Australian perspectives on global climate change and coral bleaching I:
1997-1998 sea surface temperatures at local to global scales
J. M. Lough
Australian Institute of Marine Science
Townsville, Qld 4810
Australia
1997-1998 witnessed the most intense El Nino-Southern Oscillation (ENSO) event on record, which resulted in major climatic anomalies throughout the tropics and extending into extra-tropical regions. 1998 has also been reported to be the warmest year in the past century and possibly the past millennium. For the coral reef community, 1997-1998 also witnessed a major event - an unprecedented number of reports of mass coral bleaching affecting most of the world's coral reefs. Although a variety of environmental stresses can cause corals to bleach, the evidence for 1997-1998 clearly implicates unusually warm SSTs during the respective warm seasons as the primary cause at affected reefs. Combined with mounting evidence of the reality of enhanced greenhouse-induced global warming, there is growing concern about the ability of coral reefs to withstand increased frequency of climatic conditions conducive to coral bleaching.
For coral reefs we need to know how unusual SSTs were during 1997-1998? Were SST patterns typical of ENSO events? Are SSTs in the vicinity of coral reefs warming in association with the global temperature rise? These questions are addressed at the local scale - the Great Barrier Reef (GBR), Australia and at the global scale - 47 coral reef sites affected by bleaching during 1997-98. Various sources of SST observations (in situ data loggers, automatic weather stations, satellites and “ships of opportunity”) are combined to address these questions. Two indices of warm season SSTs are used: 1) maximum monthly mean SST and 2) a omonth index which combines both the intensity and duration (over months) of unusually warm SSTs.
For the GBR, SSTs during early 1998 were found to be the warmest of the past 95 years of instrumental records; the pattern of SST anomalies was typical of ENSO events in the region; and SSTs have significantly warmed over the past century. This warming is greatest in the south and to the south of the GBR and is weakest in the northern GBR.
For the 47 globally distributed coral reef sites, the omonth index for 1998 was four times higher than in any other year during the period 1903 to 1999. This year was also anomalous in that warm SST anomalies were found in all ocean basins containing coral reefs. With the exception of the Caribbean and parts of the Pacific Ocean, the SST anomalies observed during 1997-1998 were typical of ENSO events. The majority of the coral reef sites examined have also experienced significant warming over the 20th century. The omonth index for 1999 returned to near “normal” levels.
