Earth & Sky

Interview with Rich Aronson:

RA: Coral reefs cover about 2/10 of 1% of the area of the world’s oceans, but they’re incredibly important resource, at least in human terms, because at least half a billion people live within 100 km of reefs. So that is, about eight percent of the population lives very close to coral reefs. And so, tens of millions of people depend on reefs to supply them with food. That’s one thing that’s important about reefs. The second thing is, coral reefs protect shorelines from tropical erosion. So, if you remove reefs, that means that storm waves are lapping up against the shore – goodbye beach, and goodbye habitat for a lot of people. That, in economic terms, very conservatively, the total annual income from the world’s reefs is somewhere in the range of 30 billion dollars in tourism and other sorts of services. And, of course we all know that coral reefs have great aesthetic value, as well as enormous biodiversity of reef organisms – some estimates running into millions and millions of species. So for all these reasons, we have to be concerned about coral reefs. Coral reefs have been degrading, particularly in the Caribbean over the last 30 years or so. And similar signs are showing up in the Pacific. The main issue that we’re facing in the coming decade is how global climate change is going to interact with some of the other things that are going on in reefs, like overfishing and coral disease and pollution. And how these interactions are going to affect reefs, particularly in the Pacific, because as I say, reefs in the Caribbean are already quite badly damaged. So with all of that background in mind, we were asked to put together a summary of global climate change and coral reefs – of course, that’s the title of the report – in terms that are readily understood by policy makers and the public. So our task is to pull together a lot of disparate sources and try to put together a coherent story.

RA: The original conception of the report was more geological and climatological, and the other two authors, Bob Buddemeier and Joanie Kleypas decided that it would be a nice idea to have an ecologist on board. So they asked me, as they began the report, to participate. And it sounded like a wonderful project. And so I jumped at the opportunity.

RA: I’m an ecologist who dabbles in geology. I use geological methods to try to solve ecological questions. So I segue very well the sorts of concerns that Bob Buddemeier and Joanie Kleypas have. Joanie is into ocean chemistry and interactions between the ocean and the atmosphere. And Bob has similar interests, but he’s also interested in the geology. I’m interested in the ecology – the kind of nuts and bolts of community ecology of coral reefs, but again, with an interest in historical changes. So I do paleontology on reefs, but again, with a little bit of an ecological angle. So we conscious of each other’s interests and talents. And so that’s how we were able to work together on this project. For example, one of my big interests recently has been coral disease, and whether outbreaks of coral disease that we’ve seen over the last several decades had any sort of historical precedence. So I drive core tubes into coral reefs, pull out samples, and actually reconstruct the history of the reefs. To answer the precedence of whether these coral reefs have historical precedence – these outbreaks of disease have been enormously important, they’ve killed vast amounts of coral throughout the Caribbean, and diseases are just starting up in the Pacific. So, the question is what does this have to do with climate change? Well, as it turns out, a number of coral diseases actually grow better under slightly higher temperatures, and those are the kinds of higher temperatures that we would be experiencing in the next century. So these diseases are going to be more virulent in the near future. And the other side of this, corals do less well at slightly higher temperatures. So it’s a double whammy. Coral diseases will be worse, and the corals will be more susceptible. So this is the sort of situation where geology and climatology, biology and ecology all come together.

RA: Corals live at the upper limit of the their thermal tolerance. And in order to understand what that statement means I’ve got to tell you about the basic biology of corals. Corals are colonies of genetically identical animals that are a lot like sea anemones, and they’re all stuck together by sheets of tissues. And those little sea anemone-like animals are called polyps. The polyps lay down a limestone skeleton. So, when you look at a coral head, the living tissue is a very thin sheet over a massive dead skeleton, which represents the previous generations of polyps. The coral animals, live in a tight relationship with small, single celled algae called zooxanthellae. The zooxanthellae live inside the tissue of the coral host, and they give the coral their nice, greenish-brown color. And like other plants, these little micro-algae convert sunlight and carbon dioxide into carbohydrates, which the corals use. So that’s how the corals get their energy. The corals get their protein from catching animal plankton, which are floating around in the water at night. So they use their tentacles to catch animals. So they digest their proteins, and the waste products then are used to fertilize the little micro-algae. So it’s a very tight recycling. And this is how corals are able to survive and be so incredibly productive in clear, tropical waters. The reason why those waters are clear is that they don’t have any nutrients, and they don’t have a whole lot to eat in them. And that’s how corals survive. A critical piece of this theory is that in tropical regions, corals live in the upper limit of this tolerance. So, an increase in temperature, of only a degree or two centigrade, can have absolutely devastating effects. What happens is that during hot summers, when the temperature goes just a couple of degrees too high for just a little bit too long, this symbiotic relationship between the corals and their microalgae breaks down, and the corals literally puke out most of their microalgae. So, they’re actually vomiting out their algae, the physiology of the corals has gone sour, the physiological relationship between the corals and the algae, the corals are essentially sick, they throw up and get rid of the those algae, and they lose their color. Coral tissue itself is translucent. So what you see then is the white coral skeletons transmitting through the coral skeletons, so they bleach. They turn white. And in some cases the coral can recover. But if it’s too hot for too long, the corals get killed. And that’s what happened during the El Nino event in 1997-98, when the hottest temperatures ever recorded in the world’s oceans, where it showed up in the Tropics and about 15% of the world’s reef area was bleached and killed. So, how does this relate to global warming? As global temperatures increase, the extreme highs of high temperature events like El Nino will be ratcheted higher and higher. And eventually, they’ll poke above the thermal tolerance limit of corals on a regular basis, which will almost certainly lead to mass bleaching and therefore mass coral kills.

