Severe and widespread coral reef damage during the 2014-2017 Global Coral Bleaching Event

Domain: neuroscience
Published: 2026-02-10
Editorial outcome: accept

Abstract
Ocean warming is increasing the frequency, extent, and severity of tropical coral bleaching and mortality. During 2014–2017, marine heatwaves caused the Third Global Coral Bleaching Event. We analyze data from 15,066 reef surveys globally during 2014–2017. Across all surveyed reefs, 80% and 35% experienced moderate or greater (affecting >10% of corals) bleaching and mortality, respectively. We assess the global extent of coral bleaching and mortality by applying bleaching response curves calibrated from surveyed reefs to predict bleaching globally, based on comprehensive remote-sensing of heat stress. These models predict that 51% and 15% of the world’s coral reefs suffered moderate or greater bleaching and mortality, respectively, during one or multiple years, surpassing damage from any prior global coral bleaching event. Our findings demonstrate that the impacts of ocean warming on coral reefs are accelerating, with the near certainty that ongoing warming will cause large-scale, possibly irreversible, degradation of these essential ecosystems. With heat stress levels during this event surpassing those observed previously, the National Oceanic and Atmospheric Administration developed more extreme Bleaching Alert levels that are now being used during the ongoing Fourth Global Coral Bleaching Event.

Introduction
Before the 1980s, mass coral bleaching and mortality events due to heat stress were rare1. In the last four decades, these events have become increasingly frequent and severe. Ocean warming is now the foremost threat to coral reefs worldwide2–7, and recurrent strong marine heatwaves are causing mass bleaching of corals on regional and global scales2,3,8,9. Coral bleaching occurs when the relationship between corals and their photosynthetic symbionts breaks down10. Bleached corals are physiologically damaged, nutritionally compromised, and may die if the bleaching is severe or prolonged. From June 2014 to May 2017 (hereafter referred to as 2014–17), reefs around the world experienced the third global-scale coral bleaching event11–13. This event was, at that time, the most severe global heat stress event recorded on coral reef ecosystems2,8,11,14, surpassing the two prior global coral bleaching events recorded in 199815 and 20103,16. Moreover, 2014–17 was the first record of a global coral bleaching event lasting much beyond a single year2,8,11,14. Specifically, the event spanned 3 years, with bleaching at some locations continuing after the global event concluded17–19. Numerous studies have revealed how this event has impacted coral reefs locally at sites around the globe, including the most severe impacts on record in many locations (see 20 and references therein).

Here, we analyze the heat stress affecting coral reefs during 2014–17 and the resultant bleaching and mortality to derive statistical relationships between remotely-sensed heat stress and on-site surveys of coral bleaching and mortality at a global scale. We then use these relationships to estimate the global impact of this mass coral bleaching event, accounting for the actual distribution of heat stress across the world’s reef areas. We find substantial variation in the temperature sensitivity of bleaching and survival among years and basins. Overall, we estimate that over half the world’s reefs experienced moderate or greater bleaching, and 15% experienced moderate or greater mortality. The estimated levels of accumulated heat stress, bleaching, and mortality during this event exceeded the severity from all prior global coral bleaching events.

Results
Three years of heat stress on reefs
Satellite remote sensing was used to identify cumulative heat stress on coral reefs, using the Degree Heating Week (DHW) product from the National Oceanic and Atmospheric Administration’s Coral Reef Watch (CRW). The onset of coral bleaching and mortality was previously associated with exposure thresholds of DHW≥4°C-weeks (Alert Level 1) and ≥8°C-weeks (Alert Level 2), respectively5,8. As a result of record heat stress seen during the Third Global Coral Bleaching Event, CRW established new Alert Levels 3–5 that correspond to the risk of increasingly severe bleaching and mortality of corals across reefs21 (Supplementary Table 1). This new set of Alert Levels is now being used during the ongoing Fourth Global Coral Bleaching Event22.

During the Third Global Coral Bleaching Event in 2014–17, 65.8% of ~5×5 km2 satellite remote sensing pixels containing coral reefs (n=53,997) experienced heat stress classified as sufficient to cause moderate or greater bleaching (affecting >10% of corals) (DHW≥4°C-weeks, Alert Level 1 or higher) (Figs. 1a, 2, Supplementary Fig. 1a, Supplementary Table 223,24). This compares with 37.2% in the Second Global Coral Bleaching Event (Fig. 1b) and 20.9% in the First Global Coral Bleaching Event (Fig. 1c). Among all reef-containing pixels, 23.6% also reached or exceeded the higher threshold of DHW≥8°C-weeks (corresponding with Alert Level 2 or higher), classified as sufficient to cause severe bleaching (affecting >50% of corals) and moderate or greater mortality (affecting >10% of corals), during the Third Global Coral Bleaching Event, compared with 9.5 and 2.5% in the Second and First Global Coral Bleaching Events, respectively. Some 5.0% of reef-containing pixels reached or exceeded the new Alert Level 3 (DHW≥12°C-weeks), compared with 2.1% in the Second Global Coral Bleaching Event and 0.5% in the First Global Coral Bleaching Event, indicating a risk of multi-species mortality. In all three global coral bleaching events, less than 1% of reef-containing pixels reached or exceeded the new Alert Level 4 (DHW≥16°C-weeks), indicating a risk of severe, multi-species mortality. Few pixels (0.1%) reached the new Alert Level 5 (DHW≥20°C-weeks), indicating a risk of near complete mortality.

Repeated heat-stress exposure was widespread during the Third Global Coral Bleaching Event. Half (47.2%) of reef-containing pixels that reached Alert Level 1 and one-fifth (20.8%) of reef-containing pixels that reached Alert Level 2 did so at least twice during 2014–17.

Heat stress on reefs during the Third Global Coral Bleaching Event (Fig. 1a) was higher than that seen in either the Second (Fig. 1b) or the First Global Coral Bleaching Events (Fig. 1c).

Global coral bleaching events have followed a consistent spatial progression that was observed anecdotally during all three global coral bleaching events10,11 and is now discernable analytically14. Driven by ocean heating due to El Niño and its teleconnected impacts25, the highest sea surface temperature anomalies are typically first seen in the equatorial eastern Pacific Ocean, then anomalies appear sequentially across the equatorial central Pacific Ocean, in the western South Pacific Ocean, southern Indian Ocean, northern Indian Ocean, Middle Eastern seas and Southeast Asia, then the Caribbean Sea and western Atlantic Ocean (Supplementary Fig. 1c–f, Supplementary Video 1). During some past global coral bleaching events, as well as during the Third Global Coral Bleaching Event, bleaching was seen in additional areas such as Hawai’i26,27, Micronesia28,29, the Marianas17, and the Great Barrier Reef30.

