are atmospheric concentrations of greenhouse gases rising but
also the rate of increase is accelerating (Canadell et al., 2007).
This increase in atmospheric greenhouse gases results
in significant positive radiative forcing of the global climate system and global warming attributable to human
activities. The most recent Intergovernmental Panel on
Climate Change Fourth Assessment (IPCC-AR4) report
provides observational and paleoclimatic evidence for significant recent warming of global climate that matches
theoretical and modeled consequences of increased greenhouse gas concentrations (IPCC, 2007c). The relatively
modest global warming observed to date has already been
associated with changes in the global climate system such
as more intense rainfall, more frequent droughts, sea-level
rise, loss of Arctic sea ice, melting of land-based ice, and
a widening of the tropical climate belt (IPCC, 2007a,
Chap. 3; Seidal et al., 2007). The rate of warming is about
twice as fast for land masses compared to oceans and for
high latitudes of the Northern Hemisphere compared with
that for low latitude regions. Observed changes in climate
are driving changes in the world’s biological and physical
systems that are all consistent with a rapidly warming climate (IPCC, 2007b; Rosenzweig et al., 2008).
The observational record shows that the average global
land and sea temperature climate of the most recent 30 years
(1979–2008) is significantly warmer (þ0.55
C) than the climate at the end of the nineteenth century (1871–1900;
Figure 1a). The tropical oceans, home to the world’s coral
reefs, have also significantly warmed (at about 70% of the
global average value) by þ0.40
C between the same periods
(Figure 1b). The rate of warming has accelerated from 0.05
C
(global) and 0.04
C per decade (tropical oceans), for the
period 1871 to 2008, to 0.12
C (global) and 0.08
C per
decade (tropical oceans) for the recent period 1950–2008.
Coral reef ecosystems, which occur in the naturally
warm tropical oceans and are one of the largest sources
of global marine diversity (Sala and Knowlton, 2006),
are considered among the “most vulnerable ecosystems”
to global climate change (IPCC, 2007b, Chap. 4, p. 214).
This global-scale threat is occurring against a backdrop
in which many of the world’s coral reef ecosystems have
already been degraded by direct human pressures. These
local and regional stressors include overfishing, destructive fishing, and decline in water quality due to increased
sediment, nutrient, and chemical pollution from changes
in coastal land use (Hughes et al., 2003; Buddemeier
et al., 2004). Fifteen years ago, a group of coral reef
experts concluded that such “human pressures pose
a far greater immediate threat to coral reefs than climate
change, which may only threaten reefs in the distant
future.” (Wilkinson and Buddemeier, 1994, p. VIII).
Climate factors affecting coral reefs
Physical climatic environment of coral reefs: defining
the envelope
To assess the consequences of change requires the understanding of present-day environmental controls on tropical
coral reefs (see also Corals: Environmental Controls on
Growth). They are largely confined to shallow, warm,
clear, and well-lit waters with 18
C identified as the minimum annual sea surface temperature (SST) necessary for
reef growth and although upper thermal limits are less
clear, nowhere, at present, is considered too warm for coral
reef development (Achituv and Dubinsky, 1990). Comparing present-day distribution of nearly 1,000 reefs and
a range of environmental variables, Kleypas et al.
(1999a) found that the most important factors were warm
SSTs (averaging 27.6
C), high aragonite saturation (ranging from 3.28 to 4.06 and 3.83 marking the transition from
coral reefs to non-reef-forming coral communities), and
high available light. In general, present-day coral reefs live
within a relatively narrow range of these three variables
(Hoegh-Guldberg, 2005).
Other weather and climatic factors affecting
coral reefs
Water temperatures, ocean chemistry, and available light
thus broadly define, given suitable bathymetry, where
coral reefs occur. Salinity and nutrient supplies are more
regionally specific controls which are modulated by the
amount of rainfall and freshwater flow into nearshore reef
environments. Also regionally important are ocean circulation patterns that control larval supplies between reefs
(connectivity) and upwelling of nutrients. Coral reefs span
a range of environments from clear oceanic waters to low
salinity and turbid waters near land (Fabricius, 2005).
Tropical cyclones (see Tropical Cyclone/Hurricane),
the most destructive of the world’s weather systems, are
natural sources of disturbance to many coral reefs poleward
of $10
from the equator (Emanuel, 2003). Tropical
cyclones, through the large waves they generate, can
directly affect and even decimate reef structures and coral
assemblages as well as reducing salinity through heavy
rainfall and causing coastal destruction associated with
storm surges (Dollar, 1982; Massel and Done, 1993). Given
time ($10–20 years), and no other sources of environmental stress, coral reefs can recover from such local physical
disturbances (Done, 1999; Hughes and Connell, 1999;
Coles and Brown, 2007). In addition to the immediate
physical impacts of tropical cyclones, their local occurrence
has been shown in the Caribbean to have longer term consequences by limiting subsequent coral recruitment, survival,
and compounding coral cover declines due to other factors
(Gardner et al., 2005; Crabbe et al., 2008).
El Niño-Southern Oscillation (ENSO) events are the
principle source of interannual global climate variability.
This highly coupled ocean–atmosphere phenomenon is
centered in the tropical Pacific producing significant climate and societal impacts throughout the tropics and some
extratropical regions (McPhaden et al., 2006). ENSO fluctuates between two phases, El Niño and La Niña, each
associated with distinct and different atmospheric and oceanic climate anomalies. From the perspective of coral
reefs, the most significant of these anomalies are widespread warming of much of the tropical oceans during
CLIMATE CHANGE AND CORAL REEFS
199
also the rate of increase is accelerating (Canadell et al., 2007).
