Quarterdeck, Volume 6, Number 2, Summer 1998
Coral and climate
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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Last updated August 1, 1998