Coral reefs in peril in a record-breaking year.

Climate change and its impacts on coral reefs have reached unchartered territory.

Unprecedented early-summer heat stress and forecast of coral bleaching on the Great Barrier Reef, 2021-2022.

The Great Barrier Reef (GBR) is predicted to undergo its sixth mass coral bleaching event during the Southern Hemisphere summer of 2021-2022. Coral bleaching-level heat stress over the GBR is forecast to start earlier than any previous year in the satellite record (1985-present). The National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch (CRW) near real-time satellite-based heat stress products were used to investigate early-summer sea surface temperature (SST) and heat stress conditions on the GBR during late 2021. As of 14 December 2021, values of instantaneous heat stress (Coral Bleaching HotSpots) and accumulated heat stress over a 12-week running window (Degree Heating Weeks) on the GBR were unprecedented in the satellite record. Further, 89% of GBR satellite reef pixels for this date in 2021 had a positive seven-day SST trend of greater than 0.2 degrees Celsius/week. Background temperatures (the minimum temperature over the previous 29 days) were alarmingly high, with 87% of GBR reef pixels on 14 December 2021 being greater than the maximum SST over that same 29-day period for any year from 1985-2020. The GBR is starting the 2021-2022 summer season with more accumulated heat than ever before, which could have disastrous consequences for the health, recovery, and future of this critical reef system.

Climate refugia on the Great Barrier Reef fail when global warming exceeds 3°C.

Increases in the magnitude, frequency, and duration of warm seawater temperatures are causing mass coral mortality events across the globe. Although, even during the most extensive bleaching events, some reefs escape exposure to severe stress, constituting potential refugia. Here, we identify present-day climate refugia on the Great Barrier Reef (GBR) and project their persistence into the future. To do this, we apply semi-dynamic downscaling to an ensemble of climate projections released for the IPCC's recent sixth Assessment Report. We find that GBR locations experiencing the least thermal stress over the past 20 years have done so because of their oceanographic circumstance, which implies that longer-term persistence of climate refugia is feasible. Specifically, tidal and wind mixing of warm water away from the sea surface appears to provide relief from warming. However, on average this relative advantage only persists until global warming exceeds ~3°C.

The importance of 1.5°C warming for the Great Barrier Reef.

Tropical coral reefs are among the most sensitive ecosystems to climate change and will benefit from the more ambitious aims of the United Nations Framework Convention on Climate Change's Paris Agreement, which proposed to limit global warming to 1.5° rather than 2°C above pre-industrial levels. Only in the latest Intergovernmental Panel on Climate Change focussed assessment, the Coupled Model Intercomparison Project phase 6 (CMIP6), have climate models been used to investigate the 1.5° warming scenario directly. Here, we combine the most recent model updates from CMIP6 with a semi-dynamic downscaling to evaluate the difference between the 1.5 and 2°C global warming targets on coral thermal stress metrics for the Great Barrier Reef (GBR). By ~2080, severe bleaching events are expected to occur annually under intensifying emissions (shared socioeconomic pathway SSP5-8.5). Adherence to 2° warming (SSP1-2.6) halves this frequency but the main benefit of confining warming to 1.5° (SSP1-1.9) is that bleaching events are reduced further to 3 events per decade. Attaining low emissions of 1.5° is also paramount to prevent the mean magnitude of thermal stress from stabilizing close to a critical thermal threshold (8 Degree Heating Weeks). Thermal stress under the more pessimistic pathways SSP3-7.0 and SSP5-8.5 is three to fourfold higher than the present day, with grave implications for future reef ecosystem health. As global warming continues, our projections also indicate more regional warming in the central and southern GBR than the far north and northern GBR.

Cumulative bleaching undermines systemic resilience of the Great Barrier Reef.

Climate change and ENSO have triggered five mass coral bleaching events on Australia's Great Barrier Reef (GBR), three of which occurred in the last 5 years.1-5 Here, we explore the cumulative nature of recent impacts and how they fragment the reef's connectivity. The coverage and intensity of thermal stress have increased steadily over time. Cumulative bleaching in 2016, 2017, and 2020 is predicted to have reduced systemic larval supply by 26%, 50%, and 71%, respectively. Larval disruption is patchy and can guide interventions. The majority of severely bleached reefs (75%) are predicted to have experienced an 80%-100% loss of larval supply. Yet restoration would not be cost-effective in the 2% of such reefs (∼30) that still experience high larval supply. Managing such climate change impacts will benefit from emerging theory on the facilitation of genetic adaptation,6,7 which requires the existence of regions with predictably high or low thermal stress. We find that a third of reefs constitute warm spots that have consistently experienced bleaching stress. Moreover, 13% of the GBR are potential refugia that avoid significant warming more than expected by chance, with a modest proportion (14%) within highly protected areas. Coral connectivity is likely to become increasingly disrupted given the predicted escalation of climate-driven disturbances,8 but the existence of thermal refugia, potentially capable of delivering larvae to 58% of the GBR, may provide pockets of systemic resilience in the near-term. Theories of conservation planning for climate change will need to consider a shifting portfolio of thermal environments over time.

