Coral Reef Futures Research Articles

Systematic review of the uncertainty of coral reef futures under climate change

Climate change impact syntheses, such as those by the Intergovernmental Panel on Climate Change, consistently assert that limiting global warming to 1.5 °C is unlikely to safeguard most of the world’s coral reefs. This prognosis is primarily based on a small subset of available models that apply similar ‘excess heat’ threshold methodologies. Our systematic review of 79 articles projecting coral reef responses to climate change revealed five main methods. ‘Excess heat’ models constituted one third (32%) of all studies but attracted a disproportionate share (68%) of citations in the field. Most methods relied on deterministic cause-and-effect rules rather than probabilistic relationships, impeding the field’s ability to estimate uncertainty. To synthesize the available projections, we aimed to identify models with comparable outputs. However, divergent choices in model outputs and scenarios limited the analysis to a fraction of available studies. We found substantial discrepancies in the projected impacts, indicating that the subset of articles serving as a basis for climate change syntheses may project more severe consequences than other studies and methodologies. Drawing on insights from other fields, we propose methods to incorporate uncertainty into deterministic modeling approaches and propose a multi-model ensemble approach to generating probabilistic projections for coral reef futures.

Coral reef state influences resilience to acute climate‐mediated disturbances

AbstractAimThe aim of this study was to understand the interplay between resistance and recovery on coral reefs, and to investigate dependence on pre‐ and post‐disturbance states, to inform generalizable reef resilience theory across large spatial and temporal scales.LocationTropical coral reefs globally.Time Period1966–2017.Major Taxa StudiedScleractinian hard corals.MethodsWe conducted a literature search to compile a global data set of total coral cover before and after acute storms, temperature stress and coastal run‐off from flooding events. We used meta‐regression to identify variables that explained significant variation in disturbance impact, including disturbance type, year, depth and pre‐disturbance coral cover. We further investigated the influence of these same variables, as well as post‐disturbance coral cover and disturbance impact, on recovery rate. We examined the shape of recovery, assigning qualitatively distinct, ecologically relevant, population growth trajectories: linear, logistic, logarithmic (decelerating) and a second‐order quadratic (accelerating).ResultsWe analysed 427 disturbance impacts and 117 recovery trajectories. Accelerating and logistic were the most common recovery shapes, underscoring non‐linearities and recovery lags. A complex but meaningful relationship between disturbance impact, the state of a reef pre‐ and post‐disturbance, and recovery rate was identified. Fastest recovery rates were predicted for intermediate to large disturbance impacts, but a decline in this rate was predicted when more than ~75% of pre‐disturbance cover was lost. We identified a shifting baseline, with declines in both pre‐ and post‐disturbance coral cover over the 50‐year study period.Main ConclusionsWe break down the complexities of coral resilience, showing interplay between resistance and recovery, as well as dependence on both pre‐ and post‐disturbance states, alongside documenting a chronic decline in these states. This has implications for predicting coral reef futures and implementing actions to enhance resilience.

Coral conservation requires ecological climate‐change vulnerability assessments

Climate‐driven changes to environmental conditions are driving severe declines of coral reef ecosystems. Current climate vulnerability estimates commonly focus on ocean warming and typically overlook ecological responses or use broad proxies to represent responses, leading to management decisions based on incomplete views of coral reef futures. We explore four underdeveloped aspects of climate vulnerability assessments and make the following recommendations: (1) use climate projections based on changes in global warming as future scenarios in place of the more common emissions scenarios; (2) include available high‐resolution projections for climate variables in addition to thermal stress; (3) combine projected climate stressors accounting for uncertainty in future outcomes; and (4) quantitatively assess historical and project future ecological sensitivity and adaptive capacity of corals to multiple stressors. We demonstrate how this framework can be used to reduce uncertainty in projected climate vulnerability and facilitate targeted investment in managing reefs most likely to endure climatic disturbances.

The past century of coral bleaching in the Saudi Arabian central Red Sea.

