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Coral Reefs Trace Australia's Historic Rainfall

The torrential rains that have triggered flooding across northern Australia may become an increasingly familiar occurrence, new research suggests.
/ Source: OurAmazingPlanet

The torrential rains that have triggered flooding across northern Australia may become an increasingly familiar occurrence, new research suggests.

Growth rings in corals have shown which summers brought more rain than others, creating a centuries-long rainfall record for northern Australia, the study found.

"This reconstruction provides a new insight into rainfall in northeast Queensland," said study author Janice Lough, a climate scientist at the Australian Institute of Marine Science, in Queensland, Australia. "These coral samples, which date from 1639 to 1981, suggest that the summer of 1973 to 1974 was the wettest in 300 years. This summer is now being compared with that record-setting one."

In Australia, summer occurs from December to February.

Record rains

Eastern Australia is recovering from Cyclone Yasi, which struck last week and added to damage from serious flooding due to record rainfall during the austral summer, which has slowly spread to the south along the coast.

Australia might be in for more weather extremes, the coral record suggests. Following a steady period of relatively little rainfall from the mid-18th to mid-19th centuries, average rainfall for the region has significantly increased and become more variable since the late 19th century, with wet and dry extremes becoming more frequent, Lough said. Coral cores from long-lived, massive Porites coral found along Australia's Great Barrier Reef revealed the region's rainfall history. Porites form large dome-shaped colonies that can be up to 26 feet (8 meters) in height and hundreds of years old. The coral colonies secrete layers of calcium-carbonate skeletons that can be counted like tree rings to calculate the colony's age. Rings with degraded plant matter and a mix of compounds called humic acids in them indicate a wet summer.

Annual records from multiple coral cores were calibrated against the instrumental rainfall record of the 20th century and used to reconstruct summer rainfall records back to the start of the coral colonies' growth.

Climate past and present

The records show that the frequency of extreme events has changed over the centuries, and is currently at a peak. During the earliest part of the reconstructed record, wet years occurred on average every 12 years, and very dry years every nine. Then the frequency dropped: very wet years occurred about every 25 years, and very dry years every 14 years. From 1885 and 1981, the extremes increased dramatically in frequency, with very dry years taking place every 7.5 years on average, and very wet years occurring about once every three years.

The coral records' agreement with other climate indicators is mixed, but the increase in rainfall variability since the late 19th century is evident in two independently-derived records of a recurrent tropical climate pattern known as the El Niño-Southern Oscillation.

A record of Australia's past climate is particularly valuable, Lough says, as there is an overall lack of data on long-term climate variability in the tropics and the southern hemisphere. Such data is needed to place the current variability of the region's climate in an historical context to better project what will happen in the current state of global warming.

The records derived from the Great Barrier Reef corals support predictions that tropical rainfall variability will increase in a warming world.

The new study will be published in a future edition of the journal Paleoceanography.