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Published: Friday 16 August, 2013

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Is a mysterious new weather system causing the drought in southern Australia?

Climatologists are desperately trying to explain the mystery of where southern Australias winter rainfall is going. Theyve known the rain is being pulled south by an unexplained force. Now theyve devised a revolutionary new theory to explain why. It appears that the circulation of the entire Southern Hemisphere is changing to suck our rain away. The reason is the Antarctic Vortex a natural tornado of 30km high, supercold, superfast winds spiralling around Antarctica. The vortex is not new; its one of the engines that drive climate in the Southern Hemisphere. But now it appears the vortex is shifting gear, and is spinning faster and faster, and getting tighter. As it does its pulling the climate bands further south dragging rain away from the continent out into the southern ocean. Most disturbing of all we might be responsible for shifting the speed of the vortex. Scientists at the US Office of Oceanic and Atmospheric Research believe the speeding up of the vortex is caused by the combined effect of global warming and the depletion of the ozone layer over Antarctica. If their theory is true it will have devastating consequences for our southern cities the drought may not go away. They fear that something mysterious at the bottom of the world might be sucking away our rain. If the theory is right, prime agricultural land and our southern cities may face permanent drought.

Dr David Jones, Bureau of Meteorology: Were just going to have to accept that life in the future may not provide us with as much water as we had in the past.

Narration: So whats happening to the rain? Is the dry spell just natural variation or is it being caused by us?

Rain started disappearing from Perth back in the mid 1970s. Annual rainfall suddenly dropped by 20%, and it hasnt recovered. Now the same thing is happening to Melbourne. Rainfall has fallen by nearly 20% in the last 7 years.

Dr James Risbey, Monash University: This is one of the worst droughts in the past hundred years probably comparable with the Federation drought in Australia at the turn of the century in terms of low rainfall. Its also probably one of the worst droughts weve seen in terms of very high temperatures, very high evaporation rates and so the impacts of the drought have been quite severe.

Narration: The water supplies of Melbourne and Adelaide are well below 50% capacity and in Perth their reservoirs are less than a quarter full. If the next 18 months are as dry as the last, these cities and their six million residents face a water crisis.

Dr James Risbey: The worst case scenario for our cities would be that we start to run out of water and in that case wed have to think seriously about moving some of the water that we currently use for agriculture out of agriculture and into urban uses.

Narration: To understand how dramatic the change in rainfall has been, we need to wind the clock back 30 years. Right along the bottom of the continent we could count on reliable rain arriving in late autumn and early winter. But as Kevin Hennessy from the CSIRO explains, the once reliable rainbearing cold fronts have started to move.

Kevin Hennessy, CSIRO: What weve seen in the past 30 years or so is that many of these cold fronts have moved further south so fewer cold fronts are bringing rain to places like Perth, Melbourne and Hobart.

Narration: The winter rains that would once have fallen on the land are now increasingly falling out over the sea. The big question is: why? Kevin Hennessys climate models lead him to think its connected with increasing temperatures.

Kevin Hennessy: When we look at the global average temperature over the past 100 years or so its become significantly warmer since 1970 and natural factors alone dont account for that. When we put increasing greenhouse gas levels in the atmosphere all of a sudden we start to get a decent explanation for whats been happening.

The heatwaves and fires th canada goose expedition jacket at we experienced in Australia recently are indeed a glimpse of the future. Its very clear from the consensus of many thousands of scientists that global warming is real, its actually under way now, its not just something thats going to happen in the future. We would expect more heatwaves, more droughts and of course a greater stress on people living in cities.

Narration: But the predictions dont fully explain what were already seeing in the real world. The rain has decreased much more rapidly than the models predict. Some other factor must be at work.

And its years of data collected from Antarctica thats the missing piece of the puzzle. David Jones, head of Climate Analysis at the Bureau of Meteorology, thinks that changes in the Antarctic atmosphere may be responsible for stealing our southern rain.

Dr David Jones: Weve always had a tendency to believe that the upper atmosphere was really being driven by what happened at the surface and here was a new set of ideas which were saying well in fact it can happen the other way round too.

Narration: This revolutionary idea is all to do with an amazing natural climate engine called the Antarctic polar vortex. Around Antarctica and the ozone hole swirls a continentwide tornado, a wall of 200 kilometre per hour, supercold westerly winds.

The vortex is an important driver of the weather in the southern hemisphere, including the storm fronts that bring winter rain to southern Australia.

But while the rest of the world gets warmer due to greenhouse gases, Antarctica is cooling due to ozone loss. So theres a bigger difference in temperature between the equator and the poles. The combined effect of ozone and greenhouse, it seems, is making the polar vortex spin faster.

Dr David Jones: We tend to spin the winds over the Southern Ocean more quickly. Initially the winds are spinning faster at higher levels but theyve got a tendency to slowly move down to the lower levels and even down to the surface

Narration: That increased wind speed seems to be tightening the polar vortex, and this is pulling the winds and the pressure belts that deliver our winter rains southward, closer to Antarctica. They dont need to shift very far to completely miss the Australian continent.

The dangerous combination of a tightening vortex and global warming has climatologists worried about the future.

Dr James Risbey: That means in southern Australia wed see more or less permanent drought conditions due to a contraction of the storm systems on the one hand so wed be gett canada goose expedition jacket ing less rainfall and then the rainfall we do get well get less of that moisture available due to the high temperature and high evaporation rates. So wed see a situation of more or less permanent water stress.

Narration: So, whats Australias forecast for the future? Unfortunately, if the modellers are right, Australia in winter will show the largest reduction of rainfall of any region in the world.

While we responded swiftly to the threat of the gases that eat ozone, Australia seems to be less willing to bite the bullet on greenhouse gases.

Kevin Hennessy: The atmosphere has a very long memory. Greenhouse gases stay in the atmosphere for many decades so the greenhouse gases we put into the atmosphere 50 years ago are still there now and the greenhouse gases we put up today will be still there in 50 years time. So the first response is to reduce greenhouse gas emissions but we recognise that there will still be climate change so we also have to adapt.

Narration: That means therell be no way of slowing the vortex for decades to come. So one thing looks certain wed better get used to living with less water. canada goose expedition jacket