10 – The Future of Coral Reefs in a Rapidly Changing World
99
current concentration of carbon dioxide is 380 ppm,
about 80 ppm above that of 150 years ago (Fig. 10.1B).
Scientists quite appropriately question whether these
high concentrations are part of a larger cycle that will
eventually decrease as rapidly as they appeared. However, the Vostok ice core tells us that the concentration
of greenhouse gases (like CO 2 ) has never exceeded 300
ppm in the past 720 000 years, despite repeated cycles
of warm (like the one we are in currently) and cold periods (ice ages). When other data sets are taken into account, there is growing evidence that the concentration
of greenhouse gases such as CO 2 may not have exceeded
300 ppm for the past 20 million years.
The immediate question for biologists is whether
these seemingly small changes in CO 2 really matter.
Global Circulation Models that allow us to understand
the connectivity and relationships between elements
of the climate system on planet Earth suggest that they
do. Global Circulation Models are complex computer
models that simulate the interplay between the ice,
land, sea and atmosphere. They show how even such
small changes to CO 2 and other components of the
Earth’s atmosphere could radically affect the climate
of the Earth. Over 40 of these models have been built
by the majority of technological nations and demonstrate a highly accurate ability to hind caste changes in
global temperature (Fig 10.1C). These models all tell a
similar story: recent changes in global temperature can
only be explained by taking into account the recent
changes in the greenhouse gas content of the atmosphere. Moreover, they tell us that projected increases
in atmospheric greenhouse gases through the 21st century will drive major changes in Earth’s weather systems in general. The Earth has already warmed by
0.6°C over the past 100 years, and is set to rise by between 1.4°C and 6°C by the end of this century. To
gauge the significance of these projected changes, one
has only to remember that the temperature difference
between the modern climate and the glacial periods on
the planets was only 5–7°C. The 5–7°C temperature
increase from the last Ice Age to the present (over a
period of 20 000–30 000 years) increased sea level (180 m)
and changed the shape of the world map as continental margins were inundated and Earth’s climatic zones
reconfigured (see Chapters 2 and 3). Worst-case IPCC
scenarios suggest that an equivalent changing temperature to that which occurred as the Earth warmed
up and out of an ice age may take place in the next
100 years. Even in less extreme cases, almost every
aspect of the climate will likely change, notably sea
temperature, ocean currents, rainfall, and storm intensity. These in turn will shift the conditions for life on the
planet, driving changes in the extent, abundance and
types of ecosystems at all locations across the globe. As
we will see in the next few sections, coral reefs are one
of the ecosystems whose response to climate change
has demonstrated that the impacts are anything but
subtle, even with minor enhanced greenhouse forcing.
N MASS CORAL BLEACHING AND DISEASE
Australia’s tropical waters have warmed significantly
over the past 150 years, with much of the increase being
seen over the past 50 years. Rates of warming are similar
to that observed globally for tropical/subtropical waters
(0.73°C from 1951–1990). These changes have brought
corals on reefs closer to their thermal limits, with the
result that warmer than average years (arising due to
natural variability) now push corals beyond their upper
thermal thresholds. Corals respond to stress by losing
their normal brown colour (‘bleaching’). Coral bleaching occurs when the symbiosis between coral and their
dinoflagellate symbionts (Fig. 10.3A, B) disintegrates,
with the rapid movement of the brown symbionts out of
the otherwise translucent tissues of the coral (Fig. 10.3C).
As a result, corals change rapidly from brown to white.
Bleaching is a generic response that occurs in response
to a wide array of stresses, including reduced salinity,
high or low irradiance, some toxins like cyanide and
many herbicides, microbial infection and high or low
temperatures (see Additional reading).
Coral bleaching has been known for over 70 years
from reports in which individual colonies or small
patches of reefs were observed to have bleached.
However, more recently ‘mass’ coral bleaching events
have affected coral reefs over hundreds and even
thousands of square kilometres. Mass coral bleaching
events have only been reported since 1979. Work
done during the 1980s and 1990s revealed that mass
bleaching events are triggered by warmer than normal
99
current concentration of carbon dioxide is 380 ppm,
about 80 ppm above that of 150 years ago (Fig. 10.1B).
