A new high-resolution climate model shows that southwestern Australia’s long-term decline in fall and winter rainfall, which began around 1970 and has increased over the last four decades, is caused by increases in man-made greenhouse gas emissions and ozone depletion, according to research published in Nature Geoscience.

Simulating both natural and man-made climate effects, scientists showed that the decline in rainfall is primarily driven by human activity. Rises in greenhouse gas emissions and thinning of the ozone hole have led to changes in large-scale atmospheric circulation, including a poleward movement of the westerly winds and increasing atmospheric surface pressure over parts of southern Australia. This has led to decreased rainfall, the study said.

The drying is most severe over southwest Australia, where the model forecasts a 40 percent decline in average rainfall by the late 21st century, with significant implications for regional water resources. Several natural causes were tested with the model, including volcanic eruptions and changes in the sun’s radiation, but none of these natural climate effects reproduced the long-term observed drying.

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Changes in fall-winter precipitation over Australia from observations (top panel), a model simulation of the past century (middle panel), and a model projection for the middle of the 21st century (bottom panel). The agreement between observed and model simulated precipitation changes supports the idea that human activity has contributed to the observed drying in southwestern Australia, and that this drying trend will amplify and expand in the 21st century. (Image credit: T.L. Delworth and F. Zeng, Nature Geoscience)

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