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Predicted recurrences of mass coral mortality in the Indian Ocean


In 1998, more than 90% of shallow corals were killed on most Indian Ocean reefs1. High sea surface temperature (SST) was a primary cause2,3, acting directly or by interacting with other factors3,4,5,6,7. Mean SSTs have been forecast to rise above the 1998 values in a few decades2,3; however, forecast SSTs rarely flow seamlessly from historical data, or may show erroneous seasonal oscillations, precluding an accurate prediction of when lethal SSTs will recur. Differential acclimation by corals in different places complicates this further3,7,8. Here I scale forecast SSTs at 33 Indian Ocean sites where most shallow corals died in 1998 (ref. 1) to identify geographical patterns in the timing of probable repeat occurrences. Reefs located 10–15° south will be affected every 5 years by 2010–2025. North and south from this, dates recede in a pattern not directly related to present SSTs; paradoxically, some of the warmest sites may be affected last. Temperatures lethal to corals vary in this region by 6 °C, and acclimation of a modest 2 °C by corals could prolong their survival by nearly 100 years.

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Figure 1: Coral reef sites in the western Indian Ocean where 1998 SSTs caused mass coral mortality.
Figure 2: Historical and forecast SST data from two sites before and after the transformations.
Figure 3: Probabilities of the warmest months of four sites reaching the lethal 1998 temperatures over time.
Figure 4: ‘Extinction dates’ plotted for coral reef sites on the three transects, showing the date when a probability of 0.2 is reached.
Figure 5: Recession of time to extinction date with imagined acclimation of corals by up to 2 °C with Kenya as the example.


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The HadCM3 data were provided by the Hadley Centre for Climate Research through D. Viner, who also provided information on the data's characteristics. I thank M. Keeling and G. Medley for advice on analyses; N. Rayner of the Hadley Centre for information on the HadISST1 data and for communicating results before publication; and O. Langmead and A. Edwards for assistance with data extraction.

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Sheppard, C. Predicted recurrences of mass coral mortality in the Indian Ocean. Nature 425, 294–297 (2003).

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