nutrient supply), ocean circulation patterns, and extreme
events such as tropical cyclones and freshwater flood
plumes (see various chapters in Johnson and Marshall,
2007).
Although we have much better knowledge of the global distribution and health of coral reefs (http://www.
reefbase.org/main.aspx), it is still difficult to clearly identify a significant increase in coral bleaching events due to
confounding factors of greater awareness of the problem
and hence more people looking for bleaching events
(Oliver et al., 2009). Objective, large-scale methods are
necessary for observing, for example, where and when
bleaching occurs. These need to be supported by detailed
and continuous local-scale observations that track coral
reef mortality and recovery from such stress events
(Spalding, 2009). It is also important to identify both
bleaching-resistant (i.e., reduced impacts despite high
thermal stress) and bleaching-resilient (i.e., rapid recovery after stress) coral reefs (McClanahan et al., 2007a).
There have been dramatic improvements, through remote
sensing, in our capabilities to observe coral reefs across
large spatial scales but we still cannot routinely identify
bleaching occurrences (Andrefouet and Riegl, 2004;
Elvidge et al., 2004; Mumby et al., 2004). Satellite-based
observations since the 1980s have also dramatically
improved our ability to detect anomalies in surface ocean
climate, and a range of products, based on “oceanic hotspots” (Goreau and Hayes, 1994), now routinely identify
potential bleaching conditions in near real time (Eakin
et al., 2009; http://coralreefwatch.noaa.gov/). Although
such monitoring cannot prevent bleaching, it now allows
scientists and reef managers to document the intensity,
impacts, and recovery of reefs from such disturbances
(see various chapters in Phinney et al., 2006; van Oppen
and Lough, 2009).
Projected future climates for coral reefs
Predicting future climate
Several factors must be considered to understand and
document the potential consequences and impacts of a
rapidly changing climate. First, high-quality environmental observations are needed to determine the climatic envelope of particular organisms. Second, we need sufficient
understanding of the complex physics of the global climate system, with the various interactions between the
atmosphere, ocean, land, cryosphere, and biota, to realistically model current climate. Such GCMs then provide the
basis for projecting future changes as a consequence of
radiative forcing by greenhouse gases (IPCC, 2007a,
Chap. 10). GCMs still, for example, have difficulties in
correctly simulating certain components of tropical climate (Neale and Slingo, 2003; Reichler and Kim, 2008).
The spatial resolution of GCMs is also relatively coarse
which makes projecting to regional scales, most relevant
to coral reefs, a challenge (IPCC, 2007a, Chaps. 8 and
11). Third, although based on the same physical laws, different GCMs vary in how they handle (parameterize) key
small-scale processes. This can lead to slightly different
results both for present and future climate simulations
but such parameterizations are necessary to keep computational costs down to manageable levels. There is,
therefore, no single “perfect” GCM and the most recent
IPCC-4AR uses multimodel averages of a large number
of independent climate projections to account for intermodel variability (Pierce et al., 2009).
Finally, projecting future climates depends on predicting future greenhouse gas concentrations. These depend on a variety of socioeconomic factors that determine
the global response and level of commitment to reduce
and stabilize greenhouse gas emissions in the atmosphere
(mitigation) and, hence, the magnitude and timing of future
climate changes. The IPCC constructed a number of plausible scenarios to specify the concentrations of greenhouse
gases as input to GCMs (Nakicenovic and Swart, 2000).
These range from very carbon-intensive futures with high
emission rates (750–800 ppm CO 2 by 2100) to scenarios
where emissions are reigned in very quickly (450–
500 ppm CO 2 by 2100). Many scientists consider that
greenhouse gas reductions well below those of the low
emissions scenario are necessary to avoid dangerous climate change (Hansen et al., 2008). We are currently tracking above the high emission scenario (Canadell et al.,
2007; Raupach et al., 2007) and, without significant mitigation, we are committed to ongoing, rapid, possibly intensifying climate changes for the foreseeable future, and there
is the specter of irreversible changes on the scale of thousands of years (Solomon et al., 2009). Reducing greenhouse
gas emissions by 70% by 2100 would, however, halve the
magnitude of temperature changes compared to the high
emissions scenario and would confine warming of the tropical oceans to 0.5–1.0
C (Washington et al., 2009).
Projected global changes
Average global temperatures are projected to be $2–4
C
warmer (1.1–6.4
C maximum range from different scenarios) by 2090–2099 compared to 1980–1999 and
tropical SSTs are $1–3
C warmer. An intensified hydrological cycle will increase rainfall in tropical high rainfall
regions, reduce rainfall in the subtropics, and the intensity
of rainfall extremes will increase. Ocean pH will decrease
by 0.1–0.3 pH units. There may be fewer tropical
cyclones, but those that do occur are likely to be more
intense. It is unclear from global model projections as
towhat will happen to ENSO events but they are likely
to continue as a significant source of interannual climate
variability affecting coral reefs (IPCC 2007a, Chap. 10).
A conservative estimate is that sea level will rise 20–
60 cm by the end of this century, but this value underestimates the contribution of accelerated melting of land ice
(Howat et al., 2007; Meier et al., 2007).
A changing climate for coral reefs: future impacts
Several aspects of the current and ongoing changing climate
are of significance for corals reefs, and the environmental
CLIMATE CHANGE AND CORAL REEFS
203
events such as tropical cyclones and freshwater flood
plumes (see various chapters in Johnson and Marshall,
2007).
