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Rainfall decline in south-west Australia linked to climate change

Dry west Declines in rainfall over south-west Australia over the last 40 years are linked to greenhouse gases and ozone depletion, new climate models show.

And the trend is likely to continue according to research, published today in Nature Geoscience , that predicts a 40 per cent reduction in autumn and winter rains by the end of this century.

The finding has implications for maintaining reliable water resources in the area, which has already seen declines in the annual stream flow into dams that supply Perth over the last century.

Using a new, high-resolution climate model that can forecast down to an area just 50 kilometres wide, US researchers from the Geophysical Fluid Dynamics Laboratory examined the impact of variation in both human and natural influences on climate.

Natural influences on climate include volcanic activity and solar irradiance, while human factors include increasing levels of greenhouse gases such as carbon dioxide, decreasing ozone levels, aerosols and land use change.

The model shows both greenhouse gas emissions and stratospheric ozone depletion contribute roughly the same amount to the drying. Anthropogenic aerosols, however, were not linked with drying.

The results show the impact of human factors will be felt much more strongly in south-western Australia than elsewhere in Australia, says lead author Dr Tom Delworth, also a lecturer at Princeton University.

"We do find a drying signal over southern and south-eastern Australia as well, but the signal of that drying in response to human factors was not as large relative to the natural climate variability we see going on there," says Delworth.

While the signal is currently not as strong in south-eastern Australia, it begins to emerge more in the future, he adds.

South-west Australia has a Mediterranean climate with the bulk of its annual rainfall in winter, associated with the passage of cold fronts that bring moist air over the Southern Ocean.

Delworth says the reduction in ozone levels over Antarctica leads to a cooling of the upper atmosphere over the southern pole, which in turn strengthens the westerly winds that blow around the Southern Ocean and shifts them towards the poles.

"Those are the winds which really guide where the rain-bearing storms go, so as all those winds and the storms move poleward, there are less of those winter-time storms that hit the southern coast of Australia, particularly the southwest of Australia," he says.

Rainfall declines

The study is "one of the very few instances where regional rainfall changes have been linked with human-induced climate change", writes Professor David Karoly from the ARC Centre of Excellence for Climate System Science at the University of Melbourne in an accompanying commentary.

Karoly says while the large scale changes in winds and pressure patterns have been linked to increasing levels of greenhouse gases, it had not been possible before now to formally attribute the observed decline in rainfall in this area to human activities.

"Most climate models have a spatial resolution that is too coarse to represent the topography of the Australian south-west [and] as a result, they generally underestimate its mean rainfall as well as the variability of precipitation," he writes.

But the new higher-resolution model provides greater confidence in projections that show continuing future reductions in winter rainfall across southern Australia, "a prospect that poses increasing risks to sustainable water resources".