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If you’ve ever had an aquarium, you know pH is everything. One slight fluctuation can send fish belly up.

Scientists say a precise pH level is also critical for the health of the world’s oceans — particularly any creature with a shell.

Yet human activities are sending unprecedented levels of carbon dioxide into the water, and making it more acidic, scientists reported at the American Association for the Advancement of Science meeting recently in San Diego.

Marine scientists are not yet sure what effect ocean acidification is having on local marine life such as shellfish found in San Diego’s waters. They do believe a more acidic pH is harming coral-rich areas like Australia’s Great Barrier Reef.

Crustaceans such as oysters and clams will also be vulnerable as the waters become more corrosive — researchers say they’ve grown 30 percent more acidic since the start of the industrial revolution.

The underlying problem causing the global rise in acidity is carbon dioxide, which gets absorbed in seawater and makes it more corrosive. According to Stanford University estimates, industry pumps 1,000 tons of carbon dioxide into the atmosphere each second.

A third of the CO2 in the atmosphere dissolves into the oceans. The West Coast is especially susceptible because of upwelling currents that carry high volumes of carbon dioxide from deeper waters, said Vicky Fabry, a Cal State San Marcos biologist who researches ocean acidification.

Oceanic researchers have already found waters corrosive enough to harm shellfish reaching a Northern California shoreline during a field study several years ago.

Fabry is part of a research team launching a three-year field study off that same stretch of coastline near Humboldt County this month. They’ll be monitoring how several key species, like red abalone, respond to the upwelling waters.

She will also collaborate with researchers at the Scripps Institution of Oceanography on a lab experiment exposing commercially viable species like sea urchins, oysters and abalone to fluctuating levels of carbon dioxide.

Anything with a shell, Fabry suspects, could be in trouble if waters continue to acidify off the California coast. Shells are made of calcium carbonate, which dissolves in the presence of acid.

“If you go to a rocky inter-tidal beach at low tide and you look at all the different organisms, most of them are calcified,” she said. “Think about that. Think about what if none of them could live in a high CO2 ocean. What would be left?”

Fabry said sand granules would essentially suffocate filter feeders. They would be easy prey for predators too.

If the trend continues, “a lot of stuff begins to come apart,” because it can’t form skeletons, said Edward Miles, a University of Washington researcher, speaking at the American Association for the Advancement of Science’s recent conference in San Diego.

If smaller organisms die off, there could be even larger changes at the top of the food chain, he said.

But it’s not clear what could be done to slow or reverse the changes that are taking place. The oceans have absorbed more than half a trillion tons of carbon dioxide in the last 150 years and that can’t really be undone, said Andrew Dickson, a marine chemist with the Scripps Institution of Oceanography.

“If you say, ‘Oh, can’t we add some magic fairy dust to the oceans to undo this problem?’ The answer is yes, but you’d probably have to put in at least half a trillion tons of fairy dust,” Dickson said, referring to the impossibility of grinding up enough calcium carbonate to counteract the carbon dioxide’s acidity.

The only thing, however, that can spare the ocean catastrophic change is lowering carbon dioxide emissions, marine experts agree. By a lot.

“It’s a centennial-scale problem,” said Miles, who deemed ocean acidification the evil twin of climate change. “Major decisions have to be made now to reduce emissions by 80 to 100 percent by 2100.”

— REBECCA TOLIN

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