Global warming and recurrent mass bleaching of corals

Journal name:
Nature
Volume:
543,
Pages:
373–377
Date published:
DOI:
doi:10.1038/nature21707
Received
Accepted
Published online

Abstract

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.

At a glance

Figures

  1. Geographic extent and severity of recurrent coral bleaching at a regional scale, Australia.
    Figure 1: Geographic extent and severity of recurrent coral bleaching at a regional scale, Australia.

    a, The footprint of bleaching on the Great Barrier Reef in 1998, 2002 and 2016, measured by extensive aerial surveys: dark green (<1% of corals bleached), light green (1–10%), yellow (10–30%), orange (30–60%), red (>60%). The number of reefs surveyed in each year was 638 (1998), 631 (2002), and 1,156 (2016). b, Spatial pattern of heat stress (DHWs; °C-weeks) during each mass-bleaching event. Dark blue indicates 0 DHW, and red is the maximum DHW for each year (7, 10 and 16, respectively). Orange and yellow indicate intermediate levels of heat exposure on a continuous scale. c, Frequency distribution of maximum DHWs on the Great Barrier Reef, in 1998, 2002 and 2016. White bars indicate 0–4 °C-weeks; grey bars, 4–8 °C-weeks; black bars, >8 °C-weeks. d, Locations of individual reefs that bleached (by >10% or more) in 1998, 2002 and/or 2016, showing the most severe bleaching score for reefs that were surveyed more than once. Yellow, 10–30% bleaching; orange, 30–60%; red, >60%. e, Location of reefs that were surveyed in all three years that bleached zero (white), one (light grey), two (dark grey) or three times (black). f, Frequency distribution of aerial bleaching scores for reefs surveyed in 1998 (left bars), 2002 (middle), and 2016 (right bars). Colour bleaching scores as in a. g, Bleaching severity during March to early April 2016 on both sides of Australia, including the Coral Sea and the eastern Indian Ocean. Colour bleaching scores as in a. Bar graphs show mean sea surface temperatures during March for each year from 1980 to 2016 for northern and southern latitudes on either side of Australia. The red bar highlights the north–south disparity in 2016. Map templates provided by Geoscience Australia under licence from Creative Commons Attribution 4.0 International Licence.

  2. Recurrent severe coral bleaching.
    Figure 2: Recurrent severe coral bleaching.

    a, Aerial view of severe bleaching in Princess Charlotte Bay, northeast Australia, March 2016. Close to 100% of corals are bleached on the reef flat and crest. Bleaching occurs when algal symbionts (Symbiodinium spp.) in a coral host are killed by environmental stress, revealing the white underlying skeleton of the coral. b, Severe bleaching in 2016 on the northern Great Barrier Reef affected even the largest and oldest corals, such as this slow-growing Porites colony. c, Large, old beds of clonal staghorn corals, Acropora pulchra, on Orpheus Island, Queensland photographed in 1997 were killed by the first major bleaching event on the Great Barrier Reef in 1998. d, Eighteen years later in May 2016, corals at this site have never recovered, with the original assemblages still visible as dead, unconsolidated and muddy rubble that is unsuitable for successful colonization by coral larvae. e, f, Mature stands of clonal staghorn corals were extirpated by heat stress and colonized by algae over a period of just a few weeks in 2016 on Lizard Island, Great Barrier Reef. Before (e) and after (f) photographs were taken on 26 February and 19 April 2016. Photo credits: a, J.T.K.; b, J. Marshall; c, B.W.; d, C.Y.K.; e, f, R. Streit.

  3. The relationship between heat exposure (satellite-based DHWs in 2016) and the amount of bleaching measured underwater (per cent of corals bleached) in March/April.
    Figure 3: The relationship between heat exposure (satellite-based DHWs in 2016) and the amount of bleaching measured underwater (per cent of corals bleached) in March/April.

    Each data point represents an individual reef (n = 69). The fitted line is y = 48.6ln(x) – 21.6, R2 = 0.545.

  4. Spectrum of bleaching responses by coral taxa on the Great Barrier Reef in 2016, with relative winners on the right, and losers on the left.
    Figure 4: Spectrum of bleaching responses by coral taxa on the Great Barrier Reef in 2016, with relative winners on the right, and losers on the left.

    Individual species or genera (58,414 colonies) are plotted in rank descending order along the x axis from high to low levels of bleaching, for different severities of reef bleaching. Reef-scale bleaching severities are: blue, 1–10% of all corals bleached; green, 10–30%; yellow, 30–60%; orange, 60–80%; and red, >80% bleached. See Extended Data Table 2 for taxonomic details.

  5. A generalized linear model to explain the severity of coral bleaching.
    Extended Data Fig. 1: A generalized linear model to explain the severity of coral bleaching.

    Curves show the estimated relationships between probability of severe bleaching (>30%) on individual reefs of the Great Barrier Reef in 2016 and three explanatory variables (DHWs, chlorophyll a, and reef zoning, see Extended Data Table 1). The DHW-only model is shown in black. For the DHW plus chlorophyll a model, the blue threshold shows the estimated relationship between probability of severe bleaching and DHW for the 25th percentile of chlorophyll a, and the brown threshold shows the same for the 75th percentile of chlorophyll a. For the DHW plus reef zoning model, the red threshold shows the relationship for fished reefs, and the green for unfished reefs. Water-quality metrics and level of reef protection make little, if any, difference.

