Oceanic pH levels have dropped from 8.2 [represented in blue] in 1885 to the more acidic level of 8.0 [orange] in 1985. By 2085, that number could reach 7.8 [red]. Graham Murdoch

In the past two centuries, at a pace unprecedented in the geological record, the chemistry of the oceans has changed. Seawater has become approximately 30 percent more acidic. It won’t burn human flesh (at least not anytime soon), but it has begun to dissolve the shells and exoskeletons of many sea creatures, with potentially disastrous consequences for the entire marine biosystem.

In recent years, for instance, the shells of pteropods, swimming sea snails that anchor the polar ocean food chain, have already grown measurably thinner. In the more acid waters of the near future, tank experiments show, their shells will begin to disappear, scrolling up at the edges like melting candle rims. In the Pacific Northwest, meanwhile, acidic seawater has proved fatal to oyster spawn. And if the oceans continue to acidify, other fisheries and ecosystems, including already stressed coral reefs, could be next.

The cause of acidification is well understoodA tax of just $12.50 per ton of CO2 would reduce carbon emissions by 30 percent, keeping some 214 million tons of pollution out of the ocean.. Since the beginning of the industrial era, the oceans have absorbed more than a quarter of the CO2 we’ve released into the atmosphere. This absorption was long thought to be a good thing, a brake on global warming. But that brake comes at a high price. When CO2 mixes with seawater, it forms carbonic acid. The last time the oceans acidified so dramatically, 65 million years ago as a result of volcanic activity, a mass extinction occurred. Even then, the species that survived had centuries to adapt—far more time than those inhabiting today’s oceans will have if nothing is done.

There is no easy solution. Geoengineers have proposed dumping 300 billion cubic feet of limestone, a megadose of geological Alka-Seltzer, into the sea. But even setting aside concerns about unintended side effects, the toll in dollars and emissions to produce that much alkaline antidote is prohibitive, and the benefits would be temporary. The best and perhaps only cure? Reduce CO2 production. The problem is, in part thanks to subsidies in the U.S. tax code, the dirty burning of fossil fuels remains cheaper than cleaner alternatives, a “savings” that does not account for the cost of environmental damage caused by carbon pollution.

How to reckon the real price? “A carbon tax is the preferred method,” says Hauke Kite-Powell, a researcher at Woods Hole Oceanographic Institution in Massachusetts. By one estimate, a tax of just $12.50 per ton of CO2 would reduce emissions by 30 percent, keeping some 214 million tons of pollution out of the ocean. That wouldn’t bring acidification to a halt, but it would buy the oceans—and us—some badly needed time.

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