Modelling the climate change possibilities

29 Aug, 2009 02:00 AM
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What's common to Asian factories, Australian land clearing and the subtropical ridge? They have all been implicated in decreased rainfall over the Murray-Darling Basin.

Climate researchers are grappling with the decade-long rainfall decline over the MDB, particularly in south-eastern Australia where lack of runoff has crippled irrigated agriculture and threatens urban water supplies.

When your subject is something as vast as the planet's climate, a way to try and understand all the variables at work is to use computer simulations. Three separate computer modelling exercises have thrown up some interesting possibilities to explain the south-east's rainfall deficit.

The Bureau of Meterology's Dr Bertrand Timbal and colleagues have previously reported on a drought pattern linked to intensification of a band of high pressure called the subtropical ridge. They have now linked the ridge's behaviour to global warming.

As of May 2009, south-eastern Australia had recorded its driest 12-year period on record, with an annual average of 506mm since October 1996. Previously, the driest 12-year period was from 1935-1947, when the annual average was 511mm.

Dr Timbal's team identified a link between the two dry spells: they both occurred during a period when the subtropical ridge intensified.

The high pressure band sits roughly between 30-35 degrees of latitude, in NSW defined by Wagga Wagga in the south and Walgett in the north. When it intensifies, Dr Timbal said, the ridge forces rain-bearing low pressure systems off the continent and out to sea.

The ridge's patterns of intensity tend to peak in autumn, matching patterns of rainfall decline in the south-east, where about 60 per cent of the loss of rainfall has occurred in the autumn period.

Through computer simulations, his team has made strong links with another influence: the subtropical ridge's patterns of intensification closely match the rise and fall of average global temperatures throughout the 20th Century.

"We don't have a definitive answer," Dr Timbal said. "But we suspect that it is linked to the fact that the global system, when you change the balance by putting more energy into the system such as greenhouse gases, has to balance itself."

Dr Leon Rotstayn of the Centre for Australian Weather and Climate Research has found another possibility in aerosols—particles of dust, smoke and human-generated pollution that float in the atmosphere.

In 2007 Dr Rotsayn published a paper suggesting the cloud of smog coming off Asia was cooling land and water to the extent that it changed the temperature gradients in the western Pacific, so that rain-bearing systems moved toward Australia's north-west where rainfall has more than doubled in recent decades.

He has since thrown the Northern Hemisphere's aerosols into fresh modelling, and come up with the further possibility that pollution halfway around the globe may be affecting patterns like the Southern Annullar Mode (SAM).

A phenomenon driven by ocean temperature gradients, SAM describes the way that rain-bearing lows moving across southern Australia seasonally shift north and south.

In summer, the fronts are further south, and rain falls out to sea. In winter, traditionally the fronts have moved north to provide southern Australia’s winter rainfall.

That pattern has grown less reliable, and Dr Rotstayn thinks he detects in computer simulations a possible cause across the equator, in the dust and smog of the north.

"It means we have another candidate for what might be driving trends in the Southern Annullar Mode," Dr Rotstayn said.

"No matter how far away things are, they can potentially affect our climate."

To complicate matters, Dr Clive McAlpine of the University of Queensland has suggested the answer might lie on the ground as well as in the atmosphere. He has made a link between drought in the MDB and land clearing.

Ecos magazine reported that Dr McAlpine's team used the CSIRO MARK 3 climate model to compare climate under pre-European vegetation with the climate after 200 years of land clearing. The simulation kept carbon dioxide levels constant.

The result was an increase in temperature of 0.4–2.0°C, and a 4–12 per cent rainfall decline across south-eastern Australia, plus amplified droughts during El Nino events and more hot days.

Dr McAlpine told Ecos that the impact of land clearing could be as significant to Australia’s bread basket as climate change.

"…climate change is a multi-dimensional process and we need to look at more than just reducing greenhouse gas emissions—we also need to look at how we’re managing our land surface in the future," he told the magazine.

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