California’s coastal waters are acidifying twice as fast as the rest of the oceans, a study published Monday shows. And some of California’s most important seafood — including the spiny lobster, the market squid and the Dungeness crab — are becoming increasingly vulnerable.

The carbon dioxide emissions that contribute to the planet’s rapidly warming climate are also changing the chemistry of the world’s oceans, which have absorbed roughly 27 percent of the carbon dioxide emitted worldwide.

Ocean water is ordinarily slightly basic, or alkaline, but is becoming more acidic as it absorbs carbon dioxide. This can harm marine life, especially shellfish, because they struggle to make their shells in acidic waters.

Emily Osborne, a scientist in the National Oceanic and Atmospheric Administration’s ocean acidification program, with her colleagues studied the fossil record of planktonic foraminifera — tiny simple organisms which, like shellfish, build their shells from calcium carbonate. They have been around for millions of years, but each individual organism only lives for roughly a month.