Recent news about coral reefs and climate change has not been good. In 2009, Glenn De’ath and colleagues at the Australian Institute for Marine Science had reported, on the basis of analysis of growth increments in a large sample of colonies of the massive corals, Porites spp., that growth rates across the Great Barrier Reef had declined by 13%, and calcification rates by 14% since 1990. They attributed this to the effects of ocean acidification. A smaller study by Jani Tanzil and colleagues that sampled corals from Phuket, Thailand, provided support for this claim. In 2012, on the basis of the robust data collected under the AIMS long-term monitoring program, Glenn De’ath and others reported that the Great Barrier Reef had lost 50% of its coral cover since 1985 because of storms, outbreaks of crown-of-thorns starfish, and coral bleaching. In 2012 also, Jeremy Jackson and colleagues began reporting the results of their detailed analysis of trends in coral cover in the Caribbean, a study being released by IUCN late in 2013 or early in 2014. According to the analyses, average levels of coral cover have declined about 80% since 1973, from a combination of causes including coral disease and bleaching. Jackson and colleagues stress that there are pronounced differences within the Caribbean with some locations still retaining reasonable levels of coral cover while others are severely altered. Still, reasonably large coral reef regions, such as the Great Barrier Reef and the Caribbean basin cannot sustain continuing average losses of coral cover at rates approximating 50% every 2-3 decades, and be expected to be present into the future.
The evidence continues to mount, showing that the global community is not making adequate headway in the struggle to put in place meaningful curbs on releases of CO2 and that our efforts to curb other human impacts, such as overfishing, and pollution continue to falter. Under these circumstances it becomes very difficult to remain optimistic about the future for coral reefs, and I lost almost all my optimism about this some time ago. Still, today I am talking about some glimmers of hope being generated by recent scientific reports. Sort of, “the glass is not half empty, it’s half full”. If any of us are going to continue to fight for a world in which coral reefs regain their former glory, we need these “it’s half full” stories once in a while.
Science Can Be Messy
Science proceeds step by step into the unknown. The path trodden by any scientist worth her weight is towards the unknown, the not-yet-understood; there is rarely a map, and the path is forged as the scientist progresses. Because science progresses by means of the largely independent struggles of individuals and groups of colleagues, each of which works for a while, then publishes their findings, then works some more, and publishes again, science as a whole progresses along numerous, far from straight paths that tend to converge on specific goals but frequently arrive from very different directions. The nature of scientific discovery is messy with plenty of dead ends and paths abandoned. It’s an exciting race towards answers for questions that are not always clearly specified until the race is well under way. People who want to understand what the scientists are learning must either wait for the dust of discovery to settle, or accept that stories can become more complex as time goes by and more details are learned. Over the past decade or so, numerous scientists have tackled the challenge of what is happening (if anything) to coral reefs, and what is likely to happen as climate change and other human stresses increase. The importance of these questions creates a need for the wider public to learn about new results even as the dust of discovery is still obscuring vision. As the dust clears, initially simple stories tend to become fleshed out with details that often also make them more complicated – and it’s in this complication that I find a few glimmers of good news.
Geography and Calcification on the Great Barrier Reef
Take the example of the Great Barrier Reef. The AIMS long-term monitoring data tell us that on average, for the sites sampled, coral cover has fallen 50% since 1985, for three primary reasons – predation by the crown-of-thorns starfish, physical damage due to cyclonic storms, and the effects of bleaching caused by the stress of periods of unusually warm water. The earlier study by Glenn De’ath and colleagues tells us that a substantial sample of colonies of Porites corals from a broad range of the Great Barrier Reef reveals a significant fall in growth rate and in rate of calcification. These two results do not contradict each other, because slower growth means that the effects of damage by starfish, storms or bleaching will become more apparent than in the past, and slower calcification may even mean that corals are more susceptible to damage than previously. Even though they do not contradict each other, however, these two results both focus on the average pattern of change. The Great Barrier Reef is over 1500 km long and up to 300 km wide at its widest place, and conditions are very different from place to place over this immense region. It’s worth exploring how things have been changing in the various localities that make up the Great Barrier Reef.
