Coral Reefs Under Rapid Climate Change and Ocean Acidification

Abstract

Atmospheric carbon dioxide concentration is expected to exceed 500 parts per million and global temperatures to rise by at
least 2°C by 2050 to 2100, values that significantly exceed those of at least the past 420,000 years during which most extant
marine organisms evolved. Under conditions expected in the 21st century, global warming and ocean acidification will compromise
carbonate accretion, with corals becoming increasingly rare on reef systems. The result will be less diverse reef communities
and carbonate reef structures that fail to be maintained. Climate change also exacerbates local stresses from declining water
quality and overexploitation of key species, driving reefs increasingly toward the tipping point for functional collapse.
This review presents future scenarios for coral reefs that predict increasingly serious consequences for reef-associated fisheries,
tourism, coastal protection, and people. As the International Year of the Reef 2008 begins, scaled-up management intervention
and decisive action on global emissions are required if the loss of coral-dominated ecosystems is to be avoided.

Full text

DOI: 10.1126/science.1152509
, 1737 (2007); 318Science et al.O. Hoegh-Guldberg,
Ocean Acidification
Coral Reefs Under Rapid Climate Change and
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Coral Reefs Under Rapid Climate
Change and Ocean Acidification
O. Hoegh-Guldberg,
1
*P. J. Mumby,
2
A. J. Hooten,
3
R. S. Steneck,
4
P. Greenfield,
5
E. Gomez,
6
C. D. Harvell,
7
P. F. Sale,
8
A. J. Edwards,
9
K. Caldeira,
10
N. Knowlton,
11
C. M. Eakin,
12
R. Iglesias-Prieto,
13
N. Muthiga,
14
R. H. Bradbury,
15
A. Dubi,
16
M. E. Hatziolos
17
Atmospheric carbon dioxide concentration is expected to exceed 500 parts per million and global
temperatures to rise by at least 2°C by 2050 to 2100, values that significantly exceed those of at
least the past 420,000 years during which most extant marine organisms evolved. Under conditions
expected in the 21st century, global warming and ocean acidification will compromise carbonate
accretion, with corals becoming increasingly rare on reef systems. The result will be less diverse reef
communities and carbonate reef structures that fail to be maintained. Climate change also exacerbates
local stresses from declining water quality and overexploitation of key species, driving reefs increasingly
toward the tipping point for functional collapse. This review presents future scenarios for coral reefs that
predict increasingly serious consequences for reef-associated fisheries, tourism, coastal protection, and
people. As the International Year of the Reef 2008 begins, scaled-up management intervention and
decisive action on global emissions are required if the loss of coral-dominated ecosystems is to be avoided.
Coral reefs are among the most biologically
diverse and economically important eco-
systems on the planet, providing ecosys-
tem services that are vital to human societies and
industries through fisheries, coastal protection,
building materials, new biochemical compounds,
and tourism (1). Yet in the decade since the in-
augural International Year of the Reef in 1997 (2),
which called the world to action, coral reefs have
continued to deteriorate as a result of human in-
fluences (3,4). Rapid increases in the atmospheric
carbon dioxide concentration ([CO
2
]
atm
), by driv-
ing global warming and ocean acidification, may
be the final insult to these ecosystems. Here, we
review the current understanding of how anthro-
pogenic climate change and increasing ocean acid-
ity are affecting coral reefs and offer scenarios for
how coral reefs will change over this century. The
scenarios are intended to provide a framework for
proactive responses to the changes that have
begun in coral reef ecosystems and to provoke
thinking about future management and policy
challenges for coral reef protection.
Warming and Acidifying Seas
The concentration of carbon dioxide in Earth’s
atmosphere now exceeds 380 ppm, which is
more than 80 ppm above the maximum values
of the past 740,000 years (5,6), if not 20 million
years (7). During the 20th century, increasing
[CO
2
]
atm
has driven an increase in the global
oceans’average temperature by 0.74°C and sea
level by 17 cm, and has depleted seawater car-
bonate concentrations by ~30 mmol kg
−1
seawater
and acidity by 0.1 pH unit (8). Approximately
25% (2.2 Pg C year
−1
)oftheCO
2
emitted from
all anthropogenic sources (9.1 Pg C year
−1
)cur-
rently enters the ocean (9), where it reacts with
water to produce carbonic acid. Carbonic acid
dissociates to form bicarbonate ions and protons,
which in turn react with carbonate ions to produce
more bicarbonate ions, reducing the availability of
carbonate to biological systems (Fig. 1A). De-
creasing carbonate-ion concentrations reduce the
rate of calcification of marine organisms such as
reef-building corals, ultimately favoring erosion
at ~200 mmol kg
−1
seawater (7,10).
