SCIENCE AT THE SHINE DOME canberra 7 - 9 may 2008
Symposium: Dangerous Climate Change: Is it inevitable?
Friday, 9 May 2008
Professor Ove Hoegh-Guldberg
Centre for Marine Studies, University of Queensland
Ove Hoegh-Guldberg leads an active research group that is focused on the impacts of global warming and ocean acidification on coral reef ecosystems. He has produced over 120 peer-reviewed scientific articles, runs the blog www.climateshifts.com, is a reviewing editor at Science magazine and chairs the Coral Reefs and Climate Change working group within the Coral Reef Targeted Research Program for the World Bank. In 1999 he was awarded the Eureka Prize for scientific research into the mechanisms underpinning coral bleaching and climate change.
Stresses on coral reefs: Can they adapt?
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Thank you, Amanda.
First I would like to say how grateful I am to the Academy of Science and the organisers of this important symposium for the invitation I have received to talk at the symposium. I am going to talk about the impact of climate change on another of the systems, in addition to the Arctic, which have become the signature of the ecological changes in scale that we are expecting to see in the next few decades and century.
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The impacts on coral reefs have come, really, in two forms. This is a picture that looks quite beautiful, but actually it is a picture of death. What we are looking at is a reef on the southern Great Barrier Reef in 2006 which has bleached. Bleaching involves the breakdown of the symbiosis between corals and tiny photosynthetic organisms called dinoflagellates. The net effect of this breakdown is that the corals are no longer photosynthetic, they can't trap enormous amounts of energy, and they can't deposit calcium carbonate in the same quantities and so on.
Coral bleaching has swept across the planet several times. Since 1979 there have been about six major events. In those events we have seen mass mortalities of corals, and I will talk a little bit about that.
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In addition to the bleaching problem, corals are also facing another issue, the effect of ocean acidification, which was touched on a few minutes ago by Michael Raupach. This is the effect of CO2 rising in level in the atmosphere, dissolving in the ocean and interfacing with water such that you get a proton that likes to find carbonate ions and turn them into bicarbonate ions. The net effect is that as you raise the CO2 in the atmosphere, you decrease the carbonate ion concentration.
The problem with that is that it makes the resources for corals that are precipitating calcium carbonate very rare. Recently, at the end of last year, there was a paper by Cooper et al, from the Australian Institute of Marine Sciences, suggesting that the Great Barrier Reef is now calcifying at 20 per cent less than it was in the 1980s. This is probably a combination of dwindling resources for calcification and increased stress from warming seas.
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These sorts of changes make you to start to wonder whether we are already experiencing dangerous climate change, and I will come back to that at the end because of the definition that the Framework Convention on Climate Change has set. I think we may be there, and I will try and find my argument here by going through the following issues.
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I am going to talk first about the issue of climate change and coral reefs, and I hope I will convince you that the impacts are already here and now and that the future is bleak for coral reefs if we don't amend our current trajectory. I will be skipping over a lot of data which was actually published behind this argument in December 2007, in Science; I will try not to laden you with graphs and figures, but hope you will trust me on this. I will go to the data if there are any questions.
The limit that comes up (and this really is when it is all over) is 450 ppm carbon dioxide in the atmosphere. Crucial to this argument is whether or not biological systems that are operating today will change as the environment changes – that is, will they evolve? This has recently come up in some articles in Science, it is being bandied around as a potential scenario. And of course it raises a question about whether or not evolution can be fast enough to keep pace with climate change. I will talk a little bit about the evidence for and against that.
That will bring me finally to the issue that we are discussing here today: whether or not we are avoiding, or confronting, or actually experiencing dangerous climate change now.
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Firstly, how are coral reefs doing on the planet today? There was a study published last year which was a meta-analysis of almost all studies of corals in the Indo-Pacific over the past 40 years. It went back and asked how much coral cover was being reported in 1980 versus today. The results are quite shocking.
If you look at coral cover as an indicator of the extent of coral reefs – literally how much of the bottom is covered by corals – you find that we have shifted from a mean that might have been around 45 per cent of the bottom, on the average reef, to one that is more like 20 per cent. And there have been studies done on the Great Barrier Reef independently of this that have shown a similar downward trend in coral cover.
