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Yesterday I read an article on the Huffington Post entitled “Seaweed Might Have The Power To Make The Oceans Less Acidic. The article explained that the increase in carbon dioxide concentration in the atmosphere since the beginning of the Industrial Revolution from 280 to 400 parts-per-million (ppm) – an increase of 43% – had indirectly driven a significant increase in the acidity of the oceans. It described an upcoming experiment in the Puget Sound in which a large bed of kelp, a species of seaweed, was to be planted in an attempt to see if the acidity of the sound could be reduced. While it was an interesting article, in my opinion it contained the two most common flaws in mass-media science reporting: it presumed that the underlying science was either too complicated or too uninteresting for the reader, and it failed to address the larger questions that motivated the specific study. So let’s roll up our sleeves and address those omitted items.

The concentrations of carbon dioxide in the atmosphere and carbon dioxide dissolved in the oceans exist in equilibrium with one another. Therefore, as the amount of carbon dioxide increases in the atmosphere, the amount dissolved in the oceans will increase as well.(1) In an analogous manner, dissolved carbon dioxide exists in equilibrium with dissolved carbonic acid (H2CO3). So more dissolved carbon dioxide results in more carbonic acid and, thus, ocean water that is more acidic. Since the start of the Industrial Revolution, the average pH of the oceans has fallen – the direction of increased acidity – from 8.18 to 8.07. At first glance, that does not sound like a big deal. However, since pH is expressed in a logarithmic scale, this small difference in numbers corresponds to a 30% increase in the acidity of the oceans. While you, as a human being, would not notice the difference during a swim at Wrightsville Beach, if you happened to be a krill, you most certainly would.

Krill, like other small crustaceans, have exoskeletons made of calcium carbonate. Calcium carbonate is formed when dissolved calcium ions (Ca2+), which are plentiful in salty ocean water, meet up with dissolved carbonate ions (CO32-) and then precipitate to form a solid. You likely made some calcium carbonate precipitate in high school chemistry lab, since there is a nice “gee whiz” factor when you combine two clear liquids, one with calcium ions and the other with carbonate ions, and a white, powdery solid forms. When the oceans are at a pH of 8.18, there are plenty of carbonate and calcium ions floating around. Thus, if you are an organism that uses calcium carbonate for your exoskeleton, life in these conditions is idyllic. Unfortunately, if the pH of the water starts to fall, things rapidly go awry.

As ocean water becomes more acidic, dissolved carbonate ions (CO32-) start to be converted into bicarbonate ions (HCO31-). Bicarbonate ions do not react with calcium ions to form calcium carbonate, reducing the supply of calcium carbonate in the water. Without sufficient calcium carbonate, crustaceans are limited in their ability to grow and reproduce. To make matters even worse, increased acidity also tends to dissolve the existing crustaceans’ exoskeletons. In many parts of the ocean, increased acidity has caused krill and other small crustacean populations to fall by more than 50% over the past several decades. Because krill and other small crustaceans are a vital part of the bottom of the food chain, a drop in their population reduces the overall production of biomass for the entire oceanic ecosystem. This is a critically important issue, but further explanation will have to wait until another column, because we need to get back to the kelp.

kelp

Like all plants in the ocean, kelp extracts carbon dioxide from the water via photosynthesis. In the reverse of the chemistry described above, if you reduce the concentration of the carbon dioxide in the water, you also reduce the concentration of carbonic acid, thereby making the oceans less acidic. The specific interest in kelp as a potential carbon dioxide remover stems from its remarkable growth rate and size. A kelp plant can grow up to 2 feet per day and reach heights of up to 250 feet. These features imbue kelp with the potential to remove a lot of carbon dioxide from the ocean water.

The next question we need to address is how much carbon dioxide would need to be removed from the oceans in order to return its pH to the pre-Industrial Revolution level of 8.18. That question is a bit more complicated than it sounds. Since 1850 we have added approximately 125 gigatons of carbon to the oceans. So our first thought might be that we just have to take that 125 gigatons back out. However, due to the equilibrium between the carbon dioxide in the atmosphere and in the oceans, as we remove carbon dioxide from the water, some additional carbon dioxide would dissolve in from the air. Therefore, in order to restore the pH of the oceans to the pre-industrial level of 8.18, we would need to remove more than 125 gigatons of carbon. I don’t have all of the data needed to precisely calculate this amount, but for the sake of our discussion let’s set a removal target of 150 gigatons, which should at least get us in the ballpark. Next we need to decide how long we want this effort to last. Since it is a rather enormous undertaking, I decided to allow 200 years.

A bed of kelp, provided that it is harvested on a regular basis, can remove 4,500 lb of carbon from the water per acre per year. Given this removal rate, we could extract 150 gigatons of carbon from the oceans in 200 years by farming 366 million acres of kelp. That is a prodigious amount of kelp that would fill an area equal to the combined sizes of Texas, Oklahoma, New Mexico, and Kansas. That sounds a bit daunting.

Now that we have grown all of this kelp, we need to make sure that the carbon it contains does not find its way back into the atmosphere where it would be able to dissolve back into the oceans. This turns out to be a challenging task. If we used the kelp for food or fuel, nearly all of the carbon would re-enter the atmosphere as carbon dioxide. If we used the kelp as compost and/or fertilizer, some of the carbon would be sequestered over the long term in the soil and in the biomass of long-lived plants such as trees, and some would be converted to carbon dioxide by bacteria. In fact, in almost every potential use for the harvested kelp, some of the carbon would cycle back to the atmosphere relatively quickly. Therefore, our project would take even longer than 200 years.

So is the Puget Sound kelp experiment going to find us a silver bullet which will save us all from climate change, or is it a hopeless pipe dream? Now that we have examined the chemistry and reviewed the larger questions, the answer seems to be somewhere in the middle. Kelp farming alone does not appear to the capacity to solve the problems of our increasingly acidic oceans. However, since the removal of even a “small” amount, say 10 gigatons, of carbon from the oceans would result in tangible improvements, kelp farming could be an important component of an all-hands-on-deck approach to saving our oceans. Seaweed farming can also help to clean our waters and makes some rather effective fertilizer. Given North Carolina’s long ocean coastline and large area of protected brackish bays, I’d like to see more research efforts on the benefits of seaweed farming here at home.

I hope you found this more in-depth analysis of seaweed farming and ocean acidity to be both approachable and informative. Perhaps I’ll send a link for this column to the Huffington Post!

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1) There is a limit to the amount of carbon dioxide that can be dissolved in the oceans. This is known as the saturation limit. Once the water becomes saturated with carbon dioxide, further increases in the amount of carbon dioxide in the atmosphere would not add more dissolved carbon dioxide to the water. If we were to reach the saturation level of carbon dioxide in the oceans, we would have problems much more significant than those I have described above.