like climate change."
Creating more protected areas for coral reefs may help
them better withstand the rigors of too-warm water and be
less vulnerable to mass bleachings. Kelp forests seem to
be able to cope with warmer water better in marine
reserves, explains Fujita. But even so, cautions Fujita, "the
Threats: Coral Reef Bleaching number of corals that can adapt to or withstand such
dramatic, rapid changes may be just a tiny fraction — coral
Posted: 08-Aug-2005; Updated: 16-Aug-2007 reefs may likely prove to be the first ecological victims of
unchecked global warming."
Coral reefs are taking hits around the world from many
Besides the incalculable loss of these "rain forests of the
stresses — pollution, destructive fishing practices, disease
ocean" and the colorful reef fish, turtles, sharks, lobsters,
— but perhaps the most damaging is warmer water.
shrimp, sea urchins, sea stars, anemones and sponges
that depend on them, the economic losses would be
How coral loses its color and starves
enormous in regions that depend on reefs for food and
tourism income. Coral reefs provide an estimated $375
Corals host tiny algae called zooxanthellae that give them
billion in economic benefits each year globally.
their color and a food source. When stressed by excessive
heat or cold, many corals expel their algae and "bleach." More danger lurks—acidic water
Corals are very sensitive to temperature changes and
thrive within a narrow band of heat and cold: a temperature Coral reefs face yet another threat induced by carbon
increase of one degree Celsius (1.8 degree F) can trigger dioxide pollution. A June 2005 report by the U.K.'s Royal
them to bleach. After severe bleaching, they often die. Society found that the increased carbon dioxide being
absorbed by the ocean over the last two centuries is
A mass bleaching of corals occurred during the warmest making it more acidic.
12-month period on record (1997-1998). About 16 percent
When carbon dioxide dissolves into the ocean, it produces
of the world's reefs were seriously damaged. Coral reefs
carbonic acid, which corrodes the limestone structures of
that had persisted for as long as a thousand years,
coral reefs and shells of marine organisms. "The world's
perished. Continued warming could make mass bleachings
seas are naturally alkaline," comments Fujita. "And
an annual event. Environmental Defense scientist Doug
thousands of marine animals have evolved and flourished
Rader says that "within a century, very large portions of
for eons in this environment. Many of these creatures will
coral reefs could be gone."
not survive in an acid sea."
A sliver of hope In acidic water, "There is a greater tendency for seashells
to dissolve, like putting them in vinegar, but not quite as
Rader has spent years diving and snorkeling in the dramatic," said Environmental Defense climate scientist Dr.
Caribbean and has seen firsthand the decimation to reefs: James Wang.
"The damage is bad enough from coral and sea urchin
disease, which has nearly destroyed elkhorn and staghorn As waters become more acidic, coral reefs and other
reefs throughout the region, then you add to that more marine ecosystems could suffer. The report's panel of
frequent bleaching events and the fact the seas are unable scientists said that acidification hurt tropical and subtropical
to support reef-forming coral because waters are absorbing reefs the most, but that cold water corals are also in
too much carbon dioxide — it seems hard to believe that it danger. Since acidification is "irreversible in our lifetimes,"
is happening — and happening on our watch." the authors say, "the only practical step is to reduce
emissions of carbon dioxide as quickly as possible to
But Environmental Defense marine ecologist Rod Fujita minimize large-scale, long-term harm to the world's oceans
offers a sliver of hope. "Corals are sensitive but also very and marine ecosystems."
resilient — if conditions are right. If we can reduce some of
the other direct stresses from human activities on coral Available online:
reefs, like pollution from nonpoint sources, perhaps that http://www.edf.org/article.cfm?contentID=4709
may also enable reefs to cope better with indirect threats
The 1998 mass bleaching was coincident with
anomalously high sea surface temperatures. That year
was the warmest of this century (NOAA, 1999), and
tropical sea surface temperatures were the highest in
the modern record (Strong et al., 1998). For many
parts of the Pacific, the 1997-98 mass bleaching has
been linked to the strong El Niño-induced seawater
warming. The relationship between El Niño and coral
bleaching is less clear, however, for the Indian
Early Warning Signs of Global Ocean, Arabian Gulf, and some parts of the Pacific
Warming: Coral Reef Bleaching (Wilkinson, 1999; ISRS, 1998). The geographic
extent, regional severity, and increasing frequency of
Coral reefs are one of the most productive recent mass bleaching events point to an underlying
ecosystems on Earth, providing many critical services global cause namely a trend of increasing sea surface
to fisheries, shoreline protection, tourism, and to temperatures in some of the tropical oceans, driven
medicine. They are also believed to be among the by global warming (US Department of State, 1999).
