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Predicting Coral Bleaching by Hotspot Analysis

 GCRA researchers originated and developed the Coral Bleaching Hotspot method that has been used to successfully forecast all major coral bleaching events since 1990, and to hind cast all major bleaching events since 1982. Typically we are able to predict when and where bleaching will take place and how bad it will be using this method, often before the first signs are visible in the field, allowing field researchers to be warned to catch the early stages of such events.

Large scale coral bleaching events were shown to be predicable from in-situ water temperature measurements in Jamaica in 1989 (Goreau, 1990, http://www.globalcoral.org/Coral%20Bleaching%20in%20Jamaica.htm). This approach was then applied to NOAA satellite sea surface temperature measurements from 1981-1990 in reefs around the Caribbean (Goreau, Hayes, Clark, Basta, & Robertson, 1993, http://www.globalcoral.org/elevated_sea_surface_temperature.htm). It was found that at all sites the threshold corresponded to temperatures in the warmest month that were only about 1 degree C above normal. The Hotspot method was formally defined and then successfully applied to coral reefs worldwide (Goreau and Hayes, 1994a, http://www.globalcoral.org/Coral%20Bleaching%20&%20Ocean%20Hot%20Spots.pdf), and it was shown in the South Pacific that while many environmental stresses correlated with decreased live coral cover, only elevated temperatures correlated with mass bleaching (Goreau & Hayes, 1994b, http://www.globalcoral.org/coral_reef_bleaching_in_the_sout.htm).

The Hotspot method has continued to be developed to predict the timing and location of bleaching, as well as the degree of mortality, for example in 1998, the hottest year in history when coral reefs were devastated across the Indian Ocean (Goreau, Hayes, Strong, & McClanahan, 2000, http://www.globalcoral.org/1998%20Global%20Bleaching%20Event.pdf), 2002, the second hottest year in history, when the South Pacific was devastated (http://www.globalcoral.org/GLOBAL%20WARMING%20KILLS%20SOUTH%20PACIFIC%20CORAL%20REEFS.htm). The threshold for bleaching has now been revised downwards to around 0.9 degrees above average for the warmest month. If temperatures remain above 1 degree above average for two months or reach 2 degrees above for one month, serious mortality follows bleaching, but if it is less than this most corals recover. Of course this is a simplification in that these are not sharp thresholds but graded ones, and each species has a slightly different threshold and response, but the simplest version works sufficiently accurately to need very little modification.

The GCRA Hotspot method was so successful that NOAA set up a site to show instantaneous distributions of sea surface temperature anomalies and hotspots (http://www.osdpd.noaa.gov/PSB/EPS/SST/climo&hot.html).  This site shows three maps. The top map shows the current anomaly, the difference between current temperatures and the long term average value at the current time. The bottom two maps show the Hot Spots in the Western and Eastern hemispheres, the difference between current temperatures and the long term average for the warmest month of the year. These maps must be interpreted with caution, because they only show the instantaneous Hotspots, but of course these anomalies must remain for a month before serious bleaching results. Temperatures can rise above the threshold levels for short periods, days or weeks, without noticeable bleaching. The critical factor is the product of the intensity of the Hotspot and its duration. This is shown on yet another NOAA site (http://www.osdpd.noaa.gov/PSB/EPS/SST/dhw_retro.html).