Coral bleaching is not a new phenomenon due to global
warming.
Corals live within 1–2
C of their upper thermal threshold, beyond which bleaching occurs (Coles et al., 1976;
Jokiel and Coles, 1977). Currently maximum summer SSTs
(the time of year when corals are most at risk from thermal
stress) on coral reefs averages 29.5
C and ranges between
28.2 and 34.4
C (Kleypas et al., 1999a). There is not an
absolute temperature at which corals bleach, rather (which
is evidence of adaptation over the long term) the threshold
varies with ambient SSTs (Berkelmans, 2002).
Observations off the Pacific coast of Panama in 1983
presented one of the earliest “most alarming” reports of
“large-scale” bleaching of corals which was tentatively
linked to the 1982–1983 El Niño (Glynn, 1983). Subsequent studies clearly linked these unusual events to
warmer-than-usual SSTs (Brown, 1987; Glynn, 1996;
Jokiel and Brown, 2004). What is new and now clearly
associated with warming of the tropical oceans are
mass coral bleaching events where entire reefs are
affected and which can be locally attributed to unusually
warm maximum SSTs (Smith and Buddemeier, 1992;
Brown, 1997).
The real wake-up call regarding the sensitivity of corals
to warmer waters was during the major El Niño event of
1997–1998 (McPhaden 1999) which coincided with the
warmest year in global instrumental records (Hansen
et al., 2006). Bleaching was reported from nearly every
coral reef region and 16% of the world’s reefs was estimated to have been damaged (Wilkinson, 1998, 2004).
The scale and magnitude of this event, during which
impacts could be tracked round the world’s coral reefs as
each reached its unusually warm seasonal SST maximum,
catalyzed efforts both to understand and monitor conditions conducive to coral bleaching.
In an influential paper (Hoegh-Guldberg, 1999), mass
coral bleaching events were firmly linked to warming of
tropical seas as a result of climate change and, using global
climate model (GCM) projections for selected locations,
the author suggested that thermal thresholds for coral
bleaching could be regularly exceeded within a matter of
decades. Various studies have demonstrated that the level
of thermal stress on coral reefs is closely linked to coral
bleaching (e.g., Berkelmans et al., 2004) and that the level
of this stress has been increasing as the tropical oceans
warm (Lough, 2000; Barton and Casey, 2005; Sheppard
and Rioja-Nieto 2005).
Several of the recent mass coral bleaching events
have been associated with El Niño events (Williams
and Bunkley-Williams, 1990). These do not cause coral
bleaching but they increase the likelihood in many coral reef
regions of thermal conditions conducive to coral bleaching
(Lough, 2000; Eakin et al., 2009). Some reef locations, e.g.,
along the South Pacific Convergence Zone in the western
tropical Pacific, are more likely to be unusually warm during the La Niña phase of ENSO [bleaching was observed
here, e.g., during the 1998–1999 La Niña (Wilkinson,
2004)].
Consequences of coral bleaching
A recent comprehensive review paper (Baker et al., 2008)
summarizes much that we have learned and much that we
still need to understand about the mass coral bleaching
events that have affected many of the world’s coral
reefs since the 1980s. As a consequence of bleaching,
corals may fully recover, partially recover, or die, and
there are a range of short- and long-term impacts on coral
reef ecosystems. Examples include declines in abundance of coral- and reef-associated species, reduced coral
growth rates, increased susceptibility of corals to diseases,
enhanced rates of bioerosion, and impaired reproduction
and recruitment.
The intensity and occurrence of coral bleaching associated with widespread thermal stress shows considerable
variability. Bleaching is frequently more intense at the surface than on the sides of individual colonies, which implicates high light levels as a contributing factor (Salm and
Coles, 2001; Brown and Dunne, 2008). Local weather
conditions of slack winds, low cloud amount, and little
water motion all favor both warming and increased light
penetration through the water column. Different coral taxa
show different thermal susceptibility, with branching
corals often showing greater sensitivity than massive species (Marshall and Baird, 2000; McClanahan et al., 2004,
2005). Large-scale field surveys clearly demonstrate spatial clusters of more or less bleaching (Berkelmans et al.,
2004; McClanahan et al., 2007a, b). Experimental studies
suggest that high water motion can reduce the incidence
of bleaching (Nakamura and Van Woesik, 2001; West
and Salm, 2003; Smith and Birkeland, 2007). This is
supported by field observations of reduced bleaching in
regions of strong water motion due to tides, upwelling,
mixing and wave energy (Salm and Coles, 2001; Skirving
and Guinotte, 2001; Reigl, 2003), though in some locations high water flow has been suggested to reduce the
corals ability to deal with thermal stress (McClanahan
et al., 2005). Rapid cooling of SSTs by tropical cyclones
can also locally reduce the occurrence and intensity of
coral bleaching (Manzello et al., 2007).
Recovery from a bleaching disturbance is also variable
across all spatial scales and across different coral reef
communities (Golbuu et al., 2007; Baker et al., 2008).
Some reefs recovered relatively rapidly from the catastrophic 1997–1998 event (e.g., Indian Ocean reefs),
whereas others (e.g., Caribbean/western Atlantic reefs)
continue to decline, due to ongoing local stressors. Lack
of other local stressors is an important factor in determining how well coral reefs recover from bleaching
(Sheppard et al., 2008). However, even with recovery of
hard coral cover, there is evidence of differential recovery
of various components of the original coral reef ecosystem, which results in a different community structure after
the event (Smith et al., 2008). Key effects include loss of
structural complexity and habitat, local extinctions, loss
of biodiversity and key functional groups (Graham et al.,
2006), and both immediate and long-term consequences
CLIMATE CHANGE AND CORAL REEFS
201
warming.
