“Earth's Acidifying Oceans”

JOHN RAFFERTY: So, what you're saying is that, just to clarify, is that, under the worst case scenario RCP 8.5, the models predict a 7.7 average pH across the ocean?

CHRIS LANGDON: The pH is measured on a logarithmic scale. So, a three tenths of a unit pH decrease may sound small, but because it's measured on a logarithmic scale, it's actually a 250% increase in the acidity of the water. A 250% increase in the hydrogen ion concentration. So, that's a whopping big change, and we can expect big changes, you know, in the organisms that have to live in that water. So, it's not a small change at all.

RAFFERTY: Hi, I’m John Rafferty, I am the editor for Earth Sciences at Encyclopaedia Britannica. The oceans are wide swaths of water that separate continents from one another, move heat between the tropics and the poles, and provide many forms of wildlife with places to live and make their living. For people, the ocean is a source of beauty, a source of income, and, most-importantly, a source of food. As we’ve seen in other Postcard’s episodes, today’s oceans are plagued by overfishing, chemical pollution, and plastic pollution. In this episode, we’ll explore another emerging threat, ocean acidification.

The pace and severity of ocean acidification, we’ll find, is driven by physical and chemical processes that respond to rising levels of carbon dioxide in the atmosphere. As ocean waters become more acidic—in other words, as their pH declines—plants, animals, and other forms of life, which are used to less-acidic conditions, become stressed. While some organisms have ways to cope with this stress, at least up until a point, other organisms do not, and there are signs that their survival is at risk.

For some help along the journey today, I spoke with Dr. Chris Langdon, one of the leading experts in ocean acidification.

LANGDON: My name is Chris Langdon. I'm with the Rosenstiel school of Marine and Atmospheric Science at the university of Miami, where I'm a professor in the department of Marine biology and ecology.

What is ocean acidification? It’s the lowering of the pH of seawater throughout the world’s oceans.

Ocean acidification is largely the result of loading Earth’s atmosphere with large quantities of carbon dioxide (or CO2), so, in essence, ocean acidification and global warming are caused by the same thing.

Since the beginning of the Industrial Revolution about 1750, roughly one-third to one-half of the CO2 released into Earth’s atmosphere has been absorbed by the oceans—which react chemically with the gas resulting in the production of carbonic acid that mixes with seawater. Since 1750, the average pH of seawater declined from 8.19 to 8.05, which corresponds to a 30% increase in acidity.
The pace of ocean acidification since the beginning of the Industrial Revolution has been approximately 100 times more rapid than at any other time during the most recent 650,000 years. Concentrations of atmospheric CO2 between the year 1000 and 1900 ranged between 275 and 290 parts per million by volume.

By 2020 the average concentration had grown to about 413 parts per million, and climate models project that CO2 concentrations will rise to between 450 and 1,100 parts per million by the year 2100, depending on the level of greenhouse gas emissions we humans produce.

With additional CO2 transferred to the oceans, pH would decline even further; under worst-case scenarios, seawater pH would drop as low as 7.7 by the year 2100.

LANGDON: The IPCC, which makes these quadrennial predictions have different emissions scenarios and RPC 8.5 is called the worst-case scenario. And it's assuming that the number of people on earth continues to increase, and that will be up to 9 billion by the end of the century. So, pretty much a lot of our CO2 emissions just scale with how many people are on Earth.
Marine scientists are concerned that the process of ocean acidification constitutes a threat to sea life and to the cultures that depend on the ocean for their food and livelihood.

Dr. Langdon has been studying ocean acidification for more than 20 years. One of his earliest experiences with the phenomenon took place at Biosphere 2, a self-contained scientific research facility located in Oracle, Arizona, which was designed to mimic Earth’s environment.

LANGDON: I got into this field back in the late 1990s when Columbia University got involved with the Biosphere 2 facility out in Arizona, and they had a large seawater aquarium. And because of the close nature of the biosphere, the CO2 levels inside had built up to a high level and, uh, as a result, it has acidified the large seawater aquarium. When I came out there, I just innocently measured the pH of the water, and I noticed that the pH was quite low, like 7.7, and then I just took the steps needed to raise the pH back to normal.

