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Ocean acidification

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Ocean acidification is the decrease in pH of the oceans, primarily due to increased CO2 concentrations in the atmosphere, and can be expressed as chemical equations. As more CO2 is released into the atmosphere, the oceans, which absorb a large part of that CO2, become more acidic. Anthropogenic activities that contribute to ocean acidification are those that lead to increased CO2 concentrations in the atmosphere: burning of fossil fuels, vehicle emissions, and deforestation. Created by Sal Khan.

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  • blobby green style avatar for user Amy Ishmael
    What do you feel causes the oceans to absorb this? Are you saying that this "acidic" rain falls into the ocean and doesn't leave?
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Video transcript

- [Instructor] In this video, we're going to talk a little bit about ocean acidification. And as we'll see, it's all related to increased carbon dioxide concentrations in the atmosphere. And 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. And this axis right over here, it's 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. And when you do that, this graph is showing us two things. In blue, we're seeing the actual emissions of carbon dioxide. And if we go pre-industrial revolution, or the early stages of the industrial revolution, our emissions of carbon dioxide were very low and fairly flat, and we have seen that they have gone up dramatically, especially over the last 100 or so years. And that carbon dioxide doesn't just immediately leave the atmosphere. It stays in the atmosphere for a while. So 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 so to get us to this place that is far out of that range. And to 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. So if I were to have some H2O, or water, in reaction with carbon dioxide, that is going to react, or it could be in equilibrium, to form what is known as carbonic acid, which is H2CO3. If you wanna know how the bonds are structured, it looks like this, where each of the oxygens are attached to a hydrogen. And the reason why this is called carbonic acid, is because it can easily release a hydrogen ion. So this can be in equilibrium with bicarbonate, which is HCO3 minus. So it's really just our carbonic acid minus a hydrogen ion, plus a hydrogen ion. So 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, and especially more of the carbon dioxide. And 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. And 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. So this change, if you really wanna get into the math, pH is the negative log of the hydrogen ion concentration, and so the hydrogen ion concentration here relative to there, if we wanted to compare, if we wanted see how much it grew, you would say 10 to the negative 8.1 over 10 to the negative 8.2. And if you look at this analysis, you'll see that this is approximately 1.26, or another way of thinking about it, over the course of the industrial revolution, because of the trends we have seen in this graph that our oceans are about 26% more acidic. And 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. In calcium carbonate, you have a positively charged calcium ion forming ionic bond with a carbonate ion, and carbonate looks like this, which looks a awful lot of what we see right over here in the carbonic acid, or the bicarbonate. And so if all of a sudden you have a lot more of these hydrogen ions in the water and dissolved, everything's more acidic now, it might disrupt this process of formation. Some of this carbonate might go and nab some of these hydrogen ions, less likely to form an ionic bond with the calcium. It also 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 actually also antacid. TUMS is mainly calcium carbonate, but this acidity in general is going to throw all sorts of organisms off of their homeostasis. Organisms are highly, highly sensitive to changes in pH, to changes in acidity. But 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, but 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.