North Dandalup dam in Western Australia: idyllic, but lacking water (Image: Auscape/UIG via Getty Images)

Australia is drying out, and it’s largely our fault. The south-west of the country can expect to see average annual rainfall drop by 40 per cent compared with the mid-20th century, and a new model suggests that the main cause is human greenhouse gas emissions.

Water from the skies is the stuff of life but the expectation is that many parts of the world will see less of it with climate change. But predicting how much rain will fall where is devilishly difficult.

It is an important question, because it affects water supplies. Since 2000, the average annual amount of water flowing into reservoirs in Perth, the capital of Western Australia, has dropped to less than a quarter of the yearly average between 1911 and 1974, says David Karoly of the University of Melbourne, citing national figures. As a whole, the south-west of Australia has seen a 20 per cent decline in winter rain since the 1960s, says Nerilie Abram of the Australian National University in Canberra.


Sucking vortex

Studies suggest this drop in rainfall is because westerly winds around Antarctica are moving closer to the pole, sucking moisture away from Australia, New Zealand and South America. The culprit was initially thought to be the southern ozone hole, until it was shown that greenhouse gas emissions probably shared the blame. But quirks in the models have made it difficult to definitively blame the long-term drying in south-west Australia on climate change. The resolution on climate models tends to be too low for this.

Now, Thomas Delworth and Fanrong Zeng at Princeton University have tested a high-resolution climate model on Australian climate. The great detail of the model substantially improves their ability to simulate localised rainfall.

They found they were able to faithfully reproduce rainfall levels over the continent for the last century, but only if greenhouse gas emissions were included.

The picture was very different when the pair stripped human emissions out of the model, effectively simulating a world without climate change. “The observed drying over south-west Australia does not occur in our model without human greenhouse gas emissions,” says Delworth.

Prepare for the worst

Delworth and Zeng then looked to the future. They show that if no further action is taken to curb our emissions – which models predict will result in global warming of up to 4.8 °C by 2100 – average annual rainfall in south-western Australia will drop by about 40 per cent compared with the period between 1911 and 1974.

“This research paints a very worrying picture for future rainfall,” says Abram. “It adds another compelling piece of evidence to the idea that we can expect southern parts of Australia to keep getting drier over the coming century unless strong action can be taken to reduce greenhouse gas emissions.”

Severe drying near Perth is not the only way in which climate change is hitting Australia. Several reports have warned that the extreme events that have battered the country in recent years, including catastrophic mega-droughts, floods, heatwaves and bushfires, have been linked to climate change. The Australian Climate Council says the country needs to prepare for more of the same in future.

Journal reference: Nature Geoscience, DOI: 10.1038/NGEO2201