Saturday, August 29, 2009

It's not drought, it's climate change, say scientists


Sunday Age, August 30, 2009

SCIENTISTS studying Victoria's crippling drought have, for the first time, proved the link between rising levels of greenhouse gases and the state's dramatic decline in rainfall.

A three-year collaboration between the Bureau of Meteorology and CSIRO has confirmed what many scientists long suspected: that the 13-year drought is not just a natural dry stretch but a shift related to climate change.

Scientists working on the $7 million South Eastern Australian Climate Initiative say the rain has dropped away because the subtropical ridge - a band of high pressure systems that sits over the country's south - has strengthened over the past 13 years.

These dry, high pressure systems have become stronger, bigger and more frequent and this intensification over the past century is closely linked to rising global temperatures, they found.

Climate data from across the past century shows the subtropical ridge has peaked and waned, often in line with rising global temperatures.

But to see what role greenhouse gases played in the recent intensification, the scientists used sophisticated American computer climate models.

When they ran simulations with only the ''natural'' influences on temperature, such as changing levels of solar activity, they found there was no intensification of the subtropical ridge and no decline in rainfall.

But when they added human influences, such as greenhouse gases, aerosols and ozone depletion, the models mimicked what has occurred in south-east Australia - the high pressure systems strengthened, causing a significant drop in rainfall.

''It's reasonable to say that a lot of the current drought of the last 12 to 13 years is due to ongoing global warming,'' said the bureau's Bertrand Timbal.

''In the minds of a lot of people, the rainfall we had in the 1950s, 1960s and 1970s was a benchmark. A lot of our [water and agriculture] planning was done during that time. But we are just not going to have that sort of good rain again as long as the system is warming up.''

But not all experts agree. Murray-Darling Basin Authority chief Rob Freeman told a water summit in Melbourne last week he believed the extreme climate patterns that have dried out south-east Australia would not prove to be permanent.

''Some commentators say this is the new future. I think that is an extreme position and probably a position that's not helpful to take,'' he said, expressing confidence that wetter times would return.

Dr Timbal believes 80 per cent of the rain loss in south-east Australia can be attributed to the intensification of the subtropical ridge. If the next phase of the study is approved, the scientists hope to work out exactly how rising temperatures result in a stronger subtropical ridge.

The research program, supported by the Murray-Darling Basin Authority, the federal Department of Climate Change and the Victorian Department of Sustainability and Environment, was set up in 2006 to solve the puzzle of why south-east Australia had experienced such a dramatic loss of rain.

The program covers the Murray-Darling Basin, Victoria and parts of South Australia.

Monash University's Neville Nicholls, a lead author on the Intergovernmental Panel on Climate Change who has also published on the subtropical ridge, said he believed the program's results were right.

''We did think that the loss of rain was simply due to the [rain-bearing] storms shifting south, off the continent,'' Professor Nicholls said.

''Now we know the reason they have slipped south is that the subtropical ridge has become more intense. It is getting bigger and stronger and that is pushing the rainstorms further south.''

The scientific results have implications for many state government water programs and drought funding, some of which factor in climate change. Projections for the water coming to Melbourne in the north-south pipeline are based on the assumption that Victoria will return to rainfall levels of last century.

Melbourne's dams get roughly a third less water than they did before the drought began in 1996.

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