Climate change dries out WA

ARCCSS Chief Investigator and a world leader in climate attribution studies Prof David Karoly has responded in Nature Geoscience News and Views to a paper in Nature Geoscience that directly links the decline rainfall in the southern area of Western Australia to climate change.

The paper by Delworth and Zeng, Regional rainfall decline in Australia attributed to anthropogenic greenhouse gases and ozone levels (doi: 10.1038/ngeo2201) uses, for the first time, a high-resolution model to tease out the greenhouse gas and ozone component in the rainfall decline for this region.

 Prof Karoly’s commentary, Human-induced rainfall changes (doi: 10.1038/ngeo2207) explores those changes in rainfall over south-west Western Australia and suggest implications of rainfall for all of Southern Australia.

Aside from Antarctica, Australia is the driest continent on Earth. It is expected that as global warming takes hold, the southern part of the continent will become drier.

According to Prof Karoly, the changes shown in the paper have serious ramifications for the largest city in Western Australia, Perth.  

“This new study by Delworth and Zeng is a significant improvement over earlier attempts to understand the causes of the rainfall decline in southwest Australia.  Their global climate model is better able to simulate the regional rainfall in southwest Australia and its variability,” Prof Karoly said.

“This gives greater confidence to their conclusions that human-caused increases in greenhouse gases and decreases in stratospheric ozone are the main causes of the observed rainfall decline. This also gives greater confidence in substantial future declines in rainfall in southwest WA.

“These future declines will lead to further reductions in natural inflows into Perth’s reservoirs, meaning that Perth’s water resources will not be able to support its population and industry in the future without substantial further investment in desalination plants. However, any investment in desalination plants would be a maladaptation to global warming unless all the electricity needed can be produced by zero-carbon, renewable energy sources.”

Estimated annual streamflow (in gigalitres) into dams supplying Perth, Australia. Data obtained from the Water Corporation of Western Australia

 

South-west Western Australia has already seen two dramatic step down events leading to declines in rainfall in the late 1970s and in early 2000. As a result, in the 21st Century, compared to 1911-1974, less than a quarter of the average stream flow has made its way into the water reservoirs of the City of Perth.

A key reason for this decline in rainfall has been the southward shifts of winter weather systems that formerly delivered much of the rain to this region. This movement is directly attributed to greenhouses gases by the Delworth/Zeng case study.

Prof Karoly said the case study could not be applied more broadly worldwide because the Australian southwest is a special region, “with highly seasonal rainfall, where the changes in the winds and weather systems in response to climate change have already affected precipitation”.

However, and very importantly for Australia, the Delworth/Zeng model was also successful in showing the decline in rainfall that has occurred over south-eastern Australia – although the amplitude does not entirely match.

“This agreement gives greater confidence in model projections that show continuing future reductions in winter rainfall across southern Australia, a prospect that poses increasing risks to sustainable water resources,” Prof Karoly said.

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