We used global [CO
2
]
atm
and temperature
data from the Vostok Ice Core study (5)toex-
plore the ocean temperature and carbonate-ion
concentration (10) seen today relative to the re-
cent past for a typical low-latitude sea maintain-
ing a mean temperature of 25°C during the past
420,000 years (Fig. 1B). The results show a tight
cluster of points that oscillate (temperature ±3°C;
carbonate-ion concentration ±35 mmol kg
−1
)be-
tween warmer interglacial periods that had lower
carbonate concentrations to cooler glacial pe-
riods with higher carbonate concentrations. The
overall range of values calculated for seawater
pH is ±0.1 units (10,11). Critically, where coral
reefs occur, carbonate-ion concentrations over
the past 420,000 years have not fallen below
240 mmol kg
−1
. The trends in the Vostok ice
core data have been verified by the EPICA study
(6), which involves a similar range of temperatures
and [CO
2
]
atm
values and hence extends the con-
clusions derived from the Vostok record to at least
740,000 years before the present (yr B.P.). Con-
ditions today ([CO
2
]
atm
~380 ppm) are significantly
shifted to the right of the cluster points represent-
ing the past 420,000 years. Sea temperatures are
warmer (+0.7°C), and pH (−0.1 pH units) and
carbonate-ion concentrations (~210 mmol kg
−1
)
lower than at any other time during the past
420,000 years (Fig. 1B). These conclusions match
recent changes reported for measurements of ocean
temperature, pH, and carbonate concentration (8).
In addition to the absolute amount of change, the
rate at which change occurs is critical to whether
organisms and ecosystems will be able to adapt or
accommodate to the new conditions (11 ). Notably,
rates of change in global temperature and [CO
2
]
atm
over the past century are 2 to 3 orders of mag-
nitude higher than most of the changes seen in
the past 420,000 years (Table 1). Rates of change
under both low (B1) and high (A2) Intergovern-
mental Panel on Climate Change (IPCC) emission
scenarios are even higher, as are recent measure-
ments of the rate of change of [CO
2
]
atm
(9). The
only possible exceptions are rare, short-lived
spikes in temperature seen during periods such
as the Younger Dryas Event (12,900 to 11,500 yr
B.P.) (12). Given that recent and future rates of
change dwarf even those of the ice age transitions,
when biology at specific locations changed dramat-
ically, it is likely that these changes will exceed the
capacity of most organisms to adapt.
Ocean Acidification and Reef Accretion
Many experimental studies have shown that a
doubling of pre-industrial [CO
2
]
atm
to 560 ppm
decreases coral calcification and growth by up to
40% through the inhibition of aragonite formation
(the principal crystalline form of calcium carbonate
deposited in coral skeletons) as carbonate-ion con-
centrations decrease (13). Field studies confirm that
carbonate accretion on coral reefs approaches zero
or becomes negative at aragonite saturation values
of 3.3 in today’s oceans (Fig. 4), which occurs
when [CO
2
]
atm
approaches 480 ppm and carbonate-
ion concentrations drop below 200 mmol kg
−1
in
most of the global ocean (10,13). These find-
ings are supported by the observation that reefs
with net carbonate accretion today (Fig. 4, 380 ppm)
are restricted to waters where aragonite saturation
REVIEW
1
Centre for Marine Studies, The University of Queensland,
St. Lucia, 4072 Queensland, Australia.
2
Marine Spatial
Ecology Laboratory, School of BioSciences, University of
Exeter, Prince of Wales Road, Exeter EX4 4PS, UK.
3
AJH
Environmental Services, 4900 Auburn Avenue, Suite 201,
Bethesda, MD 20814, USA.
4
University of Maine, School
of Marine Sciences, Darling Marine Center, Walpole, ME
04573, USA.
5
The Chancellery, University of Queens-
land, St. Lucia, 4072 Queensland, Australia.
6
Marine Science
Institute, University of the Philippines, Diliman, Quezon City,
Philippines.
7
Ecology and Evolutionary Biology, E321 Corson
Hall, Cornell University, Ithaca, NY 14853, USA.
8
International
Network on Water, Environment and Health, United Nations
University, 50 Main Street East, Hamilton, Ontario L8N 1E9,
Canada.
9
School of Biology, Ridley Building, University of
Newcastle,NewcastleuponTyne,NE17RU,UK.
