This file photo shows Santa Barbara Harbor in California. Scientists recently used fossils from the sea floor off Santa Barbara to show that California's ocean waters are acidifying at a rate twice as high as the rest of the world.

(Stephen Dunn/Getty Images)

At a Glance

Ocean acidification is a chemical process in which pH levels decline.It happens naturally over time, but scientists say it's happening faster now.The study looked at more than 2,000 shell fossils dating back more than a century.

Ocean waters off California are becoming more acidic twice as fast as any others in the world, threatening the state's economically valuable seafood and fishing industries, according to a new study by NOAA researchers.

– a chemical process in which pH levels decline – occurs when excess carbon monoxide is absorbed by sea water. Acidification happens naturally over time, but scientists say it's accelerating as carbon dioxide becomes more prevalent in Earth's atmosphere, mostly from the burning of fossil fuels. Carbon dioxide .

Among other effects, ocean acidification reduces the amount of calcium carbonate minerals in the ocean, key building blocks for the skeletons and shells of many marine organisms, including oysters, clams, sea urchins and coral.

The new study, in the journal Nature Geosciences, looked at the mass of more than 2,000 tiny fossilized shells spanning more than a century, pulled up in core samples from the Pacific Ocean floor off Santa Barbara, California.

"By measuring the thickness of the shells, we can provide a very accurate estimate of the ocean’s acidity level ," lead author Emily Osborne, a scientist with NOAA’s Ocean Acidification Program, said in a news release.

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The researchers calculated that the shells had gotten thinner over time, a sign that acidification was inhibiting growth.

These colorful spots are microscopic foraminifera shells seen under a microscope. Scientists studied the shells to determine the rate of ocean acidification off California's coastline.

(NOAA)

"It's a shift in what is usable by the organisms. So that they have fewer of those building blocks ," Osborne told the New York Times.

"I could just watch the shells literally getting thinner as I moved up through the record and got closer to the present day."

California's coastal waters contain valuable fisheries that feed the seafood market nationwide, including salmon, crabs and shellfish, which are some of the most vulnerable to acidification, according to the NOAA news release.

Osborne and her colleagues created the longest record so far of ocean acidification using directly measured marine species; The oldest samples dated back to 1895.

The acidity levels of oceans has risen, on average, 30% since the beginning of the Industrial Revolution.

Ocean acidification in California's waters varies due to natural seasonal upwelling, which distributes more acidic waters up from the sea floor, Osborne said. But that doesn't account for the rise in acidification seen in the study.

"It's like a double whammy, increasing ocean acidification in this region of the world," she said.

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