In the December 2013 issue of Coral Reefs, Juan D’Olivo and colleagues at University of Western Australia report the results of a study of calcification and growth rates obtained from a sample of 41 cores of Porites corals on seven reefs of the central Great Barrier Reef – three inner shelf, three mid-shelf and one outer-shelf reef. Their results are compared with the results for 151 cores from this portion of the Great Barrier Reef analysed by De’ath and colleagues for their 2009 paper. The two sets of results match, but not quite. In particular, D’Olivo and colleagues have focused on the possible differences among inner-, mid-, and outer-shelf corals, and the De’ath team have reanalyzed their data finding some errors caused by the inclusion of incomplete years in the year of collection of the cores. These incomplete years led to spuriously low rates of calcification and growth because partial growth bands were treated as annual bands in their analyses. This is the first piece of good news. Calcification rates have not fallen by 14% as initially reported.
The seven reefs from which coral cores were obtained by D’Olivo and colleagues, 3 inshore, 3 mid-shelf and 1 outer-shelf reef Patterns in coral growth and calcification differ among them in ways that suggest water quality plays an important role. Map © J. D’Olivo, Coral Reefs 32: 999-1012, 2013
The second piece of good news is that calcification and growth rates have actually risen slightly since 1930 in corals from the mid- and outer-shelf reefs, likely in response to rising temperature (which increases metabolic rate and therefore rates of growth and calcification. That trend stalled in the late 1970s, and the most recent years (2003-2005) show a reduction in rate of calcification which may indicate that the increasing ocean acidification is now beginning to counteract the effects of warmer temperatures as would be expected.
These graphs, edited from Fig 8 in their paper, compare D’Olivo’s results and De’ath’s earlier results (corrected for some errors) for the same part of the Great Barrier Reef. There is broad agreement. Calcification and growth rates vary greatly through time, but the overall trends are positive until very recently on outer-shelf and mid-shelf reefs, and negative for the inner-shelf reefs.
Image modified from J D’Olivo Coral Reefs 32: 999-1012, 2013
A third piece of news that could be good is that while the inner-shelf corals do show reduced growth and calcification rates (-9.5% and -4.6% respectively), these seem to be far more strongly influenced by patterns of river discharge in the region than by temperature or pH, and by progressive deterioration in water clarity and water quality. That growth and calcification are declining is not good news, but the fact that pollution from upland agriculture could be playing a major role IS good news – it is far easier to act to improve patterns of land use and fertilizer management in the agricultural community of Queensland than it is to gain global support for efforts to reduce CO2 emissions and curtail ocean acidification.
The study by D’Oliva and colleagues says nothing about the high rate of loss of coral cover reported by De’ath and colleagues in 2012. That individual coral colonies are not growing more slowly (except on inner-shelf reefs) tells us nothing about whether total coral cover is declining. However, the differences in calcification and growth rates reported among inner- mid- and outer-reef zones suggests that the reported average rate of decline in coral cover also needs to be explored further. There has already been some suggestion that inner-shelf reefs have shown particularly marked declines in coral cover, while outer-shelf reefs have shown less overall change.
If the smaller study by D’Oliva and colleagues stands up to future scrutiny, it means that coral growth on the mid-shelf and outer-reef portions of the Great Barrier Reef has continued strong until recently, although acidification may now be beginning to have an impact. Coral growth in the innermost portion of the Great Barrier Reef is in trouble, but it appears that remediation requires a concerted local effort rather than a global agreement. That is perhaps as good as the news is going to get – I only promised a glass half full!
Calcification on Reefs in South-East Asia
Three papers in recent issues of Global Change Biology add to our understanding of the ways in which reefs might be responding to global change. In a follow-up to their 2009 paper for corals at Phuket, Jani Tanzil and colleagues at National University of Singapore and other institutions in Malaysia and the UK have reported on growth and calcification rates for 70 Porites corals from 6 locations around the Thai-Malay Peninsula. This is a region of consistently warmer water than the Great Barrier Reef, and here the results confirm the initial Phuket data. Average rates of calcification and growth declined 18.6% and 15.4% respectively over a 31 year period. The results are attributed mainly to temperature increases – warming initially increases metabolic rate in corals, but above a certain range, warmer temperatures lead to reduced metabolism.