Corals live within 1–2
C of their upper thermal threshold, beyond which bleaching occurs (Coles et al., 1976;
Jokiel and Coles, 1977). Currently maximum summer SSTs
(the time of year when corals are most at risk from thermal
stress) on coral reefs averages 29.5
C and ranges between
28.2 and 34.4
C (Kleypas et al., 1999a). There is not an
absolute temperature at which corals bleach, rather (which
is evidence of adaptation over the long term) the threshold
varies with ambient SSTs (Berkelmans, 2002).
Observations off the Pacific coast of Panama in 1983
presented one of the earliest “most alarming” reports of
“large-scale” bleaching of corals which was tentatively
linked to the 1982–1983 El Niño (Glynn, 1983). Subsequent studies clearly linked these unusual events to
warmer-than-usual SSTs (Brown, 1987; Glynn, 1996;
Jokiel and Brown, 2004). What is new and now clearly
associated with warming of the tropical oceans are
mass coral bleaching events where entire reefs are
affected and which can be locally attributed to unusually
warm maximum SSTs (Smith and Buddemeier, 1992;
Brown, 1997).
The real wake-up call regarding the sensitivity of corals
to warmer waters was during the major El Niño event of
1997–1998 (McPhaden 1999) which coincided with the
warmest year in global instrumental records (Hansen
et al., 2006). Bleaching was reported from nearly every
coral reef region and 16% of the world’s reefs was estimated to have been damaged (Wilkinson, 1998, 2004).
The scale and magnitude of this event, during which
impacts could be tracked round the world’s coral reefs as
each reached its unusually warm seasonal SST maximum,
catalyzed efforts both to understand and monitor conditions conducive to coral bleaching.
In an influential paper (Hoegh-Guldberg, 1999), mass
coral bleaching events were firmly linked to warming of
tropical seas as a result of climate change and, using global
climate model (GCM) projections for selected locations,
the author suggested that thermal thresholds for coral
bleaching could be regularly exceeded within a matter of
decades. Various studies have demonstrated that the level
of thermal stress on coral reefs is closely linked to coral
bleaching (e.g., Berkelmans et al., 2004) and that the level
of this stress has been increasing as the tropical oceans
warm (Lough, 2000; Barton and Casey, 2005; Sheppard
and Rioja-Nieto 2005).
Several of the recent mass coral bleaching events
have been associated with El Niño events (Williams
and Bunkley-Williams, 1990). These do not cause coral
bleaching but they increase the likelihood in many coral reef
regions of thermal conditions conducive to coral bleaching
(Lough, 2000; Eakin et al., 2009). Some reef locations, e.g.,
along the South Pacific Convergence Zone in the western
tropical Pacific, are more likely to be unusually warm during the La Niña phase of ENSO [bleaching was observed
here, e.g., during the 1998–1999 La Niña (Wilkinson,
2004)].
Consequences of coral bleaching
A recent comprehensive review paper (Baker et al., 2008)
summarizes much that we have learned and much that we
still need to understand about the mass coral bleaching
events that have affected many of the world’s coral
reefs since the 1980s. As a consequence of bleaching,
corals may fully recover, partially recover, or die, and
there are a range of short- and long-term impacts on coral
reef ecosystems. Examples include declines in abundance of coral- and reef-associated species, reduced coral
growth rates, increased susceptibility of corals to diseases,
enhanced rates of bioerosion, and impaired reproduction
and recruitment.
The intensity and occurrence of coral bleaching associated with widespread thermal stress shows considerable
variability. Bleaching is frequently more intense at the surface than on the sides of individual colonies, which implicates high light levels as a contributing factor (Salm and
Coles, 2001; Brown and Dunne, 2008). Local weather
conditions of slack winds, low cloud amount, and little
water motion all favor both warming and increased light
penetration through the water column. Different coral taxa
show different thermal susceptibility, with branching
corals often showing greater sensitivity than massive species (Marshall and Baird, 2000; McClanahan et al., 2004,
2005). Large-scale field surveys clearly demonstrate spatial clusters of more or less bleaching (Berkelmans et al.,
2004; McClanahan et al., 2007a, b). Experimental studies
suggest that high water motion can reduce the incidence
of bleaching (Nakamura and Van Woesik, 2001; West
and Salm, 2003; Smith and Birkeland, 2007). This is
supported by field observations of reduced bleaching in
regions of strong water motion due to tides, upwelling,
mixing and wave energy (Salm and Coles, 2001; Skirving
and Guinotte, 2001; Reigl, 2003), though in some locations high water flow has been suggested to reduce the
corals ability to deal with thermal stress (McClanahan
et al., 2005). Rapid cooling of SSTs by tropical cyclones
can also locally reduce the occurrence and intensity of
coral bleaching (Manzello et al., 2007).
Recovery from a bleaching disturbance is also variable
across all spatial scales and across different coral reef
communities (Golbuu et al., 2007; Baker et al., 2008).
Some reefs recovered relatively rapidly from the catastrophic 1997–1998 event (e.g., Indian Ocean reefs),
whereas others (e.g., Caribbean/western Atlantic reefs)
continue to decline, due to ongoing local stressors. Lack
of other local stressors is an important factor in determining how well coral reefs recover from bleaching
(Sheppard et al., 2008). However, even with recovery of
hard coral cover, there is evidence of differential recovery
of various components of the original coral reef ecosystem, which results in a different community structure after
the event (Smith et al., 2008). Key effects include loss of
structural complexity and habitat, local extinctions, loss
of biodiversity and key functional groups (Graham et al.,
2006), and both immediate and long-term consequences
CLIMATE CHANGE AND CORAL REEFS
201