About 10 percent of US oysters come from the mild, shallow waters of Apalachicola Bay off Florida’s northwest coast. But the industry has suffered severe losses in recent years – partly because the bay is warming and its water is becoming more acidic due to rising carbon dioxide levels. The situation is so bad that in 2020 the state banned oyster harvesting for five years. Soon after, authorities encouraged climate scientists to conduct an unusual experiment.
In May, researchers injected about 2,000 liters of lime-enriched seawater into an estuary of the Apalachicola River. The alkaline powder is obtained from chalk or limestone, it is the main component of cement. It was found to neutralize some of the acid while removing carbon dioxide (CO₂) from the atmosphere.
As far as the researchers know, this is the first practical test of this technique, known as ocean liming. “It’s very valuable to see this response in a real system,” says Wade McGillis, an engineer and climate scientist at the University of Notre Dame, Indiana, who co-led the work. The team recently presented the results at a conference of the American Association of Geoscientists AGU.
The attempt is also one of the few tests so far for geoengineering – the controversial idea of artificially altering the atmosphere or the ocean to counteract the effects of climate change. For ocean geoengineering, “it’s really good when experiments like this become the norm,” says Ken Caldeira, a climate scientist at the Carnegie Institution for Science Foundation. Such demonstrations could allay fears, Caldeira says, by proving that small-scale disruptive influences don’t cause lasting environmental problems or ecological damage.
“We’ve got a really nice little glitch”
The ocean is already mitigating the effects of climate change by naturally absorbing 30 percent of annual carbon dioxide emissions. But when the CO₂ dissolves in the water, it combines with calcium and other ions, lowering the pH of the water. This harms marine life and the oceans’ uptake of CO₂ slows down. “Alkaline enrichment” aims to restore water chemistry.
The added lime dust raises the pH of the water and allows it to bind more CO₂ in the form of calcium bicarbonate. In addition, the shells of sea creatures can accumulate more carbonate. Liming improves the way the ocean removes CO₂, says Harald Mumma, a doctoral student in environmental engineering at Notre Dame. “We just speed up the natural processes and make sure that this doesn’t happen in geological time, but in human time.”
A 2021 report from the National Academies of Sciences, Engineering, and Medicine (NASEM). calls for 2.5 billion US dollars for the coming decade for research into geoengineering of the oceans, including field tests for increasing alkalinity. According to Débora Iglesias-Rodriguez, an oceanographer at the University of California, Santa Barbara and co-author of the NASEM report, scientists are increasingly pushing the limits of what can be studied in the laboratory. In the lab, you can’t see how an alkaline plume spreads in the ocean, how added particles clump together, or how the chemicals affect marine life. For all these reasons, according to McGillis, “we urgently need to do field research.”
McGillis had worked with Apalachicola Conservation Area officials for several years to study the oyster die-off. When he raised the possibility of an experiment, officials readily agreed. The team then performed several releases, using a non-toxic dye to track the plume in the water. Taking water samples showed the pH didn’t rise too much – a relief to researchers who feared it could disrupt sea life. “We’ve got a really nice little glitch,” says McGillis. One release occurred deeper in the estuary, at a long pier, where microbes had already reduced dissolved CO₂ levels to about 200 ppm, compared to more than 400 ppm in the atmosphere. The lime lowered the CO₂ levels by another 70 ppm, allowing the water to absorb even more carbon. The researchers monitored the metabolism of the oysters and the microorganisms during the experiment and could not find any abnormalities.
Liming is just one possible technique for storing carbon in the sea. In April, researchers from the Center for Climate Repair at the University of Cambridge, along with India’s Institute of Maritime Studies, distributed iron-coated rice husks in the Arabian Sea. The researchers hoped that adding the nutrient iron would encourage a bloom of photosynthetic algae, which then take up carbon. As soon as the algae die and sink, this would also be stored. Unfortunately, shortly after the application, a storm blew up and shook up the shells, making it difficult to follow their effects. “The result was inconclusive,” says Hugh Hunt, a climate engineer on the Cambridge team. Since February, researchers have also been trying to bind carbon in giant kelp algae off the coast of Namibia – in this way they have cultivated a carbon-hungry underwater forest.
The Florida trial is not the first field trial of alkaline fortification. In 2014, Caldeira and colleagues added sodium hydroxide, also known as lye and a component of many soaps and detergents, to part of Australia’s Great Barrier Reef. They found that this raised the pH to almost pre-industrial levels, which increased the reef’s natural calcification. However, the great advantage of lime is that it is already being produced in huge quantities for the cement industry, says McGillis.
However, Caldeira’s team wrote that the approach was “inadequate” as a global solution. This is because it is difficult to produce alkaline additives without CO₂ emissions. For example, heating limestone to make lime releases enough of the gas to partially offset the increased uptake by the ocean. Even if a low-emission lime could be produced, it would probably be too expensive to dump into the ocean.
But liming the ocean has a distinct advantage over other geoengineering proposals, such as releasing light-reflecting particles into the atmosphere. “Changing the chemistry of seawater is much more controllable than throwing particles into the air,” says McGillis. Such particles could remain in the stratosphere for months or years. Additives in the sea typically only last a month before being diluted and dispersed, he says. “It gives a lot better control if something goes wrong.”
This post comes from the science magazine Science. It is not an official translation of the Science-Editorial staff. In case of doubt, the English original, published by the AAAS, applies. German editing: cvei