10 – The Future of Coral Reefs in a Rapidly Changing World
103
temperature, for example, is greatest in the polar regions where it affects the melting of the ice caps in the
spring and summer months. The increased melting of
the ice caps increases the volume of the global ocean
that, in addition to the thermal expansion of the ocean,
increases sea levels across the globe. Mean sea level
across the planet has risen by almost 17 cm over the past
100 years and is currently rising at 1–2 mm a year. This
is an order of magnitude higher than the average rate
over the previous several millennia. These changes are
causing concern given the enormous human population and associated infrastructure (e.g. cities, ports) located in low lying coastal areas of the world. Some low
lying countries in the Pacific such as Tuvalu, Kiribati
and the north coast of Java (Indonesia) are already being affected by spring tides and storm surges that reach
higher than ever before and now inundate towns and
human dwellings on a regular basis.
How rising sea levels will impact coral reefs is unclear at this point. On one hand, projected changes in
sea level are relatively slow compared to the rates of
coral growth (up to 20–30 cm per year) and hence are
unlikely to present major problems for healthy coral
populations. Coral reefs have also generally kept up
with even the most rapid changes in the past, as discussed in Chapter 3. The problem comes, of course,
with a combination of sea level rise and reduced growth
as a result of warmer and more acidic oceans, which
raises the prospect of coral populations that literally
get left behind (drown) as sea level rises.
Changing weather patterns also have the potential to
affect coral reefs through impacts on river flows and
drought (and hence the amount of sediment running off
coastal areas), and through the physical impact of storms.
Climate change is expected to increase the intensity of
storms via its influence on sea-surface temperature, a
fact emphasised by recent events such as hurricanes
Katrina, Wilma, Dean and the other record storms in the
Gulf of Mexico in 2005. On the GBR, cyclones, which are
relatively rare in the north and more common in central
and southern latitudes, are important drivers of the life
expectancies of corals as well as reef and island building processes. Changing cyclone strengths, wave
heights and return periods along the GBR will change
coastal inundation and the wave climates on reefs,
land runoff regimes and coastal salinities. These
changes represent potential threats, although the precise details of how they will affect coral reefs, or their
overall importance, is not entirely clear at this point.
Changes to weather patterns may have other important synergies. Recent trends on the Australian mainland, for example, show a drying trend along the eastern
seaboard over the past 70 years as a result of removal of
vegetation, causing destabilisation of sediments in river
catchment areas. Coupled with more intense storms,
destabilised sediments are likely to be increasingly
washed down river catchments and into coastal areas
where they may have impacts on coral reefs. Given that
increased sediment runoff is a major factor affecting corals in coastal regions, these changes have the potential to
have similar impacts to those seen when hard-hoofed
cattle were introduced into river catchments over 100
years ago along the north-east coastline of Australia.
Sediment flows in this case increased by 20 fold within a
few decades of cattle farmers arriving, leading to the loss
of inshore coral reefs in many parts of coastal Australia.
N GENETIC ADAPTATION TO
CLIMATE CHANGE
Some of the future scenarios outlined above (e.g.
responses to rising sea temperature and ocean acidity)
are predicated on the assumption that genetic change
within populations of organisms does not occur rapidly
enough to keep up with climate change. In terms of
reef-building corals, thermal thresholds have been relatively stable over several decades and have not shown
any tendency to drift upwards. The very fact that bleaching and mortality are increasing is circumstantial evidence that the threshold of corals for stress is not
changing rapidly enough to neutralise the effects of increasing stress from the Earth’s rapidly warming seas.