most sensitive ecosystems to long-term climate
change (Nurse et al., 1998). Elevated sea surface Global mean sea-surface temperatures are projected
temperatures can cause coral to lose their symbiotic to increase by about 1-2°C in the next century
algae, which are essential for the nutrition and color (Kattenberg et al., 1996). If the overall warming is
of corals. When the algae die, corals appear white accompanied by more frequent periods of sustained
and are referred to as "bleached." Water temperatures high temperatures, mass bleaching events will
of as little as one degree Celsius above normal become more frequent and widespread. Increasing
summer maxima, lasting for at least two to three human stresses such as pollution, overfishing, soil
days, can be used as a predictor of coral bleaching erosion, and physical damage from boats and other
events (Goreau and Hayes, 1994). Studies indicate recreational activities will also weaken corals,
that most coral are likely to recover from bleaching if limiting their ability to adapt to climate change
the temperature anomalies persist for less than a (Hodgson, 1999; Nurse et al., 1998). Furthermore, as
month, but the stress from sustained high ocean warming coincides with sea-level rise and
temperatures can cause physiological damage that perhaps more frequent tropical storms and El Niños
may be irreversible (Wilkinson et al., 1999). (e.g., Timmerman et al., 1999), reefs are likely to
experience greater coastal erosion, sedimentation,
In 1998 coral reefs around the world experienced the and turbidity, which would add to their demise.
most extensive and severe bleaching in recorded
history (ISRS, 1998; Wilkinson et al., 1999). Coral Available online:
bleaching was reported in 60 countries and island http://www.ucsusa.org/global_warming/science_and_impacts/i
nations at sites in the Pacific Ocean, Indian Ocean, Last Revised: 11/10/03
Red Sea, Persian Gulf, Mediterranean and Caribbean.
Indian Ocean corals were particularly severely
impacted, with greater than 70 percent mortality
reported in the Maldives, Andamans, Lakshadweep
Islands, and in Seychelles Marine Park System.
Unlike most previous bleaching events in which
severe impacts were limited to less than 15 m water
depth the 1998 bleaching affected corals at up to 50
m water depth. This mass bleaching followed similar
but less severe events in 1987 and 1990. Prior to the
early to mid 1980s, bleaching tended to be rare and
localized, and corals generally recovered.
Helping Coral Reefs Survive
resistant to climate change by, for instance,
reducing local sea surface temperatures, and
ScienceDaily (July 25, 2003) — While the those that make reefs resilient to climate change
high ocean surface temperatures during by helping them recover from bleaching.
the 1997-98 El Nino bleached coral reefs in
more than 50 tropical countries worldwide, The researchers found that the factors that
patches of coral did survive in or near the confer resistance to bleaching include local
damaged reefs. A new study of these upwellings of cold water, and natural exposure
patches identifies factors likely to protect to heat stress. For instance, in an area of Binh
Thuan, Vietnam, upwelling of cold water brought
these threatened marine ecosystems
surface temperatures down from 39 degrees C
during climate change. to 29 degrees C within days, and corals there
recovered better than elsewhere in the country.
"As baseline sea surface temperatures continue
In addition, corals that emerge at low tides may
to rise, climate change may represent the single
be more tolerant of heat stress. For instance, in
greatest threat to coral reefs worldwide," say
Palau's Rock Islands, the reef flats that emerge
Jordan West of the U.S. Environmental
during the low tide were bleached less than the
Protection Agency in Washington, DC, and
parts of the reef that are in deeper waters.
Rodney Salm of The Nature Conservancy in
Honolulu, Hawaii, in the August issue of
The factors that confer resilience to bleaching
include having diverse populations of corals that
produce lots of larvae, surface currents that
Coral reefs have among the greatest biodiversity
spread the larvae, herbivorous fish that graze
of any ecosystem worldwide and provide key
the algae that otherwise grow on top of
services to people, from food to coastal
damaged reefs and prevent the establishment of
protection to tourism. Reef-building corals
new corals, and management that decreases
depend on symbiotic algae to photosynthesize
stresses such as pollution and fishing methods
much of their food, and surface waters that are
that destroy reefs.
warmer than normal can "bleach" corals by
depleting their photosynthetic pigments or even
make them expel their algae. West and Salm recommend that coral reef
managers use this work to identify and protect
patches of coral reef that are most likely to
To help conserve coral reefs during climate
persist during continuing climate change.
change, West and Salm assessed factors that
Establishing reserves that protect networks of
may have protected the coral patches that
these patches will help ensure that the corals
survived the 1997-98 bleaching. The factors fell
that survive a major bleaching event will be able
into two categories: those that make corals
to replenish those that do not. The Nature
Conservancy is currently applying this work to
help coral reefs
bleaching in the
Palau, which is