10
Department of
Global Ecology, Carnegie Institution of Washington, 260
Panama Street, Stanford, CA 94305, USA.
11
National Museum
of Natural History, Smithsonian Institution, Washington, DC
20013, USA.
12
National Oceanic and Atmospheric Administra-
tion, Coral Reef Watch, E/RA31, 1335 East West Highway, Silver
Spring, MD 20910–3226, USA.
13
Unidad Académica Puerto
Morelos, Instituto de Ciencias del Mar y Limnología, Universidad
Nacional Autónoma de México, Apdo. Postal 1152, Cancún
77500 QR, México.
14
Wildlife Conservation Society, 2300
Southern Boulevard, Bronx, New York, NY 10460, USA.
15
Resource Management in Asia-Pacific Program, Australian
National University, Canberra, 0200 Australia.
16
Institute of
Marine Sciences, University of Dar es Salaam, Tanzania.
17
Envi-
ronment Department, MC5-523, The World Bank, 1818 H
Street,NW,Washington,DC20433,USA.
*To whom correspondence should be addressed. E-mail:
oveh@uq.edu.au
www.sciencemag.org SCIENCE VOL 318 14 DECEMBER 2007 1737
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exceeds 3.3 (10). Geological studies report a no-
table gap in the fossil record of calcified organisms,
including reef-building corals (14) and calcare-
ous algae (15), during the early Triassic when
[CO
2
]
atm
increased dramatically and reached levels
at least five times as high as today’s(16). Phylo-
genetic studies suggest that corals as a group
survived the Permian-Triassic extinction event (14)
but may have done so through forms lacking cal-
cified skeletons (17,18). Although Scleractinian
(modern) corals arose in the mid-Triassic and lived
under much higher [CO
2
]
atm
, there is no evidence
that they lived in waters with low-carbonate
mineral saturation. Knoll et al. succinctly state that
“it is the rapid, unbuffered increase in [CO
2
]
atm
and not its absolute values that causes impor-
tant associated changes such as reduced [CO
32−
],
pH, and carbonate
saturation of sea wa-
ter”(19). The rate of
[CO
2
]
atm
change is
critical given that
modern genotypes
and phenotypes of
corals do not appear
to have the capacity
to adapt fast enough
to sudden environ-
mental change.
Reef-building
corals may exhibit
several responses
to reduced calcifi-
cation, all of which
have deleterious consequences for reef ecosys-
tems. First, the most direct response is a decreased
linear extension rate and skeletal density of coral
colonies. The massive coral Porites on the Great
Barrier Reef has shown reductions in linear ex-
tension rate of 1.02% year
−1
and in skeletal den-
sity of 0.36% year
−1
during the past 16 years (20).
This is equivalent to a reduction of 1.29% year
−1
or a 20.6% drop in growth rate (the product of
linear extension rate and skeletal density) over the
16-year period. While at present it is not possible
to confidently attribute the observed decrease in
growth and calcification to ocean acidification, it
is consistent with changes reported in oceanic pH
and carbonate-ion concentrations.
Second, corals may maintain their physical
extension or growth rate by reducing skeletal
density. However, erosion could be promoted
by the activities of grazing animals such as
parrotfish, which prefer to remove carbonates
from lower-density substrates. Increasingly
brittle coral skeletons are also at greater risk
of storm damage (21); thus, if rates of erosion
outstrip calcification, then the structural com-
plexity of coral reefs will diminish, reducing
habitat quality and diversity. A loss of struc-
tural complexity will also affect the ability of
reefs to absorb wave energy and thereby impairs
coastal protection.
Third, corals may maintain both skeletal growth
and density under reduced carbonate saturation
by investing greater energy in calcification. A
likely side effect of this strategy is the diversion
of resources from other essential processes, such
as reproduction, as
seen in chronic stress
(21),whichcouldul-
timately reduce the
larval output from
reefs and impair the
potential for recolo-
nization following
disturbances.