Refugia under threat: Mass bleaching of coral assemblages in high-latitude eastern Australia.

Environmental anomalies that trigger adverse physiological responses and mortality are occurring with increasing frequency due to climate change. At species' range peripheries, environmental anomalies are particularly concerning because species often exist at their environmental tolerance limits and may not be able to migrate to escape unfavourable conditions. Here, we investigated the bleaching response and mortality of 14 coral genera across high-latitude eastern Australia during a global heat stress event in 2016. We evaluated whether the severity of assemblage-scale and genus-level bleaching responses was associated with cumulative heat stress and/or local environmental history, including long-term mean temperatures during the hottest month of each year (SSTLTMAX ), and annual fluctuations in water temperature (SSTVAR ) and solar irradiance (PARZVAR ). The most severely-bleached genera included species that were either endemic to the region (Pocillopora aliciae) or rare in the tropics (e.g. Porites heronensis). Pocillopora spp., in particular, showed high rates of immediate mortality. Bleaching severity of Pocillopora was high where SSTLTMAX was low or PARZVAR was high, whereas bleaching severity of Porites was directly associated with cumulative heat stress. While many tropical Acropora species are extremely vulnerable to bleaching, the Acropora species common at high latitudes, such as A. glauca and A. solitaryensis, showed little incidence of bleaching and immediate mortality. Two other regionally-abundant genera, Goniastrea and Turbinaria, were also largely unaffected by the thermal anomaly. The severity of assemblage-scale bleaching responses was poorly explained by the environmental parameters we examined. Instead, the severity of assemblage-scale bleaching was associated with local differences in species abundance and taxon-specific bleaching responses. The marked taxonomic disparity in bleaching severity, coupled with high mortality of high-latitude endemics, point to climate-driven simplification of assemblage structures and progressive homogenisation of reef functions at these high-latitude locations.

Social-environmental drivers inform strategic management of coral reefs in the Anthropocene.

Without drastic efforts to reduce carbon emissions and mitigate globalized stressors, tropical coral reefs are in jeopardy. Strategic conservation and management requires identification of the environmental and socioeconomic factors driving the persistence of scleractinian coral assemblages-the foundation species of coral reef ecosystems. Here, we compiled coral abundance data from 2,584 Indo-Pacific reefs to evaluate the influence of 21 climate, social and environmental drivers on the ecology of reef coral assemblages. Higher abundances of framework-building corals were typically associated with: weaker thermal disturbances and longer intervals for potential recovery; slower human population growth; reduced access by human settlements and markets; and less nearby agriculture. We therefore propose a framework of three management strategies (protect, recover or transform) by considering: (1) if reefs were above or below a proposed threshold of >10% cover of the coral taxa important for structural complexity and carbonate production; and (2) reef exposure to severe thermal stress during the 2014-2017 global coral bleaching event. Our findings can guide urgent management efforts for coral reefs, by identifying key threats across multiple scales and strategic policy priorities that might sustain a network of functioning reefs in the Indo-Pacific to avoid ecosystem collapse.

High-resolution characterization of the abiotic environment and disturbance regimes on the Great Barrier Reef, 1985-2017.

This data compilation synthesizes 36 static environmental and spatial variables, and temporally explicit modeled estimates of three major disturbances to coral cover on the Great Barrier Reef (GBR): (1) coral bleaching, (2) tropical cyclones, and (3) outbreaks of the coral-eating crown-of-thorns starfish Acanthaster cf. solaris. Data are provided on a standardized grid (0.01° × 0.01° ~ 1 × 1 km) for reef locations along the GBR, containing 15,928 pixels and excluding the northernmost sections (<12° S) where empirical data were sparse. This compilation provides a consistent and high-resolution characterization of the abiotic environment and disturbance regimes for GBR reef locations at a fine spatial scale to be used in the development of complex ecosystem models. Static estimates of environmental variables (e.g., depth, bed shear stress, average temperature, temperature variation) originally developed by the Commonwealth of Australia's Environment Research Facility (CERF) Marine Biodiversity Hub were provided by Geoscience Australia. Annual (1985-2017) disturbance estimates were either interpolated from empirical data (A. cf. solaris), predicted from proxy indicators (e.g., degree heating weeks [DHW] as a proxy for bleaching severity), or explicitly modeled (e.g., wave height model for each cyclone). This data set synthesizes some of the most recent advances in remote sensing and modeling of environmental conditions on the GBR; yet it is not exhaustive and we highlight areas that should be expanded through future research. The characterization of abiotic and disturbance regimes presented here represent an essential tool for the development of complex regional scale models of the GBR; preventing redundancy between working groups and promoting collaboration, innovation, and consistency. When using the data set, we kindly request that you cite this article and/or the articles cited in the reference section, recognizing the work that went into compiling the data together and the original authors' willingness to make it publicly available.

