Janice M.
Lough
Australian Institute of Marine Science, PMB 3, Townsville MC, Queensland, 4810, Australia. E-mail: j.lough@aims.gov.au; www.aims.gov.au
First published on 7th December 2007
Tropical coral reefs are charismatic ecosystems that house a significant proportion of the world’s marine biodiversity. Their valuable goods and services are fundamental to the livelihood of large coastal populations in the tropics. The health of many of the world’s coral reefs, and the goods and services they provide, have already been severely compromised, largely due to over-exploitation by a range of human activities. These local-scale impacts, with the appropriate government instruments, support and management actions, can potentially be controlled and even ameliorated. Unfortunately, other human actions (largely in countries outside of the tropics), by changing global climate, have added additional global-scale threats to the continued survival of present-day coral reefs. Moderate warming of the tropical oceans has already resulted in an increase in mass coral bleaching events, affecting nearly all of the world’s coral reef regions. The frequency of these events will only increase as global temperatures continue to rise. Weakening of coral reef structures will be a more insidious effect of changing ocean chemistry, as the oceans absorb part of the excess atmospheric carbon dioxide. More intense tropical cyclones, changed atmospheric and ocean circulation patterns will all affect coral reef ecosystems and the many associated plants and animals. Coral reefs will not disappear but their appearance, structure and community make-up will radically change. Drastic greenhouse gas mitigation strategies are necessary to prevent the full consequences of human activities causing such alterations to coral reef ecosystems.
Janice M. Lough | Janice Lough was born in Newcastle, UK in 1955. She has a BSc in Environmental Sciences (1976) and a PhD in tropical climate variations, undertaken at the Climatic Research Unit (1982), both from the University of East Anglia, UK. She was a Research Associate at the Tree-Ring Laboratory, University of Arizona, USA from 1982 to 1986 applying dendroclimatic reconstructions to understanding climate variation and change over North America and the North Pacific. She joined the Australian Institute of Marine Science in 1986 where she is now a Principal Research Scientist leading the Responding to Climate Change Research Team and affiliated with the ARC Centre of Excellence for Coral Reef Studies at James Cook University. Her current research activities include obtaining high-resolution proxy climate and environmental information from annually banded massive corals over the past several centuries and assessing the nature of regional climate changes due to the enhanced Greenhouse effect and their impacts and consequences for tropical coral reefs. |
Global warming due to the enhanced greenhouse effect is already occurring, with observed and projected temperature changes greatest at high latitudes.1 Yet ecosystems in the naturally warm tropical ocean waters—coral reefs—are already showing evidence of global warming impacts. It is now 200 years since Matthew Flinders provided the above evocative description (that resonates with modern coral reef experiences) of the world’s largest coral reef ecosystem, Australia’s Great Barrier Reef. Why are these ecosystems now considered as one of the “most vulnerable” to climate change?2 In this article I consider the mounting body of scientific evidence (though in the space available it is not possible to do full justice to the burgeoning literature) that a rapidly changing climate due to human activities is a major threat to the maintenance of present-day coral reefs as we know and rely on them.