Then I was observing the behavior, actually with something called the calcification rate of the corals in the tank. And I just happened to notice that when I raised the pH back to the normal levels, the health of the corals as measured by their calcification rates immediately rebounded. I let the pH relax again to a low value, and then I made some observations and then raised it by making some chemical additions to the seawater that you do to raise the pH. And in doing that, I did that actually eight times. And every single time I raised the pH, the corals looked happier and started to calcify at a more normal rate. And whenever it was lowered, they did much more poorly.

So that was my entry into this field. And, I had a great deal of trouble getting the results published. People just were not prepared to believe that this was a real phenomenon. It was sort of the pH changes, uh, sounds small, and people thought the corals, other organisms were able to regulate their internal pH and wouldn't be so sensitive to small changes in the environment around them.

Several years later, I had an opportunity to come to the University of Miami and accept an academic position there, and I've been there ever since. It's 15 years now. So, I built a facility for, for doing these kinds of ocean acidification experiments. And I've looked at the effects of acidification on a bunch of different aspects of the life of a coral.

So, some of my earliest studies were on the effects on reproduction, on egg fertilization success, on the next step in life for coral, a successful coral polyp needs to settle on the seafloor. And, uh, both these early life history steps were very sensitive to small changes in pH. And then I've switched more recently to looking at the effects on adult corals. And today, um, the last couple of years, I've been looking at how ocean acidification will affect the bleaching response of corals.

RAFFERTY: But how does pH work?

LANGDON: Ocean acidification refers to the reduction in the pH of the ocean. So, just to clarify, the pH of 7is our definition of neutral, the historical pH the ocean is about 8.4 to a 8.1 or zero today. So, it's actually slightly on the basic side of neutral. So, when we say acidification, we're talking about the change or the direction of change of the pH. So, pH, as a result of CO2 building up in our atmosphere, the pH of the ocean is reducing, but it has reduced from 8.4 to 8.0. And by the end of the century, it could get further down to 7.7, but in no case, is it ever actually acidic by the definition of being less than 7.

As far as organisms and chemistry are concerned, there's nothing magical about a pH of seven. And it's really only the direction of, just like temperature. When we say cooling, you know, that doesn't specify what the background temperature is. It's just really telling us about the direction the temperature is becoming less. It doesn't tell us if we're above or below the freezing point. And acidification is the same.

RAFFERTY: If the pH of seawater is declining, and carbon dioxide gas in the atmosphere is responsible, how does this work? After all, the atmosphere and the ocean are separate media; how is atmospheric carbon dioxide gas absorbed by the ocean?

LANGDON: Dissolved gasses in the ocean, get there by a physical process called diffusion. So, there's a number of gases that are in our atmosphere, and the dominant one is nitrogen. The second, most dominant one is oxygen. And then way, way down on the scale is CO2. Diffusion is a process where anything--it could be salt, but in this case, a gas molecule--moves from one medium to the other based on a concentration gradient. So, in our case, if the concentration of a gas is higher in the atmosphere than it is in the water, then this gas will naturally want to move from the atmosphere into the water until a balance is reached, where the concentration and the water is the same as it is in the air above. And that, in fact, is how CO2 gets into the ocean, but it's also how oxygen--everything in the ocean needs oxygen to breathe--and that oxygen, or a lot of it, comes by the fusion from the atmosphere. Of course, plants in the water photosynthesize and produce oxygen there, as well.

RAFFERTY: The oceans are becoming more acidic as more-and-more carbon dioxide is absorbed by seawater, so what happens to living things in an acidifying environment?

LANGDON: All life--the cells in our bodies and in any, uh, plant or animal in the ocean--they like to keep a constant pH in their cytoplasms. So, just like our bodies maintain a very constant body temperature--for humans, it's 98 degrees Fahrenheit--cells of all living things work super hard to maintain a constant pH inside their cells of about 7.4.

But if you reduce the pH of the water in which these things are living, then a lot of organisms--and particularly in the more, the more primitive ones--um, aren't able to control their internal pH perfectly. So, as the oceans acidify what's happened is, in fact, the cells of corals and sea urchins and oysters and clams and mussels and a lot of other things begin to acidify as well. And that has negative consequences for their metabolism.

RAFFERTY: So, is it a shock? Is that what kind of happens, or is it kind of a slow boil?

