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The Ocean’s Big pHat Problem

July 21, 2008

Oceans are great hiding spots. When the mob wants to make someone disappear, they toss ‘em in the sea. When the Navy takes an old boat out of commission, they sink it—burying it in the murky depths. But it turns out that the ocean isn’t as good a hiding spot for carbon dioxide (CO2) as we thought it was.

Every day, more than 25 million tons of CO2 (the most notorious greenhouse gas) dissolve in seawater. We used to think this was a good thing because CO2 in the ocean isn’t in the atmosphere, contributing to the greenhouse effect. But like plastic, CO2 never really goes away. When it mixes with seawater, it forms carbonic acid—the stuff that makes carbonated beverages bubbly. Carbonic acid doesn’t make the ocean fizzy, but it totally screws up the ocean’s pH.

The pH scale measures acidity: values above 7 are basic and values below 7 are acidic. Using samples from ice cores, scientists have found that the ocean’s pH held steady at 8.2 for the 600,000 years before the industrial revolution. But since 1800, the ocean’s pH has been dropping. Today it is 30% more acidic (pH = 8.1) than it was in the 18th century and some scientists predict that—if we continue emitting CO2 at the current rate—the ocean will be 150% more acidic (pH = 7.8 ) by 2100 than it was in 1799.

This won’t necessarily make the ocean uncomfortable for humans (the average pH of a chlorinated swimming pool is 7.8), but it will make things a whole lot more than uncomfortable for a lot of other critters. Zillions of marine animals depend on carbonate to build their protective shells, but as the ocean’s acidity increases, the amount of carbonate in the ocean will decline.

With less carbonate to go around, crustaceans like lobsters, crabs and mussels will have to devote more energy to forming their shells and less to foraging and reproducing. What little body armor they are able to produce will likely be thin and brittle—and in some cases the shells may dissolve.

Coral reefs will be hit hard as well since they’re built by thousands of coral polyps that secrete carbonate exoskeletons (or shells). Without carbonate, coral reef growth will slow significantly and existing coral will vanish as its protective layer disintegrates. Of course, loads of marine creatures depend on coral reef ecosystems for habitat and food, so without coral, they’ll be SOL.

Scientists have also found that the shells of teeny tiny mollusks called pteropods dissolve in acidic seawater. A shell-less pteropod is pretty much screwed. Salmon, herring, cod and pollack without pteropods to eat will be hungry. And commercial fishermen trying to make a living off these fish may end up screwed and hungry. 

Right now, the ocean swallows about a third of the CO2 we puff out into the atmosphere, but scientists predict that the oceans may actually begin emitting CO2 in the next few centuries. That’s because most of the CO2 socked away in the ocean is stored in shelled-phytoplankton. And like other shelled marine creatures, phytoplankton shells are becoming increasingly fragile as the ocean becomes more acidic. Once again, a shelled-organism without a shell isn’t very likely to survive. And so, with fewer plankton in the water and more CO2 in the atmosphere, the ocean will soon become saturated and the CO2 will have nowhere else to hide.

6 Comments leave one →
  1. June 5, 2009 5:26 pm

    The feedback inherent in Gaia may indeed come back to haunt us. The point you make about balanced food food webs (via shelled plankton) balancing atmospheric CO2 is well taken. This means that there very well could be a threshold level of CO2 followed by dramatic climate change. James Lovelock seems to be saying that we can’t really do anything about it and we all better just enjoy life because we may only have a few more generations left. A little nihilistic for my tastes but something to ponder. I write a little more about the Gaia hypothesis here –


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