for corals themselves (McClanahan et al., 2009) and associated reef organisms (Pratchett et al., 2009).
An important question is to what extent corals and their
symbionts maybe able to increase their tolerance to thermal stress with continued rapid global warming. Some
argue that the potential for such successful adaptation on
the time scales of observed and projected warming is limited (Hoegh-Guldberg, 1999, 2005). There is, however,
experimental and observational evidence that some corals
in some circumstances can and have increased their thermal tolerance by switching to a more thermally tolerant
type of algal symbiont (Berkelmans and van Oppen,
2006; Goulet, 2006; Maynard et al., 2008; Oliver and
Palumbi, 2009).
Warmer water temperatures and coral diseases
Increased numbers of reports of diseases affecting marine
organisms, including corals, have been tentatively linked
to warming waters and El Niño events (Harvell et al.,
1999). Coral disease outbreaks on the Great Barrier Reef
(GBR) have been linked to temperature stress (Jones
et al., 2004; Bruno et al., 2007), and experimental and
observational studies show that coral black band disease
is enhanced in warmer waters and high light conditions
(Boyett et al., 2007).
Warmer water temperatures: other effects
Warmer water temperatures can also directly affect physiological processes and distributions of corals and associated organisms. The northward range of two branching
coral species has recently expanded in the western tropical
Atlantic (Precht and Aronson, 2004). Coral community
structure may change as juvenile corals grow more slowly
and appear to die faster in warmer waters (Edmunds,
2004, 2007). Warmer temperatures have also been shown
experimentally to affect larval supply, settlement, and survival (Negri et al., 2007; Nozawa and Harrison, 2007).
Observational evidence of recent slowing in coral growth
rates have also been linked to the combined effects of
warmer waters and ocean acidification (De’ath et al.,
2009; Tanzil et al., 2009).
Changing ocean chemistry
A more insidious consequence, and potentially catastrophic (Veron, 2008), of increasing greenhouse gases
for marine calcifying organisms is ocean acidification.
About 30% of the extra CO 2 human activities have
injected into the atmosphere has been absorbed by the
oceans (Feely et al., 2004; Sabine et al., 2004) – if this
had not happened, the earth would have warmed more
than it has. Absorption of CO 2 lowers the pH, decreases
the availability of carbonate ions, and this lowers the saturation state of the major shell and skeleton forming carbonate minerals (Kleypas et al., 2006) Observational and
modeling evidence demonstrates ongoing decline in the
aragonite saturation state of Caribbean waters over the
period 1996–2006 (Gledhill et al., 2008). Lowering ocean
pH essentially shifts the balance of the geochemical equations whereby marine organisms, such as corals, calcify.
Various modeling and experimental studies also demonstrate the reduced ability of corals to form their skeletal
structures in more acidic waters (Kleypas et al., 1999b;
Langdon and Atkinson, 2005; Orr et al., 2005; Doney
et al., 2009).
Weaker coral reef structures reduce their structural
resilience to the natural forces of erosion and a slower
growth rate sets back recovery after disturbances. A recent
experimental study suggests that ocean acidification can
itself, and in combination with higher water temperatures,
induce coral bleaching (Anthony et al., 2008). Ocean
acidification affects not only corals (Guinotte and Fabry,
2008; Kuffner et al., 2008). Crustose coralline algae are
particularly sensitive, and any setback to them may significantly undermine their vital role of cementing reef components together (Littler and Littler 1984; see also
Chapter Algae, Coralline). The poorly cemented reefs of
the eastern tropical Pacific (where aragonite saturation of
waters is naturally low) may provide a picture of future
coral reefs as the oceans continue to acidify (Manzello
et al., 2008).
Sea level
Global average sea level has risen by about 20 cm over the
past century, primarily due to thermal expansion of the
oceans and, to a lesser extent, melting of land and sea
ice (IPCC, 2007a, Chap. 5), and the rate of rise has accelerated in recent decades (Church and White, 2006).
Although continued rising sea levels are of significance
for many densely populated, low-lying tropical communities adjacent to coral reefs (McGranaham et al., 2007),
a steady rise in sea level is not considered a major threat
to present day coral reefs. Global sea level has been relatively stable for the past several thousand years and some
reefs are limited by today’s levels and the rates of rise are
considered well within the ability of corals to keep up
(Smith and Buddemeier, 1992; Done and Jones, 2006).
Linking the physical environment with biological
processes
Determining how coral reefs and associated organisms
have and will respond to changing conditions depends
upon good observational studies of both the physical environment and biological responses. Ocean climate clearly
controls many aspects of coral reef ecosystems but even
for the one of the best studied coral reef ecosystems, the
Great Barrier Reef, our ability to determine the biotic
responses of its many component organisms to climate,
climate variation, and climate change is limited. The myriad organisms that make up a tropical coral reef such as
microbial assemblages, plankton, macroalgae, seagrass
beds, intertidal mangrove, salt marshes and wetlands, benthic invertebrates, sharks and rays, marine mammals,
marine reptiles, fishes, and corals are variously sensitive
to water characteristics (temperature, chemistry, and
202
CLIMATE CHANGE AND CORAL REEFS
An important question is to what extent corals and their
symbionts maybe able to increase their tolerance to thermal stress with continued rapid global warming. Some
argue that the potential for such successful adaptation on
the time scales of observed and projected warming is limited (Hoegh-Guldberg, 1999, 2005). There is, however,
experimental and observational evidence that some corals
in some circumstances can and have increased their thermal tolerance by switching to a more thermally tolerant
type of algal symbiont (Berkelmans and van Oppen,
2006; Goulet, 2006; Maynard et al., 2008; Oliver and
Palumbi, 2009).
