large sheets of clouds over those amenable regions by as little as five percent, says Doherty, and much of the globe could be cooled. At least this’s what computer simulations suggest.
“Field studies where we spray sea salt particles into clouds at a very small scale would allow for deeper insights to key physical processes and therefore to improve models,” she says. Small-scale experiments with prototype equipment were meant to start as early as 2016 at a site near Monterey Bay, California, but they have been postponed due to lack of funding and public opposition over the experiment’s possible environmental impact.
“We would not be directly testing marine cloud brightening at any scale that would affect climate,” says Doherty. Critics, including environmentalist organizations and advocacy groups such as the Carnegie Climate Governance Initiative, however, fear that even small experiments could inadvertently affect the global climate due to its complex nature. “The idea that you could just do this on a regional scale and very limited scale, is pretty much a fallacy because the atmosphere and ocean are importing heat from other places all the time,” says Ray Pierrehumbert, professor of physics at the University of Oxford.
There are also technical challenges. Developing a sprayer that can reliably brighten clouds is no easy feat, because seawater tends to clog up as salt builds up. To solve this challenge, the MCBP has enlisted the help of Armand Neukermans—the inventor of the earliest inkjet printers, who worked at Hewlett-Packard and Xerox until his retirement. With financial backing from Bill Gates and a group of other tech industry veterans, Neukermans is now designing nozzles that will spray just the right size (120 to 400 nanometers in diameter) of saltwater droplets into the atmosphere.
As the MCBP group prepares for outdoor testing, a team of Australian scientists has adapted an early prototype of MCBP’s nozzles and tested it over the Great Barrier Reef. Australia has experienced warming of 1.4°C since 1910, more than the global average of 1.1°C, and the Great Barrier Reef has lost more than half of its corals due to ocean warming.
Cloud brightening can offer some support for the reefs and their residents. To achieve that, engineering oceanographer Daniel Harrison from Southern Cross University and his team fitted a research vessel with a turbine to pump water out of the ocean. The turbine, which resembles a snow cannon, refines water and, through its 320 nozzles, sprays trillions of tiny droplets high into the air. The water droplets dry in the air, leaving behind a salty brine, which in theory would mix with the low-altitude stratocumulus clouds.
The team’s proof-of-concept experiments in March 2020 and 2021—at the end of the Australian summer, when corals are most at risk of bleaching—were too small to significantly alter the clouds. Harrison was nevertheless surprised at how quickly the salty plume drifted skywards. His team flew drones equipped with lidar instruments up to 500 metres high to map the movement of the plume. This year, an aircraft will cover the remaining meters in altitude to assess any response in the clouds beyond the 500-meter mark.
The team will also use air samplers on a second research vessel, and weather stations on the reefs and on shore to study how particles and clouds mix naturally in order to improve their models. “Then we can start to look at how cloud brightening, if it was done on a larger scale, could potentially impact the ocean in desirable as well as potentially unexpected ways,” Harrison says.
Cutting light over the reef by about six percent would equate to a 0.6°C temperature reduction in the mid-shelf reefs of the Great Barrier Reef, according to modelling done by Harrison’s team. Harrison says that scaling the technology to cover all the reefs (the Great Barrier Reef is made up of over 2,900 individual reefs and spans 2,300km) would be a logistical challenge, as it would require around 800 spray stations operating for months ahead of expected heat waves. The Great Barrier Reef is so large it can be seen from space, but it only covers 0.07 percent of Earth’s surface.
Harrison acknowledges that this new approach has potential risks that need to be better understood. Cloud brightening could break up clouds or change local weather and rainfall patterns, which is also a major concern with cloud seeding. This is a technique that involves aircraft or drones adding an electrical charge or chemicals such as silver iodide to clouds, in order to create rainfall. The United Arab Emirates and China have experimented with the technique to tackle heat or air pollution. But such measures are hugely controversial—and many consider them downright dangerous.Cloud seeding and brightening both fall under so-called “geoengineering” interventions. which critics consider too risky or a distraction from emissions reductions.
In 2015, the physicist Pierrehumbert co-authored the US National Research Council’s report on climate interventions and warned of political and governance issues. But a new report by the academy, published in March 2021, takes a more supportive stance on geoengineering and advises that the US government invest $200 million in research. Pierrehumbert welcomes research into marine cloud brightening but finds it problematic that spraying equipment is being developed as part of the ongoing research projects. The technology could get out of hand, he says. “The scientists who say that this is not a substitute for emissions control, they're not going to be the ones making the decisions.”
The Australian government, which has been heavily criticized for its inaction in response to the climate crisis and its reliance on coal-fired power, sees potential in marine cloud brightening. In April 2020 it launched a $300 million program in April 2020 to restore the Great Barrier Reef—this pot of money funds research, technological development and testing of more than 30 interventions, including marine cloud brightening. Despite substantial investment measures such as cloud brightening remain controversial. Environmental groups argue that it could carry ecological risks and be a distraction from efforts to limit greenhouse gases.
But even if cloud brightening proves effective, Harrison doesn’t think it will be a long-term solution for saving the Great Barrier Reef. “There's only a limited amount of cooling that you can get from cloud brightening,” he says—and as the climate crisis likely worsens, any effects of brightening will quickly be overcome. Rather, Harrison argues, the aim is to buy time while countries bring emissions down. “It's too late now to hope that we're going to be able to reduce our emissions quickly enough to save the reef without any intervention.”
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