Resilience and
Tipping Points
Maintaining ecologi-
cal resilience is the
central plank of any
strategy aiming to
preserve coral reef
Fig. 1. (A) Linkages between the buildup of atmospheric CO
2
and the slowing
of coral calcification due to ocean acidification. Approximately 25% of the
CO
2
emitted by humans in the period 2000 to 2006 (9)wastakenupbythe
ocean where it combined with water to produce carbonic acid, which releases a
proton that combines with a carbonate ion. This decreases the concentration of
carbonate, making it unavailable to marine calcifiers such as corals. (B)Tem-
perature, [CO
2
]
atm
, and carbonate-ion concentrations reconstructed for the past
420,000 years. Carbonate concentrations were calculated (54)fromCO
2atm
and
temperature deviations from today’s conditions with the Vostok Ice Core data set
(5), assuming constant salinity (34 parts per trillion), mean sea temperature
(25°C), and total alkalinity (2300 mmol kg
−1
). Further details of these
calculations are in the SOM. Acidity of the ocean varies by ± 0.1 pH units
over the past 420,000 years (individual values not shown). The thresholds for
major changes to coral communities are indicated for thermal stress (+2°C) and
carbonate-ion concentrations ([carbonate] = 200 mmol kg
−1
,approximate
aragonite saturation ~W
aragonite
=3.3;[CO
2
]
atm
= 480 ppm). Coral Reef
Scenarios CRS-A, CRS-B, and CRS-C are indicated as A, B, and C, respectively,
with analogs from extant reefs depicted in Fig. 5. Red arrows pointing
progressively toward the right-hand top square indicate the pathway that is
beingfollowedtoward[CO
2
]
atm
of more than 500 ppm.
Table 1. Rates of change in atmospheric CO
2
concentration ([CO
2
]
atm
, ppm/100 years) and global temperature
(°C/100 years) calculated for the past 420,000 yr B.P. using the Vostok Ice Core data (5) and compared to changes
over the last century and those projected by IPCC for low-emission (B1) and high-emission (A2) scenarios (8). Rates
were calculated for each successive pair of points in the Vostok Ice Core record by dividing the difference between two
sequential values (ppm or °C) by the time interval between them. Rates were then standardized to the change seen
over 100 years. Ratios of each rate relative to the mean rate seen over the past 420,000 years are also calculated.
Period [CO
2
]
atm
(ppm century
−1
)
Ratio (relative to
past 420,000 years)
Temperature
(°C century
−1
)
Ratio (relative to
past 420,000 years)
Past 420,000 years (99%
confidence interval; n= 282)
0.07 + 0.223 1 0.01 + 0.017 1
Past 136 years (1870–2006) 73.53 1050 0.7 70
IPCC B1 scenario: 550 ppm
at 2100
170 2429 1.8 180
IPCC A2 scenario: 800 ppm
at 2100
420 6000 3.4 420
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ecosystems. Ecological resilience (4)
isameasureoftherateatwhichan
ecosystem returns to a particular state
(e.g., coral-dominated communities)
after a perturbation or disturbance
(e.g., hurricane impacts). Recent
changes to the frequency and scale
of disturbances such as mass coral
bleaching, disease outbreaks, and
destructive fishing, coupled with a
decreased ability of corals to grow
and compete, are pushing reef ecosys-
tems from coral- to algal-dominated
states (4,22). If pushed far enough,
the ecosystem may exceed a “tipping
point”(22) and change rapidly into
an alternative state with its own in-
herent resilience and stability, often
making the possibility of returning
to a coral-dominated state difficult.
To examine the ecological impli-
cations of the 20.6% reduction in
coral growth rate that Cooper et al.
measured in Great Barrier Reef
Porites (20), we simulated a similar
reduction in the growth of massive
brooding and spawning corals on exposed Carib-
bean forereefs specifically to investigate what hap-
pens to the balance between corals and macroalgae
in a system of high primary production (Fig. 2).
The ecological model (22) simulated a 50-year time
series for a wide range of initial coral cover and
grazing rates by fish on benthic algae while hold-
ing all other factors (e.g., nutrient concentrations)
constant. Each time series revealed the underlying
trajectory of coral recovery, stasis, or degradation
between major disturbances, and the final equilib-
rium values of coral cover were plotted to illustrate
potential resilience (Fig. 2). The unstable equilibria
represent thresholds, and for recovery to outweigh
mortality reefs must lie either above or to the right
of the threshold. For example, if coral cover is low
(<5%), the intensity of fish grazing on benthic algal
competitors needed to shift the reef into a state
where recovery is possible (i.e., to the right or
above the unstable equilibrium) moves from 30%
to almost a half of the reef having to be grazed.
This implies that in the absence of invertebrate
grazers like the sea urchin, Diade-
ma antillarum, which essentially
disappeared from Caribbean reefs
in the early 1980s after a massive
disease outbreak, highly produc-
tive reefs would likely require the
highest levels of parrotfish grazing
(i.e., ~ 40% of the reef being grazed)
for a reef to be able to recover from
disturbance. The loss of ecological
resilience occurs because coral
cover increases more slowly after
disturbance and competitive inter-
actions with macroalgae become
more frequent and longer in dura-
tion (Fig. 3) (23) (table S1). Al-
though the ecological model only
represents a single Caribbean reef
habitat in a very productive physical
environment and has not incor-
porated several other putative con-
sequences of acidification such as
a loss of rugosity, sensitivity analy-
ses reveal that changes to coral
growth rate have a relatively large
impact on model predictions (22),
and therefore the conclusions of a reduction in
resilience appear to be robust.
