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 dissolved CO2 reacts in the water, it creates bicarbonate and hydrogen ions. Those excess hydrogen ions lower the pH of the water, 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.”