How Could We Have Oceans of Acid?

As far as we can tell, the ocean has been slightly basic for hundreds of millions of years. However, that seems to be changing, and the culprit may surprise you. Everyone has heard of carbon dioxide. It’s the thing we breathe out, what plants crave, and there’s this thing called the greenhouse effect which you’ve probably heard of by now.
CO2 concentrations have certainly changed in the past based on our examinations of ice core samples. However, they are certainly on the rise now. Credit: NASA, UCR, RUSD
What we do know for sure, is that carbon dioxide is on the rise. While the most often talked about consequence of this is a trend of increasing global temperature, there is another important concern, and it has to do with our water. There are many ways that we can measure water including temperature, pollutant content, clarity, and salinity, but this particular issue focuses on acidity.
Water is made up of two parts hydrogen and one part oxygen, leading the the chemical formula H2O. In pure water, these molecules occasionally break apart due to the following reaction:
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On the left side, we’ve got our old pal water. In the middle, the double sided arrow means that this reaction can go both ways, depending on the ratio of water to its parts. Pure water is neutral, meaning that this reaction is the only source of H+ and OH ions1 in the water. If anything throws this balance out of whack, the water will become basic if more OH ions are added, or acidic if more H+ ions2 are added.
So what does all of this have to do with carbon dioxide (CO2)? When the CO2 gets in contact with water–which covers most of the Earth–it combines with the H2O by the following reaction.
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This forms Carbonic acid, which then donates its extra hydrogen to the water like this:
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In the previous paragraph, we talked about how anything that can add H+ to the water will cause it to become acidic. The resulting molecule above is known as carbonic acid because it has these extra hydrogen ions to donate.
While this all seems pretty abstract, you can watch it happed below. This water is poured over dry ice or solid carbon dioxide. Solid carbon dioxide turns directly into a gas at room temperature. The water also has an indicator solution that will cause it to turn from neutral green to red if it is acidic, and blue to deep purple if it is basic.
CO2 Carbonic Acd
We measure this using the PH scale, which runs from zero to 14 with low numbers indicating higher acidity. It is also a logarithmic scale, which means that changing from a seven to a six on the scale corresponds to an increase of ten times to the acidity! For the past 300 million years, the ocean has been at a steady–and slightly basic–PH of 8.2. Over the past 200 years, it has dropped to about 8.1, which doesn’t seem like much on this scale, but corresponds to an increase of about 30% to the number of acidic ions in water. Even without further increase to atmospheric CO2, this trend should accelerate to a PH of 7.8 by the year 2100, an increase in acidic ions of 126%!
What does all of this mean for us living on Earth? No one is quite sure. Most organisms can only tolerate a narrow range of acidity, and there are many species that live in the ocean. Simulating these conditions in a lab is pretty easy, so we can identify some at risk species right away. Coral reefs, in particular, are known to be very sensitive to acidification.
Dissolved carbon dioxide can interfere with the calcification process that allows shells and exoskeletons to grow. Over the course of 45 days, this sea butterfly slowly dissolves in a lab simulation of acidic ocean water. Courtesy of David Littschwager/National Geographic Society. Similar research continues today.
Ions are just atoms or groups of atoms with a net charge, meaning they have an extra electron if negative, or lack an electron if positive. Ions are much easier to dissolve in water than neutral molecules.
Note: The freed hydrogen ions will actually combine with surrounding water molecules, forming Hydronium (H3OH+) which is usually denoted H+.
Written By: Scott Alton