LANGDON: Yeah. A slow boil, because, you know, first of all, the oceans are acidifying extremely slowly. I mean, it's fast on a geological timescale, but even year on year, it's a very slow. So, it's like, you know, a frog in a pot, and if you increase the water slowly enough, supposedly the frog never has the sense to hop out and ends up getting cooked. It's the same here. The acidity is increasing very slowly, and in the simple organisms like corals, there's a parallel change in the pH inside their cells. So, it's not a shock response. It's a long-term chronic response.

This is opposed to higher forms of life, like fish and squid and stuff, which are more evolved. When they sense that they are acidifying, they have organs and mechanisms to compensate. So, then you see just a little blip in the pH, and then they're able to re-establish it at a healthy level. So, just like we have cold-blooded animals whose body temperature is whatever the environment is, there's these simple forms of life; their internal pH as a slave of whatever it is in the water they're living in.

RAFFERTY: Corals are invertebrate marine animals that are known for their elaborate skeletons. The term coral is also applied to the skeletons of those animals, particularly to those of the stonelike corals.
The body of a coral animal consists of a polyp—a hollow cylindrical structure attached at its lower end to some surface. At the free end is a mouth surrounded by tentacles. The tentacles, which gather food, often extend out and are armed with specialized stinging structures, called nematocysts, that paralyze prey.

During the coral’s reproductive phase, fertilization results in the production of a larva, which swims about for several days or as long as several weeks, then settles onto a solid surface and develops into a polyp. Reproduction continues in the polyp stage as new polyps bud off of the original one. The bud remains attached to the original polyp. A colony develops by the constant addition and growth of new buds. As new polyps develop, the old ones beneath die, but the skeletons remain.

These skeletons accumulate and build on each other, creating the coral branches and other structures we’re all familiar with. Corals are important in marine ecosystems because they provide habitat for shellfish and other marine life.

LANGDON: So, corals are one of the more primitive forms of invertebrates. They're in a family called the Cniderians. So, basically the anatomy of a coral is just two to really thin layers of cells, two epithelia. They really don't have any, uh, organs yet evolved inside them. So, they're basically just a two-layer bag. So, it’s microns separate the inside of a coral from the ocean and outside of it. And they don't have blood, they don't have lungs, any kind of circulatory system. So, they just don't have the wherewithal to control their internal conditions very much. But as you move up the evolutionary scale to vertebrates, like fish, they have organisms--you know, organ systems. They have, you know, livers and hearts, so they can do a lot to regulate their internal conditions, and they do. What they would do is increase the bicarbonate concentration of their, uh, blood. So, it's the same. It's interesting. It's the same buffering system as the ocean, but they're doing it inside their bodies.

RAFFERTY: What kinds of species are benefiting from these changing conditions?

LANGDON: They, so there’s some things that actually do better in an acidified ocean, and sea grasses are one of them. And then there’s some forms of bacteria that fix nitrogen--nitrogen fixation. These are blue-green algae, or cyanobacteria. So, there are actually bacteria that are able to photosynthesize, and some of them also fix nitrogen, and some of them also actually do better under acidified conditions.
So, in ocean acidification research, we’ve learned that it's very difficult to generalize. And as soon as you do, someone can point to an exception. People have been looking at different species of sea urchins, and like the purple sea urchin doesn't like acidification, but the green one does. Tiny little gradations within the same basic organism have very different responses, so it's a very complicated business. And ocean acidification, as a science, you know, really is quite young compared to a lot of other fields, so we're still learning a lot every day.

RAFFERTY: The coral reef community is made up of different types of corals, shellfish, other invertebrates, fishes, and (in some cases) marine mammals. The coral reef ecosystem is a complex labyrinth of branching corals, pits, and cubby holes that provide habitat for many other forms of life.
Dr. Langdon noted that stressed reef ecosystems are becoming simplified, both in terms of biodiversity—and in terms of habitat complexity. He notes that reef ecosystems in some places are even becoming flatter.

LANGDON: So, if you've studied the Florida reefs in the 1970s, you would have been able to find maybe 40 different species of corals. Over time due to disease and bleaching and overfishing, we've lost a lot of those species. So, we're maybe down to maybe 10 species that are prevalent today. And, a lot of their growth forms, they make them more massive or brain coral shapes. And some of the species that we've lost are the ones that make up--we call them branching forms.

So, the staghorn coral and the Elkhorn coral or branching species. We’ve lost maybe 90% of those species. And they're super important because they are--first, they're fast growing, and second, they make a three-dimensional structure, kind of like a raspberry brambles, that create just perfect hiding places for little fish. So, they're really important in creating a habitat for a lot of the other life that forms on the reef.