Bleaching and mortality
In response to the widespread and severe heat stress, we assembled coral bleaching and mortality data from 15,066 in-water and aerial surveys from teams around the globe—inclusive of observations from a pre-existing database covering the 1960s through 201031,32 (Supplementary Data 1, Supplementary Fig. 1b). During the Third Global Coral Bleaching Event, 80% of surveys reported a moderate or greater level of bleaching (affecting >10% of corals—either as a percentage of the total coral cover or a percentage of colonies, see Methods), while 31% of all surveys reported that the bleaching was severe (affecting >50% of corals). Additionally, 35% of surveys reported moderate or greater (>10%) recent coral mortality, with 6% of surveys reporting severe mortality (>50%).

Our global analyses showed a strong positive relationship of bleaching and mortality with satellite-derived, accumulated heat stress (DHW), while the rate of biological response to heat stress varied among ocean basins and bleaching years (June-to-May periods). Comparing observations of moderate or greater reef-level bleaching (Fig. 2, Supplementary Fig. 3) across all basins, bleaching sensitivity to heat stress was generally highest in 2015–16 and lowest in 2016–17 (Fig. 2a), suggesting acclimatization or adaptation, due to selective mortality of vulnerable host genotypes and changes in the genetic makeup of their endosymbiotic communities33, loss of heat-sensitive phenotypes34 or compositional changes due to selective mortality of particular genotypes or taxa35,36. Across all years, bleaching sensitivity was highest in the Caribbean/Atlantic basin and lowest in the Asia-Pacific (Fig. 2b).

The models that best fit the bleaching data included interactions between DHW and both regions and years (Supplementary Tables 4, 5). The best-fit model for moderate or greater bleaching (Supplementary Fig. 3) had a prediction accuracy of 83% and showed interactions between years and basins. Corals in all years and basins showed moderate to high probabilities of bleaching in response to heat stress at levels above the 4°C-weeks threshold normally considered predictive of moderate bleaching24. The Caribbean/Atlantic showed greater bleaching sensitivity than the Asia-Pacific in 2014–15 and 2015–16 but not in 2016–17. Additionally, the Indian Ocean/Middle East also showed greater bleaching sensitivity than the Asia-Pacific in 2016–2017 but was intermediate to and not clearly statistically distinguishable from the other regions’ thresholds in the first two bleaching years (Supplementary Fig. 3).

The best-fitting model for severe bleaching had a predictive accuracy of 88%; and spatiotemporal patterns of severe bleaching (Supplementary Fig. 4) were similar to those seen in moderate bleaching, including greater sensitivity in 2015–16 than 2016–17 in the Asia-Pacific (Supplementary Fig. 4a), and reduced sensitivity in 2015–2016 and in 2016–17 than 2014–2015 in the Caribbean/Atlantic (Supplementary Fig. 4b). However, estimated thresholds for severe bleaching (Supplementary Fig. 4) had greater uncertainty than those for moderate bleaching (Supplementary Fig. 3), likely due to the much smaller number of observations of severe bleaching, and an uneven spatial distribution of severe bleaching. Although observations from the Caribbean were, on average, a few meters deeper than those from the other two basins (Supplementary Fig. 6), sensitivity analyses incorporating depth as a covariate showed that the among-basin and inter-annual variations in bleaching thresholds were robust to differences in depth; though, for severe bleaching, broader confidence intervals reduced the statistical evidence for some of the differences observed in the original analysis (Supplementary Figs 7 and 8).

Records of moderate or greater (>10%) coral mortality again varied among basins and years, with the risk of mortality clearly increasing with heat stress level in the two Indo-Pacific basins (Fig. 3, Supplementary Fig. 5). Because the number of mortality reports was less than half the number of bleaching reports, mortality response curves (Fig. 3, Supplementary Fig. 5) were less well constrained than those for bleaching (Fig. 2, Supplementary Fig. 3). Nevertheless, the best-fitting model had a prediction accuracy of 88% and showed a probability of moderate mortality exceeding 0.2 at heat stress levels beyond 8°C-weeks in both the Indian Ocean/Middle East and Asia-Pacific basins, consistent with use of this threshold as a global predictor for moderate mortality onset5,8. In contrast, the lack of a positive relationship between mortality and heat stress in the Caribbean/Atlantic in any year (Supplementary Fig. 5b) suggests that heat stress was probably not a primary driver of mortality in the Caribbean/Atlantic during 2014–17 (discussed below). As with bleaching, the among-basin and interannual patterns in mortality were robust to the slight differences in the distribution of observation depths across ocean basins (Supplementary Fig. 9).

In 2016, over 15% of global reef containing pixels, including large areas of the tropical Pacific, the Indo-Pacific, and some of the Northwestern Hawaiian Islands, reached or exceeded heat stress of 16°C-weeks (Fig. 1). Surveys conducted in these areas showed most reefs suffered rapid and severe mortality of many coral species37. This heat stress corresponded with very high probabilities of moderate bleaching (Fig. 2, Supplementary Fig. 3), and greater than 50% probabilities of severe bleaching (Supplementary Fig. 4) and moderate mortality (Fig. 3, Supplementary Fig. 5). Establishment of new Alert Levels up to Level 5 (DHW≥20°C-weeks) will help capture the effects of increasingly long and intense marine heatwaves8,9 that became especially apparent during the Third Global Coral Bleaching Event and are predicted to become more frequent in the future38. In contrast to the recent past, the tropical marine heatwave analyzed herein spanned multiple seasons and persisted for more than a year on some equatorial reefs (e.g., record heat stress in 2014–17 at Jarvis Island in the central Pacific Ocean resulted from over 12 months of heat stress exposure, with DHW≥4°C-weeks spanning March 2015–May 201639,40).

Assessing the global footprint of the 2014–17 bleaching
The prediction accuracy obtained in our analyses was high overall (83–88%), and much of that prediction accuracy was due to the fixed effects component of our models (that is, the variation predicted by DHW, ocean basin, and bleaching year: 73% moderate or greater bleaching, 83% severe bleaching, and 81% moderate or greater mortality). These values compare favorably with earlier analyses of regional-scale mass bleaching events7,30,41,42. However, simply reporting percentages of observed bleaching or mortality from surveyed reefs, as in previous global studies6,43,44, could lead to overestimates of severity if reefs experiencing higher heat stress were disproportionately surveyed. Therefore, to correct for such a bias, we used our statistically modeled bleaching and mortality relationships for each basin and year to predict bleaching and mortality on each reef-containing pixel based on the maximum satellite-measured heat stress at that pixel. From this analysis, we estimated that 51% of global coral reef locations suffered moderate or greater bleaching and 15% suffered moderate or greater mortality. While the Third Global Coral Bleaching Event progressed globally for three full years (June 2014–May 2017), bleaching and mortality varied in time and space in relation to heat stress across the 21 GCBE3 regions in each year (Fig. 4, Supplementary Table 3). For example, the Coral Triangle (region #6) experienced relatively low-level heat stress in 2014–15 (DHW=4.7°C-weeks, yellow diamond) with an estimated 7% bleaching, compared with the highest heat stress (10.3°C-weeks, dark red) and 32% bleaching in 2015–16, and an intermediate level of heat stress (9.9°C-weeks, red) and 22% bleaching in 2016–17. In contrast, peak heat stress in the Great Barrier Reef (region #18) increased through the three bleaching years—4.4, 8.3, and 10.8°C-weeks, respectively—whereas bleaching was highest in the second year (5, 49, and 35%, respectively), potentially revealing acclimatization and/or mortality of sensitive taxa.

