麻豆传媒

A SPRINKLING of iron ore 鈥済lued鈥� onto rice husks using goo from plants hardly sounds like a recipe for saving the planet. Not to mention the fact that the mixture is designed to mimic whale faeces.

And yet if a team of researchers backed by a former chief scientific adviser to the UK government crack this, it could be coming to an ocean near you soon. Theirs is just one of several projects across the world, small in scale but big in vision, looking at a new way to stave off the worst effects of climate change: engineering the oceans.

Similar 鈥済eoengineering鈥� proposals are highly controversial, and this idea is no different, horrifying those who warn of the potential unintended consequences of fiddling with sensitive marine environments. But the world鈥檚 lack of progress on curbing carbon emissions might make it necessary. A recent report from the Intergovernmental Panel on Climate Change (IPCC) on how to tackle climate change made clear that deploying techniques to remove carbon dioxide from the atmosphere will be 鈥渦navoidable鈥� if humanity is to achieve net zero carbon emissions around the middle of the century. On land, there are plenty of schemes to do that, from planting trees to machines in Iceland that chemically capture CO₂ so it can be buried deep underground. But getting any of them to the scale we need in time to really make a difference is a tough ask. It could be that we need the oceans, too.

Using the oceans as a solution to climate change is hardly a new idea. A horizon studded with nearly 200-metre-tall wind turbines is already a familiar sight for many people living along the coast of the UK and other countries around the North Sea. Other parts of the world will soon become used to them too. Last year, as the rest of the world did in the past five years.

Yet Earth鈥檚 great blue expanses could play a far more active role. To avoid global warming breaching 1.5掳C, the target the world agreed in Paris in 2015, the that all forms of CO₂ removal would need to suck 584 billion tonnes of the stuff out of the atmosphere between 2020 and 2100, or just over 7 billion tonnes every year.

For now, planting trees is the cheapest method. But there is only so much land for trees: at most we can squeeze in around a trillion more alongside the 3 trillion we have already, according to one estimate. Terrestrial options for removing CO₂ also often compete with the need to feed almost 8 billion people. The approach considered by the IPCC to have the greatest potential 鈥� planting crops that remove CO₂ as they grow and then burning them for energy and capturing the carbon emissions 鈥� would require vast tracts of land and huge amounts of water that could have been used for food crops.

In that context, looking to the seas makes sense. 鈥淭he ocean covers 70 per cent of the Earth鈥檚 area, so you get a lot more area to work with and can potentially avoid some of the issues that pop up with land-based CO₂ removals,鈥� says at the World Resources Institute, a US-based non-profit organisation.

The oceans already contain . 鈥淚f you look at the numbers of CO₂ that need to be sequestered to become carbon neutral by 2050 [globally], that鈥檚 when people realise, OK, we need to include the ocean in this picture,鈥� says at GEOMAR, a research institute in Kiel, Germany.

According to some estimates, ocean-based technologies might remove as much as 100 billion tonnes of CO₂ a year. Constraints such as feasibility, or only using renewable energy to power schemes, shrink that figure closer to around 5 to 10 billion tonnes a year. But that would still be a major contribution 鈥� 10 billion tonnes represents around a quarter of humanity鈥檚 current annual carbon emissions. 鈥淚t鈥檚 massive,鈥� says Riebesell.

A in December 2021 by the US National Academies of Sciences, Engineering, and Medicine (NASEM) called for multimillion-dollar research programmes into how ocean-based CO₂ removal might be done safely and without unintended negative consequences. For the moment, those do remain multimillion-dollar questions.

One obvious method would be the equivalent of afforestation on land: restoring natural carbon sinks such as kelp forests, seagrass and mangroves (see 鈥�Greening the blue鈥�, page 48). But their potential is probably limited, hence the interest in engineering approaches.

One of the original ideas is iron fertilisation. It involves seeding areas of the ocean with iron, a key nutrient that stimulates the growth of carbon-absorbing phytoplankton on the ocean surface. The idea is fish and other organisms eat the plankton and lock the carbon away in the ocean鈥檚 depths when they die and sink to the bottom. At least 13 experiments using iron have taken place since 1993. , a GEOMAR researcher who is one of the committee members behind the NASEM report, says it is the method that researchers have the most field experience with. Despite some initial signs that it could be effective, it has had 鈥渕ixed success鈥�, he says. The image of the technique has also been tarnished after a 2012 large-scale test off Canada was accused of disregarding international treaties on the dumping of material at sea.

