Coral reefs have been in the international spotlight as regards climate change for over 30 years. Our understanding of the changes, both present and future, is quite well developed. For the rest of the lecture, I want to discuss the impacts of ocean warming and ocean acidification on coral reefs. Well, coral reefs are the most biologically diverse ecosystems in the ocean and include well over a million species. And they’ve been with us for tens of millions of years. Seemingly resilient and robust, you might be forgiven for thinking that these ecosystems would be able to survive anything that humans might throw at them. Well unfortunately, this is not the case. Coral reefs across the tropics and subtropics have been rapidly retracting at a rate of around 1-2% per year.
A recent meta-study by Bruno and Selig in 2007 revealed that the amount of coral on reefs in Southeast Asia and the Western Pacific has declined by around 50% over the past 30 years. A recent study by the Australian Institute of Marine Science has revealed that reefs across two thirds of the Great Barrier Reef have lost around 50% of their reef building corals since the early 1980’s. And sites like the Caribbean Sea have lost over 90% of their corals since the 1970’s. There are now many other studies from the Indian Ocean and other sites where coral has been disappearing rapidly. And while local factors such as overfishing and coastal pollution are important, rapid ocean warming and acidification are fast becoming the major drivers of change, especially as one projects how conditions are likely to change in the future. I want to now look at the phenomenon of mass coral bleaching and mortality, and explore why scientists are so concerned about ocean warming and acidification for the world’s coral reefs. The first signs that corals were being impacted by warming occurred in the early 1980s in the Caribbean.
Reef resource managers such as Billy Causey in the Florida Keys began to report a sudden whitening of coral colonies across large sections of their coral reefs. These changes literally occurred in a few weeks and seemed to coincide with prolonged periods of doldrums in which the skies were clear and the oceans still and local seas were above the normal temperature for that time of the year. In the years that followed, mass coral bleaching and mortality began to appear in other parts of the world. Always, it seemed, during the warmer years, and often coinciding with warmer phases of long-term climate patterns such as the El Niño Southern Oscillation or ENSO. While there was growing evidence from field studies, experimental studies were important to verify the mechanism associated with mass coral bleaching and mortality.
It became clear from these studies that small changes in temperature could trigger the symptoms that are identical to mass coral bleaching in the field. From these studies, it would be demonstrated that small changes in temperature were all it took to destabilise the all-important symbiosis between corals and dinoflagellates. The result was that corals went from being brown coloured, as a result of abundant concentrations of the symbiotic dinoflagellates, to becoming a bleached white colour as the symbionts left the tissues of the coral host. While many stressors can cause corals to bleach, such as toxins, too much light and sudden salinity shocks, the role of temperature in driving mass coral bleaching was confirmed by satellite remote sensing. In this case, satellites flown by the National Oceanic and Atmospheric Administration in the United States were able to predict when and where bleaching would occur based on small positive anomalies in sea temperature alone.
These two panels show the anomalies, shown here in orange, that occurred when the Great Barrier Reef experienced the worst cases of mass coral bleaching in 1988 and again in 2002. Basically, when temperatures reach around 0.9°C above long-term summer maximum for a particular region, bleaching begins to occur. If temperatures were even warmer for longer, more intense coral bleaching and mortality would occur. In some cases, very hot periods would trigger all of the corals on coral reef to bleach, with a subsequent mortality that often came to near 100% of the corals in an area. One of the most dramatic periods of mass coral bleaching and mortality, globally, occurred over nine months in 1998. This is a map that shows the dates when mass coral bleaching and mortality was first observed. Starting in January 1998, in the eastern Pacific, reef scientists began to report mass coral bleaching across the eastern Pacific. This was associated with one of the strongest El Niño conditions reported, with a massive reduction in upwelling, increasing temperatures across this region which includes the Galapagos Islands.
Soon afterwards, 50% of the Great Barrier Reef underwent mass coral bleaching. This was at this point, the most severe case of mass bleaching reported for the Great Barrier Reef so far. Mass coral bleaching was then reported throughout the central Pacific, including remote reefs of French Polynesia, such as the Tuamotu Archipelago, which has few other local human influences. By April/May, reports of mass coral bleaching and mortality were being generated in Southeast Asia and the Indian Ocean. Well, as the year progressed, mass coral bleaching began to be reported in the northern hemisphere, in Okinawa, Palau and the Caribbean. And by the end of 1998, most coral reefs around the world had experienced mass coral bleaching. Now each of the reports tend to be associated with observations of very warm seas. And a global survey done by the Global Coral Reef Monitoring Network or GCRMN reported on average that 16% of the world’s coral reefs had died over the nine months in 1998.