Figure 3 in Tanzil’s paper shows the long-term trend in coral calcification and growth rate averaged across all sites. This is NOT positive news for reefs in this region.
Figure © Tanzil Global Change Biol 19: 3011, 2013.
Interestingly, Tanzil and colleagues have explored variation among locations. While the trends in growth and calcification are negative at all but one location, the rates of decline vary quite a bit. In other words, and this is the tiny glimmer of good news here, in this region also there are some places where corals are doing a bit better than others. By examining more carefully the causes of slowed growth, it should be possible to determine why this variation occurs, and perhaps devise remedial actions.
Figure 4 in Tanzil’s paper shows results for three of his six locations. Note the differences in rates and trends, particularly for the Port Dickson site. Image © J Tanzil, Global Change Biol 19: 3011, 2013.
Tanzil’s data re-plotted against temperature rather than year, showing the way in which calcification and growth rates fall off abruptly once temperatures reach about 29.3oC.
Image © J Tanzil, Global Change Biol 19: 3011, 2013.
Glimmers of Good News from Modeling Studies
Places for Coral Reefs to Occur
The other two Global Change Biology articles use modeling techniques to ask ‘what if’ questions about the future effects of climate change on coral reefs. In the first of these, Elena Couce and colleagues at the University of Bristol have modeled the anticipated global pattern of change in sea surface temperatures and ocean acidification up to 2070, under accepted IPCC scenarios. They operated globally (from 60oN to 60oS) and at a 1olat x 1olong grid scale. By looking at present day conditions of such variables as temperature, irradiance, depth, pH (measured as Aragonite saturation value), salinity, nutrient levels, current speed, and frequency of cyclonic storms at all 1ox 1o locations where coral reefs occur, they were able to train their model to recognize environments that were suitable for reef development. Then by using the expected trends in temperature and pH under specific IPCC scenarios (they mostly used A2, the scenario which largely mirrors our current high-CO2 path), they were able to specify how locations around the world would likely change in their suitability to support coral reefs.
Couce and colleagues anticipated that warming ocean waters would favor some range extensions as currently marginal waters became warm enough to support coral reef development, and some losses, as more equatorial waters became too hot for corals to persist. They anticipated that changes in pH would tend to slow patterns of coral growth, and their primary goal was to explore how these factors would interact. What they found is a mixture of good and bad news. As we approach 2070 following the particular trajectories for CO2 emissions that are likely, there are some opportunities for improved survival and range extension for coral reefs both north and south of their current limits, and in places like the East Equatorial Pacific. However, conditions favoring coral reefs decline throughout most of the region between 20oS and 20oN, and decline very substantially in the western Pacific, the region that currently supports some of the most biodiverse coral reefs on the planet. Whether or not reefs will be able to become established in places outside their current range will depend on the ability of larval stages to colonize those places, as well as on the suitability of those locations for coral reefs. Larval transport depends on such things as the duration of larval life, the behavior of larvae, and the current regime in the region where the larvae are located.
Figure 5 from Couce’s paper showing how the conditions favoring coral reefs are expected to change by 2070 under the IPCC’s A2 scenario for CO2 emissions. The green dotted line marks the location of no net change from 1990 to 2070. Blue regions outside this line could begin to support coral reefs if other factors (especially shallow water, and sources of larvae) are favorable. More red regions are less favorable for continued coral reef presence. It is gratifying to see locations such as
the East Equatorial Pacific becoming more favorable for coral reefs, but disturbing to
see the trend against reef persistence at the very center of current reef diversity.
Figure © E Couce, Global Change Biol 19: 3592, 2013.
Unlike most articles in Global Change Biology, this one is on open access for down-loading. One important message from Couce and her colleagues is that it may become important to care for certain currently marginal reefs close to the current limits of range of reef formation, even if they do not appear very promising, because these could become much more robust as conditions change. While their study is necessarily preliminary, it does open up the possibility of identifying sites for special attention because they have the potential to become the coral reefs of the late 21st Century.
