Concluding Thoughts

Over the past three months this blog has attempted to uncover some of the most significant climate related threats to coral reefs.  These are presented in the poster below (click to view full size). 




 

This exercise has revealed the fragile nature of these globally important marine ecosystems, as well as the potential loss in natural and economic wealth that a changing climate could bring about. It is evident that anthropogenic carbon dioxide emissions are creating conditions that test the resilience of coral reefs, due to the impact of increasing sea temperatures, acidifying oceans, rising sea levels and increasing storm intensity. Although it is apparent that coral reefs have adapted to dynamic climate conditions for thousands of years, the speed of contemporary climate change combined with direct human pressures poses a worrying threat. In order to ensure that the natural resilience of coral reefs enables these ecosystems to adapt to a changing climate it is imperative that detrimental human activities, such as overfishing, coral mining and sedimentation, are effectively managed. Ensuring that an effective balance between conservation practices and sustainable resource use for local people is met will help to maintain these ecosystems. As well as attempting to mitigate the effects of anthropogenic climate change, efforts need to be made to stop the problem at source. This means stricter and better enforced international policies to reduce anthropogenic environmental degradation, as well as investment in renewable energy sources and more efficient technologies. Ultimately, the future of coral reefs is in our hands, as is the case with many other important habitats. Being able to protect these ecosystems and prevent widespread extinctions is, in my mind, the most demanding challenge human’s face.

Tropical Storms, Climate Change and Reef Resilience

The increasing number, duration and intensity of tropical storms in response to a warming climate is another possible threat to coral reef ecosystems. This issue has only been discussed briefly in this blog, and since tropical storms have the potential to cause widespread damage to coral reefs it is a threat that should be examined. While coral reefs act as protection for coastal settlements, they can often be severely damaged during tropical storms. Global Circulation Models predict rising global average sea surface temperatures, increasing the likelihood of tropical storms (IPCC, 2007). Recovery from tropical storm damage can take many years, depending on the resilience of the reef. While the prediction of future changes to storm intensity inevitably involves uncertainty, data from the last thirty yeas suggests that the number of intense storms is increasing (Figure 1).




Figure 1: Total number of intense hurricanes, showing a significant increase in category four and five storms over the period 1970-2004 (Webster et al, 2005).




The assessment of coral damage following hurricanes and cyclones has been carried out for many years. Rogers et al (1991) monitored coral reef off the south coast of St John, US Virgin Islands, at a depth of 11-13m. Nine months after permanent transects were set up Hurricane Hugo struck, allowing the analysis of storm destruction. The category 5 hurricane caused patchy damage to the reef, with large waves moving and damaging significant quantities of coral and depositing sediment on the sea floor. Following the storm there was a decrease in the percentage of living coral by 40%, 12 months later there had been no significant increase in the amount of live coral. There was also a decline in species diversity following the storm. The ability of a coral reef to recover following storm damage is highly influenced by the extent of human activities, with overfished ecosystems showing poor recovery compared with less disturbed reefs. Similar results were found in a more recent study by Guillemot et al (2010), following Cyclone Erica in New Caledonia, South Pacific, 2003. The cover of live coral decreased by 45% in the two year period following this storm. After four years the coral reef displayed good signs of recovery, with a return to pre-cyclone fish community. It is suggested that the moderate anthropogenic pressure in this region was conducive to good resilience and recovery.




In order to predict how future changes in tropical storm intensity are likely to affect coral reef ecosystems it is useful to assess long-term variability as well as their role in ecosystem disturbances. Through topographical surveys and geological dating of coral ridges and terraces (which are produced by storms) it is possible to estimate historic storm surge and wave heights. Nott and Hayne (2001) determine the intensity of tropical cyclones along the Great Barrier Reef over the past 5,000 years using this method. Their results indicate a long history of intense tropical cyclones in this region, with the majority leading to widespread coral damage, including a weakening of the substrate which enables severe damage to occur during less intense storms. This study suggests that storm events are central to coral community structure and function, including the life span of individual corals. Therefore, if the frequency and intensity of these events is to change it may have significant negative repercussions for coral reef ecosystems. The authors of this study consequently believe that coral reef ‘vulnerability, exposure and risk is much higher than previously estimated’ (Nott and Hayne, 2001 p.511).




Once again it appears that coral reefs are likely to be negatively affected by climate change. Although there is strong evidence that coral reefs are resilient to storm damage, and are able to recover in the long-term, the ability of these ecosystems to do so is significantly reduced in regions where human pressures are having damaging impacts. The need to reinforce reef resilience through effective policies and management appears a necessity if these fragile, yet globally important, habitats are to be maintained.

In the news this week . . .