Tropical coral reefs are the result of a mutually beneficial relationship (symbiosis) between the coral animal (Phylum Cnidaria, Class Scleractinia) and single-celled photosynthetic plants called zooxanthellae (genus Symbiodinium). Photosynthetic products of the algae provide the coral host with cheap energy.5–9 The zooxanthellae also play a role in light-enhanced calcification of scleractinian corals, allowing the rapid calcification necessary to form reef structures.10 In return, the algae obtain protection and essential nutrients from their coral host. The photosynthetic pigments within the algae give corals their colours. Together these organisms produce the extraordinary variety of coral skeletal forms and structures that form massive carbonate structures that can withstand the natural forces of erosion and provide the basis of coral reef ecosystems.11
Coral reefs are not just corals. They provide complex habitats that support a great diversity of reef-associated fauna and flora. Australia’s Great Barrier Reef (GBR), for example, which extends ∼2000 km along the northeast coast of Australia and covers ∼35 million hectares (an area larger than Italy), contains nearly 3000 coral reefs built from over 360 species of hard coral. These reefs harbour ∼1500 species of fish, ∼4000 species of molluscs (shells), ∼400 species of sponges, ∼800 species of echinoderms (starfish, urchins etc.), ∼500 species of macroalgae (seaweeds), 23 species of marine mammals, extensive sea grass beds (home to internationally endangered dugongs), 6 of the world’s 7 species of marine turtle species, and 30% of the world’s soft coral species, several hundred species of seabirds, breeding grounds for humpback whales from Antarctica…………..and the list continues (http://www.gbrmpa.gov.au/). No ecosystem is an island and coral reefs, such as the GBR, are intimately linked with coastal ecosystems, such as mangroves, wetlands and estuaries. The international significance of the GBR is reflected in its inscription on the World Heritage List in 1981 (http://www.environment.gov.au/heritage/worldheritage/sites/gbr/values.htm). In addition, coral reefs only occupy about 10% of the GBR shelf and we are only beginning to understand the rich biodiversity of the 90% of inter-reefal areas.12 Internationally, it has been estimated that we only know of ∼10% of the total number of species living on coral reefs.13
The global distribution of coral reefs has long been considered to be constrained to shallow, warm, well-lit, clear, low nutrient and low sediment waters, as well as by their geological and climate history and local bathymetry.18,19 A more recent comprehensive analysis has clarified the environmental limits to coral reef development using up-to-date data on the geographic locations of coral reefs (ReefBase http://www.reefbase.org/main.aspx) and improved instrumental environmental data sets.20 Temperature, salinity, nutrients, light availability and the aragonite saturation state of seawater were considered as the “first-order determinants of reef distribution” with regional-scale reef distribution being affected by factors, such as waves, ocean currents, larval sources etc. The aragonite saturation state of seawater is a measure of how easily aragonite, the main form of CaCO3 created by reef-building corals, can form and depends on the concentration of calcium and carbonate ions in seawater, i.e. the carbonate chemistry of seawater.21 This analysis found water temperature at reef locations averages 27.6 °C and ranges from a seasonal minimum of 16.0 °C to a seasonal maximum of 34.4 °C (both in the northern Arabian Gulf) and salinity averages 34.8 PSU and ranges from 23.3 to 41.8 PSU. As noted in many earlier studies, these two variables are important controls on coral reef formation, and minimum water temperatures of ∼18.0 °C have long been considered the lower limit for coral reef formation.22 In contrast to earlier studies, low concentrations of nutrients were found to be of lesser importance as a limiting factor to coral reef development. The aragonite saturation state and light penetration, which both covary with water temperature, were likely limiting at higher latitudes. These perspectives on the environmental controls of coral reef distribution are important in assessing the potential responses of coral reefs to changes in their physical environment due to the enhanced greenhouse effect. Although an important factor, light is unlikely to change on large spatial scales but significant changes in water temperature and aragonite saturation state are highly likely (discussed below).
The conclusion, that climate change was not a near-term threat to coral reefs, was drawn only 14 years by an international team of coral reef experts. What has been termed the “coral reef crisis” was already well underway before we realised the potential fragility of these highly diverse marine ecosystems to global climate change. The significant ecosystem goods and services and direct economic benefits that coral reefs provide to the large and expanding populations of tropical coastal regions have been progressively over-exploited. As a result, even without the climate change impacts discussed below, coral reefs have been declining at an alarming rate, due to direct local and regional human pressures, such as over-fishing, destructive fishing (e.g. dynamite), decline in water quality due to increased sediment from land-use changes, nutrient and chemical pollution and development on coasts (dredging, land clearing for ports, harbours etc., mining of coral reefs etc.).23–27
Only 30% of the world’s coral reefs are considered at low risk from these increasing human stresses and 20% of reefs have, effectively, been destroyed.28 The highly biodiverse reefs of Southeast Asia and the Indian Ocean (where human pressures continue to increase) are most badly affected. Indeed, an historical study of 14 coral reef regions suggests that human exploitation of coral reefs is not a recent phenomenon but has been occurring over thousands of years.29 This exploitation, primarily through over fishing, which has impacted other coastal ecosystems,30 has resulted in a trajectory of decline in coral reefs worldwide, even before concerns of global climate change impacts. Even what are regarded as pristine, well-protected reefs, such as the outer GBR, are already on this declining trajectory due to the loss of large herbivores and carnivores.
