Coral growth rings point to bad weather ahead



Coral porites

Credit: Biogeography Team, Center for Coastal Monitoring and Assessment, NOAA.

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SYDNEY: Australia may be in for more severe and frequent droughts as climate variability intensifies, according to a new study of Indian Ocean reefs.

Using growth bands of corals, scientists at the Australian National University (ANU) in Canberra constructed a record of climate change and weather conditions dating back to 1846.

They found that tropical weather patterns in the Indian Ocean have become more variable over the last century. This strongly suggests that we are in for even more extreme weather patterns in the future, the researchers say in a study published this week in the British journal <i>Nature Geoscience</b>.

Intense and frequent

The coral growth records point to an increase in the intensity and frequency of the Indian Ocean Dipole (IOD), an oscillating weather system similar to the El Niño/El Niña system found in the Pacific Ocean.

"We've found that the IOD [flip] has lately been occurring every four years, as opposed to every 20 years around the turn of the century," said ANU earth scientist and study co-author Mike Gagan. "We're seeing a clear trend towards these events becoming stronger and more frequent."

The dipole, first described in 1999, is characterised by a switch in sea surface temperatures on opposite sides of the Indian Ocean. The change tends to bring drought to western Indonesia and southern Australia and heavy rains to eastern Africa and southern India.

To reconstruct historic climate conditions, the scientists examined growth bands from common species of Porites corals found in the Seychelles, Bali, and the Mentawai Islands.

In a similar way to how tree rings are used to reconstruct historic climate on land, annual growth bands of corals correspond to environmental factors such as water temperature, salinity and wave action.

The ANU team compared two different verities of oxygen - isotopes known as oxygen-18 and oxygen-16 - that were locked into the bands as the corals grew over time.

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