There is talk of extending the network of Australian Marine Protected Areas in order to prevent commercial fishing in a region of the Coral Sea. There is debate over whether a no-take MPA would be more economically viable than a multiple use MPA, due to the cost involved with enforcement and the loss of income for local people, compared with the potential environmental benefits that would occur with full protection. This issue links in nicely with this week’s post which looked at the economic valuation of nature, and the potential cost of climate change. The full news article can be viewed here.




This week scientists from around the world travelled to Belize for a conference, hosted by the Intergovernmental Panel on Climate Change and the Caribbean Community Climate Change Centre, aimed at discussing the impact of climate change on small nation islands. These nations are likely to be some of the worst affected by climate change, due to their dependence on fragile ecosystems, such as coral reefs. Sea level rise is also a very real threat to these regions, with potential impacts including a reduction in island size, a reduction in the level of freshwater and increased soil salinity. In order for their survival adaptation is a necessity, and workshops like these will help focus scientific investigation and international action on these issues.




Finally, here is a first-hand account of the damage that both human exploitation and climate change are doing to coral reefs in Bora Bora, French Polynesia. Provided by Jon Bowermaster, a National Geographic journalist, the article offers a fascinating insight into some of the main concerns in this region, as well as some interesting solutions.

The Economic Cost of Climate Change

This blog has presented the major climate related threats to coral reef ecosystems. While the focus has primarily been on the ecological consequences, there are also important economic costs that should be taken into consideration. The economic costs associated with the impacts of climate change on coral reefs comprise both direct costs, such as lower incomes for fishers due to decreased fish populations, and indirect costs, such as the cost of decreased biodiversity and coastal protection. In many cases attaching an economic cost to an environmental issue has proved to be the catalyst for action, as it enables mechanisms to be established that incorporate the environmental cost into activities which have detrimental environmental impacts. Most of the literature regarding this issue examines the economic cost of direct human impacts, such as overfishing and tourism, rather than the potential cost of climate change. However, it is possible to get an understanding of the economic value which has been placed on coral reefs by examining studies that focus on human exploitation. From this it is possible to infer the potential economic cost of a changing climate.




A brief introduction on the subject of valuing nature is provided by Pavan Sukhdev, Special Advisor and Head of UNEP’s ‘Green Economy Initiative’:







A study by White et al (2000) investigates the economic losses caused by reef destruction in the Philippines. Here coral reef fisheries are worth around US$1billion per year, providing a living to around one million small-scale fishers. Coral reef damage in the Philippines is extensive, with less than 5% in ‘excellent’ condition. In Bacuit Bay, Palawan it has been estimated that fisheries and tourism would produce US$41 million more than forestry operations in the watershed (which is causing sedimentation in the bay) over a ten year period (Hodgson and Dixon, 1988). The rise in ecotourism in Bacuit Bay indicates the potential for the sustainable use of this resource, enabling both economic benefits as well as achieving conservation goals. The biodiversity and aesthetic value of Mabini, a marine sanctuary in the Philippines, is estimated at US$300,000 per year based on an average visitor’s willingness-to-pay for entrance. This paper highlights the potential economic cost of a changing climate, if it leads to ecosystem degradation, providing economic justification for coral reef management in this region.




The ‘environmental economics of coral reef destruction in Sri Lanka’ are considered by Berg et al (1998). The total quantifiable economic value of these reefs (which includes the fish-habitat function, the tourist-attraction function and the physical-structure function) is estimated at between US$13,000 – US$4,404,000km² annually. From this, the net economic cost of coal mining in highly developed areas over a 20 year period is estimated to amount to around US$6,615,000. Although climate related impacts may not be as severe as this activity, it provides evidence to support the view that investment should be made in order to mitigate the effects of a changing climate.




While many studies attempt to value coral reefs based on the potential income provided, Pendleton (1995) examines the economic benefits of a marine park based on what it would cost to provide the ecosystem services that the coral reef ecosystem currently supplies. It is estimated that the cost of operating Bonaire Marine Park in the Caribbean Islands to protect the reef would be around 0.7% of the net benefits. Evidently the potential cost of coral reef degradation far outweighs the cost of taking action, assuming that conservation is effective in preserving the reef.




As mentioned above, although these papers are useful in exploring the economic value of coral reef ecosystems, they are mainly concerned with estimating the cost of human exploitation. Brander et al (2009) explore the economic impact of ocean acidification on coral reefs. Their study examines reefs which are considered both economically and ecologically important, and models the cost attached to acidifying oceans based on the four IPCC emission scenarios. Figure 1 presents the estimated annual economic cost of coral reef loss in response to ocean acidification, indicating the escalating costs associated with this environmental issue. It is interesting to note that the B1 scenario is predicted to produce economic benefits by 2085. This is because the B1 scenario relates to an integrated and ecologically friendly world, which utilises clean, efficient technologies and relies on global solutions to provide environmental stability (IPCC, 2007).




