Increased acidity reduces the abundance of the right chemical forms of a mineral called calcium carbonate, which corals and other sea animals need to build shells and skeletons. It also slows the growth of the animals within those shells.
Even slightly acidified seawater is toxic to the eggs and larvae of some fish species. In others, including amberjack and halibut, it can cause heart attacks, experiments show. Acidified waters also tend to asphyxiate animals that require a lot of oxygen, such as fast-swimming squid.
The pH scale, a measure of how acidic or alkaline a substance is, ranges from 1 to 14, with 7 being neutral. The lower the pH, the greater the acidity. Each number represents a tenfold change in acidity or alkalinity.
For more than a decade, teams led by Richard Feely, a chemical oceanographer at the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, have traveled from Antarctica to the Aleutian Islands, taking tens of thousands of water samples to gauge how the ocean's acidity is changing.
By comparing these measurements to past levels of carbon dioxide preserved in ice cores, the researchers determined that the average pH of the ocean surface has declined since the beginning of the Industrial Revolution by 0.1 units, from 8.16 to 8.05.
Geological records show that such a change has not occurred in 650,000 years, Feely said.
In April, Feely returned from a cruise to the North Pacific, where he took pH measurements at locations the team first sampled in 1991. This time, Feely's group found that the average pH in surface waters had dropped an additional 0.025 units in 15 years -- a relatively large change for such a short time.
The measurements confirm those taken in the 1990s and indicate that forecasts of increased acidity are on target, Feely said.
If CO2 emissions continue at their current pace, the pH of the ocean is expected to dip to 7.9 or lower by the end of the century -- a 150% change.
The last time ocean chemistry underwent such a radical transformation, Caldeira said, "was when the dinosaurs went extinct."
Until recently, the ocean was seen as a potential reservoir for greenhouse gases. Scientists explored the possibility that carbon dioxide could be trapped in smokestacks, compressed into a gooey liquid and piped directly into the deep sea.
Then the results of Jim Barry's experiments started trickling in.
A biologist at the Monterey Bay Aquarium Research Institute, Barry wanted to know what would happen to sea creatures in the vicinity of a large dose of carbon dioxide.
He anchored a set of small plastic rings onto the seafloor to create an enclosure and sent a robot down to squirt liquid carbon dioxide into the surrounding water. Then he waited to see what would happen to animals in the enclosures and those that happened to swim through the CO2 cloud.
Sea stars, sea cucumbers and sea urchins died immediately. Eighty percent of animals within three feet of the carbon dioxide died. Animals 15 feet away also perished in large numbers.
"When they were adjacent to the CO2 plume, pretty much, it killed everything," Barry said.
Experiments in Germany, Norway and Japan produced similar results. The evidence persuaded the U.S. Department of Energy, which had spent $22 million on such research, including Barry's, to pull the plug . Instead, the department will study the possibility of storing carbon dioxide in the ground and on decreasing emissions at their source.
Scientists say the acidification of the oceans won't be arrested unless the output of CO2 from factories, power plants and automobiles is substantially reduced. Even now, the problem may be irreversible.
"One thing we know for certain is it's not going to be a good thing for the ocean," Barry said. "We just don't know how bad it will be."
Scientists predict the effect will be felt first in the polar oceans and at lower depths, because cold water absorbs more carbon dioxide than warm water. One area of immediate concern is the Bering Sea and other waters around Alaska, home to half of the commercial U.S. fish and shellfish catch.
Because of acidification, waters in the Bering Sea about 280 feet down are running short of the materials that corals and other animals need to grow shells and skeletons. These chemical building blocks are normally abundant at such depths. In coming decades, the impoverished zone is expected to reach closer to the surface. A great quantity of sea life would then be affected.
"I'm getting nervous about that," Feely said.
The first victims of acidification are likely to be cold-water corals that provide food, shelter and reproductive grounds for hundreds of species, including commercially valuable ones such as sea bass, snapper, ocean perch and rock shrimp.