The most pressing example of climate change's impact is not monster hurricanes, retreating glaciers or water wars. It's the humble swimming sea snail.

The tiny pteropod has difficulty growing a shell in a warmer planet's acidified ocean waters. Given the snails' role at the base of the cold-water food chain, its struggle threatens the entire polar ecosystem, through salmon to seals and whales.

The problem is one of many associated with ocean acidification. That change is well underway -- a consequence of warming that has already happened and fossil-fuel emissions that have long since been dumped into the atmosphere.

In absorbing those emissions the oceans have buffered humanity from the worst effects of climate change. But in doing so ocean chemistry has changed, acidifying to levels not seen in 800,000 years.

The result, according to a new report issued by Oceana, is that today's ocean chemistry is already hostile for many creatures fundamental to the marine food web. The world's oceans -- for so long a neat and invisible sink for humanity's carbon dioxide emissions -- are about to extract a price for all that waste.

The effects are not local: Entire ecosystems threaten to literally crumble away as critters relying on calcium carbonate for a home -- from corals to mollusks to the sea snail -- have a harder time manufacturing their shells. Corals shelter millions of species worldwide, while sea snails account for upwards of 45 percent of the diet of pink salmon.

To avoid the most serious problems associated with acidification, Oceana and other scientists warn, society must hold atmospheric carbon dioxide levels at 350 parts-per-million, roughly 25 percent higher than the pre-industrial mark.

The rub is that the globe has already passed 385 ppm. And many economists and climatologists figure the peak will lie somewhere north of 570 ppm before society figures out how to curb emissions.

"Climate change has been happening for a long time," said Jackie Savitz, Oceana's senior director of pollution campaigns and co-author of the report, Acid Test: Can we save our oceans from CO2? The oceans "are so big, so vast, and everyone thought they were untouchable. But the fact is we've been touching them all along."

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What alarms scientists most is the rate of change: The transformation has happened over 250 years, faster than anything in the historical record. And if emissions remain unchecked, Oceana warned, the oceans in 40 years will be more acidic than anything experienced in the past 20 million years.

Over the next several centuries the pH changes may be larger than any inferred from the geologic record of the past 300 million years, with the exception of a few rare extreme events, scientists predict.

The process is fairly simple. For eons prior to the Industrial Revolution, oceans were at equilibrium with the atmosphere, absorbing as much carbon dioxide as they released.

As humanity started burning fuel, atmospheric carbon dioxide levels started to rise, and the oceans responded, taking in more and more carbon each year and increasing acidity by nearly 30 percent.

The oceans so far have absorbed some 30 percent of the carbon dioxide that humans have added to the atmosphere since the beginning of the Industrial Revolution and nearly 80 percent of the heat generated by those gases, according to Oceana.

Today the world's oceans absorb some 30 million metric tons of extra carbon dioxide every day, according to scientists -- roughly twice the amount of carbon dioxide emitted each day by the United States.

The ocean has a number of natural buffers to help with change -- ocean sediments and deep water represent two enormous potential reservoirs -- but they all work on vastly slower time scales, said Richard Zeebe, associate professor of oceanography at the University of Hawaii at Manoa.

"It's very difficult to find a nice analogue in the past that's going to show what we're going to experience over the next 200 to 300 years," he said. "It's pretty much outrageous what we've done."

"We are overwhelming the system," he added. "The system is not quick enough to react. It takes thousands of years to do this."

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Scientists are already seeing harm as the oceans acidify. Reefs are struggling in many parts of the world, shell growth rates are slowing, life phases -- particularly reproductive maturity -- are being thrown out of whack.

Even the healthiest reefs in the most optimum conditions today face a daily struggle to grow faster than reef dwellers and the ocean can erode them, and the effects grow more dire as atmospheric carbon dioxide levels rise.

Somewhere between 450 ppm and 500 ppm atmospheric carbon dioxide, for instance, lies a tipping point where, scientists suspect, reefs become "rapidly eroding rubble banks." Much beyond that, Oceana reported, "reefs as we know them would be extremely rare." Current projections show that by the end of this century no adequate conditions for coral will remain in the world's oceans.

But the chemistry is complex and the variables myriad. Atmospheric carbon dioxide alone does not determine acidity.

"We cannot look into the past and say atmospheric carbon dioxide was highest in the Cretaceous (65 to 145 million years ago), therefore this is what the ocean is going to look like," Zeebe said. "Time scale is key. Rate of change is key."

A frequently touted example of rapid change in the geologic record is the so-called Paleocene-Eocene Thermal Maximum. About 55 million years ago the Earth abruptly warmed 6°C, the oceans acidified, atmospheric and oceanic circulation patterns shifted and a large number of bottom-dwellers died off.

That change happened over perhaps 10,000 years -- not even close to today's pace.

"This is hard for many people to understand," Zeebe said. "You need to separate the different time scales."

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Oceana maintains that holding atmospheric carbon dioxide at 350 ppm would prevent the most dire problems but still represents a concentration above the safe threshold for today's ocean life.

But for many scientists, that mark is history; in fact current industrial emissions exceed even the highest scenario -- 850 ppm by century's end -- mapped by the Intergovernmental Panel on Climate Change, said Stanford University climatologist Stephen Schneider.

There's no question 350 ppm represents the safest level, Schneider said. But society will be lucky to peak at 450 ppm, he said, with a more likely crest north of 550 ppm before emissions stabilize.

"We're going to have an overshoot," he said. "The only question is how bad is that overshoot going to be."

"Our objective has to be to prevent a 'much worse,' rather than pretend we can roll the clock back to an impossibility."

The question then becomes how much acidification can reefs handle before they start to crumble. Unfortunately as scientists learn more, the threshold keeps dropping.

"We're pretty sure that 560 is too high and we're almost certain that 700 is too high, but we just plain don't know much about whether 350 or 450 would be OK," said Joanie Kleypas, a marine scientist studying coral at the National Center for Atmospheric Research in Boulder, Colo.

Marine scientists have gradually concluded that world carbon dioxide levels will eventually peak at some higher-than-desired threshold no matter what happens, Kleypas said, and hold hope that some technology or solution will bring concentrations back down to the threshold level or lower.

There are hazards with this approach, or course, notably the increased likelihood of passing dangerous tipping points in climate, ocean circulation or general ecological response.

That's why Oceana's Savitz believes the line must be held at 350 ppm. It is a realistic goal, she said. "The good news is it's from lack of trying. We really haven't done the obvious things or picked the low-hanging fruit."

Conservation, for instance, can erase big chunks of projected emissions.

The Oceana report outlines five approaches that together would help drop atmospheric carbon dioxide concentrations to 350 ppm and preserve coral, including stopping deforestation and overfishing, promoting energy efficiency and low-carbon fuels, and regulating carbon releases.

"The better job we do at limiting ourselves, the less (harm) we'll see," Savitz said. "But we're going to see some impacts. We're not going to get out of this unscathed."