Global warming transforms coral reef assemblages.

Global warming is rapidly emerging as a universal threat to ecological integrity and function, highlighting the urgent need for a better understanding of the impact of heat exposure on the resilience of ecosystems and the people who depend on them 1 . Here we show that in the aftermath of the record-breaking marine heatwave on the Great Barrier Reef in 2016 2 , corals began to die immediately on reefs where the accumulated heat exposure exceeded a critical threshold of degree heating weeks, which was 3-4 °C-weeks. After eight months, an exposure of 6 °C-weeks or more drove an unprecedented, regional-scale shift in the composition of coral assemblages, reflecting markedly divergent responses to heat stress by different taxa. Fast-growing staghorn and tabular corals suffered a catastrophic die-off, transforming the three-dimensionality and ecological functioning of 29% of the 3,863 reefs comprising the world's largest coral reef system. Our study bridges the gap between the theory and practice of assessing the risk of ecosystem collapse, under the emerging framework for the International Union for Conservation of Nature (IUCN) Red List of Ecosystems 3 , by rigorously defining both the initial and collapsed states, identifying the major driver of change, and establishing quantitative collapse thresholds. The increasing prevalence of post-bleaching mass mortality of corals represents a radical shift in the disturbance regimes of tropical reefs, both adding to and far exceeding the influence of recurrent cyclones and other local pulse events, presenting a fundamental challenge to the long-term future of these iconic ecosystems.

Global warming and recurrent mass bleaching of corals.

During 2015-2016, record temperatures triggered a pan-tropical episode of coral bleaching, the third global-scale event since mass bleaching was first documented in the 1980s. Here we examine how and why the severity of recurrent major bleaching events has varied at multiple scales, using aerial and underwater surveys of Australian reefs combined with satellite-derived sea surface temperatures. The distinctive geographic footprints of recurrent bleaching on the Great Barrier Reef in 1998, 2002 and 2016 were determined by the spatial pattern of sea temperatures in each year. Water quality and fishing pressure had minimal effect on the unprecedented bleaching in 2016, suggesting that local protection of reefs affords little or no resistance to extreme heat. Similarly, past exposure to bleaching in 1998 and 2002 did not lessen the severity of bleaching in 2016. Consequently, immediate global action to curb future warming is essential to secure a future for coral reefs.

Seasonal and spatial heterogeneity of recent sea surface temperature trends in the Caribbean Sea and southeast Gulf of Mexico.

Recent changes in ocean temperature have impacted marine ecosystem function globally. Nevertheless, the responses have depended upon the rate of change of temperature and the season when the changes occur, which are spatially variable. A rigorous statistical analysis of sea surface temperature observations over 25 years was used to examine spatial variability in overall and seasonal temperature trends within the wider Caribbean. The basin has experienced high spatial variability in rates of change of temperature. Most of the warming has been due to increases in summer rather than winter temperatures. However, warming was faster in winter in the Loop Current area and the south-eastern Caribbean, where the annual temperature ranges have contracted. Waters off Florida, Cuba and the Bahamas had a tendency towards cooling in winter, increasing the amplitude of annual temperature ranges. These detailed patterns can be used to elucidate ecological responses to climatic change in the region.

Reserve design for uncertain responses of coral reefs to climate change.

Rising sea temperatures cause mass coral bleaching and threaten reefs worldwide. We show how maps of variations in thermal stress can be used to help manage reefs for climate change. We map proxies of chronic and acute thermal stress and develop evidence-based hypotheses for the future response of corals to each stress regime. We then incorporate spatially realistic predictions of larval connectivity among reefs of the Bahamas and apply novel reserve design algorithms to create reserve networks for a changing climate. We show that scales of larval dispersal are large enough to connect reefs from desirable thermal stress regimes into a reserve network. Critically, we find that reserve designs differ according to the anticipated scope for phenotypic and genetic adaptation in corals, which remains uncertain. Attempts to provide a complete reserve design that hedged against different evolutionary outcomes achieved limited success, which emphasises the importance of considering the scope for adaptation explicitly. Nonetheless, 15% of reserve locations were selected under all evolutionary scenarios, making them a high priority for early designation. Our approach allows new insights into coral holobiont adaptation to be integrated directly into an adaptive approach to management.