This is where , a chemist at the University of Cambridge and a former chief scientific adviser to the UK government, and his artificial whale poo come in. Now chair of the independent Climate Crisis Advisory Group, his idea is to emulate the natural role played by whales defecating near the ocean surface, which releases nutrients such as phosphorus and nitrogen that once again stimulate phytoplankton growth. The NASEM report was 鈥渁 game changer鈥�, he says, with several universities afterwards joining his group of institutes working on the synthetic whale faeces idea. The first small-scale field trials took place off the coast of Goa, India, in April and May. Depending on the results, further field trials will take place in the northern Pacific Ocean and the Southern Ocean.

It isn鈥檛 the first experiment of its kind. An Australian team called WhaleX deposited a mix of nutrients off the coast of Sydney in December and is planning bigger demonstrations. Similar ideas include 鈥渁rtificial upwelling鈥�, using pipes to bring nutrient-rich water from the ocean depths to the surface, again to create phytoplankton blooms. Tests have taken place in a bay off Gran Canaria, a Spanish island in the Atlantic, but the idea is still largely conceptual and the NASEM report indicated low confidence in its ability to remove carbon and scale up.

The big objection to all these schemes is the possibility of negative environmental side effects. Creating phytoplankton blooms reduces oxygen availability and affects light penetration into the ocean, impacting the animals that live there, says Lebling. Deoxygenation, fuelled by warming and the effects of pollutant run-off from land, is an ongoing trend in the oceans, and anything exacerbating it would be bad news.

That is one reason much current ocean CO₂ removal research is focusing on a different technique, known as ocean alkalinity enhancement. Large-scale meddling to change the pH value of the oceans might instinctively make us squeamish, but we are already doing it. For more than a century, the oceans have been absorbing much of the CO₂ that humanity鈥檚 factories and cars have pumped into the air. When this dissolves in seawater, it creates carbonic acid, increasing overall acidity.

Increasing acidity not only diminishes the ability of the oceans to draw down further CO₂, but also reduces how fast organisms from corals to mussels to zooplankton can use calcium and carbonate to build their shells and skeletons, and in the worse cases speeds up how quickly those protective structures dissolve. The net effect is to harm the entire food web above them, including us, and in the case of coral reefs to undermine their ability to act as a buffer against climate change impacts such as greater coastal flooding.

Alkalinity enhancement aims to reverse these trends, either through adding alkaline minerals to seawater or running an electric current through it to split the water into basic and acidic ions, then removing the acidic stuff. This approach is attractive because it could store carbon for a long time in the water and is scalable, says Riebesell.

Enhancing ocean alkalinity

The technology hasn鈥檛 yet been tested on a large scale, says Lebling. But it recently acquired a high-profile and wealthy backer. In January, Mike Schroepfer, a senior fellow at Meta (formerly Facebook), launched a $100 million side project called the Ocean Alkalinity Enhancement R&D Program to look at tweaking ocean pH. 鈥淭his may be one of the most promising pathways for capturing carbon and turning back the clock on the damage we鈥檝e done to the planet,鈥� he . The project has three pillars: one awarding funding for research, a second backing technology prototypes for small-scale deployment and a final one supporting research and engineering teams, such as helping them obtain permits for field studies.

In theory, enhancing ocean alkalinity should be relatively benign, says Riebesell: you are just turning back the dial on acidification, and you can do it in a controlled way. 鈥淚f you could spread it nicely over a large area of the ocean, the likely environment impact is extremely small,鈥� he says. The problem is that that isn鈥檛 how it works in reality: the alkaline materials you are loading into the ocean will always come from a localised point source, such as a ship, and negative impacts are likely at that point, says Riebesell. These could include reducing plankton that fish rely on or physiological impacts on other marine organisms.

