Seaweed could save the world’s oceans from becoming too acidic

The CO₂ that humans are pouring into the atmosphere as we burn fossil fuels is not only raising temperatures, it’s changing the Earth’s oceans. When CO₂ is dissolved in water, it becomes carbolic acid, which increases acidity. Since the beginning of the industrial revolution, ocean acidity has increased by 30% due to this process. So long as we continue to produce CO₂, ocean acidification presents a growing problem—unless we can find a way to offset its effects. Recently, scientists in China may have done just that. Seaweed, they found, can actually remove CO₂ from the ocean, keeping its acidity level down.

It’s only been about a decade since the problem of ocean acidification was first identified, and five years since methods for studying its impacts on marine environments were standardized. Very early studies (conducted in 2007) on the impact of acidification on coral reefs indicated that a doubling of atmospheric carbon dioxide (CO₂) above the pre-industrial average (likely to occur in the next few decades) would reduce by 40% the ability of corals to form their skeletons. Recently it’s been shown that some kinds of corals are less sensitive than others, and that the net average impact of a doubling of CO₂ might be a decline of 14% or less.

A new discovery is that acidity is likely to affect every stage of coral’s life cycle, yet almost no studies have been published on the effect of acidity on coral larvae. Moreover, it turns out that you can’t really study acidification in isolation. Its impacts may be lesser or greater depending on sunlight, temperature and the presence of oxygen. Despite all the uncertainty in this rapidly developing field, one thing is clear. More acid is bad for marine animals that rely on calcium carbonate to build skeletons and shells. Carbonate ions are rarer in acidic water, which slows these organisms’ development. So from prawns to oysters, we’re likely to taste and feel the cost of those old coal-fired power plants and inefficient cars on our dinner plates, and in our food bills.

Recent experiments in China have shown that in certain circumstances seaweed can be used to restore seawater to a less acidic state. The waters of the Yellow Sea off Lidao Town in northeastern China are famous for their yields of the edible seaweed Laminaria japonica. Seaweed farms there cover about 500 square kilometers of ocean surface, and they yield around 400,000 tonnes of product annually. Because the fast-growing seaweed takes in CO₂ through photosynthesis as it grows, and is removed from the ocean at harvest, it is an excellent means of removing the acidifying CO₂ from the water. Indeed the seaweed farms of Lidao more than reverse the local acidification threat, providing a safe and nurturing environment for shelled creatures. In other parts of China, where seaweed is farmed in conjunction with scallops, the buffering of the seawater provided by the seaweed provides an environment in which the scallops thrive.

Seaweed is hugely productive, outstripping the fastest-growing land-based crops many times over in its rate of growth and CO₂ absorption. Globally, the potential scale of seaweed farming is 600 times greater than any other method of cultivating algae. Seaweed is finding many uses beyond food, from medicine to fuels, and it may be that seaweed farms will offer refuges for marine species under threat from increasing acidification. One study asserted that seaweed farming could produce 12 gigatonnes per year of biomethane, while storing 19 gigatonnes of CO₂ per year directly from biogas production, plus up to 34 gigatonnes per year from carbon capture of the biomethane combustion exhaust gas. All of this could come from seaweed “forests” covering an area equal to 9 per cent of the world’s ocean surface. This would produce enough biomethane to replace all of today’s needs in fossil fuel energy, while removing 53 gigatonnes of CO₂ per year from the atmosphere, thus more than offsetting all human CO₂ emissions. A side benefit would be an increase in sustainable fish production, providing 200 kilograms per capita per year of fish for a population of 10 billion.

While the potential of seaweed to reduce both acidification and global warming is huge, we are very far from being able to exploit it. All marine plants, from seagrass to giant kelp and even humble seaweed, help reduce acidification. Yet never have they been more threatened. All round the world coastal infill, dredging and pollution are threatening the ocean meadows. Unless we address these problems, all the aquaculture presently deployed could be insufficient to compensate for their loss.

The above text is excerpted from Tim Flannery’s latest book Atmosphere of Hope, out now.