Climate change futures: health, ecological and economic dimensions

78 | NATURAL AND MANAGED SYSTEMS
THE ROLE OF CLIMATE
Reefs are very sensitive to temperature anomalies.
Although some coral colonies inhabit deep water and
temperate water niches, reef-building corals are restricted
to shallow, near-shore waters and to a temperature
range of only a few degrees Celsius. Optimal temperature
for coral vitality and skeletal production is 25-
28°C (McNeil et al. 2004). With annual warm season
sea temperature maxima now approaching 30°C,
reef-building corals are losing their efficiency for frame
construction and repair. When sea temperature persists
for a month or more above 30°C (86°F), or reach a
1°C (1.8°F), anomaly above long-term seasonal averages
during the warmest season of the year (see map,
figure 2.31), bleaching occurs in reef organisms harboring
algal symbionts. Also, more frequent and
intense extreme events, including hurricanes, cyclones,
torrential rainfall and mudslides can cause reef frame
breakage, sedimentation, microbial and planktonic proliferation,
runoff of chemical pollutants from terrestrial
sources, and shore erosion. All of these insults degrade
reef ecosystem integrity and coral vitality. Frame damage
to coral reefs following storm activity, especially
among fragile branching corals and exposed coral
mounds, is documented (Nowlis et al. 1997).
However, subtle damage to interactive balances within
the reef ecosystem may not be as evident and can only
be appreciated through physiological data.
Figure 2.31 Sea Surface Temperatures and Coral Bleaching
Apart from warming, CO 2
causes acidification of the
oceans and depletion of calcium from coral reefs could
kill all coral organisms by 2065 (Steffan et al. 2004a
Orr et al. 2005; Pelejero et al. 2005), though some
researchers believe the ocean warming itself may alter
this projected outcome (McNeil et al. 2004).
Climate change is the principal agent forcing change
in the coastal oceans. Atmospheric warming due to the
accumulation of greenhouse gases offers the source of
excess heat that rapidly equilibrates in the world's
oceans (Levitus et al. 2000). The rate of warming in
surface waters exceeds the rate of evaporation of
humidity back into the atmosphere. The reverse gradient
of warm hypersaline surface water sinks to give
reefs exposure to high temperatures. Excessive warming
of coastal oceans, especially during the warmest
months of the year, coupled with inadequate relief from
warming during the coolest months of the year, have
pushed the coral reef ecosystems beyond their thermal
limit for survival (Goreau and Hayes 1994). In
response to warming, marine microbes that are responsible
for diseases of reef-building corals also thrive.
They proliferate, colonize and invade coral tissues, producing
tissue infection and ultimately, coral death.
CASE STUDIES
14-year cumulative sea surface temperature anomalies and areas of coral bleaching indicated by red circles. Years with El Niño events and
volcanic eruptions have been removed from the data set. Yellow indicates temperatures 1°C above the mean temperature; orange indicates
anomalies 2°C above the long-term average.
Source: (Base map) Climate Diagnostics Bulletin, NOAA AVHRR Satellite Database, 1982-2003 and (data) Ray Hayes and Tom Goreau