Coral and climate
Like tree rings, coral layers reveal climate change
by Rahilla Shatto
 Reliable information about air and ocean conditions
was not consistently reported until recent times-within the last 100 years
or so. So how do scientists obtain information about year-to-year and decade-to-decade
weather changes that occurred much earlier?
Surprisingly, reef corals in the Pacific Ocean and the
Gulf of Mexico create a good archive of past water temperature. As they
grow, the tiny coral animals deposit layer upon layer of skeletal material,
much like a tree creates annual rings. Water temperature can affect the
corals' growth rates. In the Gulf of Mexico, for example, corals grow quickly
in warm water and slowly in cool water. Unusually cold conditions stress
the corals and stunt their growth significantly. Researchers pay special
attention to "stress bands" in the coral caused by these particularly
low temperatures. The bands reflect severe winter weather, and layers that
contain them probably grew during years with cold and stormy winters.
The thickness of each coral layer tells scientists whether
it grew in favorable or unfavorable conditions, but the real key to understanding
corals' ability to record weather is knowing that temperature changes cause
predictable shifts in the chemical balance of the coral skeleton. Growing
corals take up dissolved elements and compounds along with ocean water,
and alter their internal balance of chemicals in response to temperature
changes. Thus, their continually developing skeletons encode precise information
about changing temperature at a specific location.
By analyzing the thickness, density, and chemical composition
of each layer in a very old coral head, scientists can derive a long-term
record of water temperature for the area where the coral grew. The record
reveals not only past trends lasting several years, but also exact measurements
of long-ago water temperature during each individual season.
In the central and southeastern United States, strong
winter storms that descend from Canada are not-so-affectionately known as
"blue northers." Winters with a lot of blue northers could be
caused by one of two things-either a decadal climate trend in which the
jet stream bends sharply southward from its normal location over the northern
states (and brings northern weather with it), or the single severe winter
of an El Niño year. In either case, corals in the Gulf of Mexico
suffer, and their thin, stressed layers of skeletal material form a permanent
record of the hard winter.
Scientists know that the coral reefs in the Gulf of
Mexico at Flower Garden Banks National Marine Sanctuary and off the coast
of Veracruz, Mexico, contain excellent long-term records of the decade-to-decade
changes in the path of the jet stream. The coral layers reveal this weather
pattern all the way back to the early 1700s and they have potential to go
back even further as more information is retrieved from them.
Scientists also know that corals in the Pacific contain
accurate long-term records of El Niños, and there is good reason
to suspect that corals in the Gulf of Mexico do also. With the benefit of
these centuries-long weather archives, the researchers hope to find a relationship
between the year-to-year presence or absence of El Niños and the
decade-to-decade fluctuations in the jet stream. Scientists are debating
about how the patterns interact, but many hope that the enduring colonies
of tiny coral organisms will help provide answers to this global question. |
Above, heads of brain coral at the Flower Garden Banks in the Gulf of
Mexico. (Frank and Joyce Burek)
Below, arrows indicate "stress bands" revealed in an x-ray
of coral skeletal material caused by cold, unfavorable temperatures.
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