In general, higher heat stress resulted in higher bleaching and mortality throughout the event, and most GCBE3 regions followed this pattern (Fig. 4). More complex patterns in the relationship between bleaching and mortality emerge in some areas. Heat stress-driven mortality in most basins was modeled to occur in 30–50% of pixels where bleaching occurred. However, corals in much of the Caribbean/Atlantic that bleached at similar heat-stress levels as other basins (Fig. 2, Supplementary Fig. 3) showed lower new mortality, with fewer than 15% of modeled “bleaching” pixels predicted to have experienced moderate mortality. This low mortality may have resulted from a long history and higher frequency of heat stress events in the Caribbean/Atlantic2,3,8. Caribbean/Atlantic coral losses have been quite high in recent decades44,45, including bleaching-related mortality during both prior global bleaching events15,16, the 1982–83 bleaching event46, and the 2005 Caribbean bleaching event5. Additionally, coincident with the Third Global Coral Bleaching Event, disease-related mortality due to Stony Coral Tissue Loss Disease was extensive throughout the Florida Reef Tract but was not reported elsewhere in the Caribbean/Atlantic until after May 201747. Heat stress was low in Florida relative to other parts of the basin in the Third Global Coral Bleaching Event. Thus, we suspect that the slight negative relationship seen between heat stress and mortality (Fig. 3, Supplementary Fig. 5b) was an artifact of the geographic distribution of Stony Coral Tissue Loss Disease (Florida-only during this event) overlaid upon the pattern of heat stress during the Third Global Coral Bleaching Event (low heat stress in Florida).

Together with impacts from overfishing, land-based pollution, and sea urchin diseases, bleaching- and disease-related mortality have reduced Caribbean/Atlantic coral density and cover45, particularly among acroporid corals48, as well as diversity49. Depauperate in species diversity for millennia, the decline in sensitive coral species since the 1980s left many Caribbean/Atlantic reefs with especially stress-tolerant species and genotypes that survive bleaching49 but provide diminished ecosystem function50,51. One exception to this pattern was the unique coral assemblages off the coast of Brazil. Despite being the most biodiverse reef complex in the South Atlantic, these corals experienced low bleaching and mortality during the Third Global Coral Bleaching Event, likely due to reduced irradiance afforded by turbidity in Brazil’s shallow coastal waters52. Unfortunately, these corals suffered major losses due to heat stress events after 201753.

The heat stress on coral reefs during the Third Global Coral Bleaching Event was much greater than that reported in either the 1997–1999 First or 2009–2011 Second Global Coral Bleaching Events6,31,32,44. Using DHW calculated from a dataset covering 1982 to present, heat stress during the 2014–17 event (and even in 2016 alone) was more intense and widespread than prior mass bleaching events (Fig. 5). This resulted in greater bleaching and mortality during the Third Global Coral Bleaching Event than those earlier, less-intense events, especially given that elevated levels of bleaching and mortality were reported in successive years6,20.

Some reefs damaged during June 2016–May 2017 continued to experience bleaching later in 201717,18, and others were severely bleached in subsequent years19,30,53. With so little time for recovery between consecutive heat stress events, many coral reefs worldwide are becoming increasingly susceptible to ecosystem collapse35 or severe population bottlenecks (frequent intervals of low population size, with concomitant loss of genetic diversity and increased risk of local extinctions), as well as suffering knock-on effects on herbivores and other reef-associated biota54–56. For example, recovery of coral cover on the Great Barrier Reef was reported through 2022 following recent bleaching events57. While encouraging, this was driven by fast-growing Acropora corals that are highly susceptible to bleaching during subsequent heat stress, wave damage from storms and tropical cyclones, and crown-of-thorns starfish predation. This “recovery” measured by coral cover, may be masking a concomitant loss of coral diversity and associated functional traits51. Unfortunately, bleaching in 2022 ended that recovery trend58 and in 2023–24, initial analysis reported moderate or greater (>10%) bleaching of 73% of survey reefs on the Great Barrier Reef59, with major losses expected. With projected increases in the frequency and duration of heat stress3,38, boom-and-bust cycles in coral cover may trigger the transition of reef ecosystems from marine biodiversity hotspots to new, lower diversity stable states60 characterized by only a few thermally tolerant species; similar to the current state of most Caribbean/Atlantic reefs.

The 2014–17 Third Global Coral Bleaching Event was more widespread and damaging than any prior bleaching event on record, highlighting the threat of increasingly severe and widespread marine heatwaves to coral reefs that is outpacing the capacity of corals to physiologically resist heat stress6,30,61. The extent, frequency, and severity of marine heatwaves on coral reefs2,31 and elsewhere9,62 are predicted to intensify further63,64. One consequence of increasingly frequent bleaching events is a reduction in the time available between events for coral and ecosystem recovery6, leading to a decline in the structure of reefs. As coral reefs are ecosystem builders that protect shorelines from wave-driven flooding and erosion, and provide food, medicines, cultural identity, and livelihoods for over a billion people65, these essential ecosystems urgently need protection. Our study found severe coral reef losses throughout this protracted three-year bleaching event, an acceleration of the threat that has already placed them as one of the world’s most sensitive ecosystems to temperature extremes66. While the Third Global Coral Bleaching Event was unprecedented at the time, the observed impacts may be equaled or surpassed in subsequent events, including the ongoing Fourth Global Coral Bleaching Event22. As ocean warming continues, coral loss and associated reef degradation around the world are nearly certain to accelerate.

