envelope they are used to with warming water temperatures
and ocean acidification are likely to be most important
(Kleypas et al., 2001; Fabricius et al., 2007; HoeghGuldberg et al., 2007; Lough, 2008):
Warmer waters increasing incidence of coral bleaching
and diseases; direct effects on physiological processes
of corals and other reef organisms.
Ocean acidification weakening skeletons and reef structures; direct effects on physiological processes of corals
and other reef organisms.
More intense tropical cyclones increasing incidence of
localized reef destruction.
More intense rainfall and river flow increasing frequency
of low salinity waters that extend further offshore, stress
corals osmotically, and raise ambient nutrient loadings,
producing an environment that favors phytoplankton
production, proliferation of macroalgae, increases in filter feeders, and Crown-of-Thorns outbreaks.
Changed ocean circulation patterns affecting reef connectivity, upwelling, and nutrient supplies.
Changes to ENSO activity (at present ill-defined) are
likely to continue as source of significant interannual
climate variability with El Niño events increasing probability of warmer waters throughout most of tropics
and, therefore, conditions conducive to coral bleaching
and coral diseases superimposed on warmer baseline
water temperatures.
Rising sea level may drown some deeper reefs, increase
shallow areas available to others, and wash away lowlying reef islands and cays. Likely to increase destruction associated with more intense tropical cyclones
due to higher storm surges and intensified coastal
erosion.
Combined effects of chronic acidification with increased frequency of disturbances to reefs (bleaching,
tropical cyclones) and reduced recovery intervals
between disturbances.
Compromising of physical structure of reefs with an
overall decline in building of calcium carbonate reef
structures, a shift in balance from net calcification to
net erosion, and an increased available bare substrate
for algae.
Loss of structural complexity will reduce the range of
habitats and shelter available for other reef-associated
organisms.
The net effects of this range of increasing chronic and
acute stressors associated with a changing climate (combined with, in many locations, already degraded coral
ecosystems) are likely to be much simpler and ecologically less complex coral reefs characterized by lower
biodiversity. Coral reef “ecosystems” will not respond
as a whole (Guinotte et al., 2003) and their regional
makeup will change as some species are better able to
cope while others become locally extinct (Graham,
2007; Carpenter et al., 2008) and some expand and others
contract their current distributions. There will be direct
physiological responses by taxa other than corals to
warming waters and ocean acidification. For example,
experimental evidence suggests that future temperature
and acidification scenarios will lead to reduced fitness
and biodiversity losses in coral reef fishes (Munday
et al., 2008; Przeslawski et al., 2008).
Several studies have considered aspects, singly and in
combination, of these projected climate change impacts
on coral reefs. Projections of future SSTs and aragonite
saturation state of the Pacific Ocean for the late twentyfirst century suggest that nearly all present-day coral reef
habitats are likely to be marginal for reef development
(though the projected changes are not outside the ranges
of current marginal reef habitats). The potential for poleward migration of coral reefs (which depends on temperature, ocean chemistry, and suitable substrate) appears
limited (Guinotte et al., 2003). The magnitude of future
warming is likely to vary spatially with consequent spatial variability in future bleaching impacts and frequency (Sheppard, 2003; Sheppard and Rioja-Nieto,
2005; McClanahan et al., 2007b). Increases of only 1
C
in Caribbean SSTs will expand the extent and intensity
of bleaching events to 100% of the area (McWilliams
et al., 2005). A similar increase on the Great Barrier Reef
(GBR) would increase the occurrence of bleaching from
the $50% observed in 1998 and 2002 to $80%, with
increases of 2–3
C increasing the area of the GBR that
bleaches to 97–100% (Berkelmans et al., 2004). Avoiding
near-annual coral bleaching events within 30–50 years
requires corals increase their thermal tolerance levels by
0.2–1.0
C per decade (Donner et al., 2005), and coral
recovery may require management actions that reduce
expansion of algae between disturbances (Wooldridge
et al., 2005). Modeling studies also suggest that improving
water quality through improved agricultural practices can
significantly raise the thermal threshold for bleaching
(Wooldridge, 2009).
Some studies have also attempted to identify the “tipping point” at which coral reefs can no longer sustain
themselves as carbonate structures. Cao and Caldeira
(2008) estimate that prior to the Industrial Revolution,
98% of coral reefs lived in waters with suitable carbonate
chemistry, whereas once atmospheric CO 2 levels reach
450 ppm, only 8% of reefs will be in waters with the necessary aragonite saturation level. Dramatically, HoeghGuldberg et al. (2007, p. 1741) suggest that CO 2 levels
greater than 500 ppm combined with water temperatures
2
C warmer will “reduce coral reef ecosystems to crumbling frameworks with few calcareous corals” and similarly that once CO 2 reaches 560 ppm “all coral reefs will
cease to grow and start to dissolve” (Silverman et al.,
2009, p. 1).
Combined disturbances and recovery intervals
Long-term observational studies demonstrate the sensitivity of coral reef communities to repeated disturbances.