Warmer water temperatures and coral diseases
Increased numbers of reports of diseases affecting marine
organisms, including corals, have been tentatively linked
to warming waters and El Niño events (Harvell et al.,
1999). Coral disease outbreaks on the Great Barrier Reef
(GBR) have been linked to temperature stress (Jones
et al., 2004; Bruno et al., 2007), and experimental and
observational studies show that coral black band disease
is enhanced in warmer waters and high light conditions
(Boyett et al., 2007).
Warmer water temperatures: other effects
Warmer water temperatures can also directly affect physiological processes and distributions of corals and associated organisms. The northward range of two branching
coral species has recently expanded in the western tropical
Atlantic (Precht and Aronson, 2004). Coral community
structure may change as juvenile corals grow more slowly
and appear to die faster in warmer waters (Edmunds,
2004, 2007). Warmer temperatures have also been shown
experimentally to affect larval supply, settlement, and survival (Negri et al., 2007; Nozawa and Harrison, 2007).
Observational evidence of recent slowing in coral growth
rates have also been linked to the combined effects of
warmer waters and ocean acidification (De’ath et al.,
2009; Tanzil et al., 2009).
Changing ocean chemistry
A more insidious consequence, and potentially catastrophic (Veron, 2008), of increasing greenhouse gases
for marine calcifying organisms is ocean acidification.
About 30% of the extra CO 2 human activities have
injected into the atmosphere has been absorbed by the
oceans (Feely et al., 2004; Sabine et al., 2004) – if this
had not happened, the earth would have warmed more
than it has. Absorption of CO 2 lowers the pH, decreases
the availability of carbonate ions, and this lowers the saturation state of the major shell and skeleton forming carbonate minerals (Kleypas et al., 2006) Observational and
modeling evidence demonstrates ongoing decline in the
aragonite saturation state of Caribbean waters over the
period 1996–2006 (Gledhill et al., 2008). Lowering ocean
pH essentially shifts the balance of the geochemical equations whereby marine organisms, such as corals, calcify.
Various modeling and experimental studies also demonstrate the reduced ability of corals to form their skeletal
structures in more acidic waters (Kleypas et al., 1999b;
Langdon and Atkinson, 2005; Orr et al., 2005; Doney
et al., 2009).
Weaker coral reef structures reduce their structural
resilience to the natural forces of erosion and a slower
growth rate sets back recovery after disturbances. A recent
experimental study suggests that ocean acidification can
itself, and in combination with higher water temperatures,
induce coral bleaching (Anthony et al., 2008). Ocean
acidification affects not only corals (Guinotte and Fabry,
2008; Kuffner et al., 2008). Crustose coralline algae are
particularly sensitive, and any setback to them may significantly undermine their vital role of cementing reef components together (Littler and Littler 1984; see also
Chapter Algae, Coralline). The poorly cemented reefs of
the eastern tropical Pacific (where aragonite saturation of
waters is naturally low) may provide a picture of future
coral reefs as the oceans continue to acidify (Manzello
et al., 2008).
Sea level
Global average sea level has risen by about 20 cm over the
past century, primarily due to thermal expansion of the
oceans and, to a lesser extent, melting of land and sea
ice (IPCC, 2007a, Chap. 5), and the rate of rise has accelerated in recent decades (Church and White, 2006).
Although continued rising sea levels are of significance
for many densely populated, low-lying tropical communities adjacent to coral reefs (McGranaham et al., 2007),
a steady rise in sea level is not considered a major threat
to present day coral reefs. Global sea level has been relatively stable for the past several thousand years and some
reefs are limited by today’s levels and the rates of rise are
considered well within the ability of corals to keep up
(Smith and Buddemeier, 1992; Done and Jones, 2006).
Linking the physical environment with biological
processes
Determining how coral reefs and associated organisms
have and will respond to changing conditions depends
upon good observational studies of both the physical environment and biological responses. Ocean climate clearly
controls many aspects of coral reef ecosystems but even
for the one of the best studied coral reef ecosystems, the
Great Barrier Reef, our ability to determine the biotic
responses of its many component organisms to climate,
climate variation, and climate change is limited. The myriad organisms that make up a tropical coral reef such as
microbial assemblages, plankton, macroalgae, seagrass
beds, intertidal mangrove, salt marshes and wetlands, benthic invertebrates, sharks and rays, marine mammals,
marine reptiles, fishes, and corals are variously sensitive
to water characteristics (temperature, chemistry, and
202
CLIMATE CHANGE AND CORAL REEFS