And then these other corals, even though they're absolutely gigantic or like beach balls, and they don't create as many hiding spaces. So, it seems like the reefs are heading in that direction as a result of climate change, so far.
We have a simplification and a flattening. What we used to do is we'd measure something called rugosity, where you take a chain that's 10 meters long, and then you drape it over the sea floor. So if the reef is totally flat, after you draped the reef, the chain would still measure 10 meters from end to end. But if you're draping it over like tabletops and chairs--you can imagine the chain going up and down over the corals? If you did this in the seventies, there might only be three meters between end to end, because the chain is doing so much, and going up and down as you drape it over the structure of the reef. So, this is just a super-simple way to measure the structure of a reef.
So, the rugosity is the ratio of this measured distance between the two ends of the chain, as you drape it over the reef, and the full length of the chain, or actually vice versa. So, that ratio used to be three, and today we're down to, like, 1.5. So, the reefs are very much flatter today than they were just 50 years ago, because we're losing the corals that had these more complex shapes.

RAFFERTY: Nevertheless, it turned out that while several corals were being eliminated from Florida reefs, one species in particular was thriving.

LANGDON: Yeah. So, there’s a coral called Siderastrea siderea, it's one of these massive, um, it's, it's actually not a brain coral. It looks like a golf ball--you know how you have a golf ball--has these little indentations all over it? So, Siderastrea siderea looks like a golf ball, but, you know, it could be three or four feet in diameter. That's the most common coral on Florida reefs today.

RAFFERTY: Corals partner up with algae to survive—with the coral offering the algae, called zooxanthellae, a protected environment within their tissues and nutrients that can be used for photosynthesis. In return, zooxanthellae provide the corals with oxygen and help the corals to remove waste products.
Coral bleaching is whitening of coral that results from the loss of a coral’s zooxanthellae—in which the partnership breaks down and the two organisms separate from one another (or it results from the degradation of the algae’s photosynthetic pigment, which leads to lower rates of photosynthesis). Coral bleaching has a variety of causes. It may result from increases in seawater temperature, especially in the presence of high levels of ultraviolet radiation. It may be caused by changes in seawater chemistry (due to ocean acidification or pollution), increased levels of sediment in seawater, or a coral’s exposure to sodium cyanide (a chemical used in the capture of coral reef fish).

RAFFERTY: You've got this chemical effect on the organism itself from ocean acidification, but you also have this parallel challenge of coral bleaching with temperature. How do you separate the two? Are they working together? Is this, is this creating a kind of a bad situation?

LANGDON: Yeah. So, the way I study it is--so I would do what are called factorial experiments. So, some corals I'll expose only to a temperature increase sufficient to cause bleaching. In others, I'll hold the temperature constant and only reduce the pH. And then one treatment will be a control where there's no change. And then the fourth, I increase the temperature and decrease the pH. So, by having these four corals exposed to these four different treatments, you can tease apart what's going on, and what I'm finding is that acidification seems to aggravate the bleaching response. It causes the zooxanthellae to--or, you know, the corals to bleach--to lose their zooxanthellae after fewer days of exposure to elevated temperature. So, one is making the other worse.

RAFFERTY: Still, Langdon notes that zooxanthellae are resilient, and they can recover, if given the chance.

LANGDON: That's a great point. So, I actually do recovery experiments. I don't, uh, if you carry these out too long, everything will die. But if I stop at that point, or at some earlier point, then everything recovers. And corals can be bleached for weeks and months, even sometimes. When a coral bleaches, it's losing its zooxanthellae. It's losing one source of its food, but they have polyps and those polyps have tentacles on them, and they can capture food, bacteria, and phytoplankton and zooplankton that live in the water. Some species are able to make up for the food that they lost from their zooxanthellae, by what we call heterotrophic feeding. And those species can be just absolutely bone-white and survive. And then as soon as I lower the temperature or, and, or raise the pH, they, uh, within just a few weeks, they can be back to almost a hundred percent.