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That study looked at the Asia-Pacific region, and what was really surprising about it was that when you broke it down, it didn't matter how much you were protecting your reefs from other insults, like overfishing, pollution and sedimentation, you were getting the same result.
On the left-hand side here we have the Philippines, where they have extreme pressures on coral reefs from local sources, and mainland Asia, and what you see here is a similar comparison: from 1980–81 a downward trend in coral cover, a similar thing happening in mainland Asia, where today it is a lot less. But on our Great Barrier Reef, which has a very low density of people and is a highly managed system, we have got the same downward trend occurring.
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That brings us to pose the question: is this due to global factors that go way beyond these local factors in their effects? It is looking at these issues which I think is most important at this point in time, and this is being reflected in some of the recent consensus statements.
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So what is the basis for this? Why do we think it is climate change as opposed to local factors?
Shown here is some work that I began in the 1990s. It is a very simple study done in Tahiti. It shows sea temperature data from 1981 to 1999; it shows the winter-summer oscillation of those temperatures. And here you see arrows indicating mass bleaching events – unknown anywhere, I should say, before 1979, but suddenly popping up and hitting reefs hard.
The little spikes at the top are due to ENSO variability in Pacific temperatures, and what you see is the correlation or association between the exceedance of a number – in this case something like 29.4° – and the bleaching events. This turns out to hold for most coral reefs, although threshold values are different in different places because of evolutionary adaptation to local water temperatures.
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You then ask the question: what does this mean for the future? And this is where I worked with some climate modellers from the EU community. We took, in this case, the ECAM4 [4th European Conference on Applications of Climatology] model, which is probably outdated in terms of models but hindcasts pretty well. The first place for the study is Tahiti, and we have got winter-summer variability back to 1860 and forward all the way to the end of this century. This thing simulates the ENSO effect quite well.
The line indicated as the thermal threshold is the line above which we know corals go from being brown to white, and we have mass die-offs. You can see that with a doubling of CO2 in this case pushing temperatures up, we get to a point where we exceed those conditions for mass bleaching events.
In fact, if you look at things like the length of time above that threshold, you see conditions that you don't see today anywhere, and of course this is of great concern.
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That relationship, which has now built up from many, many studies where people have looked at the issue of temperature, in the lab and so on, is the basis for a highly successful satellite prediction system from the National Oceanographic and Atmospheric Administration, in Washington.
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This is a picture of the planet that shows thermal anomalies above long-term thermal maxima, the summer maxima in sea temperature.
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This is in October 2005. The hotter the temperatures, the hotter the colour you have got, the bigger the thermal stress. The eastern Caribbean suffered a major effect. Incidentally, this is also the energy that was in the ocean that then triggered the record number of hurricanes that sped across Mexico and the southern United States.
You can look at this and ask what happened on the ground. Well, if you were in the Dutch Antilles, in the eastern Caribbean, you saw 95 per cent of corals bleaching, and the latest is a 65 per cent mortality, versus over in the western side of the Caribbean, along the coast of the Yucatan, in Mexico, where you were seeing bleaching but a much lower mortality. This relationship is very, very strong.
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We can look at this at a global level. That was the Caribbean event and we've had events on the Great Barrier Reef six times; if you look across the planet you see huge impacts on a fundamental organism that builds an ecosystem which houses one in every four marine species. There are an estimated million species that live on coral reefs.
In addition to that, we have got human dependence. Over 100 million people live off coral reefs for their daily existence. In Australia, it is our second largest industry in Queensland, drawing in $6 billion worth of international tourism.
In 1998 we had the big wake-up call to the fact that this was probably a sign of the end. This was a global impact that spread across the world – we had bleaching in almost every part of the world. By the end of 1998, which as you would know was a very warm year, we saw that 16 per cent of the corals which were found on the reefs before that year had disappeared. Now, if we woke up tomorrow and were told that 16 per cent of the trees in our forests had disappeared by the end of a year, I think we would be very alarmed. One of the problems with marine ecosystems is that they are out of the way, they are under the water, and we know very little about them. But it is quite clear that the changes there are as much as or more than what we see on land.
In this same event, in the Western Indian Ocean, where they had hotter temperatures for longer, there was a die-off of about 46 per cent of all corals.
