Climate Change, Coral Bleaching and the Future of the World's Coral Reefs

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CLIMATE
CHANGE
CORAL
BLEACHING
and the
FUTURE
of the
WORLDÕS
CORAL
REEFS
by
O V E H O E G H Ð G U L D B E R G
ASSOCIATE PROFESSOR,
SCHOOL OF BIOLOGICAL SCIENCES,
UNIVERSITY OF SYDNEY
DIRECTOR,
THE CORAL REEF RESEARCH INSTITUTE,
UNIVERSITY OF SYDNEY
ISBN 90-73361-52-4
Sea temperatures in the tropics have increased by almost
1oC over the past 100 years and are currently increasing at
the rate of approximately 1-2oC per century. Reefbuilding
corals, which are central to healthy coral reefs,
are currently living close to their upper thermal limit.
They become stressed if exposed to small slight increases
(1-2oC) in water temperature and experience coral
bleaching.
Coral bleaching occurs when the photosynthetic
symbionts of corals (zooxanthellae) become increasing
vulnerable to damage by light at higher than normal
temperatures. The resulting damage leads to the expulsion
of these important organisms from the coral host. Corals
tend to die in great numbers immediately following coral
bleaching events, which may stretch across thousands of
square kilometers of ocean. Bleaching events in 1998, the
worst on record, saw the complete loss of live coral from
reefs in some parts of the world.
This paper reviews our understanding of coral bleaching
and demonstrates that the current increase in the intensity
and extent of coral bleaching is due to increasing sea
temperature. Importantly, this paper uses the output from
four different runs from two major global climate models
to project how the frequency and intensity of bleaching
events are likely to change over the next hundred years if
greenhouse gas emissions are not reduced. The results of
this analysis are startling and a matter of great concern.
Sea temperatures calculated by all model projections show
that the thermal tolerances of reef-building corals are
likely to be exceeded within the next few decades. As a
result of these increases, bleaching events are set to
increase in frequency and intensity. Events as severe as the 1998 event could be become commonplace within twenty
years. Bleaching events are very likely to occur annually
in most tropical oceans by the end of the next 30-50 years.
There is little doubt among coral reef biologists that an
increase in the frequency of bleaching events of this
magnitude could have drastic consequences for coral reefs
everywhere. Arguments that corals will acclimate to
predicted patterns of temperature change are
unsubstantiated and evidence suggests that the genetic
ability of corals to acclimate is already being exceeded.
Corals may adapt in evolutionary time, but such changes
are expected to take hundreds of years, suggesting that the
quality of the worldÕs reefs will decline at rates that are faster than expected.
Every coral reef examined in Southeast Asia, the Pacific
and Caribbean showed the same trend. The worldÕs largest
continuous coral reef system (AustraliaÕs Great Barrier
Reef) was no exception and could face severe bleaching
events every year by the year 2030. Southern and central
sites of the Great Barrier Reef are likely to be severely
affected by sea temperature rise within the next 20-40
years. Northern sites are warming more slowly and are
expected to lag behind changes in the southern end of the
Great Barrier Reef by 20 years. In summary, the rapidity
and extent of these projected changes, if realized, spells
catastrophe for tropical marine ecosystems everywhere
and suggests that unrestrained warming cannot occur
without the complete loss of coral reefs on a global scale.
C L I M A T E C H A N G E , C O R A L B L E A C H I N G A N D T H E F U T U R E O F T H E W O R L D Õ S C O R A L R E E F S 1
Executive summary
Introduction
The environmental and economic importance of the
worldÕs coral reefs
Coral reefs are the most spectacular and diverse marine
ecosystems on the planet today. Complex and productive,
coral reefs boast hundreds of...
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