The Problem

Anticipation of future environmental and climatic change depends upon knowing and, hopefully, understanding what has happened in the past and is now happening. For example, our understanding of controls on world temperature is such that we might expect it to rise following observed increases in the 'greenhouse' gases. However, instrumental records of weather and climate go back only about 100 years and that time span is insufficient to judge whether recently observed rises in global temperature are normal or abnormal.

Similarly there is national and international concern about degradation of coral reefs, but detailed observations and measurements in coral reef environments cover only the last 20 years or so. It may be that changes to reefs that are causing concern are simply natural variations.

The problem is the same in both cases; we need to get longer records in order to know enough about the past to assess the present and decide whether anything needs to be done.

Two divers descend on a very large Porites coral on the Great Barrier Reef.

The Solution

There are biological and geological systems which leave a record of both their responses to their environment and the time at which those responses occurred. For example, annual rings are formed in trees growing in temperate and sub-polar regions. The width of an individual ring records how well the tree grew in that year. Wet years are good years for trees growing in arid regions. Warm years are good years for trees growing high on mountains. Recovery of environmental information from trees is now so sophisticated and advanced that it has become the science of dendroclimatology.

Other 'proxy' records of individual years for past environments are ice cores, sediment varves (layers) and certain documentary records. None of these gives good information about tropical ocean regions. Tropical ocean regions are the "heat engine"' for the global climate system. A source of proxy information for such regions would fill a large gap in the existing proxy records. Marine tropical records would also provide new information about the El Nino/Southern Oscillation, which is a primary source of short-term (2-10 year) climate variability in Australia and Southeast Asia, as well as in other tropical and extra-tropical regions.

Annual density bands in coral skeletons have promised to be the tropical marine proxy record since their discovery 20 years ago. Extensive records are possible because some corals live for several hundred years. Density bands offer records in themselves, analogous to tree rings, and they offer a means to analyse human impact upon coral reef ecosystems. Moreover, the bands make it possible to date the time at which skeleton was deposited. The skeleton includes a variety of trace materials taken up from the coral's environment. Long-lived massive corals are, consequently, vast store-houses of information about past marine environmental conditions.

The outermost dark density band was formed during the 1986-1987 summer. Counting backwards, the coral probably started growing in 1966.

Ruler markers indicate positions of annual density bands.

The promise of coral density bands to yield similar fabulous records to those obtained from tree rings has not been realised despite extensive work in many laboratories around the world. A large body of information has accumulated, much of it apparently contradictory. The Density Banding Research Group at AIMS has been working towards an understanding of how the density bands are formed. This has been achieved within the past year. The new understanding makes sense of the apparently conflicting reports in the scientific literature. It presents a clear, succinct picture of coral density band formation and removes much of the confusion which has surrounded the topic.

The Future

There is now international recognition of the need to understand past climatic and environmental change. International programs recognise the unique contribution that massive corals can make to understanding global changes. Work on coral records at the Institute has achieved both national and international recognition.

For further information:

Contact: - The Density Banding Research Group.

e-Mail:
Dr. Janice Lough
Dr. David Barnes

tel: 077 534248, 534236
fax: 077 725852