Summer hot snaps and winter conditions: modelling white syndrome outbreaks on Great Barrier Reef corals.

Coral reefs are under increasing pressure in a changing climate, one such threat being more frequent and destructive outbreaks of coral diseases. Thermal stress from rising temperatures has been implicated as a causal factor in disease outbreaks observed on the Great Barrier Reef, Australia, and elsewhere in the world. Here, we examine seasonal effects of satellite-derived temperature on the abundance of coral diseases known as white syndromes on the Great Barrier Reef, considering both warm stress during summer and deviations from mean temperatures during the preceding winter. We found a high correlation (r(2) = 0.953) between summer warm thermal anomalies (Hot Snap) and disease abundance during outbreak events. Inclusion of thermal conditions during the preceding winter revealed that a significant reduction in disease outbreaks occurred following especially cold winters (Cold Snap), potentially related to a reduction in pathogen loading. Furthermore, mild winters (i.e., neither excessively cool nor warm) frequently preceded disease outbreaks. In contrast, disease outbreaks did not typically occur following warm winters, potentially because of increased disease resistance of the coral host. Understanding the balance between the effects of warm and cold winters on disease outbreak will be important in a warming climate. Combining the influence of winter and summer thermal effects resulted in an algorithm that yields both a Seasonal Outlook of disease risk at the conclusion of winter and near real-time monitoring of Outbreak Risk during summer. This satellite-derived system can provide coral reef managers with an assessment of risk three-to-six months in advance of the summer season that can then be refined using near-real-time summer observations. This system can enhance the capacity of managers to prepare for and respond to possible disease outbreaks and focus research efforts to increase understanding of environmental impacts on coral disease in this era of rapidly changing climate.

Global assessment of coral bleaching and required rates of adaptation under climate change.

Elevated ocean temperatures can cause coral bleaching, the loss of colour from reef-building corals because of a breakdown of the symbiosis with the dinoflagellate Symbiodinium. Recent studies have warned that global climate change could increase the frequency of coral bleaching and threaten the long-term viability of coral reefs. These assertions are based on projecting the coarse output from atmosphere-ocean general circulation models (GCMs) to the local conditions around representative coral reefs. Here, we conduct the first comprehensive global assessment of coral bleaching under climate change by adapting the NOAA Coral Reef Watch bleaching prediction method to the output of a low- and high-climate sensitivity GCM. First, we develop and test algorithms for predicting mass coral bleaching with GCM-resolution sea surface temperatures for thousands of coral reefs, using a global coral reef map and 1985-2002 bleaching prediction data. We then use the algorithms to determine the frequency of coral bleaching and required thermal adaptation by corals and their endosymbionts under two different emissions scenarios. The results indicate that bleaching could become an annual or biannual event for the vast majority of the world's coral reefs in the next 30-50 years without an increase in thermal tolerance of 0.2-1.0°C per decade. The geographic variability in required thermal adaptation found in each model and emissions scenario suggests that coral reefs in some regions, like Micronesia and western Polynesia, may be particularly vulnerable to climate change. Advances in modelling and monitoring will refine the forecast for individual reefs, but this assessment concludes that the global prognosis is unlikely to change without an accelerated effort to stabilize atmospheric greenhouse gas concentrations.

Remote sensing of coral reefs and their physical environment.

There has been a vast improvement in access to remotely sensed data in just a few recent years. This revolution of information is the result of heavy investment in new technology by governments and industry, rapid developments in computing power and storage, and easy dissemination of data over the internet. Today, remotely sensed data are available to virtually anyone with a desktop computer. Here, we review the status of one of the most popular areas of marine remote sensing research: coral reefs. Previous reviews have focused on the ability of remote sensing to map the structure and habitat composition of coral reefs, but have neglected to consider the physical environment in which reefs occur. We provide a holistic review of what can, might, and cannot be mapped using remote sensing at this time. We cover aspects of reef structure and health but also discuss the diversity of physical environmental data such as temperature, winds, solar radiation and water quality. There have been numerous recent advances in the remote sensing of reefs and we hope that this paper enhances awareness of the diverse data sources available, and helps practitioners identify realistic objectives for remote sensing in coral reef areas.