In early 1998 a mass coral
bleaching event took place on the Great Barrier Reef
(refer Berkelmans 1998; Hyne 1998). Even though the
phenomenon of coral bleaching is not new – coral
bleaching on the Great Barrier Reef was noted as
early as the late 1920s – mass bleaching events
have been recorded since the late 1970s. Broad-scale
aerial surveys conducted in March and April 1998 of
654 reefs on the Great Barrier Reef indicated that 87
per cent of inshore reefs showed at least some
bleaching compared to 28 per cent of offshore reefs
(Berkelmans and Oliver 1999).
Post-bleaching surveys were carried
out in late 1998 to ascertain the recovery of these
reefs from the bleaching event. Preliminary results
from these surveys showed that generally corals
recovered quite well. The region with the highest
bleaching-related coral mortality was in the central
Great Barrier Reef between Townsville and Cairns.
Mortality levels were highly variable ranging from
less than five per cent to 80 per cent of living
coral cover. Of the 23 inshore reefs surveyed in the
central Great Barrier Reef 17 exhibited coral
mortality of less than 50 per cent, while six reefs
suffered high to severe coral mortality (figure 1).
Reefs in the Palm Island Group near Townsville were
the worst affected.
Offshore reefs have generally
recovered very well with only one out of eight reefs
(Otter Reef) surveyed suffering greater than 50 per
cent bleaching-related coral mortality. Coral
mortality varied widely with species, habitat, reef
zone and region. In the Keppel Island group near
Rockhampton, coral mortality was less than two per
cent despite up to 80 per cent of coral cover
bleaching in early 1998.
This bleaching event was not
confined to the Great Barrier Reef. Reports received
from many other countries made it clear that the
Great Barrier Reef bleaching event was part of a
major global bleaching episode. By comparison with
many other tropical coral reef systems, the Great
Barrier Reef was only mildly affected.
Figure 1.
Number of reefs in each of six coral mortality
categories following the 1998 coral bleaching event,
based on surveys conducted in the central Great
Barrier Reef during the bleaching event (March-April
1998) and seven months after the bleaching event
(October-November 1998).
Reefs in the Indian Ocean appear to
have been the worst affected with reports of
60–90 per cent coral mortality over large areas
of reef in the Seychelles, Maldives and the east
African coast (Wilkinson et al. 1999).
The 1998 coral bleaching event
coincided with an unusual year, meteorologically
speaking. The National Oceanographic and Atmospheric
Administration (NOAA) reports that 1998 was the
warmest year on record globally and that the warmest
three years on record all occurred in the 1990s
(Wilkinson et al. 1999). It also appears that 1998
was the most extreme year for sea temperatures on the
Great Barrier Reef based on a 95-year record (Lough,
in press).
Aerial surveys conducted during the 1998 bleaching
event showed that, of the reefs surveyed, 87% of
inshore reefs and 28% of offshore reefs were
bleached.
The 1998 bleaching event also
coincided with the end of a long El Niņo event. In
the western Pacific and Atlantic Oceans, there has
been a strong link between the ENSO cycle and coral
bleaching. This is mainly due to warming of surface
waters on both sides of the American continent.
However, on the Great Barrier Reef, the connection
between coral bleaching and El Niņo cycles is not as
strong. Three of the last six coral bleaching events
at Magnetic Island near Townsville, for example, took
place when the Southern Oscillation Index was either
positive or close to zero. This may reflect the fact
that during El Niņo years, there is generally a pool
of cool oceanic water off eastern Australia. Although
this should mean cooler temperatures on the Great
Barrier Reef, these years can also coincide with low
cloud cover and unusually dry periods in Australia.
During these times, high irradiance and warm dry
winds can increase local surface water temperatures
and cause bleaching. The meteorological signals on
the Great Barrier Reef can therefore be rather mixed.
A recent report from the Australian
Institute of Marine Science (Lough, in press)
presents an analysis of long-term sea temperature
data for the Great Barrier Reef. The results
contained in the report are the first documented
findings of climate change on the Great Barrier Reef.
The major findings of the report
are that sea surface temperatures on the Great
Barrier Reef have warmed significantly since 1903
when continuous records were first compiled from the
Global Ocean Surface Temperature Atlas Plus
(GOSTAplus) ‘ships of opportunity’ data.
These data were supplemented and calibrated with the
Integrated Global Ocean Services System (IGOSS)
blended satellite data (recorded since 1981) and
continuous in situ measurements from automatic
weather stations (recorded since the late 1980s) and
data loggers. Data from loggers was obtained through
the Great Barrier Reef Marine Park Authority's
long-term temperature monitoring program which began
in the early 1990s.
Lough (in press) reports that there
is a clear long-term warming trend in average annual
and maximum summer temperatures at all latitudes
along the north-east Australian coast. The magnitude
of the warming trend is highest in southern
Queensland waters and lowest in the far northern
Great Barrier Reef. The warming appears to have
occurred mainly since the 1950s and temperatures are
rising up to 0.1oC per decade. The cumulative average
increase in sea temperature this century ranges from
around 0.3oC near the Torres Strait to just under 1oC near Brisbane. From
the perspective of the Great Barrier Reef, the
greatest threat from changing sea surface
temperatures is an increase in the magnitude of
anomalous, or extreme, temperatures and a decrease in
the return time of such events. Maximum monthly mean
sea temperature anomalies have also increased
significantly this century at rates and in a pattern
similar to the above (Lough, in press). Moreover, the
return time of extreme events has shortened from an
average of one in 10 years during the period
1904–1933 to around one in four years between
1964 and 1994. In the last two decades, maximum sea
temperatures have exceeded the bleaching thresholds
of sensitive corals at least six times at Magnetic
Island.
To what extent climate change is
caused by greenhouse gas emissions, or is simply an
existing natural trend, is still being debated by
climatologists. Certainly, coral bleaching is
connected to climate extremes. Evidence is also
building of a consistent and statistically
significant upward trend in sea temperatures at many
places in the world.
Although management agencies such
as the Great Barrier Reef Marine Park Authority
cannot do anything about climate change itself, the
importance of documenting potential links between
climate change and bleaching is recognised. Factors
such as sediment runoff, nutrients, pollutants,
dredging etc. may also contribute to, or exacerbate,
coral bleaching and mortality.
An international research
collaboration between the Great Barrier Reef Marine
Park Authority, the Australian Institute of Marine
Science and the US National Oceanographic and
Atmospheric Administration has begun to study the
climate change issue and other stress factors which
may contribute to coral bleaching and mortality. For
further information on this collaboration please
contact Dr Jamie Oliver at the Great Barrier Reef
Marine Park Authority (telephone +61 7 4750 0700) or
Dr Terry Done at the Australian Institute of Marine
Science
(telephone +61 7 4753 4211).
References
Berkelmans, R. 1998, What’s
Out There? - Corals bleached whiter than white, but
what went wrong in the final rinse? Reef Research,
8(2): 3–4.
Berkelmans, R. and Oliver, J.K.
1999, Large-scale bleaching of corals on the Great
Barrier Reef, Coral Reefs, 18(1): 55–60.
Hyne, J. 1998, Bleaching, the great
unknown, Reef Management News, Reef Research,
8(2): 8–11.
Lough, J. in press, Sea surface
temperatures on the Great Barrier Reef: a
contribution to the study of coral bleaching,
Research Publication No. 57, Great Barrier Reef
Marine Park Authority, Townsville.
Wilkinson, C. et al. 1999,
Ecological and socioeconomic impacts of 1998 coral
mortality in the Indian Ocean: an ENSO impact and a
warning of future change? Ambio, 28(2):
188-196.