RA: Corals have some limited capacity to acclimatize to increasing temperature, or adapt to increasing temperature. And one of the things that they can do, that we’re just discovering now, is that when they bleach during those hot summers, they can actually swap out their zooxanthellae, their microalgae, for strains of microalgae that are more tolerant at higher temperatures. So they puke out the algae that they have, and take in new, more thermally tolerant algae, and then the temperatures get cooler again as fall approaches. Those new, more temperature tolerant algae are reproduced inside the corals body, and then the coral’s will have a nice compliment of more thermally tolerant algae. Now does that mean that coral reefs are going to be saved? Unfortunately not, the projected increases in temperature are too high for coral to simply keep swapping out their microalgae and adapt in that respect. So that’s one thought about corals adapting. Another thought that people have had is that coral reefs will simply migrate from tropical areas up into sub-tropical areas where it’s going to be cooler. And in fact, if you look at the predictions of climate change models, the subtropics will actually warm more than the tropics. So we can expect some corals to move in a northward direction in the Northern Hemisphere and in the southward direction in the Southern Hemisphere. And in fact we do see that happening with some coral species. But that does not mean that coral reefs as an ecosystem are going to pick themselves up and go trotting out of the tropics and wander up the east coast of the United States and spend their summer on the Jersey shore. That’s simply not going to happen. And the reason it’s not going to happen is that temperature is not the only thing that determines where coral reefs will and will not be.

Another very important consideration is the type of geological environment. And what I mean is this. In the tropics, the environment, the geological milieu (background) of the corals is based on limestone, on calcium carbonate. The ocean chemistry is very conducive to laying down limestone. So the corals lay it down, and in some cases, limestone actually precipitates spontaneously out of the water. So little tiny grains of limestone sand are actually falling out of the water. That’s not the kind of environment you see up in the subtropics and moving into temperate regions. For example, as you move up into the Florida peninsula, the east coast of Florida, it’s true that some corals are migrating up and they’re being found in northerly areas where they haven’t been found before. But there is going to be a limit, somewhere in northern Florida and southern Georgia, an area beyond which a lot of these corals simply cannot tolerate the sandy environment – an environment based more on silica rather than on calcium carbonate. Silica is this sort of glassy sand that you see if you go to the beach anywhere on the eastern seaboard, with the exception of Florida.

RA: As temperatures rise, we expect to see individual coral species moving from the tropics to higher latitudes. Whether it’s if they’re moving somewhat to the north, in the Northern Hemisphere, or somewhat to the south in the Southern Hemisphere. That’s a prediction from global climate change, and in fact we do see that. So staghorn and elkhorn coral in the Caribbean are in fact moving up the east coast of Florida into areas where they have not been seen before. However, that does not mean that coral reefs as ecosystems are going to move en masse up into the northern latitudes of the Northern Hemisphere. There are some important geological limits to how far north they can live. So, while we will see individual species moving to some degree, we will not see entire coral reef ecosystems translating themselves into more northern latitudes.

