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.

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