Scientists quite appropriately question whether these
high concentrations are part of a larger cycle that will
eventually decrease as rapidly as they appeared. However, the Vostok ice core tells us that the concentration
of greenhouse gases (like CO 2 ) has never exceeded 300
ppm in the past 720 000 years, despite repeated cycles
of warm (like the one we are in currently) and cold periods (ice ages). When other data sets are taken into account, there is growing evidence that the concentration
of greenhouse gases such as CO 2 may not have exceeded
300 ppm for the past 20 million years.
The immediate question for biologists is whether
these seemingly small changes in CO 2 really matter.
Global Circulation Models that allow us to understand
the connectivity and relationships between elements
of the climate system on planet Earth suggest that they
do. Global Circulation Models are complex computer
models that simulate the interplay between the ice,
land, sea and atmosphere. They show how even such
small changes to CO 2 and other components of the
Earth’s atmosphere could radically affect the climate
of the Earth. Over 40 of these models have been built
by the majority of technological nations and demonstrate a highly accurate ability to hind caste changes in
global temperature (Fig 10.1C). These models all tell a
similar story: recent changes in global temperature can
only be explained by taking into account the recent
changes in the greenhouse gas content of the atmosphere. Moreover, they tell us that projected increases
in atmospheric greenhouse gases through the 21st century will drive major changes in Earth’s weather systems in general. The Earth has already warmed by
0.6°C over the past 100 years, and is set to rise by between 1.4°C and 6°C by the end of this century. To
gauge the significance of these projected changes, one
has only to remember that the temperature difference
between the modern climate and the glacial periods on
the planets was only 5–7°C. The 5–7°C temperature
increase from the last Ice Age to the present (over a
period of 20 000–30 000 years) increased sea level (180 m)
and changed the shape of the world map as continental margins were inundated and Earth’s climatic zones
reconfigured (see Chapters 2 and 3). Worst-case IPCC
scenarios suggest that an equivalent changing temperature to that which occurred as the Earth warmed
up and out of an ice age may take place in the next
100 years. Even in less extreme cases, almost every
aspect of the climate will likely change, notably sea
temperature, ocean currents, rainfall, and storm intensity. These in turn will shift the conditions for life on the
planet, driving changes in the extent, abundance and
types of ecosystems at all locations across the globe. As
we will see in the next few sections, coral reefs are one
of the ecosystems whose response to climate change
has demonstrated that the impacts are anything but
subtle, even with minor enhanced greenhouse forcing.
N MASS CORAL BLEACHING AND DISEASE
Australia’s tropical waters have warmed significantly
over the past 150 years, with much of the increase being
seen over the past 50 years. Rates of warming are similar
to that observed globally for tropical/subtropical waters
(0.73°C from 1951–1990). These changes have brought
corals on reefs closer to their thermal limits, with the
result that warmer than average years (arising due to
natural variability) now push corals beyond their upper
thermal thresholds. Corals respond to stress by losing
their normal brown colour (‘bleaching’). Coral bleaching occurs when the symbiosis between coral and their
dinoflagellate symbionts (Fig. 10.3A, B) disintegrates,
with the rapid movement of the brown symbionts out of
the otherwise translucent tissues of the coral (Fig. 10.3C).
As a result, corals change rapidly from brown to white.
Bleaching is a generic response that occurs in response
to a wide array of stresses, including reduced salinity,
high or low irradiance, some toxins like cyanide and
many herbicides, microbial infection and high or low
temperatures (see Additional reading).
Coral bleaching has been known for over 70 years
from reports in which individual colonies or small
patches of reefs were observed to have bleached.
However, more recently ‘mass’ coral bleaching events
have affected coral reefs over hundreds and even
thousands of square kilometres. Mass coral bleaching
events have only been reported since 1979. Work
done during the 1980s and 1990s revealed that mass
bleaching events are triggered by warmer than normal