This increase in atmospheric greenhouse gases results
in significant positive radiative forcing of the global climate system and global warming attributable to human
activities. The most recent Intergovernmental Panel on
Climate Change Fourth Assessment (IPCC-AR4) report
provides observational and paleoclimatic evidence for significant recent warming of global climate that matches
theoretical and modeled consequences of increased greenhouse gas concentrations (IPCC, 2007c). The relatively
modest global warming observed to date has already been
associated with changes in the global climate system such
as more intense rainfall, more frequent droughts, sea-level
rise, loss of Arctic sea ice, melting of land-based ice, and
a widening of the tropical climate belt (IPCC, 2007a,
Chap. 3; Seidal et al., 2007). The rate of warming is about
twice as fast for land masses compared to oceans and for
high latitudes of the Northern Hemisphere compared with
that for low latitude regions. Observed changes in climate
are driving changes in the world’s biological and physical
systems that are all consistent with a rapidly warming climate (IPCC, 2007b; Rosenzweig et al., 2008).
The observational record shows that the average global
land and sea temperature climate of the most recent 30 years
(1979–2008) is significantly warmer (þ0.55
C) than the climate at the end of the nineteenth century (1871–1900;
Figure 1a). The tropical oceans, home to the world’s coral
reefs, have also significantly warmed (at about 70% of the
global average value) by þ0.40
C between the same periods
(Figure 1b). The rate of warming has accelerated from 0.05
C
(global) and 0.04
C per decade (tropical oceans), for the
period 1871 to 2008, to 0.12
C (global) and 0.08
C per
decade (tropical oceans) for the recent period 1950–2008.
Coral reef ecosystems, which occur in the naturally
warm tropical oceans and are one of the largest sources
of global marine diversity (Sala and Knowlton, 2006),
are considered among the “most vulnerable ecosystems”
to global climate change (IPCC, 2007b, Chap. 4, p. 214).
This global-scale threat is occurring against a backdrop
in which many of the world’s coral reef ecosystems have
already been degraded by direct human pressures. These
local and regional stressors include overfishing, destructive fishing, and decline in water quality due to increased
sediment, nutrient, and chemical pollution from changes
in coastal land use (Hughes et al., 2003; Buddemeier
et al., 2004). Fifteen years ago, a group of coral reef
experts concluded that such “human pressures pose
a far greater immediate threat to coral reefs than climate
change, which may only threaten reefs in the distant
future.” (Wilkinson and Buddemeier, 1994, p. VIII).
Climate factors affecting coral reefs
Physical climatic environment of coral reefs: defining
the envelope
To assess the consequences of change requires the understanding of present-day environmental controls on tropical
coral reefs (see also Corals: Environmental Controls on
Growth). They are largely confined to shallow, warm,
clear, and well-lit waters with 18
C identified as the minimum annual sea surface temperature (SST) necessary for
reef growth and although upper thermal limits are less
clear, nowhere, at present, is considered too warm for coral
reef development (Achituv and Dubinsky, 1990). Comparing present-day distribution of nearly 1,000 reefs and
a range of environmental variables, Kleypas et al.
(1999a) found that the most important factors were warm
SSTs (averaging 27.6
C), high aragonite saturation (ranging from 3.28 to 4.06 and 3.83 marking the transition from
coral reefs to non-reef-forming coral communities), and
high available light. In general, present-day coral reefs live
within a relatively narrow range of these three variables
(Hoegh-Guldberg, 2005).
Other weather and climatic factors affecting
coral reefs
Water temperatures, ocean chemistry, and available light
thus broadly define, given suitable bathymetry, where
coral reefs occur. Salinity and nutrient supplies are more
regionally specific controls which are modulated by the
amount of rainfall and freshwater flow into nearshore reef
environments. Also regionally important are ocean circulation patterns that control larval supplies between reefs
(connectivity) and upwelling of nutrients. Coral reefs span
a range of environments from clear oceanic waters to low
salinity and turbid waters near land (Fabricius, 2005).
Tropical cyclones (see Tropical Cyclone/Hurricane),
the most destructive of the world’s weather systems, are
natural sources of disturbance to many coral reefs poleward
of $10
from the equator (Emanuel, 2003). Tropical
cyclones, through the large waves they generate, can
directly affect and even decimate reef structures and coral
assemblages as well as reducing salinity through heavy
rainfall and causing coastal destruction associated with
storm surges (Dollar, 1982; Massel and Done, 1993). Given
time ($10–20 years), and no other sources of environmental stress, coral reefs can recover from such local physical
disturbances (Done, 1999; Hughes and Connell, 1999;
Coles and Brown, 2007). In addition to the immediate
physical impacts of tropical cyclones, their local occurrence
has been shown in the Caribbean to have longer term consequences by limiting subsequent coral recruitment, survival,
and compounding coral cover declines due to other factors
(Gardner et al., 2005; Crabbe et al., 2008).
El Niño-Southern Oscillation (ENSO) events are the
principle source of interannual global climate variability.
This highly coupled ocean–atmosphere phenomenon is
centered in the tropical Pacific producing significant climate and societal impacts throughout the tropics and some
extratropical regions (McPhaden et al., 2006). ENSO fluctuates between two phases, El Niño and La Niña, each
associated with distinct and different atmospheric and oceanic climate anomalies. From the perspective of coral
reefs, the most significant of these anomalies are widespread warming of much of the tropical oceans during
CLIMATE CHANGE AND CORAL REEFS
199