By David Fogarty, Climate Change Correspondent, Asia
SINGAPORE Feb 10 (Reuters) - Flood and storm-battered northern Australia is likely to suffer more frequent weather extremes, according to a study of coral cores that reveal a centuries-old climate record for the region.
Like pages in a book, corals can help scientists go back in time by revealing years that were unusually wet or dry. The annual changes or variations in weather are recorded in growth rings that can be studied by drilling and extracting long cores.
"The corals are providing another piece of evidence that maybe suggests that we are seeing some consequences already of global warming," Janice Lough, a senior scientist at the Australian Institute of Marine Science in Queensland state, told Reuters from Townsville on Thursday.
Lough, in a study to be published in U.S. journal Paleoceanography, examined 17 coral cores taken from reefs off Queensland's northeast coast. The rings in the cores date from the 17th century until 1981 when they were collected, yielding a 300-year climate record.
Northern Queensland typically gets most of its rain during the summer monsoon and is at the mercy of the El Nino and La Nina weather patterns that normally bring drought or floods.
The current strong La Nina is blamed for record floods that have inundated large areas of Queensland in recent months, killing dozens of people, crippling coal mines, swamping thousands of homes and damaging crops.
"It can be difficult to detect whether any changes are happening just because of that high natural variability," she said, but added that the long-term record suggested some sort of change had occurred. "That tropical rainfall variability will become more extreme."
Lough's study looked at coral rings over three periods of roughly 100 years each. She said rainfall in the late 17th century and into the 18th century was probably a bit above the long-term average and was reasonably variable. She said the next 100 years from 1785 to 1884 was a bit drier and less variable.
"Then we seem to see a signal from the end of the 19th century up until 1981 of more rainfall but also more variability, so that the wet years tend to be wetter and dry years drier and that the frequency of such extremes increases."
Subsequent weather records over the past 30 years backed up the move towards more extremes, she said, with the past several years in Townsville being very wet. Townsville is in northeast Queensland, which was badly affected by Cyclone Yasi last week.
The cores come from Porites coral domes that can grow up to 8 metres (26 feet) high and be centuries old, growing between 1 and 1.5 cms a year, she said
In her research, Lough looked into slices of the coral under ultraviolet light to study the growth bands.
During wet years, rivers flush a lot of plant matter and a mix of compounds called humic acids into the ocean and these acids are absorbed by the coral and stored in its skeleton. Under UV light, growth bands with more humic acid show up more brightly than bands from drier years.
Lough said the coral records were another piece of the climate jigsaw, given the lack of long-term historical weather data, particularly in the Southern Hemisphere and the tropics.
"Corals are natural history books," she added, revealing growth rates, changes through time and the amount of freshwater run-off from land. (Editing Yoko Nishikawa)