  6. Difference in daily sea surface temperatures between the northern and southern Great Barrier Reef, before and after ex-tropical cyclone Winston.
    Extended Data Fig. 2: Difference in daily sea surface temperatures between the northern and southern Great Barrier Reef, before and after ex-tropical cyclone Winston.

    The disparity between Lizard Island (14.67° S) and Heron Island (23.44° S) increased from 1 °C in late February to 4 °C in early March 2016.

  7. A test for the effect of past bleaching experience on the severity of bleaching in 2016.
    Extended Data Fig. 3: A test for the effect of past bleaching experience on the severity of bleaching in 2016.

    The relationship between previous bleaching scores (in 1998 or 2002, whichever was higher) and the residuals from the DHW generalized linear model (Extended Data Table 1). Each data point represents an individual reef that was scored repeatedly. There is no negative relationship to support acclimation or adaptation.

  8. Flight tracks of aerial surveys of coral bleaching, conducted along and across the Great Barrier Reef and Torres Strait in March and April 2016.
    Extended Data Fig. 4: Flight tracks of aerial surveys of coral bleaching, conducted along and across the Great Barrier Reef and Torres Strait in March and April 2016.

    Blue colour represents land, white colour represents open water.

  9. Ground-truthing comparisons of aerial and underwater bleaching scores.
    Extended Data Fig. 5: Ground-truthing comparisons of aerial and underwater bleaching scores.

    Aerial scores are: 0 (<1% of colonies bleached), 1 (1–10%), 2 (10–30%), 3 (30–60%) and 4 (60–100%) on the Great Barrier Reef in 2016 (Fig. 1a). Continuous (0–100%) underwater scores are based on in situ observations from 259 sites (104 reefs). Error bars indicate two standard errors both above and below the median underwater score, separately for each aerial category.

Tables

  1. A test for the causes of coral bleaching
    Extended Data Table 1: A test for the causes of coral bleaching
  2. Winners and losers
    Extended Data Table 2: Winners and losers

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Author information

Affiliations

  1. Australian Research Council Centre of Excellence for Coral Reef Studies, James Cook University, Townsville, Queensland 4811, Australia

    • Terry P. Hughes,
    • James T. Kerry,
    • Mariana Álvarez-Noriega,
    • Jorge G. Álvarez-Romero,
    • Kristen D. Anderson,
    • Andrew H. Baird,
    • David R. Bellwood,
    • Tom C. Bridge,
    • Sean R. Connolly,
    • Graeme S. Cumming,
    • Hugo B. Harrison,
    • Andrew S. Hoey,
    • Mia O. Hoogenboom,
    • Chao-yang Kuo,
    • Janice M. Lough,
    • Michael J. McWilliam,
    • Morgan S. Pratchett,
    • Gergely Torda &
    • Bette L. Willis
  2. College of Science and Engineering, James Cook University, Townsville, Queensland 4811, Australia

    • Mariana Álvarez-Noriega,
    • David R. Bellwood,
    • Ray Berkelmans,
    • Sean R. Connolly,
    • Mia O. Hoogenboom &
    • Bette L. Willis
  3. Commonwealth Science and Industry Research Organization, GPO Box 2583 Brisbane, Queensland 4001, Australia

    • Russell C. Babcock
  4. School of Biology, University of Leeds, Leeds LS2 9JT, UK

    • Maria Beger
  5. 24 Hanwood Court, Gilston, Queensland 4211, Australia

  6. Queensland Museum, 70-102 Flinders St, Townsville, Queensland 4810, Australia

    • Tom C. Bridge
  7. Australian Research Council, Centre of Excellence for Coral Reef Studies, School of Biological Sciences, University of Queensland, Brisbane, Queensland 4072, Australia

    • Ian R. Butler,
    • John M. Pandolfi &
    • Brigitte Sommer
  8. School of Medical Sciences, University of Sydney, Sydney, New South Wales 2006, Australia

    • Maria Byrne
  9. Australian Institute of Marine Science, PMB 3, Townsville, Queensland 4810, Australia

    • Neal E. Cantin,
    • Janice M. Lough &
    • Gergely Torda
  10. Australian Research Council Centre of Excellence in Coral Reef Studies, Oceans Institute and School of Earth and Environment, University of Western Australia, Crawley, Western Australia 6009, Australia

    • Steeve Comeau,
    • Ryan J. Lowe,
    • Malcolm T. McCulloch &
    • Verena Schoepf
  11. Fisheries Research, Department of Primary Industries, PO Box 4291, Coffs Harbour, New South Wales 2450, Australia

    • Steven J. Dalton &
    • Hamish A. Malcolm
  12. School of Environment, and Australian Rivers Institute, Griffith University, Brisbane, Queensland 4111, Australia