Accurate knowledge of the spatial and temporal patterns of coral bleaching is essential both for understanding how coral reef ecosystems are changing today and forecasting their future states. Yet, in many regions of the world, the history of bleaching is poorly known, especially prior to the late 20th century. Here, I use the information preserved within skeleton cores of long-lived Porites corals to reconstruct the past century of bleaching events in the Saudi Arabian central Red Sea. In these cores, skeletal “stress bands”—indicative of past bleaching—captured known bleaching events that occurred in 1998 and 2010, but also revealed evidence of previously unknown bleaching events in 1931, 1978, and 1982. However, these earlier events affected a significantly lesser proportion of corals than 1998 and 2010. Therefore, coral bleaching may have occurred in the central Red Sea earlier than previously recognized, but the frequency and severity of bleaching events since 1998 on nearshore reefs is unprecedented over the past century. Conversely, corals living on mid- to outer-shelf reefs have not been equally susceptible to bleaching as their nearshore counterparts, which was evident in that stress bands were five times more prevalent nearshore. Whether this pattern of susceptible nearshore reefs and resistant outer-shelf reefs continues in the future remains a key question in forecasting coral reef futures in this region.

Predicting coral-reef futures from El Ni\xf1o and Pacific Decadal Oscillation events

El Niño Southern Oscillation (ENSO) events modulate oceanographic processes that control temperature and productivity in tropical waters, yet potential interactions with low frequency climate variability, such as the Pacific Decadal Oscillation (PDO), are poorly understood. We show that ENSO and PDO together predicted (i) maximum sea-surface temperatures (SST), which were associated with coral bleaching and declines in coral cover, and (ii) maximum chlorophyll-a concentrations, which were associated with high densities of coral-predatory Acanthaster starfish, across the tropical north Pacific Ocean since 1980. Asynchrony between the positive PDO and negative ENSO (i.e., La Niña) was associated with peaks in annual SST. By contrast, synchrony between the positive PDO and positive ENSO (i.e., El Niño) was associated with peaks in chlorophyll-a. Both conditions led to ecological disturbances and significant loss of coral cover, however, spatial models revealed where impacts to reefs were expected under varying climate scenarios. The 2015/17 ENSO event was coupled with a positive PDO and resulted in high SST and Acanthaster abundances in eastern Micronesia, while positive coral growth occurred in western Micronesia. Our novel approach for forecasting coral growth into the future may be applicable to other oceanic regions with differing oceanographic modulators.

Comparative thermal performance of the reef-building coral Orbicella franksi at its latitudinal range limits

Temperature drives biological responses that scale from the cellular to ecosystem levels and thermal sensitivity will shape organismal functions and population dynamics as the world warms. Reef-building corals are sensitive to temperature due to their endosymbiotic relationship with single-celled dinoflagellates, with mass mortality events increasing in frequency and magnitude. The purpose of this study was to quantify the thermal sensitivity of important physiological functions of a Caribbean reef-building coral, Orbicella franksi, through the measurement of thermal performance curves (TPCs). We compared TPC metrics (thermal optimum, critical maximum, activation energy, deactivation energy, and rate at a standardized temperature) between two populations at the northern and southern extents of the geographic range of O. franksi. We further compared essential coral organismal processes (gross photosynthesis, respiration, and calcification) within a site to determine which function is most sensitive to thermal stress using a hierarchical Bayesian-modeling approach. We found evidence for differences in thermal performance, which could be due to thermal adaptation or acclimatization, with higher TPC metrics (thermal optimum and critical maximum) in warmer Panama, compared to cooler Bermuda. We also documented the hierarchy in thermal sensitivity of essential organismal functions within a population: respiration was less sensitive than photosynthesis, which was less sensitive than calcification. Understanding thermal performance of corals is essential for projecting coral reef futures, given that key biological functions necessary to sustain coral reef ecosystems are thermally mediated.