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The other effect which I have talked about, ocean acidification, doesn't have a good prognosis either.
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You can look at corals and ask: where do they live today, and what is the carbonate ion concentration? In this case, you can put it into a formula and get the aragonite saturation. (Aragonite is the form of calcium carbonate that corals put down and deposit.) If you take the ions, divide by the solubility product of aragonite and then plot it, you get the sort of trend shown here across the planet – very low amounts in polar waters, and more and more as you go towards the tropics.
The pink dots here show you where coral reefs are found today. You may notice that they don't extend outside the blue zones, which are where the aragonite saturation is at least 3.3.
As part of this exercise with Science, Ken Caldeira, from Stanford University, modelled what would happen to the aragonite saturation as you increased CO2.
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If it is raised to 450, you can see that we have had a contraction of the blue water, the water that we know today is associated with coral reefs. Significantly, the Great Barrier Reef goes below what we think you need to make a carbonate coral reef.
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If you go any higher, you get to a point where the conditions from a carbonate ion point of view are now no longer supporting carbonate coral reefs.
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You would be right in asking at this point, 'How do we know that that is not just coincidental, that carbonate ion concentrations are like this and reefs are like that, and so on?' So we went back and used the Vostok Ice Core data from Petit et al, with the assumption with respect to total alkalinity, to recalculate the conditions from a temperature and carbonate ion point of view over the past 420,000 years.
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What you get is this quite complicated graph, but I want you to concentrate first on these are pairs where you have got the deviation from today's temperature that you would expect in the tropics, you have got the carbonate ion concentration, and you have got the atmospheric carbon content. During the interglacial cycle we have been ratcheting back and forth from about 100 ppm up to about 280 ppm. We see the carbonate ion concentration, plotted together with the ocean temperature in tropical regions, oscillating in a tight bundle. That's where we have been for the last 420,000 years.
The shocker is that here we are today, and with really minor amounts of change in carbon dioxide we cross that threshold – expressed here as a CO2 content – where we know we drop below the 3.3 aragonite saturation. We know also that coral reefs are susceptible to small temperature increases, but if we keep going then that threshold will be eventually exceeded as well.
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What you end up having to conclude at this point is that maybe this is the world we are going to see very, very soon. This is optimistic, in my opinion. Today we have got periodic impacts and we have got a declining coral cover. It doesn't take many years before we lose it all, but today we have got something.
If we raise the CO2 content and we drop the ability for corals to calcify, and if they are also being impacted by mortality more and more, you start to see corals as being remnant members and not dominant members of communities.
If we go beyond that 450 ppm, we get to a point where reefs are in net erosion and conditions are such that they are being hammered every year by bleaching until they disappear. And that may be what we see on the Great Barrier Reef, for a large part. This is quite provocative – although after Neil Hamilton's talk I feel quite conservative.
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At the heart of this is: do those thresholds move with time? This is something I am now going to deal with.
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Last month there was a discussion of the ability of biology to change with climate change, and that was actually in response to the article shown here. The authors made the argument that we have seen rapid climate change in the past, we have also seen biology adapt, and there is some evidence that some species can change fairly quickly and keep up with the rate of climate change – so, why not corals? Why are we concerned?
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Well, apart from the empirical evidence of actually losing corals, you can get to a point where you can ask: has it really happened? This graph is done really by a biologist with some climate modelling friends taking the Petit et al Vostok Ice Core data and doing a very simple calculation of the rate of change. I say it is simplistic: it overestimates the rate of change, but it is just doing a two-point rate calculation.
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If we then plot that and look at it relative to what we have seen over the last 130 years, as a biologist I see that the rate of change in CO2, for example, is 1000-fold above the range of rates that we see in and out of glacial transitions. Temperature, with its lag, is a mere 70 times faster than the fastest we have seen over 420,000 years.
We know that if you are in France today you don't see woolly mammoths and ice. The changes we have seen in the past at much slower rates have been dramatic in terms of biology and ecosystems. So in that regard I think that, apart from things like the Younger Dryas event, we have probably got a situation which is extremely hard for biology to keep up with.