These primary and ongoing causes of coral reef decline all occur at the local- and regional-scale. They are a direct result of human-induced pressures and, therefore, are also, potentially, manageable and possibly reversible. This requires both active conservation measures, such as integrated catchment management reducing land-based pollution of reefs, elimination of destructive fishing practices, sustainable management of reef fisheries, and implementing and managing expanded Marine Protected Areas.31 Reversal of the trend also requires national and international initiatives to provide the many countries with degraded and degrading coral reefs, the majority of which are developing countries, with the capacity and necessary assistance to effectively manage their coral reef ecosystems.24,26,28 Human interference with the global climate system has now added an additional set of threats to the survival of coral reef ecosystems—threats that are occurring at the global level and cannot be effectively managed locally.
Observational evidence from all continents and most oceans shows that many natural systems are being affected by regional climate changes, particularly temperature increases.2
Global climate has varied and changed in the past on a range of timescales, and organisms and ecosystems have survived, changed their distribution and adapted to such changes. It is now, however, clear that human activities since the Industrial Revolution began in the late 18th century, primarily through burning of fossil fuels, land-use changes and agriculture, have altered and are significantly altering the composition of the earth’s atmosphere. These changes are resulting in more heat being trapped in the global climate system (the enhanced greenhouse effect) and are causing rapid and significant global warming and a changing global climate.1 The atmospheric concentration of the main greenhouse gas, carbon dioxide, is now ∼40% higher than in the late 18th century. Based on instrumental records, global average temperatures are now ∼0.7 °C warmer and sea level ∼20 cm higher than in the late 19th century1 and there are already discernible changes in many natural biological and physical systems, which are consistent with a warming climate, even with the relatively modest global warming observed to date.2
There are several aspects of anthropogenic climate change that have affected and will affect the environmental envelope of coral reef ecosystems—one to which they have been accustomed to since current sea level was reached ∼8–10 thousand years ago (Table 1). The most significant of these appear, at present, to be warming ocean temperatures increasing the frequency of mass coral bleaching events and absorption by the oceans of a significant proportion of the excess carbon dioxide, altering ocean chemistry and potentially reducing the ability of corals to form their skeletons and thus the very basis of coral reef ecosystems.
Climate variable | Consequences |
---|---|
Warmer waters | •Coral bleaching |
•Coral diseases | |
•Affect other reef organisms | |
Ocean acidification | •Weaker coral skeletons and reef structures |
More intense tropical cyclones | •Physical destruction of reef |
Rising sea level | •Coastal erosion |
•Higher storm surges | |
•‘Drowning’ of some reefs | |
•Increased area for some reefs | |
More extreme rainfall & flood events | •Low salinity waters extend further out on reefs |
Changes to ocean currents | •Affect connectivity (e.g. larval supply) amongst reefs |
•Affect nutrient supply from upwelling | |
El Niño–Southern Oscillation events | •Extreme weather events that enhance probability of bleaching |
Wind fields and atmospheric circulation patterns | •Changed prevailing weather conditions |
Pioneering studies in the 1970s demonstrated just how close (within 1–2 °C) reef-building corals are living to their upper thermal tolerance limit before bleaching occurs.32,33 The threshold for bleaching becomes critical during the seasonal maximum of sea surface temperatures (SSTs) at a given locality. Maximum SSTs at 1000 reef locations averages 29.5 °C and range between 28.2–34.4 °C.20 Various studies have identified that there is not an absolute temperature at which corals will bleach but rather that the temperature threshold varies with ambient water temperatures on each reef.34 This demonstrates that over long time scales, corals have adapted to their local environmental conditions.