Rethinking coral reef functional futures

Abstract Tropical coral reefs currently face an unprecedented restructuring since their extant form and function emerged ~24 million years ago in the early Neogene. They have entered the Anthropocene—an epoch where humans have become the dominant force of planetary change. Human impacts on and interactions with coral reefs are escalating across multiple trophic levels and scales, but we have a rudimentary understanding of what this means for their functional ecology. The overall goal of this special feature is to unpack what the Anthropocene means for the functional ecology of coral reefs, laying the foundations for new approaches and research directions in coral reef science. The collection describes the functional changes and novel dynamics that characterise Anthropocene reefs, from variations in their taxonomy and geology through to the resulting shifts in ecosystem services they provide to humanity. Common changes to coral reefs are occurring that are challenging their historical functional role. These include reductions in benthic calcifiers and declining carbonate production, and benthic assemblage shifts leading to a loss of structural complexity and flattening of reef seascapes. As reefs as we know them are lost from some locations, range extensions and the “tropicalisation” of temperate locations present novel ecosystem configurations that are challenging ecological paradigms and our historical approach to ecosystem management. Hindering our progress, however, is a “functionality crisis.” Coral reef functional ecology to date has lacked a clear and universal definition of the term “function,” and many assumed links between taxa and reef processes lack empirical evidence. Moving forward, we must establish causal links between functional traits, the species that possess them, and specific ecosystem processes if we are to successfully manage Anthropocene reefs. The functional space coral reefs occupy has arguably widened, presenting ethical challenges surrounding the increasingly interventionist management practices required to achieve particular functional endpoints. For us to steer coral reefs towards a desirable functional future will require a more mechanistic understanding between ecosystem attributes and the provision of services, acknowledging that such services are coproduced by the ecosystem and society. Ultimately, this era in coral reef ecology requires a new approach to coral reef science, one that addresses the complex socio‐ecological nature of coral reefs. These works outline a path ahead for defining and studying the functional ecology of coral reefs, drive debate as to what we want their functional future to look like and call for ecosystem function to be at the heart of managing coral reef futures during this period of rapid transition.

Skeletal records of bleaching reveal different thermal thresholds of Pacific coral reef assemblages

Ocean warming is negatively impacting coral reef ecosystems and considerable effort is currently invested in projecting coral reef futures under 21st century climate change. A limiting factor in these projections is lack of quantitative data on the thermal thresholds of different reef communities, due in large part to spatial and temporal gaps in bleaching observations. Here we apply a coral bleaching proxy, skeletal stress bands, to reconstruct the history of bleaching on eight coral reefs in the central equatorial Pacific (CEP) and use this information to constrain the thermal thresholds of their coral communities. First, three genera of massive corals collected on both Pacific and Caribbean reefs are used to derive a calibration between the proportion of corals that form stress bands during a bleaching event, and the total observed bleaching incidence in the community of mixed coral taxa. The correlation is highly significant, indicating that stress bands in massive corals reflect community-level bleaching severity (R2 = 0.945, p < 0.001). We applied the calibration to stress band records from eight Pacific reefs, reconstructing their bleaching histories over the period 1982 to 2015. A percentile-based method of estimating thermal stress (Degree Heating Weeks) for CEP reefs was developed and applied. Comparing the level of thermal stress experienced by each coral community during each event with the reconstructed bleaching response, we characterized the thermal sensitivities of each reef community and quantified the thermal threshold (b½) at which 50% of the coral community bleached. Our analysis reveals a unique non-linear thermal response curve for each reef. The most thermally tolerant reefs in the study (Jarvis and Kanton Islands) experienced 50% bleaching at seven to nine times more thermal stress than did the least resistant reef in the study (Maiana Island). An exploration of the potential drivers of thermal tolerance revealed a strong correlation between b½ and the history of thermal stress events in each reef system. Thermal tolerance was also correlated with concentrations of dissolved inorganic nitrate in the water column and with estimates of coral energetic reserve.

Acclimatization of massive reef-building corals to consecutive heatwaves.

Reef-building corals typically live close to the upper limits of their thermal tolerance and even small increases in summer water temperatures can lead to bleaching and mortality. Projections of coral reef futures based on forecasts of ocean temperatures indicate that by the end of this century, corals will experience their current thermal thresholds annually, which would lead to the widespread devastation of coral reef ecosystems. Here, we use skeletal cores of long-lived Porites corals collected from 14 reefs across the northern Great Barrier Reef, the Coral Sea, and New Caledonia to evaluate changes in their sensitivity to heat stress since 1815. High-density ‘stress bands’—indicative of past bleaching—first appear during a strong pre-industrial El Niño event in 1877 but become significantly more frequent in the late twentieth and early twenty-first centuries in accordance with rising temperatures from anthropogenic global warming. However, the proportion of cores with stress bands declines following successive bleaching events in the twenty-first century despite increasing exposure to heat stress. Our findings demonstrate an increase in the thermal tolerance of reef-building corals and offer a glimmer of hope that at least some coral species can acclimatize fast enough to keep pace with global warming.