It’s interesting, a coral that looks white to the eye may still have, you know, 20% of its original complement of zooxanthellae. So, when you lower the temperature, it's probably just those remaining ones rapidly dividing and multiplying that recovers the coral. They don't necessarily have to go out and get it from the environment. I think that's why it's a pretty rapid process.
The zooxanthellae are like bacteria. They replicate just by cell division. They divide in half and then divide in half again, a couple hours later or days, so they can undergo really explosive exponential population growth. So, recovery can happen quickly.

RAFFERTY: Despite general assurances by many scientists that ocean pH won’t decline beyond 7.7, Earth’s oceans could become even more acidic. What then?

RAFFERTY: So, we're expecting it to go, based on a worst-case scenario, down to 7.7?

LANGDON: Right.

RAFFERTY: But what if it goes further than that? What if we have grossly underestimated the amount of emissions? What happens is as the pH declines to 7.4 and 7.1? What are some of the things that maybe keep you awake at night that might happen, if we don't get a handle on this?

LANGDON: We don't actually know what the lethal doses are yet, but obviously we're going to start approaching those and just--you know, instead of just slowing an organism down and causing metabolic suppression from which things can recover--we're eventually going to push things to, you know, to the tipping point where they obviously start dying.

Diversity and then numbers of individual population sizes will start to decline. And that will happen first. So, every organism, if you look geographically, there's like a geographic sweet spot where physical and chemical conditions are all optimal. And then as you move to higher and lower latitudes, we get to the fringes of their distributions. You know, just like a plant. You know, something you try to grow in your garden, you know, on the back of the seed pack, there'll be a map of where you can find it. Those bio zones are going to shrink--you know, the distributions of species will shrink. And the things that die first are the larval stages. It seems like the early life history stages of most things are most delicate. That's just like a pretty safe and general statement that we can make. So, we could end up with a lot of adults and no possibility producing babies anymore. So, then you end up with a geriatric population and no future.

We can see some of these big brain corals we were talking about. We can find the adults and the, you know, some of them, you know, they’re long-lived. Some of them are 300 years old or more, but often we can't find any babies of those species. So, that means that already, you know, the conditions in the water may not be conducive to them producing babies anymore. Obviously, some species are still successful, and they're more robust, but as we lower the pH, uh, we're gonna push ourselves in this scary direction of, uh, probably preventing re--successful reproduction of a lot of species. And then, you know, within whatever the lifespan of those organisms are, when the adults die, that'll be the end, just like the, you know, the die-off the dinosaurs.

RAFFERTY: Can we expect that we might see potential losses in commercial fishing or, uh, shellfish?

LANGDON: Oh, absolutely. Clams and oysters, in particular, their larvae are very sensitive to acidification. So, those, so that's, you know, aquaculture, and that's an important source of food in some places. And that's definitely gonna be one of the first things to take a hit, if we allow the ocean to acidify more. It's already a problem and it's going to get worse. As I said, you know, the reefs are getting flatter. So, there's certain indigenous people, you know like small islands in the Pacific, where they get a lot of their protein from the fish that they can catch on the reef. So, they're going to experience food security issues.

RAFFERTY: So, how do we get out of this situation? Ocean acidification is interacting with other environmental problems, like coral bleaching, to stress corals and other forms of life. Dealing directly with the cause of the acidification—the buildup of carbon dioxide in our atmosphere—by strengthening carbon dioxide emission regulations, while at the same time phasing out oil, coal, and other fossil fuels, is likely the best way to go.
While we are waiting for international agreements to happen, Dr. Langdon notes that we might try a few other things.

RAFFERTY: Can the effects of ocean acidification be reversed, and perhaps there is a list of things that we can do (as individuals and societies) to, uh, to help affect this change?

LANGDON: So, the first and most important thing would be to reduce CO2 emissions, because that's what's acidifying the oceans in the first place. It’s also what's making our climate warmer.
Natural processes that remove CO2: those are photosynthesis by plants in the ocean and on land and a chemical process called weathering of rocks. So, they will, those processes, um, will start to remove any CO2 that we put into the atmosphere, and the CO2 concentration will start to come back down again. And as soon as the CO2 in the air is less than it is in the ocean, the CO2 will start to escape from the ocean, and the pH will start to rise. These are pretty slow processes, but these are natural processes that will happen. Absolutely. For certain.

And then there's various things we may want to consider to accelerate things: human intervention. And some of that would be, uh, as I mentioned, the natural buffering system is to add carbonate and bicarbonate to the ocean due to weathering, but there are other chemical processes that we could undertake--that we could produce massive quantities of these chemicals--and add them to the ocean intentionally. And that would, uh, raise the pH of the ocean.

