

This is part of our series "Rebooting the Reef" on efforts to save one of the world's greatest natural wonders.

Daniel Harrison thinks he has an answer to saving Australia's Great Barrier Reef from rising ocean temperatures: a high-powered water cannon.

Harrison, a research fellow at the Sydney Institute of Marine Science, wants to shoot salt particles into the sky above the reef. To do that, a US team he collaborates with has created a nozzle that can shoot a spray comprising incredibly small droplets.

Now they need a cannon that can use these nozzles to shoot water carrying nano-sized salt particles into the air. About 5 percent of the salt, if all goes to plan, will float up into the sky. As it floats, Harrison hopes, the salt will be absorbed by the clouds above.

Filling clouds with salt will brighten them and, in turn, reflect the sun's heat away from the sea below, Harrison says. Climate change has caused Australia's ocean temperatures to rise around 0.68 degree Celsius over the last century. That may not sound like much, but it's enough to cause a coral catastrophe. Harrison says his plan can offset this change and give the reef much-needed time to heal.

Verena Scheopf/University of Western Australia

"It's like life support for the reef," Harrison told me in his Sydney office, which overlooks picturesque Chowder Bay. "Cloud brightening can buy us maybe another 10 or 20 years of temperature increase."

Harrison's efforts are driven by the sad state of the Great Barrier Reef, large swaths of which have died because of our heedlessness. He and other scientists in Australia know curbing the emissions that cause global warming is the only long-term way to save this natural wonder. Current efforts amount to little more than stopgaps. Still, they're hoping they can alter the environment just enough to give Mother Nature a helping hand in repairing herself.

The ideas include boosting the population of a reef species to control coral-eating starfish, building robots to kill the coral's predators and sinking old oil rigs to serve as artificial reefs. Some scientists are trying to give coral an evolutionary boost by encouraging heat-resistant strains of the animal that secretes what becomes the external skeleton it lives inside.

And, of course, there's Harrison's plan, which is formally known as Marine Cloud Brightening.

Warming waters threaten the reef by prompting bleachings. Algae provide coral with nutrients through photosynthesis, but if the algae become heat stressed or overexposed to sunlight, they instead produce a toxin. The coral will then expel the algae, causing the coral to bleach. Bleaching, depending on severity, can be fatal.

Two bleachings have occurred in as many years, leaving an estimated two-thirds of the Great Barrier Reef's coral bleached.

Coral can recover with time, though sometimes it takes up to a decade, and cooler temperatures. But our reliance on fossil fuels means global warming isn't likely to reverse anytime soon. The situation is compounded by other human activities, like farming and fishing, that disturb the ecosystem's delicate balance. For example, a growing population of coral-eating starfish is blamed on our behavior.

Supercoral

Verena Schoepf, a researcher at the University of Western Australia, has spent three years studying heat-resistant coral, which seem less prone to bleaching. If she can figure out why some species survive in warmer waters, she might be able to help those that don't.

The coral lives off the shores of Kimberley in northwestern Australia, a part of the country exposed to extreme temperature swings, as well as direct sunlight, on an almost daily basis. Despite the harsh conditions, the coral thrive.

Schoepf took the same species of coral from different areas of Kimberley, one where the temperatures ranged widely, with swings of up to 7 degrees Celsius, and one that had less extreme fluctuations. She placed coral from both areas in two tanks. One tank was set to summer temperatures, the other to a few degrees hotter.

"The corals from the tide pools [that] experience greater temperature fluctuations and generally more extreme conditions could cope with heat stress better," Schoepf said. "That's how we discovered the corals from the most extreme habitats are the most heat-tolerant."

Verena Schoepf

Schoepf and her three-person team want to know if this tolerance becomes part of the coral's genetics. If so, she wants to pinpoint the heat-resistant gene and splice it into other corals. In short, she wants to give evolution a boost.

She can't yet say conclusively if the resistance is genetically fixed, but she's optimistic. The team is working to get to the bottom of this, taking the heat-resistant coral from Kimberley and putting it in cooler water to see if it retains its resilience.

Even if these corals' tolerance becomes genetic, there are limits to what we can do. Tampering with ecosystems, especially through genetic engineering, comes with inherent risks. It's also hard to scale, and Schoepf thinks that at best only key parts of the reef would likely be saved. Rescuing the entire reef, which covers an area roughly the size of Germany, would cost too much and take too long. It previously cost the US government $1,620 per square metre to restore a coral reef in Florida, for instance. The Great Barrier Reef is 344,000 square kilometres.

"We live in desperate times when it comes to the reef," Schoepf said. "Sooner than later we might get to the point when these are our only options."

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A stellar enemy

If coral dreams, the crown-of-thorns starfish provides its nightmares.