Will Coral Reefs Adapt to Warming?
The second modeling paper is by Cheryl Logan of California State University at Monterey Bay, and colleagues from Princeton University, NOAA, and University of British Columbia. They explore the response of corals to warm waters, and ask what effect the capacity to adapt to warmer temperatures might have on patterns of bleaching into the future. The possibility of adaptation to bleaching has been discussed for some time, and many scientists believe it is unlikely to be an important factor because the pace at which ocean temperature is rising, and the extent of the increase likely are both well beyond the usual pattern for change in ocean features. Logan and colleagues ask the ‘what if’ question.
There are four ways that coral reefs might adapt to warmer waters. First, individual corals and/or their symbiotic algae might acclimate to warmed temperatures, just as we acclimate to summer weather, tolerating the heat better in summer and fall than we did in early spring. Second, because there is a rich community of different strains of symbiotic algae available, and because these appear to differ from one another in their tolerances to warmer conditions, individual corals might adapt by ‘shuffling’ their symbionts, acquiring new more heat-resistant strains to replace less heat-resistant strains as waters warm. Third, because coral species themselves differ in their tolerances to warm water, coral reefs might ‘shuffle’ their corals, with more heat-tolerant species becoming more abundant, at the expense of more temperature-sensitive species as conditions warm. Finally, fourth, evolutionary changes might take place within coral species and their symbiotic algae, producing genetically more heat-tolerant individuals over time as conditions warm.
Of these four possibilities, the first two would be limited in the amount of adaptation that could be achieved, because without any genetic change there would be no improvement through time in the capacity to adapt. Similarly, the third possibility allows for the persistence of coral reefs, but they will necessarily become less diverse as more and more species of coral have to drop out, unable to tolerate the warming conditions. Only the fourth possibility, involving real genetic selection could provide scope for sustained change into the future. This is where the issue of pace of warming and extent of warming comes in – many scientists believe the extent of warming over this century will overwhelm any ability to adapt using the first three pathways, while the pace at which warming is occurring is so fast, relative to the long lives and long generation times of corals, that corals will not be able to evolve quickly enough to keep up. Of course, whether or not reefs can adapt to warmer water does not prevent modeling what happens if they do adapt, and this is the point of Logan’s paper.
NOAA’s Coral Reef Watch program has used remote sensing on a global scale to follow changes in ocean surface temperature, and to identify when temperatures rise sufficiently to trigger bleaching of corals. They built a predictive model based on ‘degree heating weeks’ (DHW), which in this case measure both extent of warming and duration of events in which temperature exceeds the long-term average summer maximum temperature for that site by at least 1oC. When a warming event at a site reaches 4 DHW, bleaching usually occurs. This method has proven quite reliable, and NOAA reports the data on its web-site as a service to coral reef managers around the world. Logan and colleagues were able to use the NOAA model for predicting bleaching, and simulations of future sea surface temperature based on IPCC scenarios to predict the global frequency of bleaching events (proportion of locations containing reefs that bleach). Then they explored what would happen to the pattern of future bleaching if they built into their modeling a capacity to adapt to warmer temperatures. Their results include two interesting discoveries.
Their first discovery contains the good news. Using current climatology (1985-2004) to generate the bleaching thresholds, and assuming no capacity by the corals to adapt, their model predicted the frequency of bleaching would rise quite rapidly exceeding 50% of all reef locations every year by 2050 under typical CO2 emissions scenarios. But when they used climatology of 1900-1919 instead of current climatology, bleaching frequency rose much more rapidly, reaching 50% per year by about 2010 under most CO2 emissions scenarios. That bleaching clearly has not become that frequent already suggests that corals have already adapted to some degree, and compensated for the warming of waters during the years up until the present. Although not direct empirical evidence, this result strongly suggests that some adaptation is possible.