Figure 1: Annual economic cost of ocean acidification induced coral reef loss, under four different emission scenarios (Brander et al, 2009).

These studies have emphasised the high economic value that is placed on coral reef ecosystems, and therefore highlight the potential economic cost that climate change (in conjunction with human resource exploitation) is likely to have. The economic justification for investment in conservation and mitigation projects is evident, and should provide motivation for global action.  

Reconstructing Past Climates

As demonstrated in my post on 4^th April, the investigation of fossilised reefs allows the reconstruction of historic sea level fluctuations. This is possible due to the known ecological conditions required by certain species of coral. Similarly, it is possible to investigate other aspects of climate, such as air temperature, sea surface temperature, precipitation and salinity, by using coral reefs as a palaeoclimate proxy indicator. Since the 1970s palaeo-climatologists have realised the potential of coral reefs to monitor climate variations. Coral growth rates, density bands and isotopic composition respond to changes in climate, and can therefore be used to reconstruct past climates (Dunbar and Wellington, 1981). Dunbar and Wellington’s paper assessed the stable oxygen isotopic response to seasonal changes in temperature, salinity and growth rate of branching corals. The coral was stained at certain points in time so that visible markers facilitated the measurement of growth rates and isotopic profiles. By measuring ocean temperature, salinity and oxygen isotopic composition of seawater over the period of a year it was possible to link these changes with observed variations in the coral composition and growth rate. They conclude that variations in oxygen isotopes in coral reefs record seasonal temperature changes, as can be used to explain variations in salinity. Low growth rates were closely associated with colder temperatures and greater cloud cover.




The stable oxygen isotope technique employed by Dunbar and Wellington (1981) has been used extensively to reconstruct past climates. Evans et al (2002) reconstructed Pacific sea surface temperatures from 1607-1990 by assessing stable oxygen isotope (δ¹⁸O) in coral. Sea surface temperatures were derived from oxygen isotope time series taken from 12 tropical Pacific Ocean, Indian Ocean and Red Sea sites. By comparing this data with similar attempts and historical data, it was possible to reconstruct sea surface temperatures in the Nino 3.4 region of the Pacific, the region where sea surface temperatures have the most significant impact on shifting rainfall patterns. Figure 1 presents the results of their reconstruction, indicating the number of El Nino Southern Oscillation (ENSO) warm events, where a sea surface temperature anomaly greater than 0.5°C was inferred from their investigation of coral reefs. 




Figure 1: Coral reconstruction of the number of ENSO warm events in NINO 3.4, 1607-1990 (Evans et al, 2002).

A similar technique was employed in a study by Urban et al (2000), in order to reconstruct ENSO variability over the past 155 years.  Negative oxygen isotope anomalies can be found in central western Pacific corals when El Nino events have increased sea surface temperatures and enhanced rainfall. In contrast, La Nina events (characterised by cool and dry conditions in this region) produce positive oxygen isotope anomalies. Coral from the Maiana Atoll was sampled and the bimonthly oxygen isotope record was used to infer climatic conditions. Figure 2 presents results from this paper, showing the strong correlation between oxygen isotope records in the coral and the instrumental sea surface temperature record. This indicates the potential of this method to reconstruct past climate.




Figure 2: Tropical Pacific variability from coral and instrumental data, 1950-1995 (Urban et al, 2000).

 

A review of present knowledge of tropical palaeoclimates derived from coral reef investigation was compiled by Gagan et al (2000). This article reinforces the benefits that the growing network of coral oxygen isotope records has for paleoclimatology. Constructing climate records that go back to the last deglaciation reveals previously unknown climate variability, such as ENSO cycles that exist on times scales of decades to centuries. However, there are still uncertainties involved with this method that stem from a lack of knowledge regarding the processes that control isotope changes within corals, and the specific climate mechanisms that produce these changes. A multi-proxy approach, combining records from ice cores, tree rings and sediment records is encouraged to ensure high-resolution paleoclimatology.

In the news this week . . .

