Status of Coral Reefs of the World 2000

The 1997-98 Mass Coral Bleaching and Mortality Event
Bleaching then followed the path of the solar zenith in the far West Pacific from September
to November 1998, in: the Federated States of Micronesia in September; and Palau in
September to November when there was unprecedented coral mortality.
A consistent feature of the weather during all these bleaching events was the coincidence
of a major shift of the SOI away from zero, which apparently induced particularly calm
weather. For example, in the Indian Ocean, the monsoon winds stopped almost
completely, resulting in doldrum-calm conditions and weak currents. Under these
conditions sea surface waters warmed above the temperature tolerance limits for corals
and other symbiotic animals, without mixing with cooler oceanic and deeper waters. Warm
waters often extended down as far as 30m or more. Coral bleaching consistently started
about 4 to 6 weeks after the passage of the solar zenith.
A critical question remains; was the 1997-98 event a one-in-a-thousand year event killing off
large old corals, or will bleaching events like 1997-98 become more frequent and severe in
the future? Coral bleaching and mortality levels were closely associated with the El Niño
and La Niña switches in climate that started in 1997 and ended in early 1999, with large
deviations in the SOI. Many scientists now predict that major climate change events will
become more frequent and severe, resulting in greater bleaching. It is apparent that when a
strong La Niña (positive SOI) coincides with the northern summer, bleaching will follow;
likewise the major trigger for bleaching in the southern hemisphere is when a strong El Niño
(negative SOI) occurs in summer. Reefs closer to the equator will probably experience
bleaching under either El Niño or La Niña conditions.
Thus, these predicted scenarios mean that coral reefs will change in structure over the next
few decades with a major reduction in the branching and plate forming species that are the
fastest growers and major contributors to coral reef accretion. In their place, reefs will have
a lower profile of slow growing, massive species, with lower coral cover and lower
diversity in the medium term. This will probably reduce the capacity of reefs to act as
breakwaters for fragile shorelines and provide sand and rock as building materials on low
islands. Fish populations will probably drop as there will be reduced structural complexity
to shelter juveniles and the many small species, but changes will be gradual. The impacts
on tourism are harder to predict. Coral reef tourism, one of the fastest growing sectors of
the market, and growth will continue via the many tourists visiting coral reef destinations for
the first time. The only adverse affects may be felt with those few tourists who revisit coral
reefs for diving, and become disillusioned with large areas of dead corals. However, these
tourists will probably seek new experiences and go to the many areas that will escape
damage in the immediate future. Surveys of many tourists indicate that many are unaware
that the corals are dead, as they focus on fish and above water activities in the tourist resorts.
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