The first “most alarming” reports of mass coral bleaching events were not initially linked with unusually warm SSTs, although a connection was made with El Niño events.35,36 This was largely due to the lack of reliable long-term records of SSTs and other environmental variables on coral reefs. As more mass bleaching events occurred and observations improved, the link was made with unusually warm waters.37–40 The suggestion that these unusual occurrences on coral reefs might be linked to global climate change due to the enhanced greenhouse effect,41,42 is now no longer considered “dubious” but “incontrovertible.24
Large-scale SST anomalies in the tropical oceans create conditions that can result in coral bleaching. The most extensive coral bleaching event on record occurred during the major El Niño–Southern Oscillation event of 1997–1998,43 with 1998 being the then warmest year on record globally.44 Mass bleaching was reported from nearly every coral reef region of the world.45 Sixteen percent of the world’s coral reefs were estimated to be damaged by this unprecedented event and, of these, only 40% appear to have recovered and 60% have not.28 The scale and magnitude of this event, that could essentially be tracked around the world as each region experienced unusually warm SSTs during its annual seasonal maximum, catalysed efforts to monitor the occurrence of conditions conducive to coral bleaching. The advent of satellite-based observations of the oceans since the early 1980s has dramatically increased our capabilities to observe global-scale patterns and anomalies in ocean surface climate. A range of products, derived from the original concept of “oceanic hotspots”,46 are now routinely produced by the US National Oceanographic and Atmospheric Administration (NOAA;47http://coralreefwatch.noaa.gov) and provide the basis for identifying potential bleaching conditions in near-real time. These bleaching alerts need to be confirmed with field observations and there is some evidence emerging that for particular reefs, the thresholds for the occurrence and intensity of bleaching may be modified by the history of past bleaching events.48 Although such monitoring cannot prevent coral bleaching or mortality, timely information about bleaching potential enables scientists and reef managers to document the intensity, impact and subsequent effects more comprehensively than possible only 10–20 years ago. Improved knowledge of the links between the physical environment and biological processes of coral bleaching helps managers develop and test strategies to protect corals from bleaching and also, importantly, identify bleaching-resistant reefs. The latter are clear targets for enhanced protection efforts, as they may provide important refugia for coral reef organisms and sources for recruitment for bleaching-affected reefs as climate continues to change and increasingly stresses the world’s already compromised coral reef ecosystems.
At the local scale, the occurrence and intensity of bleaching can be highly variable, both within a coral colony, between coral colonies, within a reef and between reefs in a region.49 Such variations are in addition to the different susceptibility of different coral species to thermal stress.50 Other local physical factors can enhance or suppress the impacts of warmer than normal regional SSTs and thus the intensity of bleaching. Observations that corals often bleach more on their upper surface than at the sides of colonies clearly implicates light as an additional factor and frequently the local weather conditions that cause intense warming of the water column (calm conditions, low cloud amount, little water motion) allow increased light penetration at the coral surface.51 Increased cloudiness can also mitigate bleaching, even when SSTs are high.52 Lowered salinity due to a major flood event increased the intensity of coral bleaching on inshore reefs of the central Great Barrier Reef in 1998.53 There can also be considerable local-scale variations in water temperatures within and between reefs that can also affect the occurrence and intensity of bleaching.34,54,55 Such local-scale variations that reduce thermal stress can be linked to water movements, such as upwelling, mixing, tidal range and wave energy.51,56,57 The continued existence of such bleaching-resistant sites may be critical for recovery of nearby bleaching impacted coral populations.58
As a result of bleaching, corals may die, partially die or recover. Recovery depends on the coral rapidly regaining its zooxanthellae population soon after the bleaching stress.59 Even for corals that appear to recover fully, there is mounting evidence of long-term consequences as a result of the thermal stress associated with the bleaching event. These consequences include reduced reproduction, reduced growth rates and increased susceptibility to other disturbances, such as coral diseases.60 In addition, due to the different susceptibility of different species,50 the overall effect of a bleaching event can be to reduce species richness and diversity and change the community structure of the coral reef.61