Methods
Satellite-based heat stress
To assess heat stress that can lead to coral bleaching and mortality, we used daily global 0.05° (~5×5 km2) sea surface temperature (SST) and heat stress metrics from the U.S. National Oceanic and Atmospheric Administration’s CRW version 3.1 Satellite Coral Bleaching Heat Stress Monitoring Product Suite dataset23,24. The primary metric of heat stress, DHW, accumulates instantaneous bleaching heat stress during the most recent 12-week period and has been associated with risk of bleaching (Alert Level 1) and mortality (Alert Level 2)24 (Supplementary Table 1). As a result of record heat stress seen during the Third Global Coral Bleaching Event and in subsequent years, CRW established new Alert Levels 3–5, corresponding to progressively higher threshold DHW values21 (Supplementary Table 1 and main text). Further explanation and justification for each Alert Level is located at: https://coralreefwatch.noaa.gov/product/5km/index_5km_baa-max-7d.php.

With the Third Global Coral Bleaching Event beginning in June 2014, bleaching years, as defined for this paper, covered June through May for 2014–15, 2015–16, and 2016–17. The June–May timing also incorporates the mid-year lull in heat stress and bleaching observations that is apparent at almost all reef locations3. Maximum Bleaching Alert Level values for the entire period of the Third Global Coral Bleaching Event were plotted in Fig. 1 and Supplementary Fig. 1a (the latter containing an overlay of reef-containing satellite pixels), and a summary of the number of pixels reaching Alert Levels 1–5 by bleaching year was summarized (Supplementary Table 2). Maximum Bleaching Alert Level values for the First and Second Global Coral Bleaching Events were plotted in Fig. 1b, c. Because the First and Second Global Coral Bleaching Events did not follow the same June-May heat stress timing as the Third Global Coral Bleaching Event14, 2-year couplets of June 1997–May 1999 and June 2009–May 2011 were plotted to capture all of the peak heat stress seen during these events. Corresponding charts of maximum DHW for the global coral bleaching events were plotted in Supplementary Fig. 2. Remote sensing-derived data were extracted, calculated, and plotted using IDL 8.7 and Python 3.8.5, with the following required Python computing environment: netcdf4 version 1.5.5.1, scipy version 1.6.0, python version 3.8.5, matplotlib version 3.3.3, pytables version 3.6.1, basemap version 1.2.2, basemap-data-hires version 1.2.2, pandas version 1.2.3, xarray version 0.20.1, pooch version 1.4.0, scikit-learn version 1.0.2, numba version 0.55.0, pyts version 0.12.0.

We extracted the daily and bleaching year maximum DHW values for the pixel co-located with each field survey. Surveys undertaken prior to the maximum heat stress were compared with the contemporary DHW value, while those undertaken on or after the date of the maximum heat stress were compared with the maximum value. For observations that contained an observation month without a date, we assigned a value of the 15th of the month. For on site observations on or next to coastlines, we extracted DHW values from the nearest valid ocean pixels. For Fig. 5, global Bleaching Alert Levels were calculated from the 0.25° (~25×25 km2) Optimum Interpolation Sea Surface Temperature (OISST), Version 2.1 dataset67,68 using the same methods as CRW’s operational products24 and extracted for June–May bleaching years and plotted with Microsoft Excel. OISST was chosen for this global application as, at the time this article was written, the higher resolution CRW dataset23 did not yet extend back to 1982.

Bleaching and mortality observations
We collated field observational surveys conducted from 2014 through 2017 to document the spatial extent and severity of bleaching and mortality worldwide. The resulting 2014–17 database (Supplementary Data 1, Supplementary Fig. 1b) followed the Donner et al. 201769 format, including categories for (1) measures of coral bleaching as coral cover bleached (%), number of coral colonies bleached (n) and total number of colonies surveyed (N), or both; and/or (2) measures of coral mortality as coral cover dead (%), number of coral colonies dead (n) and total number of colonies surveyed (N), or both; (3) observation date; (4) observation location, including latitude, longitude, and reef site name; (5) data source; and (6) survey method used. We converted percentage bleached (and mortality, where available) into categorical variables following the same protocol as ReefBase (http://www.reefbase.org). While this simple method of bleaching collation has limitations (most notably, no requirement for data on bleaching by taxon), it allows for consistency in collation over time and inclusion of reports from rapid and low-technology bleaching assessments, as well as those conducted by citizen scientists through multiple programs (e.g., CORDIO-EA, ReefCheck, BleachWatch70–73). Additionally, we provided a qualitative questionnaire through which contributors could provide additional information about each observational dataset.

The database was compiled from data submitted directly by individuals and extractions from large databases. CRW sent out multiple calls for coral reef survey data (including observations of the absence of bleaching and/or mortality) conducted during the Third Global Coral Bleaching Event. We sought out a wide range of potential collaborators through direct contacts, calls for data via the NOAA Coral-List listserv, published calls for data74, and an appeal through the team who created the film Chasing Coral75. We also extracted data on coral bleaching and mortality (where available) covering June 2014–May 2017 from multiple regional/international databases/sources including Donner et al. 201769, Reef Check International, Atlantic and Gulf Rapid Reef Assessment, Coastal Oceans Research and Development—Indian Ocean (CORDIO) East Africa, NOAA’s National Coral Reef Monitoring Program (NCRMP)76, and Great Barrier Reef surveys77. After removing surveys that lacked information critical to this study, the remaining 15,066 bleaching and mortality surveys (Supplementary Data 1) spanned the period June 1, 2014–May 31, 2017.

As the multiple individuals who contributed data to this paper used a variety of methods, the work presented here can be considered an unweighted meta-analysis of surveys conducted by numerous individuals, institutions, and organizations during the 2014–17 Third Global Coral Bleaching Event. The techniques used were all highly comparable, well-accepted field methods. Past comparisons among coral reef survey methods have demonstrated that while some biases exist among methods, most provide comparable results across similar types of observations, such as percent coral cover78,79. The percentage of colonies bleached was often higher than the percentage of cover bleached because (1) small colonies bleached more often than large colonies; and/or (2) both partially- and fully-bleached colonies were counted as bleached in some survey methodologies. However, previous statistical comparison of the two methods found no significant differences when assessing the extent of coral bleaching or mortality5. Additionally, aggregating the data into broad categories minimized the need for precise estimates of bleaching. Therefore, we assumed that the different observation methods provide comparable results for this meta-analysis.

In our data, many surveys were conducted to meet multiple monitoring objectives, and the timing did not necessarily coincide with peak or even active local bleaching (e.g., established monitoring programs with pre-scheduled survey times)80. Conversely, some surveys may have been timed to coincide with predicted thermal stress events, likely informed by bleaching alerts based on CRW satellite products. In other cases, visits to particularly difficult-to-access survey locations may have been undertaken for similar reasons. However, because heat stress is an explicit covariate in our model, any such differential propensities for reefs to be surveyed would not introduce biases in our estimated thresholds; in this sense, our approach is more robust to such biases than raw reports of bleaching or mortality event frequency in survey datasets. Nevertheless, we do acknowledge that, if the actual existence of bleaching made a reef more likely to be surveyed than another reef that was unbleached but experiencing similar levels of heat stress, this could have led to overestimates of bleaching probability (as in all prior analyses of which we were aware).