This is an extraordinary number – just imagine if you were told that 16% of the trees, of the rainforest trees of the world had died overnight. Given the role that corals play within tropical coastal ecosystems, this was of great concern, not only to the corals, but all the organisms that live in and around those corals that were missing. And, of course, this a global average which hides the fact that many regions experienced much higher mortality rates. In the Maldives, North-west Australia, Okinawa and Palau, up to 95% of reef building corals on some reefs were reported as being killed by extraordinary events of 1998. Now, since 1998, sea temperatures have continued to slowly increase, and mass coral bleaching and mortality, continue to occur. In 2005, the eastern Caribbean experienced record sea temperatures and consequently experience mass coral bleaching and mortality that resulted in the loss of 30 to 40% of the corals in the Eastern Caribbean. Ironically, the same warm conditions that drove mass coral bleaching and mortality also triggered the multiple record hurricanes that impacted Mexico and United States.
One of those being hurricane Katrina. In 2010, coral reefs across large parts of Southeast Asia also bleached. The close relationship between mass coral bleaching and elevated sea temperature provides an opportunity to explore the question as to what the future holds for coral reefs if warming continues. Here we see sea temperatures in French Polynesia since 1981. As you see, temperatures oscillate between winter and summer, and every now and then temperatures are warmer than average. In this region, El Niño years are associated with warmer than normal summers. Now, each of these black arrows are when mass coral bleaching was reported. And as you can see there is a temperature threshold above which coral bleaching tends to be reported. And in this region, it is about 29.2°C.This threshold for coral bleaching lies about 1°C above the long-term summer maximum. We can take this threshold and map it over the outputs of models that simulate how sea temperature is likely to change under further increases in greenhouse gases in the atmosphere.
What we get is an idea about how the frequency and intensity of coral bleaching and mortality is likely to change. I did this in a study some time ago, the trace here represents the model outputs for a doubling of CO2 by the end of the century. As before, we see the summer/winter variability and underneath that, you see the underlying upward trend driven by the doubling of CO2. What you see from this analysis is that sea temperatures will exceed the threshold for mass coral bleaching and mortality on an annual basis by the mid part of the current century. This analysis has a series of important ramifications for coral-dominated reef ecosystems. If the current rate of climate change is not reduced, then events like 1998 will become commonplace, indeed, considerably worse than 1998. If this happens, then coral dominated reef systems will be impacted on a regular basis by these mortality events and won’t have enough time to recover between those events. At this point coral reefs, coral dominated reef systems, are likely to disappear; in this case, within the next 30 to 50 years.
This analysis has been explored by a number of different groups. In those studies the basic premise still holds, that ocean waters will soon exceed the known thermal tolerance of corals within about 30-50 years. There has been some discussion about whether or not reef building corals will be able to evolve rapidly enough to keep up the pace with changes in sea temperature. However, corals are long-lived organisms which have generation times that range between 3 and 100 years in length. Organisms that have long generation times are unlikely to be able to evolve to changes which are happening on decadal time frame. Another possibility is that coral reefs may be able to migrate to higher latitudes. However, the evidence that entire reef systems are migrating to higher latitudes is completely missing. While there have been reports of single species migrating to higher latitudes there is no evidence that entire reef systems have been to relocate with all of the complex biodiversity and ecosystem processes.
One of the other problems with reefs migrating to higher latitudes is that temperature is not the only variable that determines the distribution of coral reefs. Both light and the concentration of carbonate ions are important variables that we’ve discussed previously. They also decrease as we get to higher latitudes, probably beyond the amount required for actively growing carbonate reef ecosystems. It is important to point out that the impacts of climate change are not just restricted to reef building corals. As you know from previous parts of this course, corals form a critically important habitat in terms of making a three-dimensional framework for reefs and a habitat in which over a million species find their shelter. As corals disappear, there are huge ramifications for the many other species that live in and around coral reefs. The next video demonstrates this important point for coral reefs off Heron Island. I hope that you noticed how the branching corals on the outer reef slope at Heron Island were able to provide a complex three-dimensional structure into which many fish and invertebrates where finding habitat. Unfortunately, branching corals tend to be among the most sensitive corals when it comes to changes in sea temperature and other stresses.
One species, Acropora cervicornis, shown here, used to be the most abundant coral in the Caribbean Sea but is now so rare that it has been placed on the US Endangered Species list. Gone are the thickets of this coral and the associated habitats. This is just one of the three small pieces I saw on an extensive survey in the eastern Caribbean recently. In an earlier part of this course, we profiled a study done by Wilson and colleagues which showed the high dependence of the biodiversity of fish populations on reef building corals and the associated three-dimensional structure of coral reefs. Review the information on the impacts of climate change on coral reefs and perform the following exercise..