Carbon Dioxide and Ocean Chemistry

We have tried to avoid any more posts related to human-caused global warming; long-time Subscribers know our position on this topic.  But this past Sunday’s Los Angeles Times provides us with a chance both to revisit this topic and to look at some important water chemistry at the same time.  We couldn’t resist.  See:

http://www.latimes.com/news/local/la-me-acidic-oceans-20121007,0,7494056.story

It is a known scientific fact that the concentration of a gas dissolved in a liquid is proportional to the relative amount of that gas (called the partial pressure) in the atmosphere in contact with the liquid.  So the basic premise of this news story is valid: if global carbon dioxide levels in the atmosphere are increasing, then we will also see a proportional increase in carbon dioxide levels in the oceans.

The article goes on to indicate that significant biological changes are taking place in the oceans because of higher levels of CO2 in the oceans.  It seems that CO2 acts as an acid, and thus is increasing the acidity of the oceans.  Carbon dioxide definitely acts as an acid, and will cause the pH to drop as it is added to water.  Many water treatment plants actually use this gas to do exactly that.

Carbon dioxide is commonly added to drinking water treatment plants that engage in lime softening — a process that is called recarbonation.  Lime softening causes the pH of the water to increase to 9.5 or even higher, and water at that pH level cannot be delivered to our customers.  The secondary drinking water standards require that the water be less than 8.5.  So carbon dioxide is added to bring the pH back down to an acceptable level.

Does that mean that our oceans will turn into vast pools of acid, thanks to global warming?  We doubt it, based on ocean chemistry.

The chemical opposite of acidity is alkalinity.  When these two chemical categories come into contact with each other, the result is a “neutralization reaction,” the products of which are water and salts.  If there is enough alkalinity in the oceans to neutralize the acidity of the carbon dioxide, then the result will be more water and more salt in the oceans — and in quantities that are trivial in contrast to the size of the oceans themselves.  (That is, we will not witness any increase in sea level due to this chemistry.)  But is there enough alkalinity present to neutralize the carbon dioxide?

In drinking water we routinely measure the “total alkalinity.”  We define it as the sum of the hydroxide, carbonate, and bicarbonate ions in the water.  (There are other sources of alkalinity, but these are the dominant ones in natural waters, and the only ones we consider in drinking water quality analysis.)  Total alkalinity varies significantly, but typical southern California drinking water has a level around 100 mg/L — somewhat on the high side, as drinking water goes.

So what is the level in sea water?  About 200 mg/L.  Plenty to neutralize an incremental increase in carbon dioxide and thus keep the pH level of the oceans fairly constant.  But there may be some legitimacy to claims that ocean chemistry is being altered by increasing atmospheric CO2 levels.

The article states something that we posted a couple years ago: “… colder water can hold more CO2 …”  If the earth warms, that means the level of CO2 in the oceans will decrease!  So that means that global warming will definitely cause atmospheric CO2 levels to increase.  In short, global warming causes CO2 levels to increase.  We are being told that the converse true: an increase in CO2 causes global warming!

Another interesting note from the article is the presence of “… high levels of acidity because of carbon dioxide bubbling up from undersea volcanic vents.”  You mean that carbon dioxide naturally enters the oceans from a source other than human combustion of fossil fuels?  And how long has this been going on?  Well, slightly longer than we’ve been burning fossil fuels — in fact, about 10 million times longer.  Which of these sources of carbon dioxide would you suppose has had a more profound impact on ocean chemistry?

The researchers cited in the article pose many interesting questions which scientists should explore.  Such explorations require money.  If we were a scientist in search of grant funding, we would want to paint as dire a picture as possible, just to make sure the money keeps coming.

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