RA: The reason there is a northern limit to how far these corals will be able to migrate, is that once you reach northern Florida or southern Georgia, the geological background changes completely from limestone to the south, to silica to the north. Silica is the glassy substance that makes sand that you see when you go to the beach, as you move up the eastern seaboard. It’s different from the coral sand that you see in Florida and the Caribbean. Coral depend on an ocean chemistry that is conducive to laying down limestone, and they cannot make it in areas where there is a lot of silica type sand running off the continent. So that means that where that silica begins is going to be the northern limit of how far up corals can go – most corals from the Caribbean in any event. So what that means again is that individual coral species are going to be limited by the geology of where they are moving, and also, second point is that coral reef ecosystems are not going to move in one big package, we’re just talking about individual species.

RA: What we know right now for the Caribbean is that two very important species of coral, staghorn coral and elkhorn coral, have moved from their previous limit, which was toward Bicayne Bay just south of Miami, that’s where they used to be. They have moved up, and are now in Broward County off Fort Lauderdale and as far off as Pompano Beach. And that’s just happened in the last decade or so, as far as we can tell. And the last time those corals were there off Broward County was about 6000 years ago when the climate was several degrees warmer.

RA: In addition to massive bleaching as temperatures go up, coral reefs face another threat. And that is, a threat based on changing ocean chemistry. The reason that temperatures are increasing globally is because humans are emitting greenhouse gasses into the atmosphere. And the main greenhouse gas that we are sending into the atmosphere is carbon dioxide. As carbon dioxide accumulates into the atmosphere, it also dissolves in the ocean, and it makes the ocean more acid. And when the ocean is more acid, it’s more difficult for the little coral polyps to lay down the limestone skeleton.

RA: One of the most significant threats to coral reefs as global climate temperature increase is massive coral bleaching. Coral bleaching is the breakdown of a tight, symbiotic relationship between and the little, single-celled plants that give them their energy. When global temperatures increase, the corals actually become sick, and the relationship between the corals and their algae breaks down. The corals then literally puke up their algae. And that is very bad for them because then they have no source of energy. And if they bleach for too long, or too hot, the corals will actually die. So, for example, in 1997 and 1998, very high temperatures around the world, which were related to an El Nino Southern Oscillation (ENSO), which was enhanced by global warming caused mass coral bleaching around the world – something like 16% of the world’s coral reef area died in that single event. Clearly, coral bleaching is very bad for coral reefs, and just as clearly, we can expect massive bleaching in the future as temperatures rise.

A second aspect of global climate change has to do with the greenhouse gasses themselves, the gasses that actually cause the temperature to go up. We’re emitting enormous quantities of carbon dioxide into the atmosphere, and it’s carbon dioxide along with several other gasses that are causing our temperatures to rise. As the concentration of carbon dioxide increases in the atmosphere, more carbon dioxide is forced into solution in the ocean. That is to say, more carbon dioxide dissolves in seawater. That makes the sea water more acid, and that makes it more difficult for coral to lay down their limestone skeleton. And what that means is that if it’s more difficult for corals to lay down limestone skeletons, that means it’s going to be much more difficult for coral reefs to build up. And that is to say, to keep up with sea level, which is going start rising at an accelerating rate. But as sea level goes up, the coral reefs are less able to keep up with sea level, and so the reefs are going to fall further and further behind, the corals are going to be getting sick because they’re going to be bleaching because it’s too hot, and the reef ecosystems simply are going to fly apart.

RA: The coral bleaching events that we’ve observed over the last 20 years or so have all happened during strong El Nino events. And what happened is that the temperature on the reef that’s affected gets just above the thermal tolerance level of the corals. If it gets just a degree above that thermal tolerance level, for too long, it causes the corals to bleach. As global temperatures increase as a result of global warming, the frequency of high temperature El Nino events is going to increase. So, what that’s saying is that high temperatures of El Nino are going to be ratched up so that more frequently they will strike above the thermal tolerance limit of the coral. It’s going to be too hot for the corals, more often, as temperature increases, and that means that the frequency of these mass bleaching events is going to increase. And if these mass bleaching events keep happening year after year, they’re simply going to kill the coral.

RA: Corals are, of course, the main elements of coral reefs. It’s the skeleton of coral that provides the structure of the reefs that is the home to thousands, or perhaps millions of species around the world, from fish, sea urchins, snails, clams, sea weeds – all of these organisms depend on these structures that are created by the coral. If you kill the coral, by coral bleaching, or coral disease that is enhanced by global warming, then the corals die, and the skeletons that they have created begin to break down. The coral reef actually loses its topography, it becomes flatter. And once the coral becomes flatter, then fish and sea urchins and all the other sorts of organisms that compose the reef community will abandon it. So that means that the productivity of the reef in terms of fish that could potentially feed local populations and create a tourism value, and a lot of aspects of the reef – it’s ability to shield coastlines from erosion of storms – all of those things are going to go away as the actual structure of the reef breaks down.

