The pH of the saltwater has dropped 0.025 units since the early 1990s. The number seems unremarkable, but the pH scale is exponential, so a one-unit drop is a 10-fold decrease. The new measurement also puts the ocean on track for a dramatic decline by the end of the century.

Plankton -- tiny plants and animals that live in the ocean -- are among the creatures that could be harmed by the change. In addition to the water becoming more acidic, the extra carbon dioxide reduces the amount of chemical compounds used to construct coral and the shells of plankton.

"That's a major issue," said John Guinotte, a marine scientist with the Bellevue-based Marine Conservation Biology Institute who studies deep sea corals.

"You're likely looking at serious effects through out the marine food web across the board," he said.