Data handling
We defined a unique bleaching observation (survey) as one with a unique combination of geographic coordinates, date, and depth or depth range. Each unique bleaching observation was assigned a data ID and included as a single, independent observation in the database. Multiple observations (quadrats or transects) taken at any reef site within a few days at the same depth range (±1 m) were combined into a single survey, with bleaching by mean percent cover data or proportion of the number of colonies surveyed included in the database as the relevant observation. If percent bleached or mortality by individual colony data were provided, total coral area bleached was calculated by multiplying percent bleached or mortality of individual colonies by the size dimensions of each colony and dividing that number by the total area of all coral colonies at each site. Each survey was categorized by bleaching year and ocean basin based on coordinate boundaries: Asia-Pacific (AP, 100°E to 100°W crossing the anti-meridian), Caribbean/Atlantic (CA, 100°W to 20°E crossing the prime meridian), and Indian Ocean/Middle East (IM, 20°E to 100°E).

Because heat stress often reduces zooxanthellae (family Symbiodiniaceae) densities before bleaching becomes visible81, coral colonies showing any degree of bleaching, from pale and partially bleached to fully bleached colonies, were identified as “bleached”. Similarly, we included partial and complete mortality under “mortality”, as it either indicated a heat stress response resulting in mortality due to bleaching or other, often heat-related, diseases. Therefore, partial and complete bleaching, and partial and complete mortality were combined as observations of bleaching and mortality, respectively.

As heat stress is not the only cause of bleaching and/or mortality of corals, some observations of bleaching or mortality may have been caused by other stress or predation. For this study, we used only bleaching observations that took place after satellite-based heat stress had begun, and we used mortality data recorded after the peak of the heat stress event (i.e., the bleaching year maximum DHW) within a pixel or change in cover between repeat surveys during June 2014–May 2017. Bleaching was compared against either the heat stress at the time of the observation or that year’s maximum heat stress if the observation was after the peak; mortality data were compared against that year’s maximum heat stress. A prior analysis of reefs showed that 6% bleaching and 4% recent mortality (within the past year) normally existed as a background level during surveys in years lacking any major disturbance82. As previously established5, a value of approximately twice these (10%) was used to define a threshold for moderate levels of bleaching and mortality. Severe bleaching was identified as being at least half of the population (>50%). Onset month of observed severe (>50%) bleaching in various regions across the globe is plotted in Supplementary Fig. 1d–f for each bleaching year. This revealed the general agreement of the bleaching sequence with that seen in 199810 and is generalized across the three bleaching years in Supplementary Fig. 1c and animated in Supplementary Video 1. Bleaching onset months during the Third Global Coral Bleaching Event can be compared with typical heat stress onset months found in CRW’s Thermal History products3,21.

Mortality data from the large databases only included corals known to have recently died (within the last year) or data that expert observers determined had recently died, most likely due to heat stress. In most cases, the actual cause of mortality was not known with complete certainty. Certain data contributors provided mortality data as a function of change over time (i.e., reduction in live coral cover since the previous survey). These data were listed as mortality present on the ending date of the reporting period.

Some monitoring programs reported percent bleaching or mortality in categories based on a range of values, and a few different sets of ranges were used across the contributing programs. These were adjusted for consistency by determining the ranges of the reported categories and then entering the mid-point value of that range into the database. Similarly, where data contributors provided ranges of percent bleached or mortality rather than single values, the mid-point values of the original ranges were used in the database.

Data analyses
We fitted generalized linear mixed models (GLMMs) with binomial error structure using the library glmmTMB version 1.1.10 in version 4.4.2 of the R statistical software package83,84. We considered DHW, year, and ocean basin as the potential fixed effects, and the percent coral bleached or dead as the binomial response—i.e., whether the percentage of corals (area or number) in an individual reef survey exceeded the appropriate threshold (10% for moderate impact, 50% for severe impact) or not, for both bleaching and mortality. To check for spatial autocorrelation of residuals, we calculated between-site distances from latitude and longitude with the “geodesic” method, using the library “geodist” (version 0.1.0) in the software program R84,85; we computed semivariances using the function “Variogram” in the R package “nlme” (version 3.1.166)86. Models including only the fixed effects exhibited some evidence of spatial autocorrelation of residuals within the first several 1000 km of distance in the second and third bleaching years. This was particularly apparent within the first few 100 km in 2015–2016 for moderate or greater bleaching (Supplementary Fig. 10: note the steep initial trend in the blue circles of the top row, middle panel), and in 2016–2017 for moderate or greater bleaching (Supplementary Fig. 10: note the similar increase in the first few 100 km in the top row, right panel) and severe bleaching (Supplementary Fig. 11: note the similar increasing trend in the top row, right panel).

Spatial autocorrelation for mortality was also apparent in 2015–2016 and 2016–2017 (Supplementary Fig. 12 top-middle and top-right panels). Consequently, for all subsequent analyses, we added a random effect of “Regional Virtual Station” which, for all bleaching observations, pulls together all surveys found within the boundaries of each of CRW’s 214 Regional Virtual Stations21,87. Once these effects were included, there was much less apparent autocorrelation in either the residuals or the random effect estimates themselves (Supplementary Figs. 10–12, middle and bottom rows), and models including these random effects exhibited about 2000 AIC units better fit than the models omitting them. So, we used these mixed-effects models for further inference.

To assess the absolute performance of the best-fitting model (in addition to the relative performance of alternative models assessed by AIC), we calculated each model’s prediction error: the proportion of events correctly predicted by the model (e.g., a correct bleaching prediction would be where the predicted probability of bleaching was >0.5, and bleaching occurred). We calculated prediction error both for the full model (including the random effects of Regional Virtual Station) and for the fixed effects component of the models only (omitting the random effects), analogous to “conditional” and “marginal” R2 values from mixed-effects OLS regression. Previous bleaching studies most commonly reported prediction error, facilitating comparability of our models’ performance with those used in earlier work7,30,41,42.

To evaluate the evidence for the different possible effects and interactions among temperature, ocean basin, and bleaching year, we used Akaike Information Criterion (AIC) to compare the fit of models including full 3-way interactions (among DHW, bleaching year, and basin), with models removing all possible combinations of interactions involving bleaching year and basin. We wanted to consider models that had the same baseline level of bleaching at DHW=0°C-weeks but still differed in their sensitivity to heat stress. This corresponded to models where interactions between DHW and bleaching year, or ocean basin, or both were retained, but there was no “main effect” (i.e., no difference at the intercept where DHW=0°C-weeks) of year or basin, or no two-way interaction between year and basin.

