Chapter 9 Coral Records of the 1990s in the Tropical Northwest Pacific: ENSO, Mass Coral Bleaching, and Global Warming

The decade of the 1990s was very likely the warmest of the second millennium. The 1990s was also characterized by one of the strongest El Niño-Southern Oscillation (ENSO) events of the twentieth century, which occurred in 1997-1998 and was followed by worldwide mass coral bleaching. In this study, we examined the signal-recording ability of Porites spp. corals by comparing high-resolution coral oxygen isotope (δ¹⁸O) records from the northwestern Pacific with instrumental records of these profound events of the 1990s. Temporal changes in coral skeletal δ¹⁸O records from Ishigaki Island, Japan, showed good agreement with instrumental records of sea-surface temperature (SST) because the effect of seasonal and interannual variations in salinity on the δ¹⁸O of seawater in that region was relatively small. In the northwestern equatorial Pacific, the cooler SST and relative drought-characterizing El Niño events were particularly well recorded by corals from the Philippines. These conditions were also faithfully recorded as distinct positive anomalies in coral δ¹⁸O records from Chuuk Atoll and Pohnpei Island in Micronesia. Bleached Porites spp. corals from Ishigaki Island, as well as corals from Pandora Reef of the Great Barrier Reef, Australia, showed a growth gap, indicating a substantial decrease in skeletogenesis during the 1997-1998 mass bleaching event. At Ishigaki Island and the Pacific side of the Philippine Islands, a decreasing trend in δ¹⁸O_c was found even over the relatively short period between 1980 and 2000 which may indicate an SST rise and/or surface-water freshening related to global warming. Our results demonstrate the potential of coral isotope records to serve as indicators of climatic change in the tropical and subtropical waters of the northwestern Pacific, where the number of coral records is still limited.