鈥淚n theory, enhancing alkalinity to reverse ocean acidification should be relatively benign鈥�

One encouraging sign is that in a yet-to-be-published study conducted last autumn off Gran Canaria, Riebesell and his colleagues found no biological impacts from changing the alkalinity of seawater in nine floating 鈥渕esocosm鈥� bags, enclosed chambers that reflect the wider natural environment. In May, he started further tests south of Bergen, Norway, that will try out different mineral types, one silicate and one carbonate-based, in coastal waters. The location is near Europe鈥檚 largest mine for olivine, a silicate mineral considered a leading candidate for making oceans more alkaline.

Myriad practical and technical challenges could still pop up. Alkalinity enhancement using minerals would require about 3 to 4 tonnes of material per tonne of CO₂ removed, meaning a global scale project would require effectively doubling existing operations to mine the minerals involved, says Riebesell. The scale of that effort would be 鈥渕ind-blowing鈥�, he says.

Meanwhile, some unresolved questions dog all proposed ocean engineering schemes. One is who decides if activity such as alkalinity enhancement and nutrient boosting can take place. Most visions for such forms of ocean carbon capture are for the lightly regulated high seas, outside of the more tightly controlled Exclusive Economic Zones around the coasts of countries. 鈥淥n the governance side, the biggest issue is there isn鈥檛 an explicit framework for carbon removals,鈥� says Lebling. Some approaches could fall under two international treaties, the (UNCLOS) and the .

These have already been invoked in the case of iron fertilisation, though an amendment to the London Convention means it doesn鈥檛 block research projects. Moreover, they aren鈥檛 universal. Some countries that are actively exploring ocean-based carbon removal aren鈥檛 fully signed up: the US hasn鈥檛 ratified UNCLOS, for example. Muddying matters further, countries signed up to the Convention on Biological Diversity have passed a series of relevant non-binding resolutions, including one in 2008 saying iron fertilisation projects should be avoided unless scientifically justified. Lebling says there is disagreement over whether those existing frameworks can be amended, or whether an entirely new treaty on ocean-based CO₂ removal is needed. That means governance can seem a 鈥渧ery daunting problem鈥�, she says.

Riebesell thinks public attitudes could be a barrier, too. 鈥淧eople have this gut feeling the ocean is pristine. We know it鈥檚 not. We鈥檙e messing with it every day,鈥� he says. Lebling also thinks public perception could be a challenge. Among researchers, there is also a concern that private start-ups seeking a quick return may rush things, or be inclined to ignore negative research findings about side effects. One fear is a cowboy company ploughing ahead too fast with the likes of ocean alkalinity enhancement and triggering a backlash.

Equally, there is a risk of going too slow and never scaling projects up. The debate on governance in particular 鈥渘eeds to move a bit faster鈥�, says at the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany.

Riebesell says society needs to start weighing up the threats posed by extreme weather and unchecked global warming against the risks of ocean-based carbon removal. 鈥淎t some point, climate change and the negative effects of that will be so obvious to our societies that we鈥檒l be more willing to properly compare the relative risks,鈥� he says. 鈥淟et鈥檚 not look at risks in isolation. Doing nothing is not an option any longer.鈥�

Greening the blue

Is there a 鈥渘atural鈥� way to enhance the potential of the oceans to lock away climate-warming CO₂? Planting more trees on land can help draw down more CO₂ from the atmosphere 鈥� the basis of many schemes for carbon credits that companies buy to offset their emissions. Something similar might work in the oceans, by stimulating the growth of marine and coastal ecosystems such as kelp forests, mangroves and salt marshes.

Some regard the potential for this 鈥渂lue carbon鈥� as huge, although as yet there is no mechanism for integrating it into carbon offsetting schemes. 鈥淚t鈥檚 really hard to turn blue carbon conservation and restoration into carbon credits that you can sell,鈥� says of Duke University in North Carolina. 鈥淵ou have to go out and measure all the carbon that鈥檚 there, you have to show that it鈥檚 not going to be lost, you have to keep monitoring it.鈥� Virdin and others have proposed (reducing emissions from deforestation and forest degradation) to the ocean, but that is an idea whose ship has yet to sail.

There is still some doubt about how big the marine offsetting effect might be. In March, the UK government鈥檚 climate adviser, the Climate Change Committee, found that restoring and creating seagrass and saltmarsh ecosystems in the country would , removing 鈥渨ell below鈥� 1 million tonnes a year. Nonetheless, the committee said these are efficient carbon stores and conserving them is important given how much fishing and other activities have degraded them.