Logan’s evidence that coral reefs have already adapted to some of the warming that has already taken place. The horizontal dotted line marks the point at which 50% of coral reefs globally are experiencing two or more bleaching events per decade. The solid gray and black lines show the trend modeled in the more accurate ‘bias-corrected’ way but using 1900-1919 climatology (gray) or 1985-2004 climatology (black). The black curve reaches the 50% line about 40 years after the gray curve, suggesting strongly that during the period from 1900 to now corals have been adapting to warmer water to some degree.
Image © Logan, Global Change Biol 20: 125, 2014.
Their second discovery resulted from the modeling they did in which they built in some capacity to adapt. They used two approaches: a capacity to adapt temporarily to a 2oC warming, perhaps by symbiont ‘shuffling’ during a period of thermal stress, and a rolling capacity to adapt to continuous warming. The temporary adaptation was able to persist for from 2 to 10 years following a bleaching event, while the rolling adaptation could proceed at rates of warming of 2oC every 40, 60, 80 or 100 years. Their results showed that the temporary adaptation bought around a decade of delay before bleaching frequency rose steeply, but that the rolling adaptation led to much more dramatic delays in the increase in frequency of bleaching, especially if the capacity to adapt was powerful enough to accommodate the faster 2oC per 60 or 40 year rates of warming. In effect, these results suggest that any form of adaptation other than evolutionary change will be at best a temporary respite if we continue to warm the world.
Figure 3 from Logan’s paper shows the results for two different models of adaptation, and two different IPCC scenarios for CO2 emissions. Scenario RCP 6.0 is an optimistic scenario for transition away from use of fossil fuels while scenario RCP 8.5 is pretty much the carbon-intensive route we are currently on. Model 2 models a rolling or continuous adaptation (likely through evolutionary change) that proceeds rapidly (40 year) to more slowly (100 year). Model 3 permits a more limited form of adaptation (through such processes as symbiont shuffling following each bleaching event. The adaptation is less (2 year) or more (10 year) long-lasting. It’s clear that Model 2 with a continuous capacity to adapt is far more effective in coping with warming than is Model 3. It’s also clear that continuing on our present carbon-intensive route (RCP 8.5) makes it far more difficult for any form of adaptation by corals to have a lasting effect. And we still do not know if corals can adapt to ever warmer waters. Image © E Logan, Global Change Biol 20: 125, 2014.
Time for a Summary
So putting all these studies together, here are the glimmers of good news for coral reefs:
- The report that calcification rates by Great Barrier Reef corals had slowed by 14% appears to have been an over-estimate. Mid-shelf and outer-shelf reefs show little evidence of a drop in calcification rate until the last decade or so. However, it appears that there are real problems of calcification for corals on inner-shelf reefs. That the underlying causes may have more to do with agricultural run-off and less to do with ocean acidification provides a thin silver lining.
- In South-East Asia, on the Thai-Malay Peninsula, decline in rate of calcification by corals seems real, and greater than even the 14% rate reported earlier for Great Barrier Reef, but here also there are significant variations among locations suggesting that some reefs may be lucky (and that we might be able to learn how to make more reefs lucky).
- By modeling what coral reefs require in their environment, and then tracking how environments will change in such features as temperature and pH as climate change progresses it has been possible to identify locations outside the present range of distribution of reefs that should become suitable for colonization in the future. However, the same modeling effort has revealed the profound extent to which conditions for coral reefs are likely to degrade within the West Pacific.
- By modeling the likely pattern of coral bleaching into the future under various plausible scenarios for CO2 emissions, scientists believe they have evidence that some adaptation to warmer conditions has already occurred. This modeling study has also shown that adaptation by any means other than evolutionary change will buy only a very short period of time for coral reefs as we continue to warm the world.
We still are confronted with the large average rates of loss of coral cover on the Great Barrier Reef and in the Caribbean, and there remains a sense that coral reefs are truly the one ecosystem most savagely impacted by climate change. But maybe, just maybe, they can squeak through to a time when we no longer use vast quantities of fossil fuels and spew CO2 into the atmosphere. Like I said, the glass is half full.
A living reef at St. Croix, USVI –not the way it used to be, but better than many places are these days, and surely worth caring for. A glass-half-full kind of place. Image courtesy NOAA.