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Dr. Daniel Harrison described his cutting-edge research on Marine Cloud Brightening for the Great Barrier Reef to the Healthy Planet Action Coalition (HPAC) in May 2026.
Can we engineer the sky to save the sea? One of the world’s most ambitious environmental rescue missions may have the answer.
Leading this effort is Dr. Daniel Harrison, principal investigator for the Cooling and Shading Program of Australia’s Great Barrier Reef Restoration and Adaptation Program, and chief investigator for the project Marine Cloud Brightening in a Complex World, funded by the UK government’s Advanced Research and Invention Agency.
His presentation offered a rare, behind-the-scenes look at the technical feasibility, energy requirements, financial costs, and scalability of Marine Cloud Brightening (MCB).
He was quick to clarify, however, that this initiative—while currently the world’s most advanced practical field research program into MCB—remains in the experimental and scientific validation stage. It is not yet ready for large-scale deployment.
Instead, the team is testing a radical premise: Can microscopic sea-salt aerosols be generated, lifted into the atmosphere, and incorporated into clouds to increase their reflectivity?
If successful, this could function as a giant sunshade, reducing the devastating heat stress currently affecting coral reefs.
Harrison opened with a sobering reality check regarding the severe deterioration of the Great Barrier Reef. Over the last decade, it has experienced six severe mass bleaching events.
Today, essentially none of the vast reef system remains untouched by bleaching. These catastrophic events are driven by rising global ocean temperatures and localized marine heat waves that push corals beyond their natural thermal tolerance.
Bleaching, Harrison explained, is not merely a temporary cosmetic loss of color. When corals are stressed by heat, they expel the symbiotic algae living in their tissues, which provide them with food. If the heat stress continues, the corals eventually starve and die.
When this happens, entire reef ecosystems can collapse into permanently degraded states. Vibrant coral cities can be replaced by “rubble-dominated reefs,” where fragments of dead coral shift with currents, preventing new life from taking hold. In other areas, “macroalgae-dominated systems” emerge, where thick, fleshy seaweeds take over the space left behind by dead corals. Once these transitions occur, they are extremely difficult—if not impossible—to reverse.
Tragically, Harrison argued that the reef has now crossed a threshold where bleaching can occur in almost any given year. Surprisingly, he linked the recent acceleration in global temperatures to a sudden reduction in sulphur emissions from global shipping following the implementation of the 2020 International Maritime Organization (IMO) fuel regulations.
While cleaning up ship exhaust was a major benefit for human health, those sulphur particles had also been seeding clouds and reflecting solar heat back into space. Although the exact scientific attribution is still being studied, Harrison believes that removing these shipping aerosols effectively reduced a massive, unintended cloud-brightening effect—including locally over the Great Barrier Reef.
Faced with this crisis, the Reef Restoration and Adaptation Program (RRAP) cast a wide net for possible interventions to help the reef survive climate change.
The program initially brainstormed around 140 ideas, narrowed them down to 43, and then prioritized only the options that could be scientifically demonstrated, ecologically effective, socially acceptable, technically feasible, and economically viable at a massive scale.
The resulting Cooling and Shading Program focused heavily on engineering approaches to reduce bleaching stress by either directly cooling reef waters or reducing the amount of intense sunlight reaching corals.
At its peak, the program involved 17 research organizations and approximately 70 researchers. Among all the concepts tested, Marine Cloud Brightening (MCB) emerged as the most promising large-scale approach.
Why? Because of its exceptionally high “energetic leverage.”
The appeal of MCB can be explained through simple physics.
Imagine trying to cool a swimming pool by dumping ice into it. Directly pumping artificially cooled water across the vast Great Barrier Reef would require astronomical amounts of energy—roughly equivalent to the amount of cooling achieved.
By contrast, MCB requires a relatively small amount of energy to create nano-sized sea-salt particles (aerosols). When these tiny salt particles rise to the base of a cloud, they act as “seeds” that water Vapor can cling to these particles, forming cloud droplets. As they absorb moisture, their volume can grow by up to 500,000 times. In this process, nature performs the vast majority of the work.
