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Ocean acidification | Biodiversity and human impacts | High school biology | Khan Academy


4m read
·Nov 10, 2024

In this video, we're going to talk a little bit about ocean acidification. As we'll see, it's all related to increased carbon dioxide concentrations in the atmosphere. We have talked about this in other videos, but we can see if we look at carbon dioxide concentrations over the last 800,000 years, which is well before modern human beings existed, it has oscillated between roughly 200 parts per million and 300 parts per million.

But then, if we look at modern times, the spike has gone well beyond that range. This axis right over here is covering so much time it might not be obvious when or why the spike has happened. So let's zoom in a little bit on the last several hundred years or so. When you do that, this graph is showing us two things. In blue, we're seeing the actual emissions of carbon dioxide. If we go pre-industrial Revolution or the early stages of the industrial re-evolution, our emissions of carbon dioxide were very low and fairly flat.

We have seen that they have gone up dramatically, especially over the last 100 or so years. That carbon dioxide doesn't just immediately leave the atmosphere; it stays in the atmosphere for a while. As we increase our emissions, the cumulative concentration of carbon dioxide has gone up to those levels that we just saw before the Industrial Revolution or the early stages. We were within that range that we saw over the last 800,000 years, but then it cumulatively has increased.

To get us to this place that is far out of that range and appreciate what it's doing to our oceans, we just have to recognize that the carbon dioxide in the air is in interaction with the ocean, actually with water everywhere. If I were to have some H2O or water in reaction with carbon dioxide, that is going to react or can be in equilibrium to form what is known as carbonic acid, which is H2CO3.

If you want to know how the bonds are structured, it looks like this, where each of the oxygens are attached to a hydrogen. The reason why this is called carbonic acid is that it can easily release a hydrogen ion. So this can be in equilibrium with bicarbonate, which is HCO3 minus. It's really just our carbonic acid minus a hydrogen ion, plus a hydrogen ion. As you have more carbon dioxide in the air that reacts with water in the ocean, well then you're going to have more carbonic acid and you're going to have more of your hydrogen ions.

The reaction is going to go this way as you have more of this stuff, especially more of the carbon dioxide. We have observed that in the oceans themselves. We have seen that ocean pH, if we go to the early Industrial Revolution, it was around 8.2, and it has gone to 8.1. You might recognize that the lower the pH, the more acidic it is.

But you also might be saying, "Hey, that doesn't look like that much of a change." But it actually turns out that pH is measured on a logarithmic scale, so we're actually talking about powers of 10. This change, if you really want to get into the math, pH is a negative log of the hydrogen ion concentration.

The hydrogen ion concentration here relative to there, if we wanted to compare, if we want to see how much it grew, you would say 10 to the 8.1 over 10 to the 8.2. If you look at this analysis, you'll see that this is approximately 1.26. Another way of thinking about it is that, over the course of the Industrial Revolution, because of the trends we have seen in this graph, our oceans are about 26% more acidic.

To appreciate why this is a big deal, I will remind you that things like coral reefs or shells in sea animals, these are formed with calcium carbonate. Calcium carbonate involves a positively charged calcium ion forming an ionic bond with a carbonate ion. Carbonate looks like this, which looks an awful lot like what we see right over here in the carbonic acid or the bicarbonate.

If all of a sudden you have a lot more of these hydrogen ions in the water and dissolved, everything is more acidic now. It might disrupt this process of formation. Some of this carbonate might go and nab some of these hydrogen ions, making it less likely to form an ionic bond with the calcium.

It doesn't just directly affect things like calcium carbonate, which is everywhere; it's actually the main constituent of pearls. It's the structure of so many, especially rigid structures in life, including sea life. It's also, in general, an acid; Tums is mainly calcium carbonate.

This acidity, in general, is going to throw all sorts of organisms off of their homeostasis. Organisms are highly sensitive to changes in pH and changes in acidity. The big picture takeaway: a lot of talk is about global warming and carbon dioxide concentrations in the air, but it's also not only warming the ocean because of the greenhouse effect; it's also making the oceans more acidic, which is having some obvious consequences now and probably some follow-on consequences that we are just beginning to understand.

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