Despite the voluminous observations in the 2014–17 bleaching database (15,066 survey data points), the uneven spatial distribution of sampling (Supplementary Fig. 1b) yielded direct, observational data from only 6.6% (3592 pixels) of the total 53,997 reef-containing pixels worldwide21. To predict the probabilities of >10% (moderate or greater) bleaching and mortality for all reef-containing pixels globally, we projected the statistically-modeled bleaching and -mortality thresholds for each basin and year from data on the maximum satellite-measured heat stress (DHW) at each reef-containing pixel.

We sought to predict the extent of bleaching and mortality globally for this event in a way that would not be biased by a propensity for surveys to have been conducted disproportionately on reefs experiencing particularly high heat stress. To do this, we used our fitted bleaching and mortality thresholds, together with maximum DHW values from each reef-containing pixel and for each of the three bleaching years of the Third Global Coral Bleaching Event, to predict the probability of moderate or greater bleaching and moderate or greater mortality in each bleaching year globally. Specifically, we used the estimated coefficients from the relevant GLMM (moderate or greater bleaching and moderate or greater mortality) for the effects of DHW, bleaching year, ocean basin, and associated interaction terms. We excluded the random effects of Regional Virtual Station ID since there were reef-containing pixels in Regional Virtual Stations from which we did not have observations.

Reef locations were assigned to one of 22 GCBE3 regions (Supplementary Table 3). Within each region, the probabilities were summed across reef-containing pixels for each of the bleaching years, yielding a measure of the predicted extent of bleaching (area of colored diamonds) and mortality (area within the sub-diamonds) for each period. Regions were designated as either Northern or Southern Hemisphere for the purpose of display, in which the predicted extent of bleaching was represented, with the color of each diamond reflecting the maximum DHW during that bleaching year. Areas for each 12-month period (solid diamonds and enclosed dashed lines) were determined using fitted probabilities derived from the fixed effects of the best-fitting models of moderate or greater bleaching and moderate or greater mortality due to satellite-derived heat stress (Supplementary Fig. 3, Supplementary Fig. 5), summed across reef-containing pixels (i.e., two pixels with 30% probability of bleaching contribute the same as one pixel with a 60% probability of bleaching). The total reef area reflects the number of reef-containing pixels in each GCBE3 region (Fig. 4 inset). Summaries of all GCBE3 regional values are reported in Supplementary Table 3.

Comparisons of satellite-derived heat stress at each of the 214 NOAA CRW Satellite Regional Virtual Stations21 to in-water observations of bleaching and mortality yielded very few mismatches between satellite-measured heat stress and observed response in corals (<5% of all observations). Cases of high DHW values (≥8°C-weeks) with less than 10% bleaching or mortality, and those with no measured heat stress but more than 50% bleaching or 10% mortality in individual surveys were scattered in space and time and showed no geographic pattern. While the Third Global Coral Bleaching Event had a worldwide scope, there are many factors that confound relationships between heat stress and coral bleaching and mortality. These include poorly-timed observations that missed bleaching and mortality related to heat stress80, and variations in bleaching and mortality with depth, light penetration, or habitat. In some sites, greater water depths have provided a layer of insulation from thermal events88. However, the potential for deep reefs to serve as refugia is uncertain and varies greatly89–93. The potential impact of depth on our analysis is described in the next section.

Model selection
Model selection for moderate or greater bleaching (>10% of corals) indicated that the best-fitting model was one with all main effects and interactions, except for an interaction between ocean basin and bleaching year (i.e., baseline bleaching when DHW=0°C-weeks varied among years, and among basins, but year effects were consistent across basins and vice-versa). However, the retained three-way interaction involving DHW implies that the sensitivity of bleaching to temperature varied differently among basins in different years (see Supplementary Table 4 for model selection; Supplementary Fig. 3 for fitted thresholds of best-fitting model). For severe bleaching (>50% of coral bleached), the best-fitting model implied that all bleaching years and ocean basins had a common baseline bleaching value at DHW=0°C-weeks (i.e., no main effects of basin or year, and no interaction between basin and year), but that the severe bleaching thresholds did vary in different ways among years and basins, as in the “moderate bleaching” analysis (see Supplementary Table 5 for model selection; Supplementary Fig. 4 for best-fitting model).

For moderate or greater mortality (>10% of corals), the best-fitting model included all main effects and interactions involving DHW, ocean basin, and bleaching year (see Supplementary Table 6 for model selection; Supplementary Fig. 5 for best-fitting model). For Fig. 2 we fitted models including only fixed effects and interactions for DHW and bleaching year (for panel a), and DHW and basin (for panel b), to assist visualization of overall differences in thresholds among years and among basins. However, it is important to note that the best model for these data included interactions among all three fixed effects; those fitted relationships were plotted in Supplementary Fig. 3.