Global average temperatures have warmed less than 1 °C since the 19th century and those of the tropical oceans by about half the global average.1 Average water temperatures for the most recent 30 years on the GBR, for example, are only 0.4 °C warmer than at the end of the 19th century but could be 1–3 °C warmer by the end of this century.62 The weather conditions (1–2 weeks of calm, cloudless conditions) that allow rapid warming of the water column in the summer season do not appear to have changed but this seemingly modest increase in baseline temperatures has been sufficient to take corals over the bleaching threshold in 1998, 2002 and again in 2006. Modelling of future impacts suggest that a 1–3 °C warming of the GBR would result in ∼80–100% bleaching compared to ∼50% in 1998 and 2002.49,55 Maintenance of the hard coral at the heart of coral reefs cover requires corals to increase their upper thermal tolerance limits by 0.1–1.0 °C per decade.63–65
Is it possible that corals and coral reefs can adapt or acclimate to these changing conditions? Some evidence is emerging that corals can respond to bleaching by changing the dominant type of symbiotic algae to more thermally tolerant partners. Corals can contain different types of symbionts and may be able to change the relative abundances of these clades—“symbiont shuffling”.66 This strategy may, however, be at the expense of growth rates, competition and reproduction,67 may only occur in a few species68 and may not occur sufficiently rapidly to keep up with warming ocean temperatures.69 Changing from type C to the more thermally-tolerant D, for example, would raise the thermal tolerance by 1–1.5 °C, which only matches the most optimistic projected warming for the end of this century.70
Given that minimum water temperatures are a limitation on tropical coral reef development and the oceans are warming, why will coral reefs not simply expand into higher latitudes? Unfortunately, water temperatures are not the only limitation to coral reef development and there appears to be little opportunity for significant poleward expansion of their distribution as the world continues to warm. This is due to lack of suitable substrate combined with greater changes in ocean chemistry that are detrimental for reef development at these higher latitudes.3,26,71
Increasing frequency of mass coral bleaching events is not the only impact of warming oceans on coral reef ecosystems. There is also an increasing frequency of reports of disease outbreaks affecting corals and other marine organisms72 and evidence is mounting that these occurrences are related to warmer ocean temperatures.73 Warmer waters appear to be increasing the severity of diseases in the ocean, which will reduce the vitality of marine ecosystems, such as coral reefs.74,75
Tropical cyclones are amongst the most destructive weather systems on earth, and although rarely observed equatorward of 5° latitude, are common and natural disturbances to many coral reefs regions.83 Tropical cyclones are “agents of mortality” on coral reefs and, primarily through the large waves they generate, can directly influence the structure and local distribution of coral reef assemblages.84,85 Tropical cyclones can also result in reduced salinity due to heavy rainfall and enhanced river flows onto nearshore reefs, as well as coastal destruction due to elevated sea levels associated with destructive storm surges. Such natural events can cause significant local disturbance but, given time and the absence of other stressors, coral reefs can recover.86,87 Recent studies in the Caribbean suggest, however, that these already seriously degraded reefs are not recovering as well as they used to from tropical cyclone impacts.88 Ironically, the passage of a tropical cyclone, by rapidly cooling surface ocean temperatures, can reduce the impact of elevated water temperatures and thus the intensity of bleaching, at least on small space scales.89
There is already some evidence to suggest that the destructive potential of tropical cyclones around the world has increased in recent decades.90,91 Although warming ocean waters might be expected to increase the intensity and frequency of occurrence of tropical cyclones, their formation depends upon a number of other factors.83 Current projections give no clear indication as to whether the number and preferred locations of tropical cyclones will change in a warming world. The intensity of tropical cyclones is, however, expected to increase,1 which will result in increased localised physical destruction on coral reefs—another disturbance from which reefs need time to recover.
