An Explanation of Ocean Acidification and its Effects on Life

By David J Strumfels on http://AMedelyofPotpourri.Blogspot.com/

 

 

 

 

 

 

 

 

 

(Wikipedia, 2/28/14):  "Between 1751 and 1994 surface ocean pH is estimated to have decreased from approximately 8.25 to 8.14 (how measured in 1751?),[5] representing an increase of almost 30% in H⁺ ion concentration in the world's oceans.[6][7] Available Earth System Models project that within the last decade ocean pH exceeded historical analogs [8] and in combination with other ocean biogeochemical changes could undermine the functioning of marine ecosystems and many ocean goods and services.[9]"

8.25 to 8.14 -- what does that mean?  What it does not mean is that the oceans are acidic, for their pH would have to be below 7.0 to say that.  In fact, we would call it alkaline (the opposite of acidity).  But acidification indicates a direction, not a specific place on the pH scale (some climate warming skeptics seem not to understand the difference) .  Chemically, pH is defined as the "negative log (hydrogen ion concentration)". But what does THAT mean?

The hydrogen ion, or H⁺ (a hydrogen atom without its electron, or a bare proton), is the main acidic species in water.  If the concentration of H⁺ is 8.00E-8 = 0.00000001 Molar (moles per liter, or just M), then the log of that number is -8.00, its negative log is 8.00, and so the pH is 8.00.  As to the specific cases here:  pH 8.25 => -8.25; antilog (-8.25), or 10 to the power of the number) yields 5.62E-9 H⁺ M, while 8.16 => -8.16 => 7.14E-9 H⁺ M.  That's a difference of 1.52E-9 H⁺ M concentration, or 0.00000000152M.  Yes, it is a 29% increase in acidity, mathematically.  Of course, 29% of practically nothing is even closer to actually nothing.   But chemically, especially biochemically, it can be very important.

For example, your blood pH has to be kept within a narrow range of 8.25 and 8.35, or illness, even death, can result.  I do not know all the reasons for this, but I can tell you that many biomolecules have both basic (opposite of acidic) and acidic forms, and they are chemically different.  They may have to be kept within a very narrow equilibrium of basic and acidic forms, each running a different reaction.  Or some are all necessarily basic at this pH, while others all acidic.  At pHs near 7.0 (neutrality), these equilibria are extremely sensitive to these tiny changes I outlined above.

So it is not difficult to see how a drop in 0.09 pH units (combined with a degree or two warming) could wreak havoc on numerous sea organisms, plant, animal, protozoan, or bacterial. On the other hand, the immensity of the oceans assures that there will be pH (and temperature) variations, in time and location, so sea life should be expected to be a little more hardy. But there are still fairly strict limits. The 21'st century will surely test those limits.

A little more chemistry now. We blithely speak of CO₂ increasing water acidity, but how? First, CO₂ is mildly soluble in water, the lower the water temperature, or the higher the water pressure, the more soluble (for thermodynamic reasons we need go into here) it is. That isn't enough for acidity, however. There has to be a chemical reaction between CO₂ and H₂O first: CO₂ + H₂O <=> HCO₃^- and H⁺. The <=> sign means the reaction goes in both directions; which set of reactants/products is favored depends on various conditions. Cold leans toward the first two, pressure the opposite way. This again is thermodynamics, with enthalpy and entropy competing against each other. At the low concentrations of CO₂, in general the latter is favored. But it's still a tiny contribution of H⁺, enough, as you've seen, to lower ocean water an average of 0.1 pH units over the last two hundred years (and I would not be surprised if 0.5 – 0.7 units of that is within the last 30-40 years, and it ~doubles by 2100 – this is serious).