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Time for artificial planet coolers? A cooling “sunshade” for the planet could reduce harmful coral bleaching and the number of hurricanes, which damage reefs.

With the effects of climate change becoming increasingly apparent, the idea of squirting a cloud of sulphate aerosols into the upper atmosphere is being investigated by several groups of scientists. This would scatter some of the sun’s rays back into space, reducing the rate at which the Earth is warming.

Now a study by at the University of Bedfordshire, UK, and his colleagues examines what this form of geoengineering would do to the Caribbean region and its fragile reefs. “Corals are the rainforests of the sea, and if you lose them the impacts on ecosystems and people would be complex and far-reaching,” says Crabbe.

The team used computer models to simulate both the changing climate and rising seas between 2020 and 2069. They then modelled what would happen if solar radiation was artificially reduced. “We show very convincingly that, by injecting sulphur dioxide in the atmosphere, sea surface temperatures would decrease significantly by 2069,” says Crabbe.

Hold back the hurricanes

When the sea is too warm, corals expel the tiny algae living in their tissues, which feed their hosts through photosynthesis. The corals turn white or “bleached”. After severe bleaching, most corals starve to death. Keeping temperatures down prevented this in the model.

But bleaching is not the only threat. According to Crabbe, solar radiation management would also reduce the frequency of hurricanes. These intense storms “destroy coral reefs, not only by smashing them, but also by limiting their reproduction and recruitment”: drifting larvae cannot attach themselves to a new reef in choppy waters. Crabbe says hurricanes would still occur, but not as frequently, giving the reefs some time to recover.

“One of the main concerns with solar radiation management is not necessarily its effectiveness, but its side effects,” says at the University of Oxford. For example, “it could disrupt regional weather patterns and monsoons”, which would be a problem because many people rely on predictable seasonal cycles for tasks like farming.

Back to normal

There is also the risk that geoengineering systems could be suddenly turned off. “What if the system was stopped through a terrorist attack?” asks Bellamy. “Global temperatures would jump back to where they would have been without geoengineering.” If greenhouse gas emissions had not been cut in the meantime, “that could be a very sharp rise”.

Crabbe admits that “we don’t know… what would happen to the marine environment” in that scenario. “But the situation is currently so extreme that we have to make provisions.”

at Harvard University has suggested an alternative, less aggressive way to cool the seas: increasing the reflectivity of the oceans with tiny air bubbles that could be delivered cheaply by ships or oil rigs. “Microbubble lifetimes are measured in minutes, as opposed to the months needed to turn stratospheric aerosols on and off, so brightening the water may afford better local control than dimming the sun,” he says.

But this would be a local solution, not a global one.

Other researchers have proposed brightening the clouds over coral reefs, which would cool only that area, and reducing ocean acidification – another threat to corals caused by greenhouse gas emissions – by adding minerals to the ocean to neutralise the acid.