DROUGHT, that most undesirable element of Australian agriculture, is going to be more frequent and more severe in the decades ahead.
That’s the sombre warning of a Climate Council report that maps current trends into a future where drought frequency and severity is increased as the planet grows warmer.
The effects will be felt in agriculture, in particular, but also on urban water supplies, native ecosystems and the public purse as governments are called on to provide drought relief more frequently.
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The report, Thirsty Country: Climate Change and Drought in Australia, notes that warming has already produced substantial drying in south-west Western Australia, which has lost 15 per cent of the annual average rainfall it had in the 1970s, and in south-east Australia, where soil moisture deficits are increasingly common.
Both patterns appear related to the same global warming phenomenon, the movement south of cool-season rain-bearing fronts out into the Southern Ocean.
How these fronts track as they roll eastward off the Indian Ocean is largely determined by the subtropical ridge, a band of high pressure that tends to lie across Australia.
The intensifying of the subtropical ridge as the planet has warmed has pushed the movement of the fronts south - by some estimates, already up to 300 kilometres - so that they more frequently move across the Southern Ocean and less frequently over land.
The Bureau of Meterology and CSIRO estimate that 80pc of the rainfall decline in south-east Australia can be attributed to this shift.
Meanwhile, things are growing hotter.
Australia had its warmest year on record in 2013. Since 2001, the number of extreme heat records in Australia has outnumbered extreme cool records by almost 3 to 1 for daytime maximum temperatures, and almost 5 to 1 for night-time minimum temperatures.
By increasing soil temperatures and evaporation rates, higher temperatures increase the severity of events like the Millenium Drought (1997-2010) of south-eastern Australia.
Lack of rainfall and high temperatures reinforce each other to cause a cumulative rainfall deficit - deep drying of soils and landscapes that need several years of above-average rainfall before returning to baseline health.
The report’s author, Will Steffen, said although rainfall trends are harder to forecast that temperature, it seems possible that annual averages may not change much, but the distribution of the rain events will, leading to more drought.
“There is now pretty convincing evidence that across the southern part of the country it’s going to get drier, especially in the cooler months of the year,” said Professor Steffen.
“Pretty much everywhere can expect to spend more time in drought. Even when average rain falls across the year, the effect of climate change now appears to be more extensive dry periods, and heavier rainfall when it comes.
“You can take that old phrase, this is a land of drought and flooding rain, and say that climate change is going to intensify that pattern.”
Australia is not the only country to see a drying trend.
“There is a fairly strong drying trend in southern Africa, and there is some evidence that’s related to climate change. Drying around the Mediterranean is quite pronounced, and that’s a very consistent prediction of all the models when they are run in hindcast - when they run to see if they simulate what’s already happened. They get that pretty right.”
However, the jury is out on the role that climate change is playing in increasing the severity of the acute drought gripping California.
The Climate Council’s recommendation for halting the drying trend is familar and currently, politically implausible.
“To stabilise the climate, we must rapidly reduce greenhouse gas emissions, increase investment in clean energy, and most of the world’s fossil fuel reserves – coal, oil and gas - must remain in the ground,” the report said.
The Thirsty Country report can be read online.