The Origin and Role of Organic Matrix in Coral Calcification: Insights From Comparing Coral Skeleton and Abiogenic Aragonite

Understanding the mechanisms of coral calcification is critical for accurately projecting coral reef futures under ocean acidification and warming. Recent suggestions that calcification is primarily controlled by organic molecules and the biological activity of the coral polyp imply that ocean acidification may not affect skeletal accretion. The basis for these suggestions relies heavily on correlating the presence of organic matter with the orientation and disorder of aragonite crystals in the skeleton, carrying the assumptions that organic matter observed in the skeleton was produced by the polyp to control calcification. Here we use Raman spectroscopy to test whether there are differences in organic matter content between coral skeleton and abiogenic aragonites precipitated from seawater, both before and after thermal annealing (heating). We measured the background fluorescence and intensity of C-H bonding signals in the Raman spectra, which are commonly attributed to coral polyp-derived skeletal organic matrix (SOM) and have been used to map its distribution. Surprisingly, we found no differences in either fluorescence or C-H bonding between abiogenic aragonite and coral skeleton. Annealing reduced the molecular disorder in coral skeleton, potentially due to removal of organic matter, but the same effect was also observed in the abiogenic aragonites. The presence of organic molecules in the abiogenic aragonites is further supported by measurements of N content and δ15N. Together, our data suggest that some of what has been interpreted in previous studies as polyp-derived SOM may actually be seawater-sourced organic matter or some other signal not unique to biogenic aragonite. Finally, we create a high-resolution Raman map of a Pocillopora skeleton to demonstrate how patterns of fluorescence and elevated calcifying fluid aragonite saturation state along centers of calcification are consistent with both biological and physico-chemical controls. Our aim is to advance discussion on biological mediation of calcification and the implications for coral resilience in a high-CO2 world.

Local-scale projections of coral reef futures and implications of the Paris Agreement.

Increasingly frequent severe coral bleaching is among the greatest threats to coral reefs posed by climate change. Global climate models (GCMs) project great spatial variation in the timing of annual severe bleaching (ASB) conditions; a point at which reefs are certain to change and recovery will be limited. However, previous model-resolution projections (~1 × 1°) are too coarse to inform conservation planning. To meet the need for higher-resolution projections, we generated statistically downscaled projections (4-km resolution) for all coral reefs; these projections reveal high local-scale variation in ASB. Timing of ASB varies >10 years in 71 of the 87 countries and territories with >500 km2 of reef area. Emissions scenario RCP4.5 represents lower emissions mid-century than will eventuate if pledges made following the 2015 Paris Climate Change Conference (COP21) become reality. These pledges do little to provide reefs with more time to adapt and acclimate prior to severe bleaching conditions occurring annually. RCP4.5 adds 11 years to the global average ASB timing when compared to RCP8.5; however, >75% of reefs still experience ASB before 2070 under RCP4.5. Coral reef futures clearly vary greatly among and within countries, indicating the projections warrant consideration in most reef areas during conservation and management planning.

Guiding coral reef futures in the Anthropocene

Anthropogenic changes to the Earth now rival those caused by the forces of nature and have shepherded us into a new planetary epoch – the Anthropocene. Such changes include profound and often unexpected alterations to coral reef ecosystems and the services they provide to human societies. Ensuring that reefs and their services endure during the Anthropocene will require that key drivers of coral reef change – fishing, water quality, and anthropogenic climate change – stay within acceptable levels or “safe operating spaces”. The capacity to remain within these safe boundaries hinges on understanding the local, but also the increasingly global and cross‐scale, socioeconomic causes of these human drivers of change. Consequently, local and regional management efforts that are successful in the short term may ultimately fail if current decision making and institution‐building around coral reef systems remains fragmented, poorly coordinated, and unable to keep pace with the escalating speed of social, technological, and ecological change.

