Field:Earth Science/Geoscience/Environment
Session Topic:
CO2 and Climate change: past – present – future
Speaker:
Atsushi Suzuki , National Institute of Advanced Industrial Science and
Technology (AIST)
Title: Coral reefs and climate change
1. Introduction
The decade of the 1990s was very likely the warmest of the
second millennium. The 1990s was also characterized by one
of the strongest El Niño-Southern Oscillation (ENSO) events of
the twentieth century, which occurred in 1997–1998 and was
followed by worldwide mass coral bleaching. The IPCC report
dared to conclude that most of the warming observed over
the last 50 years can be attributed to human activities (IPCC,
2001). A very wide range of physical and biological indicators
was listed in the report, together with key observed changes
attributable to anthropogenic climate perturbations,
including a global rise in mean surface temperature,
increased El Niño frequency, and frequent coral bleaching. In
this regard, sea-surface temperature (SST) is an important
indicator of climate change, because the global mean
surface temperature is defined as the area-weighted mean
of land- and sea-surface temperatures. All three of these
indicators, SST rise, El Niño frequency, and coral bleaching,
are detectable by analysis of coral skeletal records (Fig. 1).
Fig. 1 X-ray photo
of
Porites
sp.
coral with annual
density bands.
2. Global warming reconstructed by coral skeletal records
Corals offer rich archives of past climatic variability in tropical and subtropical
regions, where instrumental records are limited. The oxygen isotopic ratio (18O/16O) of
the coral skeleton (18Oc) is a function of both SST and the oxygen isotopic
composition of seawater (18Ow), which is related to salinity (Fig. 2A). In oceanic
settings where 18Ow is constant, coral 18Oc records SST variability. High-resolution 18O
records from long-lived Porites spp. corals collected in the northwestern Pacific have
been used successfully to monitor recurrent El Niño events and a long-term rise in SST
(Fig. 2B). In the relatively high-latitude region of the NW Pacific, coral 18Oc records
are a good proxy for SST because of the limited influence from 18O-depleted rainfall.
A century-long coral 18Oc record from Ishigaki Island clearly indicates decreasing
trend of about 0.2‰, probably corresponding to about 1.5°C SST increase in the area.
Fig. 2 Coral 18O thermometry (A) and coral 18O records from the NW Pacific(B).
3. Coral mass bleaching event in 1997-1998
Coral bleaching has been described in the literature since the beginning of the
twentieth century. Since the late 1970s, however, there have been numerous reports
of mass bleaching affecting coral reefs in the Caribbean and elsewhere in the
tropics. These recurrent coral-bleaching events may be a response to global
warming. High SST destroys the symbiotic relationship between the host corals and
zooxanthellae
(symbiotic
algae).
During
1997–1998, in particular, severe coral bleaching
was reported from many coral reefs in the
tropical regions of the world. This global-scale
coral-bleaching event coincided with strong El
Niño
event
and
the
warmest
global
temperatures on record. Thermal stress and
bleaching of corals are reported to reduce
calcification and decrease skeletal extension Fig. 3 Bleached coral (right) in the
rates. Our research group also identified a Great Barrier Reef (courtesy of K.
distinct reduction in skeletal growth of a Porites Fabricius)
coral during the 1997–1998 mass bleaching
event in the Ryukyu Islands.
4. Summary
Our results are intriguing enough to warrant further data acquisition and attempts at
quantitative time-series analysis of coral records. Especially, high-resolution records
have potential for studies of the climate of the recent and distant past, including the
middle Holocene and earlier interglacial periods, which may provide partial analogs
for the future greenhouse Earth. Another recently recognized risk of coral reefs
(including cold-water corals) is ocean acidification, which is caused by ocean
uptake of the excess CO2 released by anthropogenic activities. Even small changes
in CO2 concentrations in surface waters may have large negative impact on coral
calcification. Coral skeletal records can be used for examining ocean pH histories by
the analysis of boron isotope ratio, which is a new proxy for ocean pH.