Thermal Stress, Synergies, and
Ecological Feedback Loops
The sensitivity of corals and their endosymbiotic
dinoflagellates (Symbiodinium spp.) to rising
ocean temperatures has been documented ex-
tensively (24). Symbiodinium trap solar energy
and nutrients, providing more than 95% of the
metabolic requirements of the coral host, which
0.1 0.2 0.3 0.4 0.5 0.6
0
10
20
30
40
50
60
70
80
90
Grazing (proportion of reef grazed in 6 months)
Equilibrial coral cover after 50 years (%)
Combinations of coral
cover and grazing that
permit reef recovery
between disturbance
events under reduced
coral growth
Coral−dominated stable equilibrium
Algal−dominated stable equilibrium
Unstable equilibrium with 20%
reduction in coral linear extension rate
Unstable equilibrium
with current coral
linear extension rate
Fig. 2. Reduction in the resilience of Caribbean forereefs as coral growth
rate declines by 20%. Reef recovery is only feasible above or to the right of
the unstable equilibria (open squares). The “zone of reef recovery”(pink) is
therefore more restricted under reduced coral growth rate and reefs require
higher levels of grazing to exhibit recovery trajectories.
Fig. 3. Ecological feedback processes on a coral reef showing pathways of
disturbance caused by climate change. Impact points associated with ocean
acidification (e.g., reduced reef rugosity, coralline algae) are indicated by the
blue arrows, and impact points from global warming (e.g., bleached and
dead corals) by the red arrows. Boxes joined by red arrows denote that the
first factor has a negative (decreasing) influence on the box indicated. Green
arrows denote positive (increasing) relationships. Over time, the levels of
factors in hexagonal boxes will increase, whereas those in rectangular boxes
will decline. Boxes with dashed lines are amenable to local management
intervention.
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is consequently able to maintain high calcification
rates. When temperatures exceed summer maxima
by 1° to 2°C for 3 to 4 weeks, this obligatory
endosymbiosis disintegrates with ejection of the
symbionts and coral bleaching (24). Bleaching and
mortality become progressively worse as thermal
anomalies intensify and lengthen (24). Indeed,
mass coral bleaching has increased in intensity
and frequency in recent decades (24–27). At the
end of the International Year of the Reef in 1997,
mass bleaching spread from the Eastern Pacific to
most coral reefs worldwide, accompanied by
increasing coral mortality during the following 12
months (24). Corals may survive and recover their
dinoflagellate symbionts after mild thermal stress,
but typically show reduced growth, calcification,
and fecundity (24) and may experience greater
incidences of coral disease (28,29).
To illustrate the combined effects of acidifica-
tion and bleaching on reefs, we simplified the coral
ecosystem into the nine features required to model
feedback mechanisms (Fig. 3). Although it is not
comprehensive, the model provides a theoretical
framework indicating that acidification and bleach-
ing enhance all deleterious feedbacks, driving the
coral ecosystems toward domination by macro-
algae and noncoral communities (Fig. 3) (table S1).
Trajectories in Response to Climate Change
Global temperatures are projected to increase rap-
idly to 1.8°C above today’s average temperature
under the low-emission B1 scenario of the IPCC,
or by 4.0°C (2.4° to 6.4°C) under the higher-
emission A1F1 scenario (Table 1) (8). Increases in
the temperature of tropical and subtropical waters
over the past 50 years (24) have already pushed
reef-building corals close to their thermal limits.
Projections for ocean acidification include reduc-
tions in oceanic pH by as much as 0.4 pH units by
the end of this century, with ocean carbonate
saturation levels potentially dropping below those
required to sustain coral reef accretion by 2050
(Fig. 4) (7, 10, 13). Changes in ocean acidity will
vary from region to region, with some regions,
such as the Great Barrier Reef and Coral Sea, and
the Caribbean Sea, attaining risky levels of arag-
onite saturation more rapidly than others (Fig. 4).