The crown-of-thorns starfish population has grown rapidly over the past five decades, in part spurred by run-off fertilizer. Nitrogen in the fertilizer prompts algal blooms, which in turn encourage starfish to breed, because their larvae eat algae.

The explosion is bad for the reef, says Cherie Motti, a chemical ecologist at the Australian Institute of Marine Science, because the adult starfish eat coral. The crown-of-thorns starfish will consume coral until it's all gone, she says.

"Corals are actually quite amazing in that they can recover from many events," Motti said. "The only event they truly don't recover from is being eaten by crown-of-thorns starfish."

Australian Institute of Marine Science

Motti and her seven-person team may have a solution. They want to breed the natural predator of the crown-of-thorns, the giant triton snail.

The snail's numbers have dwindled thanks to commercial fishing, because the creature's shell is a hot item for tourists. Its devastation has been so great that it's been designated a protected species.

In September, Australia's federal government committed AU$568,000 ($446,600) to their research into the snail, which is famous for its distinctive, trumpetlike shell. The snail is native to the reef and eats starfish. More snails, Motti says, could mean fewer coral-munching starfish.

The team's first goal is figuring how the snails transform from larvae to snail. If that case can be cracked, the researchers hope they can breed snails. Then, with luck, they'll be used to control the growing crown-of-thorns starfish problem.

Barrier the Reef

Harrison's cloud brightening hopes to cool the Reef's corals through sunlight reflection. Scientists at the University of Melbourne and the Australian Institute of Marine Science have a similar idea: Use a biodegradable film to literally shield the corals from the sun.

The ultra-thin film is made from calcium carbonate, the same element corals use to make their shells, and will lie on the water's surface. It's 100 percent biodegradable, so it won't further pollute the reef, and preliminary tests show it can reduce sunlight exposure by up to 30 percent.

"The 'sun shield' is 50,000 times thinner than a human hair and completely biodegradable," said Anna Marsden, managing director of the Great Barrier Reef Foundation.

The project has some decent backing. Sponsors include the Australian government, the Tiffany & Co. Foundation and the University of Melbourne USA Foundation.

Like all Reef-saving methods outside lowering emissions, though, it's got a catch.

"It's important to note that this is not intended to be a solution that can be applied over the whole 348,000 square kilometres of Great Barrier Reef," Marsden said.

"That would never be practical. But it could be deployed on a smaller, local level to protect high value or high-risk areas of reef."

Something in the air

Harrison, the researcher at the Sydney Institute of Marine Science, first got interested in cloud brightening when he began exploring technology that could help the reef. He went through a few ideas, including pumping cold water into the reef, but cloud brightening "had the most appeal."

Cloud brightening is something of a natural process. Corals release the chemical dimethyl sulfide, or DMS, when heat-stressed. DMS rises from the sea to the sky, where it helps clouds form and shade the reef below. As coral reefs die, less coral is left to produce DMS, creating harsher conditions for the remaining coral.

In the 1970s, Sean Twomey, an Irish physicist who worked in Australia and the US, figured out the mechanics of cloud brightening. Twomey discovered that aerosols carried by carbon dioxide pollution caused water in clouds to disperse, essentially turning a few big droplets into lots of little droplets.

Spreading the moisture more evenly makes the clouds more reflective.

Daniel Harrison

In 2010, Silver Lining, a San Francisco-based geoengineering group that Harrison collaborates with, received a $300,000 grant from the Bill & Melinda Gates Foundation to build machines that spray salt water into the clouds and reflect ultraviolet rays back out. If the seawater droplets can be produced in the right size range, enough of the salt crystals could hit the cloudline to allow the Twomey Effect to occur.

Harrison received a 2017 Myer Innovation Fellowship, which funded his research for this year. He's now three months deep and seeks funding to actually build the necessary machinery -- and high-powered water cannons aren't cheap. Designing and building a prototype, he estimates, could cost around AU$500,000.

But he figures it'd be worth it. Harrison envisions these cannons mounted on the boats that already ferry about the reef. Brighter clouds are doubly productive, he says, because the change in air mass around a cloud will make it hang around the same area for longer, helping cut both heat and light radiation.

"By increasing the brightness of clouds, you're cooling the water," he said. "But you're likely to get quite an extra benefit by having less sunlight hitting the corals when they're stressed."

As powerful as this may be as a tool, it's still a band-aid. An elaborate, technically impressive band-aid, but a band-aid nonetheless. To really save the Reef, the scientists agree, we need to lower climate emissions. To do that, Harrison says, Australians need to use their voting power.

"If I turn the lights off in my house it'll help a little bit, but real change will come by policy."

First published Oct. 21, 2017 at 5 a.m. PT.

Update, March 28 at 8:55 p.m.: Adds "Barrier the Reef" section.

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