Last week saw the 2011 instalment of the ‘Greenhouse Conference’ series, held in Cairns, Australia. Organised by Australia’s national science agency, CSIRO, the climate change focused conference covered a wide range of topics, including coral reefs. Speakers included Bronte Tilbrook, Ove Hoegh-Guldberg and Janice Lough who presented their findings from recent research projects. Tilbrook was speaking about the impact of ocean acidification on the Great Barrier Reef, with his research finding that the level of calcium carbonate was below the optimum level for coral reef growth. Although coral reefs appear to be growing, it is suggested that conditions in the Great Barrier Reef are not far from reaching a critical point, the full story was reported in the New Scientist. Hoegh-Guldberg told the conference that the future of the Great Barrier Reef would be determined in the next ten years, by then they will have reached a tipping point if anthropogenic degradation continues. A report on his comments can be viewed here. Janice Lough  spoke about the use of coral reefs to reconstruct past climate change. You can listen to an interview with her on Pacific Beat, courtesy of Radio Australia, talking about her research and the benefits of it. 




The benefits of assessing past coral reef response to climate change has been reinforced by a study led by James Klaus from the University of Miami. The study aims to determine the structure and function of these ecosystems during the Pliocene epoch, in order to assess how present day climate change will affect coral reefs. The findings suggest that modern day coral reefs are very different from Pliocene coral communities, as they were predominantly free living corals that were not attached to the sea floor. These species were adapted to higher sea temperatures and nutrient rich water, indicating the possible coral reef community of the future. The article was published in April’s edition of Geology. 




A ‘stress test’ has been developed by researchers from the Wildlife Conservation Society. The model aims to identify the coral reefs ecosystems that are most likely to adapt to climate change. It is hoped that by prioritising these regions then conservation and management projects will be more effective.




A project run by the Living Oceans Foundation aims to map shallow reef ecosystems around the world in order to develop an effective tool for managers. Over the next five years the team will identify shallow marine habitats, characterise the organisms that live there and develop an understanding of the key processes and interactions. It is hoped that the maps developed will aid effective management, and assist with modelling exercises. The full story can be viewed here.

Historic Sea Level Rise

As mentioned in the previous post, the potential response of coral reefs to present day climate change can be estimated through the assessment of the elevation and ages of drowned reefs. These drowned reefs also allow the investigation of historic sea level rise, which will not only improve our understanding of the changes brought about by a changing climate but will also enable more accurate predictions to be made about contemporary sea level rise. The last glacial to inter-glacial transition, which occurred around 13,000 – 10,000¹⁴C years BP, is the most studied due to the rapid climate change that occurred and the fact that it is the most recent transition (Hoek, 2008).




Figure 1: Caribbean sea-level rise during the last de-glaciation, with the shaded areas showing drowned reefs and the white areas indicating catastrophic rise events (CRE) (Source: Blanchon and Shaw, 1995). 

Blanchon and Shaw (1995) use drowned coral reefs in the Caribbean-Atlantic region to study ‘glacio-eustatic sea-level changes’ during the last de-glaciation. They emphasise the benefits of using coral reefs for studying sea level changes, referring to their suitability for radiometric dating and the way in which they maintain themselves at sea level through vertical accretion. In particular they investigate Acropora palmata, since it forms monospecific reefs and is strongly depth restricted, meaning it can be successfully and reliably used to infer the sea level. When sea levels rise faster than this species can grow vertically the reef will exhibit a mixed framework with other corals, eventually this is replaced by deep water corals. An inverse relationship has been uncovered between the rate of sea level rise and the thickness of this mixed framework layer. Figure 1 presents the sea level rise that occurred during the last de-glaciation, reconstructed from drowned reefs. The results indicate the large volumes of meltwater that entered oceans at three main points in time, and that sea level rise occurred rapidly. However, it also suggests that reefs were able to form again following the rapid sea level rise, which is good news considering the threats currently facing these ecosystems.




A similar study was carried out in Hawaii by Webster et al (2004) which attempted to investigate the reason for the drowning of the 150m submerged reef off Hawaii. Through radiometric dating techniques they were able to place the drowning of this reef at around 14,700 years ago which coincides with a sea level rise period known as 'Meltwater Pulse 1a'. The study not only provides evidence for the occurrence and timing of this meltwater phase, but also the intensity of the sea level rise (approximately 35m in 500 years, equivalent to 40-50mm per year). 

 


Figure 2: Position of drowned reefs in Hawaii. Reef 1 was subject to reef drowning following Meltwater Phase 1a (Source: Webster et al, 2004).

While the main process under investigation in studies like these is eustatic sea level rise, an understanding of  isostatic changes is required to infer eustatic sea level rise from the assessment of coral reefs. Stirling et al (1998) highlight the inconsistencies in the inferred timing and duration of the last interglacial as a result of isostatic processes that vary spatially.




From these studies it is evident that historic sea level rise caused by de-glaciation following the last glacial maximum caused the drowning of many coral reefs. However, it is also apparent that coral reefs were able to recolonize shallow regions of the ocean once sea levels were stable, which has positive implications for the consequences of present day anthropogenic changes.