Greenhouse gases have so profoundly altered the world's oceans that scientists say "corrosive" acidified water is now surfacing off the west coast of North America.

The water, capable of dissolving the shells of marine organisms, is rising up from the deep ocean along the coast from Vancouver Island to Mexico, an international team reported Thursday in the journal Science.

The corrosive water has shown up decades earlier than expected, say the researchers who warn of far-reaching impacts on the marine ecosystem.

Not only does it threaten such sensitive shelled creatures as free-swimming snails, but also the animals that feed on them.

"They're a staple for salmon," says study co-author Debby Ianson, of Fisheries and Oceans Canada, referring to pteropods, the tiny snails known as sea butterflies. They have difficulty maintaining their shells in acidified water. "They can't make it [shell] as fast as it dissolves," she says.

The study is the first to show that a large section of North America's western continental shelf, critical habitat for crab, clams, salmon and many other species, is bathed in acidified waters during the summer months.

"This means that ocean acidification may be seriously impacting marine life on our continental shelf right now," says team leader Richard Feely, of the U.S. National Oceanic and Atmospheric Administration in Seattle.

Scientists have long warned the oceans are growing more acidic. This is because they act like a sponge, soaking up vast amounts of carbon dioxide from the atmosphere, which dissolves and forms an acid in the seawater -- CO2 is one of the leading greenhouse gases associated with global warming.

It is estimated the oceans have absorbed about a third of the CO2 humans have pumped into the atmosphere through the burning of fossil fuels.

This process is "significantly reducing the greenhouse gas levels in the atmosphere and minimizing some of the impacts of global warming," Feely and his colleagues say in their Science report.

"However, the ocean's daily uptake of 30 million metric tonnes of carbon dioxide is significantly impacting its chemistry and biology," the team says. The oceans have become more acidified since beginning of the industrial revolution, and researchers say the acidification is increasing as humans pump increasing amounts of CO2 into the atmosphere.

Climate change models and studies have forecast that ocean water would become "corrosive" to organisms between 2050 and 2100 and previous studies have found acidification at deeper depths farther from shore.

The new evidence indicates the climate modellers underestimated the threat to coastal waters.

"They weren't thinking about the coast," says Ianson, explaining how acidified, corrosive water from depths of more than 200 metres is drawn up and onto the continental shelf by the strong spring and summer winds.

"What we're seeing is that the water is already getting to the surface," she says.

She and her colleagues from the U.S. and Mexico spent six weeks at sea last summer, collecting close to 2,600 bottles of water along 13 survey lines from Queen Charlotte Sound, north of Vancouver Island, to northern Mexico.

They found acidified corrosive seawater at depths of 40 to 120 metres in most areas and tracked it to all the way to the surface, within seven kilometres of the northern California coast. Off Vancouver Island, it was seen at a depth of 80 metres, says Ianson.

While the researchers describe the water as "corrosive" Ianson says it poses no danger of bathers. "It's not that children swimming at the beach will get their feet blistered," she says.

But the scientists say ocean acidification poses potentially catastrophic risks to marine ecosystems. Laboratory experiments have shown the delicate shells of pteropods become pitted when they are placed in water with the acidify levels spotted off the coast. And many species of fish and shellfish of economic importance, including salmon, oysters and clams, are sensitive to acidified waters.

They scientists say little is known about the impact of intermittent, seasonal exposure to corrosive water that is now occurring off the coast. But they say there is an urgent need to find out how the water is affecting larval, juvenile and adult stages of both shelled organisms the creatures that feed on them.

"What is scary about ocean acidification on the whole is how fast it is happening," says Ianson.