Finally, finer-scale patterns of bleaching or mortality in nearshore reefs may have resulted from localized heating, cooling, or shading at spatial resolutions not detectable by the heat stress metric used in this study. To test if depth had an impact on the models, sensitivity analyses incorporated depth as a covariate on the observations where depth was recorded. Observation depths were not consistent among basins with those from the Caribbean tending to be somewhat deeper, on average, than those from the other two ocean basins (Supplementary Fig. 9). The sensitivity analysis showed that the basin and inter-annual variations in moderate bleaching, severe bleaching, and moderate mortality thresholds were robust to these differences (Supplementary Figs 10–12), therefore we believe the analyses that included the full dataset and omitted depth provided a stronger representation of the bleaching and mortality relationships with heat stress across the three basins.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "While I was not a Reviewer of the original submission of this manuscript, I have carefully read the previous reviewers’ comments and the authors’ responses to those comments.\nIn my view, the authors have constructively and adequately addressed the previous reviewer’s critiques, and as a result the manuscript is now clearer, more rigorous, and more compelling.\nFurthermore, I reviewed the revised manuscript – and did so prior to reading the response letter (to establish my own, unbiased assessment).\nI found the manuscript to be very interesting, technically sound, and well-written (which is impressive, given both the scope and nuance of the underlying data). The authors impressive blending of global-scale in situ observations with remotely sensed data conveys a clear and important story about the severity and extent of the third global coral bleaching event and the spatiotemporal variance of this phenomenon. The revised version is solid.\nPer the prior comments and response letter, I agree you should leave Figure 1 as is (re: alert levels) – as I agree that this metric is best suited for a broad audience – but I encourage the authors to include the DHW version in supplement as well, for the benefit of scientists working on this specific topic matter.\nIn my view, this MS is ready for publication following small editorial changes (see below) and any subsequent edits deemed necessary by R2.\nMinor editorial comments:\nLine 270-271: Unclear what “these” is referring to – I presume warming and coral disease? Please clarify.\nLine 299: “and major losses on the GBR are expected for 2023–24”. Were incurred? Do we know yet?\nLine 302: As written (and given the previous few sentences), the word “dominated” gives the uninformed reader the impression that Caribbean reefs now generally display high coral cover (underpinned by a few thermally tolerant taxa). But in reality, most Caribbean reefs have very low coral cover (and are not dominated by coral at all, but rather other groups like seaweed). Thus, change “dominated” to “characterized” (or similar) to avoid confusion.", "reviewer_2": "This manuscript has been extensively reviewed and revised by the authors at least two times in this journal by other reviewers. As a new reviewer to this process in the third round, I generally agree that the authors have done as best a job they can with the previous sets of reviews (including the responses to the latest round of reviews). At an earlier stage, I would suggest a revised focus or statistical approach, but that doesn’t feel appropriate now. My major suggestion is on the framing – as Reviewer 1 notes, this bleaching event occurred over 10 years ago and another one (in 2024, named the Fourth Global Bleaching Event - so a major disturbance) was also purported as the most severe and widespread. Without a direct comparison between the 2014-2017 and the 2024 event, I don’t think the authors can justify their title or conclusions – that this is “The Most Severe and Widespread Coral Reef Damage Ever Recorded”. I recommend acceptance pending a re-framing, i.e. that this is a 10-year review of the 2014-2017 event and that similar analyses of the 2024 event deserve similar comparison, at least of the temperature / DHW / satellite data (since gathering the monitoring data from so many different sources is likely what took 10 years!) I would also take the moment to appreciate the work that many NOAA scientists and technologists have done over many years to advance this work, and that I hope the NOAA datasets remain useful to scientists for years to come. In the current political climate, it’s also worth noting that publishing this article quickly will be a record of the usefulness of NOAA’s tracking and forecasting of coral bleaching events worldwide.\nMajor suggestions:\nTitle. Change to, “The 2014–17 Global Coral-Bleaching Event: A Record of Severe and\nWidespread Coral Reef Damage”\nAbstract. Identify Fourth Global Bleaching Event and how these findings can help inform continued predictions / understanding of bleaching events.\nPersonally I find the ‘GCBE’ acronyms to be distracting and not a common acronym, I would suggest removing and replacing with ‘bleaching events’ or ‘Third Global Bleaching Event’, etc.\nFig1 - Any chance to add a panel of NOAA Alerts for the 2024 event to put these panels in context with the most recent event? I’m definitely not asking for any addition or extra analysis of the monitoring data / bleaching simulations - just simply for the thermal context of these events.\nMinor comments\nL106 - Was this ‘consistent spatial evolution’ also what you observed during this event? Or did this bleaching event follow a different spatial expansion? I suggest replacing ‘evolution’ with ‘expansion’ to be more appropriate (it’s not an evolution over many generations, per se)"}, "author_response": "Response to reviews\nWe thank the reviewers for their consideration of our manuscript, and we are grateful for\nthe opportunity to submit this revised manuscript. We have responded to each of the\ncomments provided as detailed below, in most cases leading to changes in the manuscript\nmain text.\nWe specifically note:\n we have prioritised expediting the response to these review comments in\nacknowledgement that several co-authors are U.S. Federal government employees\nwhose capacity to engage in a timely fashion may be affected in the near future\n some authors have been re-instated as their contact information had changed prior\nto the submission to Nature Communications (and following the earlier submission\nto Nature)\n the reviewers’ statements, with the revisions/reframing undertaken based on the\nreviewers’ comments, that the manuscript is “ready for publication” (R1) and to\n“recommend acceptance” (R2).\nReviewer 1\nR1.1 While I was not a Reviewer of the original submission of this manuscript, I have\ncarefully read the previous reviewers’ comments and the authors’ responses to those\ncomments.\nIn my view, the authors have constructively and adequately addressed the previous\nreviewer’s critiques, and as a result the manuscript is now clearer, more rigorous, and more\ncompelling.\nResponse: We thank the reviewer for these comments on the efforts undertaken previously.\nR1.2 Furthermore, I reviewed the revised manuscript – and did so prior to reading the\nresponse letter (to establish my own, unbiased assessment).\nI found the manuscript to be very interesting, technically sound, and well-written (which is\nimpressive, given both the scope and nuance of the underlying data). The authors impressive\nblending of global-scale in situ observations with remotely sensed data conveys a clear and\nimportant story about the severity and extent of the third global coral bleaching event and\nthe spatiotemporal variance of this phenomenon. The revised version is solid.\nResponse: We are grateful for this support of the scope and quality of our submission.\nR1.3 Per the prior comments and response letter, I agree you should leave Figure 1 as is (re:\nalert levels) – as I agree that this metric is best suited for a broad audience – but I encourage\nthe authors to include the DHW version in supplement as well, for the benefit of scientists\nworking on this specific topic matter.\nResponse: We have now included charts of maximum DHW for each of these three global\ncoral bleaching events in the Extended Data (now ED Fig. 2).\nR1.4 In my view, this MS is ready for publication following small editorial changes (see\nbelow) and any subsequent edits deemed necessary by R2.\nResponse: We thank the reviewer for this assessment.\nMinor editorial comments:\nR1.5 Line 270-271: Unclear what “these” is referring to – I presume warming and coral\ndisease? Please clarify.\nResponse: We have replaced “these” with “bleaching- and disease-related mortality”.