    • Guillermo Diaz-Pulido &
    • Emma V. Kennedy
  13. Coral Reef Watch, US National Oceanic and Atmospheric Administration, College Park, Maryland 20740, USA

    • C. Mark Eakin,
    • Scott F. Heron,
    • Gang Liu &
    • William J. Skirving
  14. School of Biological Sciences, University of Sydney, Sydney, New South Wales 2006, Australia

    • Will F. Figueira
  15. Australian Institute of Marine Science, Indian Oceans Marine Research Centre, University of Western Australia, Crawley, Western Australia 6009, Australia

    • James P. Gilmour
  16. Global Science & Technology, Inc., Greenbelt, Maryland 20770, USA

    • Scott F. Heron,
    • Gang Liu &
    • William J. Skirving
  17. Marine Geophysical Laboratory, College of Science, Technology and Engineering, James Cook University, Townsville, Queensland 4811, Australia

    • Scott F. Heron
  18. Department of Environment and Agriculture, Curtin University, Perth, Western Australia 6845, Australia

    • Jean-Paul A. Hobbs
  19. Great Barrier Reef Marine Park Authority, PO Box 1379, Townsville, Queensland 4810, Australia

    • Rachel J. Pears &
    • David R. Wachenfeld
  20. Torres Strait Regional Authority, PO Box 261, Thursday Island, Queensland 4875, Australia

    • Tristan Simpson
  21. Department of Parks and Wildlife, Kensington, Perth, Western Australia 6151, Australia

    • Shaun K. Wilson

Contributions

The study was conceptualized by T.P.H. who wrote the first draft of the paper. All authors contributed to writing subsequent drafts. J.T.K. coordinated data compilation, analysis and graphics. Aerial bleaching surveys in 2016 of the Great Barrier Reef and Torres Strait were executed by J.T.K., T.P.H. and T.S., and in 1998 and 2002 by R.B. and D.R.W. Underwater bleaching censuses in 2016 were undertaken on the Great Barrier Reef by M.A.-N., A.H.B., D.R.B., M.B., N.E.C., C.Y.K., G.D.-P., A.S.H., M.O.H., E.V.K., M.J.M., R.J.P., M.S.P., G.T. and B.L.W., in the Coral Sea by T.C.B. and H.B.H., in subtropical Queensland and New South Wales by M.B., I.R.B., R.C.B., S.J.D., W.F.F., H.A.M., J.M.P. and B.S., off western Australia by R.C.B., S.C., J.P.G., J.-P.A.H., M.T.M., V.S. and S.K.W. J.G.A.-R., S.R.C., C.M.E., S.F.H., G.L., J.M.L. and W.J.S. undertook the analysis matching satellite data to the bleaching footprints on the Great Barrier Reef.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

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Author details

Extended data figures and tables

Extended Data Figures

  1. Extended Data Figure 1: A generalized linear model to explain the severity of coral bleaching. (67 KB)

    Curves show the estimated relationships between probability of severe bleaching (>30%) on individual reefs of the Great Barrier Reef in 2016 and three explanatory variables (DHWs, chlorophyll a, and reef zoning, see Extended Data Table 1). The DHW-only model is shown in black. For the DHW plus chlorophyll a model, the blue threshold shows the estimated relationship between probability of severe bleaching and DHW for the 25th percentile of chlorophyll a, and the brown threshold shows the same for the 75th percentile of chlorophyll a. For the DHW plus reef zoning model, the red threshold shows the relationship for fished reefs, and the green for unfished reefs. Water-quality metrics and level of reef protection make little, if any, difference.

  2. Extended Data Figure 2: Difference in daily sea surface temperatures between the northern and southern Great Barrier Reef, before and after ex-tropical cyclone Winston. (60 KB)

    The disparity between Lizard Island (14.67° S) and Heron Island (23.44° S) increased from 1 °C in late February to 4 °C in early March 2016.

  3. Extended Data Figure 3: A test for the effect of past bleaching experience on the severity of bleaching in 2016. (42 KB)

    The relationship between previous bleaching scores (in 1998 or 2002, whichever was higher) and the residuals from the DHW generalized linear model (Extended Data Table 1). Each data point represents an individual reef that was scored repeatedly. There is no negative relationship to support acclimation or adaptation.

  4. Extended Data Figure 4: Flight tracks of aerial surveys of coral bleaching, conducted along and across the Great Barrier Reef and Torres Strait in March and April 2016. (217 KB)

    Blue colour represents land, white colour represents open water.

  5. Extended Data Figure 5: Ground-truthing comparisons of aerial and underwater bleaching scores. (36 KB)

    Aerial scores are: 0 (<1% of colonies bleached), 1 (1–10%), 2 (10–30%), 3 (30–60%) and 4 (60–100%) on the Great Barrier Reef in 2016 (Fig. 1a). Continuous (0–100%) underwater scores are based on in situ observations from 259 sites (104 reefs). Error bars indicate two standard errors both above and below the median underwater score, separately for each aerial category.

Extended Data Tables

  1. Extended Data Table 1: A test for the causes of coral bleaching (123 KB)
  2. Extended Data Table 2: Winners and losers (101 KB)

Additional data