Brightening the clouds touted as a way to save corals

The world's corals are in dire straits, so it will take an ambitious plan to save them. But will cooling the climate locally by making clouds brighter, the latest proposal on the table, help?

Corals face a perfect storm of threats including acidifying oceans, invasive predators and tropical storms. Warming waters also puts them under stress, making them expel photosynthetic microbes that provide most of their energy as well as their vibrant colours, leaving them a bleached white. Without symbiotic microbes, the corals will die.

"To save the coral reefs, you're going to have to do something drastic," says Alan Gadian of the University of Leeds, UK.

To try to reduce this bleaching, Gadian and colleagues simulated a form of geoengineering called marine cloud brightening. Ships would spray seawater into clouds to create extra water droplets, making the clouds whiter. This could reduce the ocean temperature locally because more sunlight would be reflected back into space.

When the model's carbon dioxide levels were double that of pre-industrial times ? a level expected to be reached by around 2050 ? bleaching events became much more common, compared with a control scenario with lower CO₂ levels, where events were rare. But when the coral beds of the Caribbean and French Polynesia, and the Great Barrier Reef ? three areas of ocean that tend to have lots of low-lying cloud ? were targeted with cloud brightening, levels of bleaching were the same as the control run.

Gadian estimates the project could cost at least $40 million a year.

Not a brilliant idea

"This is an attempt to do something good," says Tim Lenton of the University of Exeter in the UK. "But I'm not sure it's a brilliant idea." If the programme stopped for any reason ? because there were harmful side-effects elsewhere in the world, or because the ships were sabotaged by protestors, say, the ocean would heat up quickly. "The worst possible thing for the corals is rapid warming."

What's more, bleaching is only one of the threats facing corals, says Lenton. Although there were eight mass bleaching events on the Great Barrier Reef between 1979 and 2007, bleaching has only caused 10 per cent of the coral losses seen to date: storms and predatory starfish were responsible for far more. "Bleaching events are not the real reef killer, although they're very visible," says Lenton.

Naomi Vaughan of the University of East Anglia in Norwich, UK says it's not clear whether the clouds could be brightened sufficiently to have the necessary impact, in part because cloud brightening only works at certain times of day.

While most geoengineering proposals focus on controlling the global climate, Gadian's is unusual because it limits itself to specific regions ? albeit rather large regions that span oceans.

In theory, such regional geoengineering is a more nuanced approach, targeting those regions of the world that are worst affected by global warming. Other researchers have suggested planting crops with shiny, reflective leaves that reflect sunlight back into space, to cool areas like the US grain belt that might get too hot.

Journal reference: Atmospheric Science Letters, DOI: 10.1002/asl2.442

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