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But what is the evidence of biology keeping up? There is a paper by Skelly et al, in Conservation Biology, which discusses this and tries to formalise how you would assess whether biology could keep up.
They point to the fact that there is a frog that in less than 40 years had changed its thermal tolerance, they point also to studies on insects in labs where you can get change in as few as 10 generations. When a fly has a generation time of a few months, however, you really have to consider what you are dealing with.
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Corals, it turns out, have generation times from about five to as much as 30 years. For comparison I have included elephants – I am not an elephant biologist, but I know they probably have low diversity and long lives; the symbiont out of corals has got a 17-day generation time, and bacteria have got a generation time of 20 minutes.
You can evolve fast if you have short generation times and high genetic diversity. Corals, it seems, have long generation times and low genetic diversity, so their potential to track environmental change is probably pretty low.
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The recently cited evidence for coral populations tracking climate change I think is really questionable. There is a paper by Peter Glynn, from the University of Miami, which has suggested that the amount of mortality that happened in the 1982–83 ENSO event in the eastern Pacific was less than in the event in 1997–98, despite the fact, they felt, that there was more stress in 1998 [i.e. 1983?].
They also pick up on papers that several people have written about the fact that when you have bleaching events going through coral reefs, the weak die and the tough survive. But that is species, not individuals within a population. And they also pick up on studies showing that populations of corals that have experienced bleaching tend to lose the weaker varieties of symbionts. But is this really the adaptation you need to keep up with the novel circumstances of the future?
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Suppose we have a population – this is a reef, each of these little pentagons is a different species, the green ones are more vulnerable and the other red and orange ones are successively tougher – and we start to go through bleaching events that 'weed out the weaker'.
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What we are seeing is not necessarily adaptation in any real sense.
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We are seeing, essentially, a bottleneck where, as we increase conditions, we are narrowing down that population, that diversity. So – when it comes to other factors (e.g. disease ... the population will be weaker – merely due to the loss of diversity. Fewer options in a population.
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And it is not just thermal stress that we have to get through, it is all the other stresses out there.
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So, really, I think it is a nonsense when we put those sorts of data up as evidence of adaptation.
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So, can corals evolve? Even under the best circumstances, corals with the shortest generation times might be able to keep up in the short term. Corals have very long generation times, so that most of them would be out of the game. Ten generations at 15 years is 150 years. Of course, evolution is very slow.
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There have been other ideas. What about swapping the symbionts? Someone suggested that bleaching is all about changing one set of symbionts for another. So we might have a coral that spits its zooxanthellae out and then adopts a more thermally tolerant variety.
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Despite that idea being extremely interesting and so on, however, there is actually no evidence, and Tamar Goulet from the University of Mississippi had put the evidence for and against this idea together some years ago. No-one has been able to demonstrate it as happening.
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Given that you have got one cell living inside another, the symbiosis is not a trivial thing to form. You can't just form a symbiosis overnight. Let's consider what these tiny cells have to do to live inside the tissues of corals. They have to mimic the host, they have to avoid digestion, they have to integrate metabolically, they have got to do all these things. That has got to involve a lot of co-evolution. So the symbionts are quite special. You can't just take them in from the water column.
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When you look at the family trees of corals and their symbionts, you see that they overlap to a high degree. Michael Stat a student from my lab and Dee Carter's lab at the University of Sydney did a study where he looked at fidelity between the two family trees and found that they were only unfaithful, swapping their hosts, on the level of 100 to 1000 years. So this is not an ecological process. Swapping of symbionts really doesn't make any sense on the time scale of ecological event like a bleaching event.
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Some corals can shuffle existing symbionts, one variety over another. There has been some excellent work done by Ray Berkelmans and Madeline van Oppen where a coral that has two types of symbionts can experience a temperature change and take one guy out and reduce its population, and upregulate another one. But this is not the type of thing you need to deal with novel circumstances. This is essentially a phenotypic change, existing symbiotic relationships that are being up and down-regulated.
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That may be an important mechanism in the short term, but it is not the panacea that we seek.
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All of these ideas about symbionts being swapped out and everything forget the fact that if the host, which has a thermal tolerance, isn't also evolving, then it is going to be a limited strategy at the first instance.
