A simple calculation on Ocean Acidification


A recent article in Science, “Rising Acidity Brings an Ocean of Trouble” claims that post industrial human CO2 has lowered ocean pH from 8.2 to 8.1. This is an extraordinary claim since there were no preindustrial ARGO floats to establish the baseline. To get some sort of baseline we determine the molar solution if all the purported 1.1 trillion tonnes of human post industrial CO2 were dissolved in an Ocean of distilled water.

Dissolving our CO2 in all 1.3 billion cubic km of ocean would clearly do nothing at all, so to be abundantly fair we limit the dilution to the surface mixed layer which we round to 350 million cubic kilometers. We make this concession even though in reality atmospheric CO2 is actively pumped into the deep ocean by ocean circulation.

Doing some simple math with lots of zeroes we arrive at a 2.35×10-9 molar solution which would lower the pH of our imaginary distilled mixed layer from 7 to 6.95.

This is only half of the acidification claimed in Science and, andWhile it ignores sulfuric and other lesser acid contributions,  it is based on the worst possible situation where there is no buffering, the dilution is limited to the mixed layer, and all our CO2 goes into the ocean. In reality the oceans have impressive titratable alkalinity (TA), the deep oceans are a massive CO2 sink, and only about half of incremental atmospheric CO2 is absorbed by the oceans.

Advertisements
This entry was posted in Carbon Theology, Ocean Acidification, Oceanography and tagged . Bookmark the permalink.

5 Responses to A simple calculation on Ocean Acidification

  1. John Voeks says:

    There seems to be an error someplace. My calculations based on what I thought were your data and some from the handbook showed that one could not dissolve all of the CO2 in the available water. Are we using different data or did I make a mistake? Probably the latter. My old brain is not what it was sixty years ago. Maybe I don’t know what a trillion is. I think it is 10exp12 and a billion is 10exp9. The British, I understand, think a billion is 10exp12 and only they know what a trillion is, probably 10exp 24. They don’t know what ethylene is either, but talk a lot about ethene. They are pretty good scientists in spite of these “deficiencies”.

  2. John Voeks says:

    I don’t have anything intelligent to say about this. The high pH suprises me. Apparently my calculatiohns of the amount of CO2 that can be dissolved were way off. I would like to know why, but am not sufficiently energetic to sort it all out. If I understand these data , man made CO2 can’t have ny significant impact on ocean chemistry. I would like to know what is resoonsible fot the high pH. I would have guessed that it would be on the acid side of neutral.

  3. IanC says:

    You neglected the fact the pH=-log[H+], thus a drop from 7 to 6.95 corresponds to an increase in Hydrogen ion concentration from 10^(-7) to 10^(-6.95), i.e. an increase in 1.2E-8. On the other hand, if ocean pH dropped from 8.2 to 8.1, that corresponds to an increase of 1.6E-9. The observed change is in fact much smaller than what you’ve calculated.

    • gymnosperm says:

      Ian,

      Thank you. I do make mistakes, particularly as I try to keep my posts concise. You are right. I did overlook the proton disparity between the pH 8’s and 7’s and I will redact the sentence stating that the resulting pH is half what was touted. However, I believe that the thrust of the message remains true. When I get a chance I will rerun the calculation with the entire ocean. After all, the deep ocean contains 1.1-1.2x the CO2 of the surface, and the recent lagged ocean warming has amplified the disparity. The acidification of the Oregon coast reported in the Science article resulted from the increased upweling from the PDO shift. Carbon dioxide enters the ocean primarily in the north Atlantic and and around Antarctica where brine rejection creates supercooled sinking water with extraordinarily low pCO2. It takes, whatever, let’s round to a millenium for that CO2 to make the trip to Oregon. So it would be fair to say that the carbonic acid eating those shells in Oregon sank into the ocean during the Viking warm period.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s