RAFFERTY: So are we talking about, uh, dumping instant cement into the oceans, or…

LANGDON: We're talking about adding carbonate and bicarbonate. So, these are innocent chemicals that are, as I mentioned, they're what the ocean naturally uses, but there are chemical processes. There's actually thoughts of linking it to carbon. You know what carbon sequestration is?

RAFFERTY: Yes.

LANGDON: So, these are industrial processes that take CO2 out of the air, and some of them as a by-product actually produce carbonate and bicarbonate. So, the very same process that would help take CO2 out of the atmosphere and out of the ocean--one of its waste products is carbonate and bicarbonate. We have to get rid of it somewhere. We could dump it into the ocean. It would be a win-win.

RAFFERTY: Langdon also notes that some corals are more susceptible to the effects of ocean acidification than others. He and other members of the scientific community are examining the qualities and characteristics that surviving corals have for clues on how to restore damaged reef ecosystems.

LANGDON: And I'm finding that if you pre-expose the corals to reduce pH, corals actually become more sensitive to bleaching. They'll bleach in fewer days of exposure to a certain given level of a stressful temperature. But another aspect of this Is I'm looking at genotypic differences. So, I'm working with the same species, which is the staghorn coral, but I keep track of the parents that the coral pieces that I work with come from. And in my recent experiments, I had corals from 40 different parents. So, each parent we assume has slightly different genotypes, genes. And I expose them to the same level of heat stress. And I found that some genotypes can survive for almost a hundred days longer than others, under conditions of both the elevated temperature and reduced pH. So, this is a little bit of good news. The bad news is that acidification makes corals more sensitive to bleaching, but there's tremendous genotypic variability from one coral to the next. And I think that the--what I'm finding is--that there's some genotypes that already exist on reefs in Florida that have a pretty good chance to survive in until the end of this century, in terms of being able to withstand a two degree warming--a couple of tenths, a reduction in pH.

So, I’m trying to find genotypes that are more resistant. And ideally, I want to find ones that are resistant to bleaching and acidification, because in nature, they're going to happen hand in hand. And then--I'm calling these supercorals--and if I find them, then I want to raise them up like in nurseries. And then there's a big coral reef restoration effort. We're actually, we call them citizen scientists--just people that know how to scuba dive on weekends can volunteer to take these corals that have been raised in nurseries and take them out on the reefs and epoxy them to the reef to replace the corals that have died. So, we're trying to think outside the box and actually try to not just study the decline of corals forever, but, actually, do something to help improve their chances of surviving. And then, if I find these supercorals, I'm also interested in what genes do they have that make them super. And then, if I can isolate those genes, could we actually insert those genes into other corals--to increase the number of corals that have these superior qualities? Again, trying to improve corals chances of surviving.

RAFFERTY: Ocean acidification is not a highly visible environmental problem, but its pervasiveness throughout the oceans will spell deep trouble for marine life, as the pH of seawater declines and acidity increases.

As corals, algae, shellfish, and other marine species are pushed closer to their tolerance limits, their activity will slow, and they will become weaker. When these limits are breached, first individual animals, and then species, will die off in this slowly acidifying environment.

Still, there is hope. Ocean acidification is closely tied to the problem of global warming, so as international rules dealing with carbon dioxide and other greenhouse gases get stronger, the acidification will slow, and maybe—just maybe--reverse itself as conditions improve.

I hope that you were able to gain an appreciation of the close ties Earth’s ocean and atmosphere have with one another. I hope that you were also able to gain a deeper understanding of ocean acidification and the slow-burning threat it poses to marine life, aquaculture, and commercial fishing.

Don’t forget, you can catch up on anything you might have missed on Britannica.com. Learn more about extinction and its causes from our article located at mainten.top/science/extinction-biology.

There, you can also find other parts of this podcast series. More information on ocean acidification, carbon dioxide, coral, and seawater can be found at mainten.top.

Earth’s Acidifying Oceans: Story by John Rafferty; produced by Kurt Heintz. A special thanks to Dr. Chris Langdon for his contributions to this episode. This is the eleventh part of the “Postcards from the 6th Mass Extinction” series. This program is copyrighted by Encyclopaedia Britannica, Incorporated. All Rights Reserved.