RA: Corals on coral reefs provide the structure that is home to reef fish. And there are a lot of reef fish that eat plant material, and it’s those reef fish, along with some sea urchins, that also depend on reef structure, that control seaweed. If you kill the coral, and open up empty space – that is to say dead limestone skeletons – and the structure of the reefs begin to get knocked down by erosion, then the fish and the urchins are going to go away, they are not going to be eating the seaweed. Seaweed will grow enormously quickly. They will take over the reef, and they’ll prevent the corals from actually coming back. They actually can prevent the little baby corals from setting on the reef and recovering the reef. So, once you kill the corals by disease, or by coral bleaching, and again we have said that global warming potentially enhances the reef. Once you kill those corals, they’re an entry for seaweed to get in there. So the balance between corals and seaweed is completely disrupted. Seaweed takes over, and what that means is that the reefs will be limited even further, and that means that they will be able to grow toward the surface even less, and they will fall further and further behind the rising sea level, and eventually the reefs will simply drown. They’ll be out of their depths. They’ll be embedded so deep that corals, even if they are there, will not be able to manage. There’s not enough light for the little internal algae to be able to conduct photosynthesis.

RA: There’s a huge difference between a coral reef that’s dominated by coral and a coral reef where the coral has been killed and seaweeds have taken over. A coral reef that has its corals intact and healthy is a place of fantastic topography and structure. It has fish swimming in and out of it, and sea urchins, and snails, and all sorts of other reef organisms – shrimps and worms and all the wonderful things that live on a coral reef, and these organisms all interact together in ways that increase the productivity of the reef. In areas where the coral has been killed, the seaweed has taken over, and what you usually find is a flat area, covered with seaweed, not very pleasing aesthetically and all the fish have run away. And so the reefs become flatter and flatter because there are no fish to control the seaweed. And that means that there are no corals coming back. And that means that there is less structure being created, while at the same time, urchins, sponges, and burrowing clams are digging into the reef structure and degrading it, actually breaking down the reef. The reef is getting flatter and flatter for a whole variety of reasons. Again, the corals could add structure, plus there are organisms that are actually breaking down structure, and so the situation just feeds on itself, it’s a feedback loop. And as the algae increase, the algae increase. That is to say, it’s worse for the corals, it’s better and better for the seaweed, and the whole structure becomes flatter and flatter, and less and less able to recover.

RA: One of the things that we need to understand about coral bleaching is that bleaching does not always kill the coral. If the corals bleach in the summertime, and then the temperature goes down again, soon enough, as fall comes on, the coral can actually recover the microalgae that live inside them. In fact, when they bleach, they lose about 90% of their microalgae. As it gets cooler, those microalgae can reproduce and repopulate the coral, and the coral can take on additional microalgae that are floating around on the water. They can get their algae back from two sources. Once they’ve recovered their nice, greenish-brown color, however, those corals are very skinny. They haven’t had anything to eat during the summertime. And it takes about a year for them to recover to their full, plump, happy polyp state. Once it’s bleached a second time, that can kill them. Additionally, if the corals bleach for too long, or it’s too hot the first time around, that can kill them outright. So, there’s a decent possibility for them to recover if the bleaching event is not too severe. And the other possibility that scientists have talked about is the idea that some of these corals might be able to switch to more heat tolerant strains of microalgae. So these are possibilities, that the coral heads that are there, on the ground that bleach should be able to recover. Another way that you can think about reef recovery is if little coral larvae are able to come from upstream sources, from areas that have not bleached so severely, and come on to a reef where corals have been killed, settle down and form new colonies and grow up. And that depends a lot on the balance of corals and seaweed. So if the reef is badly overfished, the reef and the corals have been killed by bleaching, then when the new corals settle, there’s going to be so much seaweed because the fish aren’t there to control the seaweed. There’s going to be so much seaweed that the new corals cannot get established. But if we’re able to conserve the fish on these reefs, when they do bleach, they’ll be more scope for corals to come in from elsewhere, as larvae, and settle and grow up without being overrun by the seaweed. So all these problems – fishing, and disease, and global warming – they’re all interconnected. Clearly, it’s going to be easier to do something about fishing and pollution and some of the more localized human impacts than it is going to be do something about global warming. So we should certainly, immediately take whatever actions we can to at least preserve the integrity of these reefs as much as possible. So when mass bleaching events do occur, it is easier for those reefs to recover. In terms of issues about global warming itself, there’s absolutely no doubt that we have to curb emissions of greenhouse gasses.

