Scientists read climate change history in coral reefs. Luminescent Lines.
by Dr Janice Lough
Senior Principal Research Scientist
Australian Institute of Marine Science
We all experience weather every day, the temperature, the amount of rain (or snow), wind speed and direction, cloudy or sunny etc.
Climate is what we expect the weather to be like at a particular place and time of year.
Our current climate is defined by weather observations compiled over many years from measurements made with instruments such as thermometers, rain gauges, anemometers, barometers.
See the Australian Bureau of Meteorology .
These observational records have provided important information to climate scientists about how global and regional climate varies and, most recently, clear evidence for a rapidly warming world due to human activities changing the composition of the atmosphere.
Instrumental observations of climate for large parts of the world only date back to the late 19th century – a relatively short time period to fully understand how and why climate varies and changes.
We need longer records to better understand how the climate system works and be able to better predict how climate may change in the future.
Fortunately, the natural world provides us with several types of “proxy” climate records, i.e. substitutes for instrumental observations.
These are biological, geological or human systems that are affected by climate and leave a record of that influence that we can measure and date.
Examples include variations in tree ring widths, annual snow layers and air bubbles trapped in ice cores, human documents and diaries that mention weather events and cave deposits known as speleotherms (stalagmites and stalactites).
Shallow-water tropical coral reefs also contain natural history books which can provide proxy climate information for the tropical oceans which are the heat engine of the global climate system.
The backbone of coral reefs is the hard calcium carbonate skeletons built by corals in partnership (symbiosis) with single-celled algae (zooxanthellae) – these skeletons come in many shapes and forms and provide a diverse range of habitats for many thousands of reef-associated organisms.
Some massive coral species grow to several meters in height [1-Picture of diver and large Porites coral] and contain annual growth layers similar to tree rings.
Growing at 1-2 cm per year, such corals can contain several hundred years of growth and also record changes that have occurred in their environment.
To “read” these records, cores are removed from the centre of a colony (the core hole is plugged and the coral tissue will gradually grow over it) [2-Picture of diver taking coral core].
Slices from the core are then X-rayed to reveal annual banding in the density of the calcium carbonate skeleton and each year can be dated from the outer edge – the year when the core was collected.
From measuring the annual density bands we can find out how fast the coral is growing and also identify the timing of unusual events in the coral’s life which may have affected its growth (e.g. coral bleaching events, toppling of the coral by a tropical cyclone) [3-X-ray positive print of coral slice showing annual density bands and highlighting dark band in 1998 when coral growth slowed due to coral bleaching].
We can also measure various geochemical tracers included in the skeleton which vary with, for example, sea water temperature, salinity or the amount of sediment in the water.
One special type of records is seen when slices from corals growing close the coast on the Great Barrier Reef are viewed under ultra-violet light [4-Coral slice under UV light showing luminescent lines].
These are called luminescent lines and their occurrence and intensity is directly related to seasonal river flood events which, in turn, depends upon the amount of summer rainfall in northeast Queensland.
By dating and measuring the intensity of these annual luminescent lines in several inshore corals, we have been able to reconstruct northeast Queensland summer rainfall back to the 17th century – more than triple the length of rainfall observations made with rain gauges. This record tells us about past rainfall variability and changes and how our rainfall is influenced by El Niño-Southern Oscillation events. The coral record tells us that Queensland rainfall was less than present and also less variable from the mid-18th to mid-19th centuries. They also tell us that since the end of the 19th century rainfall has increased, become more variable from year to year and that extreme wet and extreme dry years have become more common. The summer of 1973-1974 [5-1973-74 luminescent lines in corals ~800 km apart], with which the most recent extreme summer of 2010-2011 has been compared, was the wettest in at least the past three centuries. More extreme rainfall events are one of the projected consequences of a warmer world in tropical Queensland. So, looking to the past can help us to prepare for different climates in the future.
Associated story: ABC Science Report, Winners and losers in ocean acidification.
Australian Institute of Marine Science