This remarkable leverage makes MCB highly attractive for shading a vast, shallow ecosystem like the Great Barrier Reef. The main mechanism involved is known as the “Twomey effect.”
To understand it, imagine a glass jar filled with a few large marbles compared with a jar filled with thousands of tiny grains of white sand. The sand reflects much more light. Similarly, adding extra-small salt aerosols to a cloud increases the number of smaller water droplets within it. This makes the existing cloud whiter and brighter, increasing its albedo (the ability of a surface to reflect solar energy back into space), rather than creating an entirely new cloud.
Early computer modeling suggested that the fluffy, cotton-like cumulus clouds naturally found over the reef are highly responsive to this kind of brightening, even though they are not the flat, blanket-like “stratocumulus” clouds usually targeted in global geoengineering proposals.
The modeling work for this project took place in three distinct stages:
• Atmospheric modeling:
Researchers first confirmed that adding sea-salt aerosols could increase the albedo (reflectivity) of clouds over the reef.
• Oceanographic and biogeochemical modeling:
Next, they examined whether brighter clouds would actually cool the water below. Harrison noted that to cool the reef during a summer marine heat wave, the localized cooling effect needs to be quite strong—close to 10 watts per square meter. This is equivalent to successfully blocking the heat from a 10-watt lamp over every square meter of ocean surface.
• Ecological modeling:
Finally, researchers projected coral survival under different climate scenarios.
The results were revealing. Under a low-emissions pathway, broadly consistent with the goals of the Paris Agreement, MCB could help coral cover recover and stabilize. However, under a “business-as-usual” high-emissions scenario, MCB would only buy the reef a few additional decades before rising temperatures overwhelmed the intervention.
This is because the relationship between aerosols and clouds follows the law of diminishing returns. The first addition of salt particles (or cloud condensation nuclei) produces a major brightening effect. But as more and more salt is added to the same cloud, each additional release produces progressively less extra brightness until the cloud reaches its practical limit.
The clear conclusion: MCB is not a silver bullet. It could serve as a vital emergency measure to reduce stress and buy time, but it cannot provide a long-term solution unless combined with efforts to reduce global greenhouse gas emissions.
In the real world, the field program has proceeded with extreme caution, working closely with government regulators who wanted Engagement, Harrison said, is a genuine two-way dialogue. Community panels, tourism operators, regulators, and local residents have all had direct input into where and how the research is conducted.
While this rigorous community process has occasionally slowed the pace of scientific work, Harrison argued that without it, the research could not have proceeded at all. The Great Barrier Reef is one of the most highly regulated marine environments in the world. Every field campaign requires extensive permits, risk assessments, reporting, and repeated reviews before it can be expanded.
A further question concerned whether this project should be considered “geoengineering” a controversial term usually associated with efforts to cool Earth’s climate on a global scale.
Harrison explained that he does not describe the reef project as geoengineering, even though it uses similar technology. He emphasized a sharp distinction between an intermittent, regional intervention designed to protect a specific ecosystem during periods of extreme summer heat and a planetary-scale effort intended to permanently alter Earth’s overall reflectivity.
Trying to cool a small fraction of the globe for a few weeks carries entirely different risks from attempting to cool the entire planet.
The risk-benefit equation, he argued, is fundamentally different. Trying to cool a small fraction of the globe for a few weeks carries entirely different risks from attempting to cool the whole planet.
This localized, protective focus is central to why the Australian public has generally accepted the project. While the lessons learned on the reef will undoubtedly contribute to broader research on solar radiation modification, Harrison emphasized that the reef project must be evaluated according to its own unique purpose, scale, and merits.
This project is arguably the most significant and successful direct climate-cooling research initiative in the world. However, the potential extinction of coral reefs is also a race against time. It requires immediate attention and support to protect one of Australia’s—and the world’s—most vital ecological treasures.
Unfortunately, due to limited public awareness and concern, Australian government funding for the project is expected to end in 2026.
Robert Tulip is a retired Australian who worked for many years in his country’s international development programs.