Just as carbonate coral reefs do not exist in waters
with carbonate-ion concentrations < 200 mmol kg
−1
(10), they are likely to contract rapidly if future
[CO
2
]
atm
levels exceed 500 ppm. Similarly, un-
less thermal thresholds change, coral reefs will
experience an increasing frequency and severity
of mass coral bleaching, disease, and mortality
as [CO
2
]
atm
and temperatures increase (24–27).
We have projected three scenarios for coral
reefs over the coming decades and century. In
doing so, we recognize that important local threats
to coral reefs, such as deterioration of water quality
arising from sediment and nutrient inputs associ-
ated with coastal development and deforestation,
and the overexploitation of marine fishery stocks,
may produce synergies and feedbacks in concert
with climate change (30) (Fig. 3) [supporting on-
line material (SOM)]. How quickly we arrive at or
how long we stay within each of the three sce-
narios will depend on the CO
2
emission rate, with
each scenario highlighting the context against which
management and policy actions must be devised.
If conditions were stabilized at the present
[CO
2
]
atm
of 380 ppm, that is, Coral Reef Scenario
CRS-A (Figs. 1B and 5A), coral reefs will con-
tinue to change but will remain coral dominated
and carbonate accreting in most areas of their
current distribution. Local factors—i.e., those not
directly related to global climate change, such as
changes to water quality—affecting levels of sedi-
ment, nutrients, toxins, and pathogens, as well as
fishing pressure, will be important determinants of
reef state and should demand priority attention in
reef-management programs. However, as we move
toward higher [CO
2
]
atm
, coral-community compo-
sitions will change with some areas becoming
dominated by more thermally tolerant corals like
the massive Porites (31) and others potentially dom-
inated by thermally sensitive but rapidly coloniz-
ing genera, such as the tabulate Acropora. Under
the current rate of increase in [CO
2
]
atm
(>1 ppm
year
−1
), carbonate-ion concentrations will drop
below 200 mmol kg
−1
and reef erosion will exceed
calcification at [CO
2
]
atm
= 450 to 500 ppm, i.e.,
Scenario CRS-B (Figs. 1 and 5B). The density and
diversity of corals on reefs are likely to decline,
leading to vastly reduced habitat complexity and
loss of biodiversity (31), including losses of coral-
associated fish and invertebrates (32).
Coralline algae are a key settlement substrate
for corals, but they have metabolically expensive
high-magnesium calcite skeletons that are very
sensitive to pH (33). Hence, coral recruitment may
be compromised if coralline algal abundance de-
clines. Coral loss may also be compounded by an
increase in disease incidence (34). Ultimately, the
loss of corals liberates space for the settlement of
macroalgae, which in turn tends to inhibit coral
recruitment, fecundity, and growth because they
compete for space and light, and also produce anti-
fouling compounds that deter settlement by
potential competitors. Together these factors allow
macroalgae to form stable communities that are
relatively resistant to a return to coral domination
(Figs. 2 and 3) (22,23,35). As a result of weak-
ening of coral growth and competitive ability, reefs
within the CRS-B scenario will be more sensitive
to the damaging influence of other local factors,
such as declining water quality and the removal of
key herbivore fish species.
Increases in [CO
2
]
atm
> 500 ppm (11)will
push carbonate-ion concentrations well below
Fig. 4. Changes in aragonite saturation {W
aragonite
=([Ca
2+
].[CO
32−
])/K
sp aragonite
)} predicted to occur as at-
mospheric CO
2
concentrations (ppm) increase (number at top left of each panel) plotted over shallow-water coral
reef locations shown as pink dots (for details of calculations, see the SOM). Before the Industrial Revolution (280
ppm), nearly all shallow-water coral reefs had W
aragonite
> 3.25 (blue regions in the figure), which is the
minimum W
aragonite
that coral reefs are associated with today; the number of existing coral reefs with this
minimum aragonite saturation decreases rapidly as [CO
2
]
atm
increases. Noticeably, some regions (such as the
Great Barrier Reef) attain low and risky levels of W
aragonite
much more rapidly than others (e.g., Central Pacific).
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200 mmol kg
−1
(aragonite saturation < 3.3) and sea
temperatures above +2°C relative to today’sval-
ues (Scenario CRS-C, Fig. 1). These changes will
reduce coral reef ecosystems to crumbling frame-
works with few calcareous corals (Fig. 5C). The
continuously changing climate, which may not
stabilize for hundreds of years, is also likely to
impede migration and successful proliferation of
alleles from tolerant populations owing to con-
tinuously shifting adaptive pressure. Under these
conditions, reefs will become rapidly eroding
rubble banks such as those seen in some inshore
regions of the Great Barrier Reef, where dense
populations of corals have vanished over the
past 50 to 100 years. Rapid changes in sea level
(+23 to 51 cm by 2100, scenario A2) (8),
coupled with slow or nonexistent reef growth,
mayalsoleadto“drowned”reefs (36)inwhich
corals and the reefs they build fail to keep up
with rising sea levels.