Incorporating Uncertainty in Predicting the Future Response of Coral Reefs to Climate Change

Coral reefs are considered one of the ecosystems most vulnerable to ongoing global climate change. However, geographic and taxonomic responses to climate change are highly variable, and fundamental aspects of key research approaches remain unresolved, leaving substantial uncertainty in our ability to predict the future of coral reefs. I review the ecological and evolutionary response of coral reefs to climate change in a broad temporal context, primarily focusing on tropical reef corals. I show critical gaps in our understanding that impede accurate prediction of future responses. These gaps include the response of past reefs to global change, the interpretation of coral response to thermal stress and ocean acidification, how corals and other reef organisms might respond evolutionarily, and our approach to evaluating response to climate in the context of multiple stressors. Reducing uncertainty by filling these gaps and by incorporating variation in geographic and taxonomic response will substantially improve our ability to model coral reef futures and manage coral reefs.

Tectonic subsidence provides insight into possible coral reef futures under rapid sea-level rise

Sea-level rise will change environmental conditions on coral reef flats, which comprise extensive habitats in shallow tropical seas and support a wealth of ecosystem services. Rapid relative sea-level rise of 0.6 m over a relatively pristine coral reef in Solomon Islands, caused by a subduction earthquake in April 2007, generated a unique opportunity to examine in situ coral reef response to relative sea-level rise of the magnitude (but not the rate) anticipated by 2100. Extent of live coral was measured from satellite imagery in 2003, 2006, 2009 and 2012. Ecological data were obtained from microatolls and ecological surveys in May 2013. The reef was sampled at 12 locations where dense live hard coral remained absent, remained present or changed from absent to present following subsidence. Ecological data (substratum depth, live coral canopy depth, coral canopy height, substratum suitability, recruitment, diversity and Acropora presence) were measured at each location to identify factors associated with coral response to relative sea-level rise. Vertical and horizontal proliferation of coral occurred following subsidence. Lateral expansion of live coral, accomplished primarily by branching Acropora spp., resulted in lower diversity in regions which changed composition from pavement to dense live coral following subsidence. Of the ecological factors measured, biotic factors were more influential than abiotic factors; species identity was the most important factor in determining which regions of the reef responded to rapid sea-level rise. On relatively pristine reef flats under present climatic conditions, rapid relative sea-level rise generated an opportunity for hard coral to proliferate. However, the species assemblage of the existing reef was important in determining response to sea-level change, by providing previously bare substrate with a source of new coral colonies. Degraded reefs with altered species composition and slower coral growth rates may be less able to respond to climate change-induced sea-level changes.

Predicting coral reef futures under climate change

Predicting coral reef futures under climate change

The Future of Coral Reefs

In their Review “Projecting coral reef futures under global warming and ocean acidification” (22 July, p. [418][1]), J. M. Pandolfi et al. argue that the threat of climate change for corals has been exaggerated by pointing to variability in coral heat tolerance, adaptation potential, and the

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Projecting Coral Reef Futures Under Global Warming and Ocean Acidification

Many physiological responses in present-day coral reefs to climate change are interpreted as consistent with the imminent disappearance of modern reefs globally because of annual mass bleaching events, carbonate dissolution, and insufficient time for substantial evolutionary responses. Emerging evidence for variability in the coral calcification response to acidification, geographical variation in bleaching susceptibility and recovery, responses to past climate change, and potential rates of adaptation to rapid warming supports an alternative scenario in which reef degradation occurs with greater temporal and spatial heterogeneity than current projections suggest. Reducing uncertainty in projecting coral reef futures requires improved understanding of past responses to rapid climate change; physiological responses to interacting factors, such as temperature, acidification, and nutrients; and the costs and constraints imposed by acclimation and adaptation.

Coral Bleaching in the Southern Seychelles During the 1997–1998 Indian Ocean Warm Event

Coral bleaching shows complex spatial-temporal dynamics at several scales. Recent developments in ocean surface remote sensing technologies allow for both a better appreciation of these dynamics and an opportunity to place more local studies of coral bleaching and bleaching-related coral mortality into wider oceanographic and biogeographic contexts. Coral bleaching is described at four coral reefs in the southern Seychelles (sea area 6–10°S 45–54°E) during March–May 1998. Bleaching intensity varied between locations, between environments at the within-reef scale, and between coral growth forms. These data are compared with bleaching reports and sea surface temperature statistics for eight further stations in the western Indian Ocean to establish a link between bleaching and the unprecedented warming of the Indian Ocean during 1997/98. Implications for long-term reef history, and coral reef futures, in the western Indian Ocean are discussed.