Pacific in Peril - Greenpeace

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PACIFIC IN PERIL: BIOLOGICAL, ECONOMIC AND SOCIAL IMPACTS OF CLIMATE CHANGE ON PACIFIC CORAL REEFS Figure 6. Degree Heating Months (DHM) for 13 Pacific nations (see text for explanation). intensity and frequency of mass bleaching is likely to change over the next century. Strong et al (2000b) recognise the important contribution that the magnitude of a thermal anomaly and the exposure time make to intensity of a bleaching event. The NOAA satellite-derived Degree Heating Week (DHW) is an experimental product that calculates the accumulated thermal stress experienced by coral reefs. One DHW is equivalent to one week of sea surface temperature that is 1ºC above the expected summertime maximum sea 20 | Pacific in Peril (surface) temperature. Strong et al (2000b) and Toscano et al (1999) have observed that, in the past, DHWs of greater than 10 have resulted in severe bleaching and mortality among reef-building corals. For the current study, the DHW methodology was adapted for use in the following way. Because monthly mean temperatures were the only ones available going back to 1900, Degree Heating Months (DHMs) were used instead of DHWs. In this way, as pointed out by Strong et al (2000b), a DHM represents 4.3 DHWs and the critical threshold for severe bleaching and mortality are DHMs that are greater than 2.3. The method was applied to sea temperature data from November 1981 to August 2000 (Figure 6) and successfully detected key bleaching events as discussed above. The same approach of Strong et al (1996) was used in the present study to calculate the thermal “triggers” associated with coral bleaching. Strong et al (1996) have found that deviations of 0.8ºC above the mean summertime maxima will usually result in a bleaching event. The thermal triggers for 14 Pacific locations were calculated from IGOSS data and are reported in Table 2. The occurrence of thermal stress in the periods 1987-88, 1991-92, 1994 and 1998 for certain sites is accurately represented in Figure 6. Also clear from these graphs is the extent to which the intensity of events appears roughly encoded. Severe events, such as the recent Fijian and the 1998 Palauan events, are recorded as more intense than bleaching events that have been reported to be less severe, such as those in 1998 in French Polynesia and the Cook Islands (mild bleaching, personal observations). The relative intensities of events are likely to be best within data sets. In French Polynesia, the 1994 event was widely reported to be the most intense recent event (Salvat, 1991; Hoegh-Guldberg and Salvat; 1995, Drollet et al, 1994; Hoegh-Guldberg, 1999). The event in Palau resulted in the widespread loss of corals (J Bruno, pers comm), as did similar events in Okinawa (Y Loya, W Loh and K Sakai, pers comm), Seychelles (Spencer et al, 2000) and Scott Reef off Western Australia (Smith and Heyward, in press). To
verify that DHMs greater than 2.3 resulted in severe bleaching and significant mass mortality of corals, DHMs were calculated for the 1998 event in Palau, Okinawa, Seychelles (using data of Spencer et al, 2000) and Scott Reef. IGOSS data were downloaded for these locations and the DHM associated with these mortality events calculated. The DHMs associated with these severe events were 3.9, 3.0, 3.1 and 2.6 for Palau, Okinawa, Seychelles and Scott Reef respectively. By contrast, the DHMs for the southern, mid and northern sectors of the Great Barrier Reef (calculated for SST data for sectors of the Great Barrier Reef that were away from the more affected inshore sections) were 1.7, 1.4 and 0.5 respectively. Bleaching on the Great Barrier Reef was far less severe than in the four other locations (Berkelmans Degree Heating Months (ºC month) and Oliver, 1999). The DHM approach used in the present study can detect past bleaching events and logically fits with our current understanding of how thermal stress culminates in coral bleaching (that is, the longer the exposure the greater the effect on the dark reactions of photosynthesis, Jones et al, 1998). This strengthens the grounds for using it to project future thermal stress on coral reefs and the intensity and frequency of coral bleaching events. In the next section, the frequency and intensity of coral bleaching on reefs associated with Pacific nations is analysed. Most importantly, dates for critical levels of stress are determined, which drive the economic and social analysis that is the heart of the current study. Figure 7A. Estimates of thermal stress associated with the warming trends detected in the Pacific Ocean. Sea surface temperature data from COADS data (prior to November 1981) and IGOSS data (November 1981 to August 2000) were compared to the mean summer maxima and positive anomalies above + 0.2 oC recorded. Shown are Degree Heating Months (DHMs, two month moving point averages). 21 | Pacific in Peril
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Pacific in Peril - Greenpeace

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