Expanding seaweed farming poses new risks to the ozone layer

For decades, the world has watched the ozone layer heal. This invisible shield high in the atmosphere protects life on Earth by absorbing the sun's most harmful ultraviolet rays. Its recovery followed a global agreement to stop producing specific industrial chemicals that were tearing it apart. Now, as the planet seeks new ways to fight climate change, a different solution is rising from the sea: vast farms of seaweed. These underwater crops are celebrated for their ability to pull carbon from the air and provide food and materials for a growing population. But as scientists look closer at this green solution, they are finding a hidden cost that could undo the progress made on the ozone layer. The concern centers on tiny, short-lived gases that seaweed naturally releases. While these gases vanish quickly, they are incredibly efficient at destroying ozone when they reach the upper atmosphere.

A new study brings this hidden risk into sharp focus, asking what happens if we expand seaweed farming to the massive scales currently proposed for global sustainability. The researchers did not just guess; they built a detailed computer model of the Earth's atmosphere and fed it new data on how much gas seaweed farms might release in the coming decades. They tested three different paths for the future: a steady, moderate growth that matches current trends, a faster expansion driven by blue-economy ambitions, and a maximum, highly aggressive growth scenario. The results were striking. Under the moderate path, the recovery of the ozone layer would slow down significantly by the end of the century. Under the faster scenarios, the damage would be severe and rapid. In the most aggressive case, the ozone layer over Antarctica could shrink back to levels worse than the historic "ozone hole" of the early 2000s by the 2060s. Unlike the old ozone hole, which was a seasonal event limited to the polar regions, this new depletion would spread across the globe and persist year-round, exposing more people and ecosystems to dangerous ultraviolet radiation.

The story begins with the seaweed itself. Seaweed is not just a passive plant; it is a living factory that produces a variety of gases. When the algae are stressed by bright sunlight, changes in water temperature, or being eaten by small marine creatures, they release short-lived halogens. These are gases containing bromine and chlorine that are naturally present in the ocean but are usually released in small amounts. Two of these gases, bromoform and dibromomethane, are particularly potent. Once they drift up into the stratosphere, they break down and release bromine atoms. A single bromine atom can destroy thousands of ozone molecules, acting like a tiny, relentless eraser. For years, scientists knew that wild seaweed released these gases, but the amounts were small compared to the industrial chemicals banned decades ago. The question was whether turning the ocean into a massive agricultural landscape would tip the balance.

To answer this, the team gathered data on how much gas different types of seaweed release. They looked at measurements from dozens of studies, covering many species and conditions. They found that the amount of gas released varies wildly depending on the species and the environment. Some seaweeds are much more prolific emitters than others. Using this data, they calculated how much gas would be released if seaweed farming grew at the rates currently being discussed by policymakers. They started with the historical growth rate of about six percent per year, which has been the trend for the last two decades. They then imagined a scenario where that rate doubled to twelve percent, and a third scenario where it jumped to twenty percent, a pace that would push farming to the very limits of what the ocean can support.

The researchers then plugged these emission numbers into a sophisticated Earth system model, a powerful computer simulation that mimics the complex interactions between the ocean, the atmosphere, and chemical reactions. They ran the simulation from the present day through the end of the century. The model showed that even the moderate growth scenario would have a measurable impact. By the late 2090s, the recovery of the ozone layer would be significantly delayed, with the Antarctic ozone levels failing to return to their pre-damage state. However, the faster growth scenarios painted a much darker picture. If farming expanded at twelve percent a year, the ozone layer would begin to decline again, dropping below the historical minimums by the 2090s. If the twenty percent growth rate were achieved, the damage would be swift and catastrophic. The model showed the ozone layer over Antarctica falling below its worst-ever recorded levels by the 2060s.

What makes this finding particularly alarming is the scope of the damage. The historic ozone hole was a seasonal phenomenon, appearing mostly in the spring over the South Pole. The depletion driven by these new seaweed emissions would be different. It would not be confined to the poles or to a specific time of year. The model showed the thinning of the ozone layer spreading to mid-latitudes, where billions of people live, and persisting throughout the entire year. This would lead to a significant increase in the amount of ultraviolet radiation reaching the Earth's surface. The study calculated that under the highest growth scenario, the extra radiation reaching the ground would be comparable to, or even greater than, the levels seen during the peak of the Antarctic ozone hole. This increase would not just be a number on a chart; it would translate to higher risks of skin cancer, cataracts, and damage to marine life and crops in regions that have largely been spared from such intense exposure.

The researchers were careful to note that their estimates might actually be on the conservative side. They focused only on the two most common gases, bromoform and dibromomethane, but seaweed releases other ozone-depleting gases as well that were not included in the calculation. They also assumed that the types of seaweed being farmed would remain similar to what is grown today. However, if the industry shifts toward species that are known to release even more gas, or if climate change stresses the seaweed and causes them to release more, the problem could be worse than predicted. Furthermore, the study assumed that the farming would happen in specific zones, but if it expands into new areas or if the species composition changes to include high-emitting varieties like Asparagopsis, the emissions could surge.

The paper concludes that while seaweed farming offers great promise for climate mitigation and food security, it carries a serious, unintended risk to the ozone layer. The scale of the proposed expansion is large enough to threaten the hard-won recovery of the ozone shield. The study suggests that there is a limit to how much seaweed can be farmed without causing global harm. If emissions from seaweed farming exceed two to three times the current natural background levels, the ozone layer could begin to deteriorate again. This does not mean that seaweed farming must stop, but it does mean that the industry cannot grow unchecked. Strategic choices about which species to farm, where to place the farms, and how fast to expand will be critical. The researchers call for careful monitoring and proactive planning to ensure that the solution to one environmental crisis does not become the cause of another. The ocean's green crops could feed the world, but only if we manage them with a clear understanding of the delicate chemical balance they interact with.