An ocean of opportunity: exploring the potential risks and rewards of ocean-based solutions to climate change

The ocean is a vast repository for carbon. Over the last century, it has absorbed nearly a third of the carbon dioxide that humans have pumped into the atmosphere through the burning of fossil fuels. This carbon ends up dissolved into the water, captured by organisms, folded into coastal sediments, or buried in the deep sea.

How the ocean stores carbon

The surface ocean and the air above it are constantly exchanging gases, trying to reach an equilibrium. When carbon dioxide is more plentiful in the atmosphere than in the surface waters, it diffuses into the ocean. There, physical and biological processes work to pump some of the carbon into deeper waters, making room for more absorption at the surface.

Some dissolved carbon dioxide bonds with molecules weathered from rocks or ancient shells, locking it into a new, complex form that cannot easily escape back into the atmosphere. Other carbon is dragged to the depths by major ocean currents, which pull warm surface waters laden with carbon dioxide toward the poles, where they cool and sink. Once carbon has reached the deep ocean, it can remain there for hundreds to thousands of years.

On the biological side, marine algae play a key role, using dissolved carbon dioxide to conduct photosynthesis. When those phytoplankton and other microorganisms die, they too sink to deeper waters.

carbon cycle
In various forms, carbon is continuously exchanged between Earth’s atmosphere, land, and water—an essential cycle for life and regulating the planet’s climate. Atmospheric carbon dioxide readily dissolves in the ocean’s surface waters, where some of it is taken up by living organisms or sequestered in deep-sea rocks and sediments. (Illustration by Natalie Renier, WHOI Creative, © Woods Hole Oceanographic Institution)

Just add alkalinity

Over geologic timescales, alkaline molecules weathered from rocks or released by the dissolving shells of long-dead microorganisms help the ocean lock away a significant amount of carbon and play an important role in regulating the carbon dioxide levels in our atmosphere. But since we don’t have millions of years to address climate change, some scientists are investigating the possibility of boosting the ocean’s alkalinity to mimic the effects of rock weathering over a much shorter time scale.

Alkaline molecules react with dissolved carbon dioxide to create carbonate and bicarbonate, forms of carbon that don’t diffuse easily back into the atmosphere. At the sea surface, this reaction lowers the amount of dissolved carbon dioxide relative to the adjacent atmosphere, which encourages more carbon dioxide to diffuse into the ocean. Essentially, ocean alkalinity helps make space for more carbon storage.

But alkalinity from the slow and steady weathering of rocks can’t keep pace with how rapidly we are producing carbon dioxide.

“The ocean is going to be less and less able to take up more CO2 because it’s basically becoming saturated,” says Adam Subhas, a marine chemist at WHOI. “By adding alkalinity, we could restore the ability of the ocean to continue to soak up more CO2.”

There are several potential ways to add alkalinity to the ocean. One proposal is to seed beaches with finely crushed olivine, an igneous rock that weathers relatively quickly, and let wave motion dissolve it into the sea. Other proposals involve using electric currents to induce alkalinity or scattering alkaline minerals from vessels already crossing the ocean.

“Each one of those has very different engineering challenges, costs, and risks associated with them,” Subhas says. “But from my perspective, the big question is: Does adding alkalinity to the ocean do anything to marine ecosystems?”

When carbon dioxide dissolves into the ocean, it combines with seawater to form carbonic acid. The acid rapidly breaks down, releasing hydrogen ions. This process lowers the pH of the ocean and increases its acidity. In a stable climate system, alkalinity provides a buffer by snatching up the excess hydrogen to form bicarbonate, thereby stabilizing the pH.

As human-made sources of carbon dioxide have outpaced natural sources of alkalinity, the ocean has grown increasingly acidic. Adding alkalinity to the ocean may counteract ocean acidification, in addition to helping with our carbon problems.

But the rapid addition of anything to the ocean has the potential for unintended consequences, even if it is mimicking a natural process. Subhas and his colleagues have been running experiments to investigate potential impacts. Last year, the group collected samples from two sites—one near the Bahamas and one in the central North Atlantic Ocean—to look for changes in how phytoplankton fix carbon or certain species build shells when alkalinity is added to the water.

“We’re starting to learn that we’re not going to be able to bring things back to exactly the way they were,” Subhas says. “There are going to be other effects, and the only way to understand them is to actually look and see what those effects are at a small scale, then build up to larger scales as people are ramping up on the technology side.”

But Subhas is optimistic that alkalinity enhancement could be one piece of a larger climate solution.

“The ocean has soaked up hundreds, if not thousands, of gigatons of carbon before,” Subhas says. “Every option needs to be on the table, and needs to be researched seriously, because of the scale of the problem.”

An ocean decade

This year marks the start of the United Nations’ Decade of Ocean Science for Sustainable Development. It is an opportunity to bring together global collaborations to conduct “the science we need for the ocean we want.” The effects of climate change are threatening coastal communities and ocean ecosystems alike—this is the perfect moment to look to the ocean for solutions.

Laura Castañón, Woods Hole Oceanographic Institution, 7 December 2021. Full article.


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