Tropical cyclones, bleaching events, low salinity waters
due to heavy rainfall and river flows, outbreaks of
204
CLIMATE CHANGE AND CORAL REEFS
and ocean acidification are likely to be most important
(Kleypas et al., 2001; Fabricius et al., 2007; HoeghGuldberg et al., 2007; Lough, 2008):
Warmer waters increasing incidence of coral bleaching
and diseases; direct effects on physiological processes
of corals and other reef organisms.
Ocean acidification weakening skeletons and reef structures; direct effects on physiological processes of corals
and other reef organisms.
More intense tropical cyclones increasing incidence of
localized reef destruction.
More intense rainfall and river flow increasing frequency
of low salinity waters that extend further offshore, stress
corals osmotically, and raise ambient nutrient loadings,
producing an environment that favors phytoplankton
production, proliferation of macroalgae, increases in filter feeders, and Crown-of-Thorns outbreaks.
Changed ocean circulation patterns affecting reef connectivity, upwelling, and nutrient supplies.
Changes to ENSO activity (at present ill-defined) are
likely to continue as source of significant interannual
climate variability with El Niño events increasing probability of warmer waters throughout most of tropics
and, therefore, conditions conducive to coral bleaching
and coral diseases superimposed on warmer baseline
water temperatures.
Rising sea level may drown some deeper reefs, increase
shallow areas available to others, and wash away lowlying reef islands and cays. Likely to increase destruction associated with more intense tropical cyclones
due to higher storm surges and intensified coastal
erosion.
Combined effects of chronic acidification with increased frequency of disturbances to reefs (bleaching,
tropical cyclones) and reduced recovery intervals
between disturbances.
Compromising of physical structure of reefs with an
overall decline in building of calcium carbonate reef
structures, a shift in balance from net calcification to
net erosion, and an increased available bare substrate
for algae.
Loss of structural complexity will reduce the range of
habitats and shelter available for other reef-associated
organisms.
The net effects of this range of increasing chronic and
acute stressors associated with a changing climate (combined with, in many locations, already degraded coral
ecosystems) are likely to be much simpler and ecologically less complex coral reefs characterized by lower
biodiversity. Coral reef “ecosystems” will not respond
as a whole (Guinotte et al., 2003) and their regional
makeup will change as some species are better able to
cope while others become locally extinct (Graham,
2007; Carpenter et al., 2008) and some expand and others
contract their current distributions. There will be direct
physiological responses by taxa other than corals to
warming waters and ocean acidification. For example,
experimental evidence suggests that future temperature
and acidification scenarios will lead to reduced fitness
and biodiversity losses in coral reef fishes (Munday
et al., 2008; Przeslawski et al., 2008).
Several studies have considered aspects, singly and in
combination, of these projected climate change impacts
on coral reefs. Projections of future SSTs and aragonite
saturation state of the Pacific Ocean for the late twentyfirst century suggest that nearly all present-day coral reef
habitats are likely to be marginal for reef development
(though the projected changes are not outside the ranges
of current marginal reef habitats). The potential for poleward migration of coral reefs (which depends on temperature, ocean chemistry, and suitable substrate) appears
limited (Guinotte et al., 2003). The magnitude of future
warming is likely to vary spatially with consequent spatial variability in future bleaching impacts and frequency (Sheppard, 2003; Sheppard and Rioja-Nieto,
2005; McClanahan et al., 2007b). Increases of only 1
C
in Caribbean SSTs will expand the extent and intensity
of bleaching events to 100% of the area (McWilliams
et al., 2005). A similar increase on the Great Barrier Reef
(GBR) would increase the occurrence of bleaching from
the $50% observed in 1998 and 2002 to $80%, with
increases of 2–3
C increasing the area of the GBR that
bleaches to 97–100% (Berkelmans et al., 2004). Avoiding
near-annual coral bleaching events within 30–50 years
requires corals increase their thermal tolerance levels by
0.2–1.0
C per decade (Donner et al., 2005), and coral
recovery may require management actions that reduce
expansion of algae between disturbances (Wooldridge
et al., 2005). Modeling studies also suggest that improving
water quality through improved agricultural practices can
significantly raise the thermal threshold for bleaching
(Wooldridge, 2009).
Some studies have also attempted to identify the “tipping point” at which coral reefs can no longer sustain
themselves as carbonate structures. Cao and Caldeira
(2008) estimate that prior to the Industrial Revolution,
98% of coral reefs lived in waters with suitable carbonate
chemistry, whereas once atmospheric CO 2 levels reach
450 ppm, only 8% of reefs will be in waters with the necessary aragonite saturation level. Dramatically, HoeghGuldberg et al. (2007, p. 1741) suggest that CO 2 levels
greater than 500 ppm combined with water temperatures
2
C warmer will “reduce coral reef ecosystems to crumbling frameworks with few calcareous corals” and similarly that once CO 2 reaches 560 ppm “all coral reefs will
cease to grow and start to dissolve” (Silverman et al.,
2009, p. 1).
Combined disturbances and recovery intervals
Long-term observational studies demonstrate the sensitivity of coral reef communities to repeated disturbances.
Tropical cyclones, bleaching events, low salinity waters
due to heavy rainfall and river flows, outbreaks of
204
CLIMATE CHANGE AND CORAL REEFS