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What about ocean acidification? This slide is based on some work by Stanley and others. When you look at the fossil record, looking at reef accretion and at periods of high saturation rate, what you see essentially is that any time you have had high CO2 you have had low carbonate ion concentrations, you have had low reef accretion. There is no evidence that there has been some biological shift to accommodate that.
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When you put this together and you look at the rate of change – we are getting up to 2 ppm per year at the moment – you start to wonder whether these things are fixed in the time-frame that we are dealing with them, whether evolution is not an ecological process. And then we will be dealing with vastly changed reefs.
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When you look at the United Nations Framework Convention on Climate Change you see that it specifically says that it is aiming to stabilise greenhouse gas concentrations at a level that would prevent 'dangerous anthropogenic interference with the climate system'. The thing that would pique my interest as a biologist is the words, 'Such a level should be achieved within a time-frame sufficient to allow ecosystems to adapt naturally to climate change,' and so on.
In view of all the evidence – as I think we saw with Neil's paper and some of the other bits of evidence that have been presented – we are talking about polar bears that are not going to suddenly evolve floatie wings on their backs or whatever. We are talking about something fundamentally changing. We are not going to talk about coral reefs suddenly, magically becoming robust against climate change.
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If you were to ask me, I would say of avoiding dangerous climate change that we are already there. There is nothing to avoid any more.
Thank you.
Discussion
Question (Keith Boardman): I have two questions. One is: what is known about the mechanism for the expulsion of the symbiont from the coral? Secondly, what is the temperature differential, say in the Great Barrier Reef, between the southern and the most northern parts? Why can't you get recolonisation of the southern part by spawning from the northern corals?
Ove Hoegh-Guldberg: Two very good questions. On the first question, from the cell biological point of view there is probably a range of mechanisms. There is clearly apoptosis, and one of the early career researchers here, Tracy Ainsworth, is actually working on that issue. There is also evidence that there is exocytosis, the expulsion of one outside the other.
What is really critical is that even in a bleached coral you still have about a thousand of the symbionts per square centimetre in a coral. They never get flushed out completely. So from that point of view I think we know enough to say that it is not a clean sheet and it is not a blank page for the next symbiont to come in on.
In terms of the Great Barrier Reef it is really interesting, because the southern end versus the northern end is 2° cooler. In fact, when you look at the thermal tolerance of corals in the southern Great Barrier Reef you see that they are 2° more sensitive than those in the north. Why don't they migrate down? After all, we saw the movie Finding Nemo, we saw that great transport system there, waiting to happen.
The truth of the matter is that the more we have learnt about reef connectivity, and coral 'migration' of each generation, the more it looks as if it is a 10-kilometre radius for a successful coral spawning here, for successful settlement. It is also probably a very slow process, relative to the rate of climate change.
It is occurring, and if we were going more slowly it would have some effect, but unfortunately not.
Question (Graham Farquhar): This is slightly off the topic, I suppose. Going back to the pH part of what you were discussing: can you give us some insight into what happened in the Cretaceous, when we laid down the White Cliffs of Dover and all that kind of thing? The pH must have been really low, because the CO2 was thought to be well over 1000 ppm. I just wondered how that came about.
Ove Hoegh-Guldberg: I am going to admit my complete ignorance here, but I will take a stab at it. The first thing is that, if the rate of change of CO2 in the atmosphere is slow enough, you get buffering of the ocean anyway. So the actual pH drop during that period was probably not as great as we have been seeing.
The second thing is that the hiatus that you see is, I think, 10 million years after the event, before you start to see calcification turning up again. At those slow rates – even though they were fast in the picture of the Earth – biology probably was keeping up, and I would not be surprised to find, if we went to that time in our time machine and actually looked at the properties of symbiotic organisms and calcification, that they required less calcium carbonate.
One of the issues about this is that where people study the calcification in corals is actually sealed off from the environment, and pumps are pumping in calcium and carbonate to create that supersaturated solution to get the crystals to come out. That is a biological process. So you would ask: why aren't the corals able to do that today? Well, the truth of the matter is that they haven't evolved to deal with these new conditions.
Looking back is going to be a very important part, I think, of understanding the future in this case.
Amanda Lynch: Thanks very much, Ove, for a fascinating if depressing talk.
