Update: Thinner Shells Put Ocean on Thin Ice

Some argue that more carbon dioxide is a “good thing.” I guess they just can’t get their minds around the whole climate change thing. OK, but what about ocean acidification? If you like seafood, ocean acidification is definitely not a good thing.

It may turn out that climate change is not the worst consequence of our dependence on fossil fuels. It could be ocean acidification.

Quick Review of Ocean Acidification

The ocean is a slightly alkaline, salty water solution. The alkaline description means the seas have slightly more dissolved bases than acids.

All liquids can be described in terms of how basic or acidic they are. Acids, which comes from the Latin acidus, meaning sour or sharp, have an excess of hydrogen ions. Vinegar and lemons are common acids. Basic liquids have an excess of hydrogen oxide ions. Ammonia is a common base. The alkalinity of the ocean is fairly close to that of sodium bicarbonate, an antacid taken to counteract heartburn.

So Who Will Be Affected by This Acidification? Very Likely You and Me.

Every time we add carbon dioxide (CO2) to the atmosphere, a portion of the CO2 finds its way into the ocean. But since dissolved CO2 forms a weak acid – carbonic acid – the net effect is to cause the ocean alkalinity to decrease a little bit (and the acidity to increase a bit). If you like swimming, no worries - that extra acidity is way too slight to make a difference to you. But that doesn’t mean you shouldn't be concerned.

As the ocean becomes more acidic, the process of forming calcium carbonate minerals like calcite and aragonite from dissolved calcium and carbonate becomes increasingly more difficult. For the multitude of ocean creatures that form calcareous shells and skeletons from the dissolved calcium and carbonate ions in the ocean, ocean acidification could be a huge problem. And if it’s be a problem for them, it will be a problem for people.

Calcifying ocean species include corals, mollusks (such as clams and mussels), and single-celled creatures at the bottom of the food chain called protists. (These include plankton like coccolithophores and foraminifera.) Since coral reefs are home to many of the world’s fisheries and because protists form the bottom of the ocean food web, the suffering of those calcareous species will affect all the species on up the ocean food chain – all the way up to the top: us. It is estimated that more than one billion of the world’s six billion people depend primarily on the ocean for their protein. A collapse of the ocean food web from ocean acidification could spell disaster for these one billion.

There’s another potential consequence from this chemical problem: more CO2 in the atmosphere. Come again? You read right.

Calcareous protists play a key role in moving dissolved CO2 from surface waters to the deep ocean. When these species die, they sink into the deep ocean taking with them the carbon in their shells and skeletons. This removal allows room for more atmospheric CO2 to settle into the surface ocean. So, damage to species like coccolithophores and foraminifera could slow the rate at which CO2 from fossil-fuel burning finds its way out of the atmosphere.

Not Just Some Theory

That adding dissolved CO2 to the ocean will make the ocean more acidic and destabilize calcium shells and skeletons is pretty, er, basic stuff from a scientific point of view – simple solution chemistry worked out long ago. But scientists have nevertheless been working to confirm that what happens in the lab also happens in the real world.

Late last year, The Green Grok reported on two scientific papers that found increasing levels of acidity in the Southern Ocean and in waters off the coast of Washington.

But what about the effect on the critters themselves? How do we know that ocean acidification will affect them at all? A new paper in Nature Geoscience by Andrew Moy of the Antarctic Climate and Ecosystems Cooperative Research Center in Hobart, Tasmania, and colleagues, provides very convincing evidence of a significant impact.

Moy and his colleagues focused on Globigerina bulloide, a relatively common species of foraminifera that builds its shells out of calcite, the most stable form of calcium carbonate. To identify the effect of ocean acidification on these forams, the scientists compared modern-day shells with those from pre-industrial times. The modern shells were collected from species falling through the ocean column into sediment traps in the Southern Ocean. The pre-industrial samples were gathered from sediment cores pulled out of the Southern Ocean.

The results are striking. The weight of the modern day shells was 30 to 35 percent less than those of their pre-industrial counterparts. But that’s not all. The researchers were able to reconstruct the record of shell weights of G. bulloides over the past 50,000 years. They compared those to the record of CO2 variations over the same period obtained from ice cores. They found a very tight relationship: times of high CO2 had low shell weight and vice versa.

In summary we now have very strong evidence that burning fossil fuels is having a profound impact on the ocean, an impact that may very well be undermining one of our main sources of food.