The types of synergistic impacts on coral and
reef-dependent organisms defined for Scenario
CRS-B (Fig. 5B) will be magnified substantially
for CRS-C (Fig. 5C), with probably half, and pos-
sibly more, of coral-associated fauna becoming rare
or extinct given their dependence on living corals
and reef rugosity (37). Macroalgae may dominate
in some areas and phytoplankton blooms may be-
come more frequent in others, as water quality de-
clines owing to the collateral impact of climate
change on associated coastal areas, drying catch-
ments and causing episodic heavy rainfall that
transports nutrients and sediments into coastal areas.
Whether or not one defines the transition from
CRS-B to CRS-C and [CO
2
]
atm
of 450 to 500 ppm
as the tipping point for coral reefs, it is clear that
coral reefs as we know them today would be ex-
tremely rare at higher [CO
2
]
atm
.
We recognize that physiological acclimation or
evolutionary mechanisms could delay the arrival of
some scenarios. However, evidence that corals and
their symbionts can adapt rapidly to coral bleach-
ing is equivocal or nonexistent. Reef-building
corals have relatively long generation times and
low genetic diversity, making for slow rates of
adaptation. Changes in species composition are
also possible but will have limited impact, as even
the most thermally tolerant corals will only sustain
temperature increases of 2° to 3°C above their
long-term solar maxima for short periods (24,31).
However, such changes come at a loss of bio-
diversity and the removal of important redundan-
cies from these complex ecosystems. Some studies
have shown that corals may promote one variety
of dinoflagellate symbiont over another in the
relatively small number of symbioses that have
significant proportions of multiple dinoflagellate
types (38). These phenotypic changes extend the
plasticity of a symbiosis (e.g., by 1° to 2°C) (21)
but are unlikely to lead to novel, long-lived as-
sociations that would result in higher thermal
tolerances (39). The potential for acclimation even
to current levels of ocean acidification is also low
given that, in the many studies done to date, coral
calcification has consistently been shown to de-
crease with decreasing pH and does not recover as
long as conditions of higher acidity persist (13).
Socioeconomic Impacts of Coral Reef Decline
The scenarios presented here are likely to have se-
rious consequences for subsistence-dependent so-
cieties, as well as on wider regional economies
through their impact on coastal protection, fish-
eries, and tourism. These consequences become
successively worse as [CO
2
]
atm
increases, and un-
manageable for [CO
2
]
atm
above 500 ppm.
Although reefs with large communities of coral
reef-related organisms persist under CRS-A and
CRS-B, they become nonfunctional under CRS-C,
as will the reef services that currently underpin
human welfare. Climate change is likely to fun-
damentally alter the attractiveness of coral reefs to
tourists (compare Fig. 5, A and C), which is an
important consideration for the many low-income
coastal countries and developing small island states
lying within coral reef regions. Under-resourced
and developing countries have the lowest capacity
to respond to climate change, but many have
tourism as their sole income earner and thus are at
risk economically if their coral reefs deteriorate
(40). For instance, tourism is a major foreign ex-
change earner in the Caribbean basin and in some
countries accounts for up to half of the gross do-
mestic product (40). Coral reefs in the United
States and Australia may supply smaller compo-
nents of the total economy, but still generate con-
siderable income (many billions of U.S. $ per year)
from reef visitors that are increasingly responsive
to the quality of reefs (41).
Reef rugosity is an important element for the
productivity of all reef-based fisheries, whether sub-
sistence, industrial, or to supply the aquarium trade.
The density of reef fish (32) is likely to decrease as
a result of increasing postsettlement mortality aris-
ing from a lack of hiding places and appropriate
food for newly settled juveniles (42). Regardless of
future climate-change influences, the total landing
of coral reef fisheries is already 64% higher than
Fig. 5. Extant examples of reefs from the Great Barrier Reef that are used
as analogs for the ecological structures we anticipate for Coral Reef
Scenarios CRS-A, CRS-B, and CRS-C (see text). The [CO
2
]
atm
and tem-
perature increases shown are those for the scenarios and do not refer to
the locations photographed. (A) Reef slope communities at Heron Island.