\n“Together with impacts from overfishing, land-based pollution, and sea urchin diseases,\nbleaching- and disease-related mortality have reduced Caribbean/Atlantic coral density and\ncover [44], particularly among acroporid corals [47], as well as diversity [48].” (lines 272-\n274)\nR1.6 Line 299: “and major losses on the GBR are expected for 2023–24”. Were incurred? Do\nwe know yet?\nResponse: We have rephrased to clarify and to provide a reference (in a nutshell extensive\nbleaching was documented but effects on mortality and overall coral cover are not yet\nknown). We have added information and a citation for what is known so far from the Great\nBarrier Reef:\n“ For example, recovery of coral cover on the Great Barrier Reef was reported through 2022\nfollowing recent bleaching events [56]. While encouraging, this was driven by fast-growing\nAcropora corals that are highly susceptible to bleaching during subsequent heat stress,\nwave\ndamage from storms and tropical cyclones, and crown-of-thorns starfish predation. This\n‘recovery’ measured by cover may be masking a concomitant loss of coral diversity and\nassociated functional traits [50]. Unfortunately, bleaching in 2022 ended that recovery trend\n[57] and in 2023-24, initial analysis reported moderate or greater (>10%) bleaching of 73%\nof survey reefs on the Great Barrier Reef [58], with major losses expected.” (lines 296-303)\nR1.7 Line 302: As written (and given the previous few sentences), the word “dominated”\ngives the uninformed reader the impression that Caribbean reefs now generally display high\ncoral cover (underpinned by a few thermally tolerant taxa). But in reality, most Caribbean\nreefs have very low coral cover (and are not dominated by coral at all, but rather other\ngroups like seaweed). Thus, change “dominated” to “characterized” (or similar) to avoid\nconfusion.\nResponse: We concur with the reviewer’s suggestion and have replaced “dominated” with\n“characterized”.\nReviewer 2\nR2.1 This manuscript has been extensively reviewed and revised by the authors at least two\ntimes in this journal by other reviewers. As a new reviewer to this process in the third round,\nI generally agree that the authors have done as best a job they can with the previous sets of\nreviews (including the responses to the latest round of reviews).\nResponse: We thank the reviewer for these comments.\nR2.2 At an earlier stage, I would suggest a revised focus or statistical approach, but that\ndoesn’t feel appropriate now. My major suggestion is on the framing – as Reviewer 1 notes,\nthis bleaching event occurred over 10 years ago and another one (in 2024, named the\nFourth Global Bleaching Event - so a major disturbance) was also purported as the most severe\nand widespread. Without a direct comparison between the 2014-2017 and the 2024 event, I\ndon’t think the authors can justify their title or conclusions – that this is “The Most Severe\nand Widespread Coral Reef Damage Ever Recorded”. I recommend acceptance pending a re-\nframing, i.e. that this is a 10-year review of the 2014-2017 event and that similar analyses of\nthe 2024 event deserve similar comparison, at least of the temperature / DHW / satellite\ndata (since gathering the monitoring data from so many different sources is likely what took\n10 years!)\nResponse: While the third global coral bleaching event was the most severe and widespread\ndamage to reefs when it concluded, we are sad to see that the ongoing fourth global coral\nbleaching event may turn out to be even worse. We concur with the reviewer and have\nchanged the title and re-framed this paper in light of current events.\nThat said, we did not include any quantitative analyses of heat stress for the more recent\nevent, because it requires fairly extensive additional treatment (for example, deciding when\nthe event began, and when or if it has ended). Those analyses are the subject of a separate\nmanuscript that has recently been submitted for consideration at Nature (Spady et al.,\nattached as supplement). We hope this reassures the editor and reviewer that we indeed\nconsider a comparison with the current and (we believe) ongoing global event to be\nimportant and worthwhile, but also that attempting to do so in the current manuscript\nwould not do service either to our comprehensive analysis of the bleaching data from\nGCBE3, nor the heat stress data from GCBE4.\n” While the third global coral bleaching event was unprecedented at the time, the observed\nimpacts may be equaled or surpassed in subsequent events, including the current (and\nongoing) fourth global coral bleaching event [67]. As ocean warming continues, coral loss\nand associated reef degradation around the world are nearly certain to accelerate.\n” (lines 325-328)\nR2.3 I would also take the moment to appreciate the work that many NOAA scientists and\ntechnologists have done over many years to advance this work, and that I hope the NOAA\ndatasets remain useful to scientists for years to come. In the current political climate, it’s\nalso worth noting that publishing this article quickly will be a record of the usefulness of\nNOAA’s tracking and forecasting of coral bleaching events worldwide.\nResponse: We thank the reviewer for these comments, and all authors agree that it is\nimportant that NOAA and its scientist be adequately credited for this work. To that end,\nbecause the US Administration changed since this MS was submitted, the MS has had to\nundergo additional internal review at NOAA. In response to that review, we have made\nsome minor changes in wording (e.g., “global warming” and “anthropogenic climate\nchange” to “ocean warming”), and we reworded the final sentence of the main text, so that\nit could not be read as advocating for a particular policy response to the effects of climate\nchange on coral reefs, while still emphasizing the likely consequences of ongoing warming\nfor the status of the world’s coral reefs.\nMajor suggestions:\nR2.4 Title. Change to, “The 2014–17 Global Coral-Bleaching Event: A Record of Severe and\nWidespread Coral Reef Damage”\nResponse: We thank the reviewer for this suggestion, which we have re-worded as “Severe\nand widespread coral reef damage during the 2014-2017 Global Coral Bleaching Event”.\nR2.5 Abstract. Identify Fourth Global Bleaching Event and how these findings can help inform\ncontinued predictions / understanding of bleaching events.\nResponse: Unfortunately, we were at 149 words and the abstract word limit is 150, so we\ncould not incorporate this suggestion in the abstract. However, we do make changes in the\nmain text as noted above. If the editor suggests we can exceed the 150-word limit to add\nlanguage about the fourth global coral bleaching event, we would be glad to do so.\nR2.6 Personally I find the ‘GCBE’ acronyms to be distracting and not a common acronym, I\nwould suggest removing and replacing with ‘bleaching events’ or ‘Third Global Bleaching\nEvent’, etc.\nResponse: We agree that excessive proliferation and use of acronyms and initialisms can be\ndistracting. At the same time, we believe that GCBE is in common use, having been\nestablished in Little et al. (2022), and appearing in numerous other manuscripts of which we\nare aware. Moreover, they are useful shorthand in the figures we use comparing the\ndifferent global coral bleaching events, and in sentences that refer to multiple global coral\nbleaching events (to avoid use of the same 4-word phrase 2 or 3 times in one sentence). Our\ncompromise has been to reduce use of GCBE in the main text, either writing out “third\nglobal coral bleaching event”, or, when clear from the context, “the 2015-2017 event” or\nsimilar, either in addition to or in place of the acronyms (e.g., lines 75, 86, 99, 107-116, 280,\n314, 325-327).\nR2.7 Fig1 - Any chance to add a panel of NOAA Alerts for the 2024 event to put these panels\nin context with the most recent event? I’m definitely not asking for any addition or extra\nanalysis of the monitoring data / bleaching simulations - just simply for the thermal context\nof these events.\nResponse: As noted in our response to R2.2, we have not included this, due to the need for\na more extensive and focused treatment of the fourth global event, which is found in the\nuploaded Spady et al manuscript.\nMinor comments\nR2.8 L106 - Was this ‘consistent spatial evolution’ also what you observed during this event?\nOr did this bleaching event follow a different spatial expansion? I suggest replacing\n‘evolution’ with ‘expansion’ to be more appropriate (it’s not an evolution over many\ngenerations, per se)\nResponse: We agree with the reviewer that the usage of the term ‘evolution’ is not\nconsistent with the biological definition and are using the term “spatial progression”\ninstead."}]

Decision letter