WA drought 'could be worst for 750 years'

If you thought the drought affecting south-west WA since the 1970s was extreme, you were right.
But just how extreme has been a matter of contention.

Now, scientists believe it could be the worst of its kind in 750 years, after making an unexpected discovery.
Researchers from the Australian Antarctic Division and Antarctic Climate and Ecosystems Co-operative Research Centre have identified a link between the drought, which began in the early 1970s, and snowfall at a site in East Antarctica over the same period.

In research published in Nature Geoscience, they say the relationship is inverse:  high snowfalls at the Law Dome site correlate with low rain in the South-West.

This is a result of the atmospheric circulation pattern that brings dry, cool air to Australia, while sending warm, moist air to East Antarctica.

However, the high snowfall at Law Dome was unlike any other in the past 750 years, and led the researchers to believe the drought was similarly unusual.

Since the 1970s, there has been a decline of up to 20% in winter rainfall in the South-West and, though the cause of the drought remains unclear, others have pointed to land-use changes, ocean temperatures, air circulation changes and natural variability.

But its severity has been hard to calculate, with weather records going back only about 100 years, and the oldest tree-ring record, 350 years, from a site that has not been affected by the drought.

The researchers found that the snowfall was of a severity expected only once in every 38,000 years.

Adjusting their analysis of ice cores, it still should happen only about every 5400 years.

"It also suggests ... that if the mix of factors that influence [South-West] rainfall over the past century reflects that of the longer term, then the recent drought ... may be similarly unusual," the researchers say.

Lead researcher Tas van Ommen said the results of the study were unexpected.

"We were surprised at first, given the complexity of climate processes, to find such a direct connection between our ice core and the climate of Western Australia," he said.

"By identifying new processes that influence regional Australian climate, this work offers the possibility to improve understanding and reduce uncertainty in future projections of climate change.

"This work underscores the need for long-term records of past climate from sources like ice cores and it illustrates the important role that Antarctic climate processes play globally."

It suggested human influence was likely to have played a role in the drought, Dr van Ommen said.

University of NSW professor Andy Pitman said the study was a "good and bad news story."

"It is good for those policy makers in WA who invested in alternative sources of water based on earlier research by CSIRO and Bureau of Meteorology," he said.

"This new science suggests they made a wise decision. It is, of course, less good news for the future of water dependent industries in WA and reinforces the urgent need for global cuts in greenhouse gas emissions."

Monash University professor Neville Nicholls said the researchers may even have underestimated the severity of the drought.

"Since about 1990, snowfall at their site in Antarctica appears to have decreased, but the South-West rainfall has not rebounded as we might have expected from this," he said.

"This indicates that some additional mechanism is affecting either snowfall or the drought. This is not surprising in a time of strong global warming. But we do need to work out these mechanisms."

Link:  http://www.watoday.com.au/environment/climate-change/wa-drought-could-be-worst-for-750-years-20100205-niee.html