Hakai Magazine

Coastal science and societies

Monterey Bay, California
Monterey Bay, California, absorbs excess carbon dioxide produced in nearby Silicon and Salinas Valleys. Photo by Yuval Helfman/Alamy Stock Photo

Cities Acidify the Water Next Door

Coastal communities have a stronger effect on local ocean acidification than previously believed.

Authored by

by Jess Mackie

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Sea turtles caught in fishing nets. Albatross stomachs clogged with bottle caps. Campaigns to clean up beaches and ban single-use plastics often use images of remote animals to elicit action, urging us to think global, act local. Yet when it comes to carbon emissions and their effects on ocean chemistry, a recent study suggests that if you live near the coast, you ought to think local, too.

That’s because new measurements taken in California’s Monterey Bay show that it absorbs carbon dioxide emissions from the surrounding cities and agricultural lands, making it more acidic. The finding is reminiscent of the urban heat island effect, in which cities tend to be a few degrees warmer than the surrounding countryside.

According to Francisco Chavez, a biological oceanographer at the Monterey Bay Aquarium Research Institute, the finding runs counter to the prevailing understanding of how greenhouse gases mix into the air. Scientists have typically assumed that carbon dioxide emitted by everything from cars to respiring plants rapidly combines with other molecules to form a homogenous aerial cocktail, he says.

After five years of analyzing carbon dioxide concentrations in the air and water of Monterey Bay, however, Chavez and his team found that when atmospheric carbon dioxide peaks in the early morning, it creates a dome of carbon-dioxide-laden air. When the seaward wind picks up, it carries this unmixed carbon-dioxide-rich mass offshore, where it gets absorbed by the bay. Every day, when the wind is right, the water within about 100 kilometers of shore receives a heavy dose of carbon dioxide.

There’s another reason this effect has been overlooked until now, Chavez says. Typically, scientists measuring carbon dioxide concentrations in and over the ocean do so from fossil-fuel-burning ships. If a result is abnormally high, “you’d say, ‘Oh, that’s because the wind was blowing from the smokestacks over my sensor, so it must have contaminated it,’” he says.

The Monterey Bay team, however, used sensors anchored to the seafloor or affixed to a wave- and solar-powered glider to get around this source of potential interference. That’s how they discovered that 20 percent more carbon dioxide is entering Monterey Bay than scientists had previously estimated, meaning that nearshore acidification is more intense than they thought. Chavez says other coastal areas with strong offshore winds and large carbon-emitting sources are likely having a similarly disproportionate effect on nearby water.

Wiley Evans, an oceanographer with the Hakai Institute* who was not involved in the study, says that while this process is likely playing out in other coastal regions, there is a caveat. The magnitude of the impact on coastal oceans will not be uniform, he says, but will vary due to geographical and meteorological differences.

What the study does highlight is that “local actions have local consequences on acidification,” says Shallin Busch, an ecologist with the US National Oceanic and Atmospheric Administration’s Ocean Acidification Program who was not involved in the research.

“We have this huge diversity of [life] along the coast that [is] going to be influenced by this higher uptake of carbon dioxide,” Busch says. The finding is a wake-up call for biologists who, like her, have been studying the threat ocean acidification poses to animals such as shell-building bivalves and pteropods—marine life that play critical ecological, cultural, and economic roles in coastal societies.

Now that Chavez and his team have quantified carbon dioxide, he says the next step is to examine what other contaminants are entering the bay.

“Whatever you can imagine, we’re generating it over cities,” he says. “It’s coming into the ocean in the same manner.”

*The Hakai Institute and Hakai Magazine are both part of the Tula Foundation. The magazine is editorially independent of the institute and foundation.