The present-day distribution of ocean currents and circulation in the vicinity of coral reefs are important controls on the ecosystem dynamics as amongst other factors, these can have significant effects on the connectivity between reef systems that can control larval dispersal etc. Little is known as yet as to how ocean currents will change as the world continues to warm, although there is evidence in some areas of significant changes already occurring. The East Australian Current, for example, flows southwards from its origin in the Coral Sea. There is already some evidence that its southern extension is strengthening and carrying sub-tropical marine species further south.107 Changes in ocean current strength and location will also impact other coral reef ecosystems.
The world’s coral reef ecosystems were in serious trouble before the advent of rapid climate change due to the enhanced greenhouse effect. The prognosis for their future is undoubtedly dire and their loss, severe degradation and change in community structure that will result from ongoing climate changes, would be catastrophic at many levels. These threats are in addition to ongoing localized degradation and deterioration of coral reef ecosystems due, primarily, to over exploitation.27 Although many value coral reefs for their aesthetics, the majority of people who depend on coral reefs for their livelihoods (fisheries, tourism, shoreline protection) live in poor, developing countries who contribute only a tiny part of the world’s greenhouse gas emissions. A recent estimate is ∼10% of the world’s population live within 100 km of coral reefs and ∼90% of these people live in developing nations—∼63% of people (415 million) living within 100 km of coral reefs live in countries where per capita GDP is <US$5000.109 This compares with a per capita GDP for Australia, which contains ∼22% of the world’s coral reef area, of ∼$US33000. As with many aspects of climate change, the people who are likely to suffer the greatest impacts are those from developing countries who have contributed the least to global warming.110
A plethora of scientific papers, national and international reports and scholarly books now document the nature, impacts and potential consequences of ongoing rapid climate change for coral reefs, as well as local direct human impacts.58,111–113 There is a clear consensus that the world’s coral reefs are in trouble and that the alarm bells have been ringing for decades. The future of coral reefs as climate continues to change is inextricably linked to coral reef health. Some coral reefs have shown the ability to withstand disturbances, such as coral bleaching (resistance), and some coral reefs have shown the ability to recover from such disturbances (resilience).114,115 These attributes can be the result of such coral reefs being dominated by resistant species or to their physical environment reducing the probability of stress. Reducing and reversing local, direct insults to coral reefs clearly increases their resilience to global-scale climate change—“healthy” coral reefs, for example, recovered better after the major 1997–98 world-wide coral bleaching event than those already compromised by local anthropogenic stresses or diseases.28 Increasing the resilience of the world’s coral reef ecosystems requires integrated national and international actions and a greatly expanded network of marine protected areas.24
Even with rapid global implementation of stringent mitigation strategies to stabilize and reduce greenhouse gas emissions, the world and coral reefs are committed to significant rapid climate change, increased acidification of the oceans and accelerated sea-level rise. The issue is not just a “change” from one climate regime to a new one, to which organisms have to adapt BUT that for the foreseeable future, climate will be changing and it could be a long time before a new, relatively stable climate regime is reached (i.e. one not influenced by human activities). Indeed, even if it were possible to stabilize greenhouse gas concentrations at their present levels, we are still committed to “future climate changes that will be greater than those we have already observed”.116,117 Although coral reefs have a long geological history, there is now a severe mismatch in timescales for successful organism adaptation (thousands to millions of years vs. tens to hundreds of years).
Various aspects of our current experiment with the global climate system pose significant challenges for even the most well-managed and highly protected of the world’s coral reefs, such as the GBR—a vast ecosystem that we do not even fully understand in its present form. Coral reefs are unlikely to disappear everywhere, although some are clearly already beyond recovery. We may well witness within our lifetimes a shift from coral reefs dominated by corals to reef communities dominated by algae and filter feeders. Coral reefs are also not just corals, these rich, biodiverse marine ecosystems provide habitat and food to a wealth of other animals and plants which, in turn, provided millions of people with economic and social benefits. To enter a world of “low coral cover” could be one of the earliest and most profound consequences of global climate change due to human activities.
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