O is for Ocean Acidification

We all know that CO2 emissions are warming the planet.  Or at least, most of us do.  What often goes unreported is the effect of carbon dioxide on the worlds’ oceans.  A lot of the CO2  that we pump into the air makes its way to the water and is making it more difficult for shelled creatures like sea urchins, lobsters, and coral to survive.

Lobster
This is Bob the lobster. This is his “I’m sad because of the increased levels of anthropogenic carbon dioxide that are making my life harder” face.  Image by Pedrosanch

In order to understand why this happens, we need to go back to secondary school chemistry.

Don’t worry, I’ll make sure Jared doesn’t pick on you.

Jared
No Jared! No!               Image public domain

The first lesson we need to recall is what about acids.  What is an acid?

Something that bubbles in a flask?  Image by Joe Sullivan
Something that bubbles in a flask? Image by Joe Sullivan

Acids are compounds that have free hydrogen ions floating around.  These hydrogen atoms are quite reactive, so it means the more free hydrogen you have floating around, the more reactive your compound.  Acidity is usually measured in pH, which stands for the “power of hydrogen”.  pH is measured on a scale (creatively named the “pH scale”) that ranges from 0 to 14.

Compounds that get a 0 on the scale are exceedingly acidic, meaning they are made up of pretty much just free-floating hydrogen ions.  Compounds that rate 7 are perfectly neutral, like distilled water.  Compounds on the other end, near 14, are called “basic” or “alkaline” and instead of having lots of hydrogen ions to give away, they have all sorts of space for hydrogen atoms.  This makes them reactive because they can strip hydrogen from things that don’t usually want to give it away (like Edward Norton’s hand in Fight Club).

The other confusing bit to remember is that the pH scale is logarithmic, meaning that each number you jump actually indicates a multiplication by 10.  For example, something with pH 3 (like soda) is 100 times more acidic than something with pH 5 (like coffee).  This means if a large body of water (like the ocean) shifts by even a small pH number, the effect can be very large.

Image by OpenStaxCollege
Image by OpenStaxCollege

The second lesson we need to recall is about equilibrium.

In chemistry, everything tends towards balance.  If you combine equally strong acids and bases, they will react together until the result has a pH that is in between.  You might also get a volcano-themed science fair demonstration.

When CO2 combines with water (H2O), they form carbonic acid (H2CO3).  The carbonic acid will break up (dissociate) into bicarbonate (HCO3) and a hydrogen ion (H+).  In a basic environment, the bicarbonate will dissociate further into carbonate (CO32-) and the result will be two hydrogen ions (2H+).

We can visualize this path with a chemical equation:

H2CO3  —-   H+ + HCO3–   —-   2H+ + CO32-

Where this path stops depends on the environment it is in.  In an acidic environment, the balance will tend towards the left, with more hydrogen bound up with the carbonate (because there is no space in the solution for more free hydrogen).  In a basic environment, the balance will tip to the right, releasing more hydrogen and freeing up the carbonate.

Currently, the pH of the ocean sits at about 8.1 (slightly alkaline).  Because of this, there is plenty of carbonate available for creepy-crawly-shellfish to use to build their homes.  Crustaceans and corals combine the free carbonate with calcium to form calcium carbonate (aka limestone, chalk, and Tums).  They can’t use bicarbonate (HCO3) or carbonic acid (H2CO3) and find it hard to form anything at all in an acidic environment.

This means that as we add CO2 to the water, we create more carbonic acid and contribute to the acidity of the ocean, dropping its pH.  Not only does this make it hard for the little guys down there trying to make a living, but it also endangers the big chompers that eat the little guys.

The ultimate big chomper.  This is what happens when you jokingly search for
The ultimate big chomper. This is what happens when you jokingly search for “chomper” on wikimedia.

A recent review found that even under the most optimistic emissions scenario, the ocean’s pH is likely to drop to 7.95, affecting 7-12% of marine species that build shells.  Under a high emissions scenario, the pH will go down to 7.8, affecting 21-32% of those species.

In order to keep track of the progress of this acidification, researchers from Exeter have proposed using satellites to monitor hard-to-reach bits of the ocean.

Regardless of the pace of the change, scientists agree one thing is certain: the oceans will become less hospitable for shell-builders.  The superficial impact of this for humans will be rising prices on shellfish, but there will be much deeper ramifications throughout marine ecosystems.

And I think we all know who is to blame.

Jared

Thanks Jared.

One Reply to “O is for Ocean Acidification”

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