Several authors have challenged the use of fixed
thresholds for coral reefs, suggesting that the thermal
tolerance of corals and their symbionts may evolve rapidly. Some authors have even proposed that ‘reef corals
bleach to survive [climate] change’. These arguments, if
true, would have major impacts on how coral reefs will
respond to rapid shifts in the Earth’s climate. If the thermal tolerance of corals and their symbionts could keep
103
temperature, for example, is greatest in the polar regions where it affects the melting of the ice caps in the
spring and summer months. The increased melting of
the ice caps increases the volume of the global ocean
that, in addition to the thermal expansion of the ocean,
increases sea levels across the globe. Mean sea level
across the planet has risen by almost 17 cm over the past
100 years and is currently rising at 1–2 mm a year. This
is an order of magnitude higher than the average rate
over the previous several millennia. These changes are
causing concern given the enormous human population and associated infrastructure (e.g. cities, ports) located in low lying coastal areas of the world. Some low
lying countries in the Pacific such as Tuvalu, Kiribati
and the north coast of Java (Indonesia) are already being affected by spring tides and storm surges that reach
higher than ever before and now inundate towns and
human dwellings on a regular basis.
How rising sea levels will impact coral reefs is unclear at this point. On one hand, projected changes in
sea level are relatively slow compared to the rates of
coral growth (up to 20–30 cm per year) and hence are
unlikely to present major problems for healthy coral
populations. Coral reefs have also generally kept up
with even the most rapid changes in the past, as discussed in Chapter 3. The problem comes, of course,
with a combination of sea level rise and reduced growth
as a result of warmer and more acidic oceans, which
raises the prospect of coral populations that literally
get left behind (drown) as sea level rises.
Changing weather patterns also have the potential to
affect coral reefs through impacts on river flows and
drought (and hence the amount of sediment running off
coastal areas), and through the physical impact of storms.
Climate change is expected to increase the intensity of
storms via its influence on sea-surface temperature, a
fact emphasised by recent events such as hurricanes
Katrina, Wilma, Dean and the other record storms in the
Gulf of Mexico in 2005. On the GBR, cyclones, which are
relatively rare in the north and more common in central
and southern latitudes, are important drivers of the life
expectancies of corals as well as reef and island building processes. Changing cyclone strengths, wave
heights and return periods along the GBR will change
coastal inundation and the wave climates on reefs,
land runoff regimes and coastal salinities. These
changes represent potential threats, although the precise details of how they will affect coral reefs, or their
overall importance, is not entirely clear at this point.
Changes to weather patterns may have other important synergies. Recent trends on the Australian mainland, for example, show a drying trend along the eastern
seaboard over the past 70 years as a result of removal of
vegetation, causing destabilisation of sediments in river
catchment areas. Coupled with more intense storms,
destabilised sediments are likely to be increasingly
washed down river catchments and into coastal areas
where they may have impacts on coral reefs. Given that
increased sediment runoff is a major factor affecting corals in coastal regions, these changes have the potential to
have similar impacts to those seen when hard-hoofed
cattle were introduced into river catchments over 100
years ago along the north-east coastline of Australia.
Sediment flows in this case increased by 20 fold within a
few decades of cattle farmers arriving, leading to the loss
of inshore coral reefs in many parts of coastal Australia.
N GENETIC ADAPTATION TO
CLIMATE CHANGE
Some of the future scenarios outlined above (e.g.
responses to rising sea temperature and ocean acidity)
are predicated on the assumption that genetic change
within populations of organisms does not occur rapidly
enough to keep up with climate change. In terms of
reef-building corals, thermal thresholds have been relatively stable over several decades and have not shown
any tendency to drift upwards. The very fact that bleaching and mortality are increasing is circumstantial evidence that the threshold of corals for stress is not
changing rapidly enough to neutralise the effects of increasing stress from the Earth’s rapidly warming seas.
Several authors have challenged the use of fixed
thresholds for coral reefs, suggesting that the thermal
tolerance of corals and their symbionts may evolve rapidly. Some authors have even proposed that ‘reef corals
bleach to survive [climate] change’. These arguments, if
true, would have major impacts on how coral reefs will
respond to rapid shifts in the Earth’s climate. If the thermal tolerance of corals and their symbionts could keep