Although we have much better knowledge of the global distribution and health of coral reefs (http://www.
reefbase.org/main.aspx), it is still difficult to clearly identify a significant increase in coral bleaching events due to
confounding factors of greater awareness of the problem
and hence more people looking for bleaching events
(Oliver et al., 2009). Objective, large-scale methods are
necessary for observing, for example, where and when
bleaching occurs. These need to be supported by detailed
and continuous local-scale observations that track coral
reef mortality and recovery from such stress events
(Spalding, 2009). It is also important to identify both
bleaching-resistant (i.e., reduced impacts despite high
thermal stress) and bleaching-resilient (i.e., rapid recovery after stress) coral reefs (McClanahan et al., 2007a).
There have been dramatic improvements, through remote
sensing, in our capabilities to observe coral reefs across
large spatial scales but we still cannot routinely identify
bleaching occurrences (Andrefouet and Riegl, 2004;
Elvidge et al., 2004; Mumby et al., 2004). Satellite-based
observations since the 1980s have also dramatically
improved our ability to detect anomalies in surface ocean
climate, and a range of products, based on “oceanic hotspots” (Goreau and Hayes, 1994), now routinely identify
potential bleaching conditions in near real time (Eakin
et al., 2009; http://coralreefwatch.noaa.gov/). Although
such monitoring cannot prevent bleaching, it now allows
scientists and reef managers to document the intensity,
impacts, and recovery of reefs from such disturbances
(see various chapters in Phinney et al., 2006; van Oppen
and Lough, 2009).
Projected future climates for coral reefs
Predicting future climate
Several factors must be considered to understand and
document the potential consequences and impacts of a
rapidly changing climate. First, high-quality environmental observations are needed to determine the climatic envelope of particular organisms. Second, we need sufficient
understanding of the complex physics of the global climate system, with the various interactions between the
atmosphere, ocean, land, cryosphere, and biota, to realistically model current climate. Such GCMs then provide the
basis for projecting future changes as a consequence of
radiative forcing by greenhouse gases (IPCC, 2007a,
Chap. 10). GCMs still, for example, have difficulties in
correctly simulating certain components of tropical climate (Neale and Slingo, 2003; Reichler and Kim, 2008).
The spatial resolution of GCMs is also relatively coarse
which makes projecting to regional scales, most relevant
to coral reefs, a challenge (IPCC, 2007a, Chaps. 8 and
11). Third, although based on the same physical laws, different GCMs vary in how they handle (parameterize) key
small-scale processes. This can lead to slightly different
results both for present and future climate simulations
but such parameterizations are necessary to keep computational costs down to manageable levels. There is,
therefore, no single “perfect” GCM and the most recent
IPCC-4AR uses multimodel averages of a large number
of independent climate projections to account for intermodel variability (Pierce et al., 2009).
Finally, projecting future climates depends on predicting future greenhouse gas concentrations. These depend on a variety of socioeconomic factors that determine
the global response and level of commitment to reduce
and stabilize greenhouse gas emissions in the atmosphere
(mitigation) and, hence, the magnitude and timing of future
climate changes. The IPCC constructed a number of plausible scenarios to specify the concentrations of greenhouse
gases as input to GCMs (Nakicenovic and Swart, 2000).
These range from very carbon-intensive futures with high
emission rates (750–800 ppm CO 2 by 2100) to scenarios
where emissions are reigned in very quickly (450–
500 ppm CO 2 by 2100). Many scientists consider that
greenhouse gas reductions well below those of the low
emissions scenario are necessary to avoid dangerous climate change (Hansen et al., 2008). We are currently tracking above the high emission scenario (Canadell et al.,
2007; Raupach et al., 2007) and, without significant mitigation, we are committed to ongoing, rapid, possibly intensifying climate changes for the foreseeable future, and there
is the specter of irreversible changes on the scale of thousands of years (Solomon et al., 2009). Reducing greenhouse
gas emissions by 70% by 2100 would, however, halve the
magnitude of temperature changes compared to the high
emissions scenario and would confine warming of the tropical oceans to 0.5–1.0
C (Washington et al., 2009).
Projected global changes
Average global temperatures are projected to be $2–4
C
warmer (1.1–6.4
C maximum range from different scenarios) by 2090–2099 compared to 1980–1999 and
tropical SSTs are $1–3
C warmer. An intensified hydrological cycle will increase rainfall in tropical high rainfall
regions, reduce rainfall in the subtropics, and the intensity
of rainfall extremes will increase. Ocean pH will decrease
by 0.1–0.3 pH units. There may be fewer tropical
cyclones, but those that do occur are likely to be more
intense. It is unclear from global model projections as
towhat will happen to ENSO events but they are likely
to continue as a significant source of interannual climate
variability affecting coral reefs (IPCC 2007a, Chap. 10).
A conservative estimate is that sea level will rise 20–
60 cm by the end of this century, but this value underestimates the contribution of accelerated melting of land ice
(Howat et al., 2007; Meier et al., 2007).
A changing climate for coral reefs: future impacts
Several aspects of the current and ongoing changing climate
are of significance for corals reefs, and the environmental
CLIMATE CHANGE AND CORAL REEFS
203