(B) Mixed algal and coral communities associated with inshore reefs
around St. Bees Island near Mackay. (C) Inshore reef slope around the
Low Isles near Port Douglas. [Photos by O. Hoegh-Guldberg]
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can be sustained, with an extra 156,000 km
2
of
coral reef estimated as being needed to support
anticipated population growth by 2050 (43). For
example, in Asia alone coral reefs provide about
one-quarter of the annual total fish catch and food
to about 1 billion people (43). Climate-change im-
pacts on available habitat will only exacerbate al-
ready overstretched fisheries resources.
The role of reefs in coastal protection against
storms (44) has been highlighted in analyses of
exposed and reef-protected coastlines (45,46). We
do not yet have estimates for how fast reef barriers
will disappear (47), but we can anticipate that
decreasing rates of reef accretion, increasing rates of
bioerosion, rising sea levels, and intensifying storms
may combine to jeopardize a wide range of coastal
barriers. People, infrastructure, and lagoon and es-
tuarine ecosystems, including mangroves, seagrass
meadows, and salt marshes, will become increas-
ingly vulnerable to growing wave and storm im-
pacts. Observations of increasingly intense tropical
hurricanes and cyclones in all oceans (48) suggest
that losses of beach sand from coastal zones are
likely to increase (49). Further losses may occur
from reduced sand production, formed in many
cases by coral reefs, as a consequence of ocean
acidification and thermal stress on calcareous algae
and other sand producers. Beaches are also under
threat of erosion from rising sea levels. The com-
bination of these factors will lead to less stable
beaches and impacts on other organisms, such as
turtles and sea birds that depend on beach habitats
for reproduction, as well as leading to economic
impacts on tourism and coastal fishing communities.
Opportunities for Management Intervention
The inherent inertia of the atmosphere and of our
attempts to mitigate CO
2
emissions suggest that
reef managers and coastal resource policies must
first reduce the influence of local stressors such as
declining water quality, coastal pollution, and over-
exploitation of key functional groups such as her-
bivores (4). These types of action are most likely to
assist coral reefs through the decades of stress that
inevitably face them. There may be opportunities
for using coral restoration to reduce the risk that
reefs will shift into a non–coral-dominated state
(Fig. 3); however, the efficacy of coral restoration
methods to increase rugosity and coral cover re-
mains unclear, and further evaluation of methods is
badly needed. With respect to the latter, there is a
mismatch between the feasible scale of restoration
(hectares) and that of the extent of degradation
(many thousands of km
2
). Nevertheless, new tech-
niques for the mass culture of corals from frag-
ments and spat may assist local restoration or the
culture of resistant varieties of key organisms (44).
At the 100- to 1000-km scale of coral reefs,
one of the most practical interventions is to facil-
itate grazing by fish and invertebrate herbivores.
This is likely to play an important role in situations
like that of the Caribbean where densities of one
important herbivore, the sea urchin Diadema
antillarum, were decimated by disease in the early
1980s (50). Clearly, the improved management of
reef fish, especially grazers such as parrotfish,
would be expected to result in an improved ability
of coral reefs to bounce back from disturbances (51),
as long as other factors such as water quality are not
limiting. Unfortunately, with the exception of marine
reserves, there is negligible explicit management
of herbivores in most countries, but this could be
improved by setting catch limits (52). Diversifica-
tion of the herbivore guild to include modest den-
sities of invertebrates like sea urchins will also
enhance the resilience of coral reef ecosystems.
Conclusion
It is sobering to think that we have used the lower
range of IPCC scenarios in our analysis yet still
envisage serious if not devastating ramifications for
coral reefs. Emission pathways that include higher
[CO
2
]
atm
(600 to 1000 ppm) and global temper-
atures of 3° to 6°C defy consideration as credible
alternatives. Equally important is the fact that IPCC
scenarios are likely to be cautious given scientific
reticence and the inherently conservative nature of
consensus seeking within the IPCC process (53).
Consequently, contemplating policies that result in
[CO
2
]
atm
above 500 ppm appears extremely risky
for coral reefs and the tens of millions of people
who depend on them directly, even under the most
optimistic circumstances.
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this manuscript, N. Cenacchi for assistance in rendering
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preparation of Fig. 3. The manuscript contents are solely
the opinions of the authors and do not constitute a
statement of policy, decision, or position on behalf of
NOAA or the U.S. Government. This paper is dedicated to
the memory of Kim Mitchell, who saw the value of and
worked hard for the future of the world’s natural
ecosystems.
Supporting Online Material
www.sciencemag.org/cgi/content/full/318/5857/1737/DC1
SOM Text
Table S1
References
10.1126/science.1152509
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