Global warming and recurrent mass bleaching of corals

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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.

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Figure 1: Geographic extent and severity of recurrent coral bleaching at a regional scale, Australia.
Figure 2: Recurrent severe coral bleaching.
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.
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.

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Acknowledgements

The authors acknowledge the 21 institutions that supported this research, in Australia, the UK, and the USA. Twenty-six of the authors are supported by funding from the Australian Research Council’s Centre of Excellence Program. Other funding support includes the Australian Commonwealth Government, the European Union, the USA National Oceanographic & Atmospheric Administration, and USA National Science Foundation. GlobColour data (http://globcolour.info) used in this study has been developed, validated, and distributed by ACRI-ST, France. The contents in this manuscript are solely the opinions of the authors and do not constitute a statement of policy, decision or position on behalf of NOAA or the US Government. We thank the many student volunteers who participated in field studies.

Author information

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.

Correspondence to Terry P. Hughes.

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Extended data figures and tables

Extended Data Figure 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.

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.

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.

Extended Data Figure 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.

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.

Blue colour represents land, white colour represents open water.

Extended Data Figure 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.

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

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Hughes, T., Kerry, J., Álvarez-Noriega, M. et al. Global warming and recurrent mass bleaching of corals. Nature 543, 373–377 (2017) doi:10.1038/nature21707

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