Projected changes to
coral reefs
Based on......
Outline
• Importance of coral reefs
• Different types of reefs
• Requirements for good coral growth
• Recent stresses on coral reefs
• Projected effects of climate change
• Key management measures
Importance of coral reefs
• Form complex
structures
• Provide habitat for
thousands of
species
• Support local
fisheries and
tourism
• Variation in local
pressures
Different types of reefs
• Dominant coastal
habitat
• Majority are oceanic
• Great diversity of reef
types
• Reef area in Cook
Islands is >2.7 times
land area
Location matters
• Fringing continental reefs
affected by river runoff
• Isolated oceanic reefs not
well connected e.g. larval
supplies
• Tropical cyclones > 10o
from equator
Requirements for good coral growth
• Warm water temperatures
• Shallow well-lit waters
• Low sediment and nutrients
• Right ocean chemistry
Coral reefs grow in the warmest parts of
oceans & have a narrow temperature range
Corals must build skeletons fast enough to
withstand natural forces of erosion
Waves
Cyclones & storms
Bleaching
Predators
Coral eaters
A special relationship
• Symbiosis is at the heart of tropical coral reefs
• Photosynthetic algae live within corals
• Algae give corals energy for rapid calcification
Stresses – higher water temperatures
• Stressed corals lose algae (and their pigments)
• Coral bleaching
• Corals living only ~1-2oC below upper thermal limit
• Too much freshwater can also cause bleaching
Healthy - unbleached
Stressed - bleached
Recently dead
Stresses – ocean acidification
• 30% extra CO2 entered ocean
• Changes ocean chemistry
• Harder for corals to form
skeletons
• Greater coral erosion
Ocean acidification: natural laboratory
• High CO2 volcanic seeps, PNG
• “Winners” = massive corals
• “Losers” = branching, tabulate corals
• Reduced coral diversity
• Much simpler reef with lower pH
Normal pH = now
Mid pH = 2050
Lower pH = 2100
Source: Fabricius et al 2011
Projected effects of climate change
Projected climate change
1980–1999
average
2035 A2
2050 A2
2100 A2
Air temperature (°C)
27.4
+0.5 to +1.0
+1.0 to +1.5
+2.5 to +3.0
Sea surface temperature
(°C)
27.3
+0.7 to +0.8
+1.2 to +1.6
+2.2 to +2.7
+5 to +20
+10 to +20
+10 to +20
-5 to -20
-5 to -20
-5 to -20
Rainfall: equatorial (%)
Rainfall: subtropics (%)
n/a
Sea level (cm)
+6a
+20 to +30
Ocean pH (units)
8.1
-0.1
Cyclones & storms
9
a = since 1960
+70 to +110
-0.2
+90 to +140
-0.3
Number of cyclones/storms may decrease but
likely to be more intense
Warmer water temperatures
• Increased bleaching, ~1% loss per year by 2035
More acidic ocean
• Weaker reef frameworks
Stronger storms and heavier rainfall
• Moderate vulnerability of reefs
• More disturbances = less time to recover
Higher sea level
• Some corals may keep up
• Loss of deeper corals
What climate change means for reefs
• More bleaching and diseases
• Weaker skeletons
• Physical destruction
• Less time to recover between disturbances
• Healthy reefs better able to cope
Coral reefs will not disappear entirely BUT
are likely to be
MUCH SIMPLER ECOSYSTEMS
Management of reefs
ALGAL REEFS
Mitigating local stresses
Anthony & Maynard 2011
Conclusions
• Coral reefs in Cook Islands are expected to decline in
complexity and area due to climate change
• Coastal fisheries that depend on coral reef will be
affected as coral reefs degrade
• Acting now to manage existing threats to coral reefs
will assist these habitats to adapt and reduce the scale
of degradation