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Climate: Greenhouse gases drive Australia drying trend

Since the 1970s, southern Australia has been experiencing declining rainfall in the fall and winter, creating scenes like this one in a 2007 photograph at Lake Hume. (Creative Commons/ Suburbanbloke)

Since the 1970s, southern Australia has been experiencing declining rainfall in the fall and winter, creating scenes like this one in a 2007 photograph at Lake Hume. (Creative Commons/ Suburbanbloke).

New model can resolve some climate impacts on a regional scale

STAFF REPORT

FRISCO — Increasing concentrations of greenhouse gases and ozone depletion over Antarctica are the main drivers of the long-term decline in rainfall over southwestern Australia, federal scientists said in a weekend press release.

The findings, published in Nature Geoscience, are derived from a new  high-resolution climate model that may help researchers identify more links between heat-trapping gases and regional climate trends, including here in the U.S.

“This is really the first study to explicitly show in a high-resolution model that the observed regional rainfall trends in southwestern Australia are very likely due to human activity and very unlikely due to natural variability,” said Monica Allen, communications director for the National Oceanic and Atmospheric Administration.

“This new high-resolution climate model is able to simulate regional-scale precipitation with considerably improved accuracy compared to previous generation models,” said Tom Delworth, a research scientist at NOAA’s  Geophysical Fluid Dynamics Laboratory in Princeton, N.J.,  who helped develop the new model and is co-author of the paper. “This model is a major step forward in our effort to improve the prediction of regional climate change, particularly involving water resources.”

Comparing observations with climate model Observed precipitation (top) and the model simulation (middle) showed a similar pattern, supporting the idea that human activity contributed to the fall and winter drying in southwestern Australia. The bottom figure is a model simulation showing the drying trend will increase this century. Precipitation units are millimeters per month. (NOAA)

Comparing observations with climate model
Observed precipitation (top) and the model simulation (middle) showed a similar pattern, supporting the idea that human activity contributed to the fall and winter drying in southwestern Australia. The bottom figure is a model simulation showing the drying trend will increase this century. Precipitation units are millimeters per month. (NOAA)

Understanding how global warming will change the water cycle has been a holy grail of sorts for some researchers, and making direct attributions between the buildup of heat-trapping greenhouse gases and regional climate trends is critically important for resource managers and policymakers.

As NOAA researchers ran the new global climate model, the climate change signal in Australia was striking, showing that the long-term decline in rainfall over southwestern Australia is probably caused by the poleward movement of prevailing westerly winds, which means fewer ocean storms hitting the continent.

The decline in rainfall in that region has been well-documented and the climate modeling showed that the trend is well outside the realm of natural variability. The scientists tested several natural causes, including volcano eruptions and changes in the sun’s radiation. But none of these natural climate drivers reproduced the long-term observed drying.

The decades-long drying trend started in the 1970s and shows no signs of letting up. In fact, by the end of the century, the region may be facing a nearly permanent drought, with a 40 percent drop in average rainfall.

“Predicting potential future changes in water resources, including drought, are an immense societal challenge,” said Delworth. “This new climate model will help us more accurately and quickly provide resource planners with environmental intelligence at the regional level. The study of Australian drought helps to validate this new model, and thus builds confidence in this model for ongoing studies of North American drought.”

To see the Research Highlight online at NOAA’s Geophysical Fluid Dynamics Laboratory: go to http://www.gfdl.noaa.gov/news-app/story.95/title.regional-rainfall-decline-in-australia-attributed-to-anthropogenic-greenhouse-gases-and-ozone-levels/

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