RA: There are a lot of things going wrong on coral reefs these days. Diseases are increasing – there are new diseases – and some of our paleontological work tells us that outbreaks of disease have not occurred before in at least the last several thousand years. Those diseases are killing coral. We also have pollution and global warming that lead to a whole variety of reasons for which corals are dying out there. The relationship between coral and seaweed, and how that relationship is mediated by fish and sea urchins is very important, and I’ll tell you about what is going to happen to coral reefs. Corals provide structure of the reefs. The reason they’re called coral reefs is because there are corals there. And the corals have all sorts of cavities and crevices and high towers and valleys and mountain peaks and all those kinds of structures on the reefs. And it’s that structure that provides shelter for fish and sea urchins. And some of those fish – as well as the sea urchins make their living by eating seaweed. So, in a healthy reef, seaweed is usually a very minor component of what’s on the bottom. It’s mostly coral with a little bit of seaweed. Once you kill the coral, you provide a lot of open space for new things to come in. And the new things that would come in and recolonize the open space are corals, sponges, seaweed, and lots of other sorts of bottom-dwelling animals. It’s the activities of the fish and the sea urchins that keep seaweed under control. And by keeping seaweed under control, they allow new corals to come in. And those new corals that are coming in are usually little larvae – little, baby corals – that are coming in from upstream sources. So some reef that’s upstream from a reef that’s been affected, the upstream reef has not been as severely affected, the corals are reproducing, they launch their eggs and sperm into the water. The eggs and sperm fertilize, and the larvae float on the ocean currents and come down and settle on the reef. This is all when things are going in the direction that we want them, things are going well. What happens though, in areas where the reefs are severely overfished, there aren’t any fish available to keep the seaweed under control. So if you can’t keep the seaweed under control, the seaweed goes wild, and takes over all that open space from the coral kill. And that means that the new corals can’t get in because the seaweed has monopolized the space. And that means that because the coral reef has been eliminated because of fishing pressure. If you can control fishing pressure, that’s at least one thing you can do to mitigate the problems that we’re facing with coral bleaching. At least this means that the corals that are coming in from the outside – little baby corals floating on the ocean current – have some possibility of landing and establishing themselves successfully.

RA: The big picture is that the conditions that we’re facing now are truly unprecedented. We have no really good analog in the geologic past for what’s going on now. However, we can make predictions based on experimentation and observations that we’re already able to make. And the future looks bleak, but I would certainly not say that it looks hopeless. So what we have to do right now take action. We have to limit greenhouse gas emissions. We have to take a look at limiting fishing pressures, we’ve got to limit pollution, and we’ve got to understand what is going on with coral diseases. We have so little understanding of these weird new outbreaks of these emerging diseases on coral reefs, and take what management actions we can to mitigate the effects of more overarching problems. So, for example, if we can do something locally, like control fishing pressure, then even if it takes a longer time to curb the problems associated with global climate change, at least the reefs have a little better chance of recovering. So it is a bad situation – the hour is late, we really need to get on this now, but it certainly is not a hopeless situation.

RA: There have been some very successful attempts to create marine protected areas, especially in the Caribbean, but also in the Pacific. And remember that the Caribbean is very heavily damaged. Those marine protected areas, there’s no doubt that fish abundance has increased enormously. There’s also some indication seaweed have declined in areas where fish have been protected. How that is going to play out in terms of the success of corals coming back in is not well known at this point, and it’s going to take a few years before we can track the reef for a long enough time to be able to see if recovery is even possible. So we’re in the very early stages of trying to get a handle on how effective some of these actions are. The problem is, of course, that as you protect reefs on a local scale, they’re being affected by more regional factors. Like I said before, you have to take the actions that you can. There’s strong, limited evidence that those actions are helpful. But how this is going to play out in the overall scheme of things – whether this will totally solve the problem of coral reefs – well, I certainly doubt that. The point is to take the actions that you can now, and give reefs at least a little bit of a chance to stave off what I hope is not the inevitable, and then take actions on the more regional and global issues.

A good website for background information about coral habitats:

Habitats: Coral Reefs – Characteristics (Office of Naval Research)

The following person was interviewed for today’s program. Our thanks to:

Richard Aronson
Senior Marine Scientist
Daulphin Island Sea Lab
Dauphin Island, Alabama