The balmy, shallow waters of Apalachicola Bay, off Florida’s panhandle, supply about 10% of U.S. oysters. But the industry has declined in recent years, in part because the bay is warming and its waters are acidifying because of rising carbon dioxide (CO2) levels. Things got so bad that in 2020, the state banned oyster harvesting for 5 years. Soon afterward, state officials encouraged climate scientists to perform an unusual experiment to see whether they could reverse the changes in the water.

In May, at an Apalachicola estuary, the researchers injected some 2000 liters of seawater enriched with lime, an alkaline powder and a primary ingredient in cement that’s derived from chalk or limestone. They showed it neutralized some of the acidity and, in the process, drew CO2 out of the atmosphere.

It is the first field demonstration of the technique, called ocean liming, that they know of. “It is precious getting this response in a real system,” says Wade McGillis, an engineer and climate scientist at the University of Notre Dame who helped lead the work, which was presented this week at a meeting of the American Geophysical Union.

The experiment is also a rare test of geoengineering, the controversial proposition of artificially altering the atmosphere or ocean to counteract the effects of rising CO2. For ocean geoengineering, “normalizing doing these experiments is really good,” says Ken Caldeira, a climate scientist at the Carnegie Institution for Science. Such demonstrations can allay fears by showing small-scale perturbations do not cause lasting environmental or ecological damage, he says.

The ocean already blunts the effects of climate change, naturally absorbing 30% of annual carbon emissions. But as it dissolves in the water, the CO2 combines with calcium and other ions, depleting them. As a result, the pH of the waters drops, harming marine life, and CO2 uptake slows. “Alkaline enhancement” aims to reset the water chemistry.

Liming is one approach. The added calcium hydroxide, or lime, raises the water’s pH and enables it to sequester more CO2 in the form of calcium bicarbonate or as carbonate deposited in the shells of sea creatures. In effect, the liming enhances the way the ocean naturally removes CO2, says Harald Mumma, an environmental engineering graduate student at Notre Dame. “We just speed up natural processes and make it happen not on geological time scales, but on human time scales.”

A 2021 report from the National Academies of Sciences, Engineering, and Medicine (NASEM) called for $2.5 billion in ocean geoengineering research in the next decade, including field tests of alkaline enhancement. Researchers are facing limits to what can be learned in the lab, says Débora Iglesias-Rodriguez, a biological oceanographer at the University of California, Santa Barbara, and co-author of the NASEM report. The lab can’t show you how a plume of alkali spreads through ocean waters, how added particles might clump up, or how the chemicals might affect marine life. For all these reasons, she says, “We desperately need to go in the field.”

McGillis had worked with officials at the Apalachicola National Estuarine Research Reserve for several years, studying the oyster decline. When he mentioned the possibility of a trial, they readily agreed. The Notre Dame–led team conducted several releases, using a nontoxic dye to follow the plume. Sampling the water, they first found that pH levels did not increase too drastically, a relief for researchers who feared it might disrupt marine life. “We got a really nice small perturbation,” McGillis says. They conducted one release deeper in the estuary, off a long pier, where microbial activity had already reduced levels of dissolved CO2 to about 200 parts per million, compared with more than 400 ppm in the atmosphere. The lime lowered CO2 levels by another 70 ppm, making room for more. They also monitored oyster and microbial metabolisms during the trial and saw no red flags.

Liming is only one possible technique for increasing ocean carbon storage. In April, researchers from the Centre for Climate Repair at the University of Cambridge, along with India’s Institute of Maritime Studies, spread iron-coated rice husks across the Arabian Sea. Iron, a nutrient, is scarce in much of the ocean; the researchers hoped adding it would fertilize a bloom of photosynthetic algae, which would soak up carbon and sequester it when the algae die and sink. Unfortunately, a storm hit soon after the deployment, stirring up the husks and making their effect difficult to track. “The result was inconclusive,” says Hugh Hunt, a climate engineer with the Cambridge team. Since February, researchers have also sought to capture carbon by cultivating giant kelp off the coast of Namibia—in effect creating a carbon-hungry submarine forest.

The Florida trial is not the first field test of ocean alkaline enhancement. In 2014, Caldeira and colleagues added sodium hydroxide—also known as lye and an ingredient in many soaps and detergents—to a part of Australia’s Great Barrier Reef. They found it raised pH levels nearly to preindustrial levels, allowing the natural calcification of the reef to increase. But the great advantage of lime is that it is already produced at enormous scales for the cement industry, McGillis says.

Caldeira’s team wrote that its approach would be “infeasible” as a global solution. That’s because it’s difficult to make alkaline additives without emitting CO2, he says. Heating limestone to make lime, for example, releases so much of the gas that it partially offsets the increased uptake by the ocean. Even if low-emission lime could be made, it would probably be too costly to dump into the ocean.

But as CO2 continues to rise and geoengineering a climate solution grows more tempting, ocean liming has a key advantage over other geoengineering proposals, such as schemes to release sunlight-reflecting particles in the atmosphere. “Altering the chemistry of seawater is much more controllable than throwing particles in the air,” McGillis says. Particles can stay in the stratosphere for months or years. Ocean additives tend to only last a month before being diluted and dispersed, he says. “There’s much greater control if it goes south.”

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