Saturday, July 9, 2011

Scientist: Less rain across southern Australia a long term climate trend

Increasing atmospheric carbon dioxide concentrations is increasing atmospheric temperatures changing the dynamic of the sub-tropical jet stream and thus the number and intensity of storms bringing autumn and winter rainfall to southern Australia, according to CSIRO climate scientist Dr Jorgen Frederiksen. He was addressing the International Union of Geodesy and Geophysics conference in Melbourne this week regarding recent research on changes to atmospheric circulation, storm intensity and reduction in winter rainfall in Australia.


"The drop in winter and autumn rainfall observed across southern Australia is due to a large downturn in the intensity of storm formations over at least the last three decades compared with the previous three decades, and these effects have become more pronounced with time," Dr Frederiksen said. "Our recent work on climate model projections suggests a continuation of these trends over the next 50 years."

Between the 20-year periods 1949 to 1968 and 1975 to 1994 south-west WA rainfall reduced by 20 per cent. In south-east Australia, there were reductions of 10 per cent. The CSIRO has repeatedly warned that South East Australia becoming drier, with Global Warming implicated. The CSIRO and the Bureau of Meteorology issued a joint statement in March 2010 that Australia's climate already changing. As far back as 2004 CSIRO warned that climate change would make Australia hotter, wetter, with more extreme weather.

"Our research has identified the historic relationship between the reduction in the intensity of storms, the southward shift in storm tracks, changing atmospheric temperatures and reductions in mid-latitude vertical wind shear affecting rainfall." said Dr Frederiksen "We expect a continuation of these trends as atmospheric temperatures rise based on projections from climate models forced by increasing carbon dioxide concentrations."

"Trends during the 21st Century are likely to be similar to those observed during the second half of the 20th Century, when we saw substantial declines in seasonal rainfall across parts of southern Australia. Indeed, reductions in projected southern Australian rainfall during the 21st Century, particularly over south-west WA, may be as much as, or larger than, those seen in recent decades," Dr Frederiksen said in conclusion.

The address to the conference was based on a research paper - Changes and Projections in Australian Winter Rainfall and Circulation: Anthropogenic Forcing and Internal Variability (abstract) - published in the International Journal of Climate Change Volume 2 issue 3.

If anything Dr Frederiksen appears to have downplayed in his comments the possible extent of the reduction in rainfall that might occur. The conclusions at the end of the abstract of the paper seem to indicate an upper end scenario for reductions in rainfall twice that seen at the end of the 20th century: "Our results show that the impact of further increases in anthropogenic CO₂ concentrations can lead to further large reductions in baroclinic instability, with model trends during the 21st century similar to those simulated during the second half of the 20th century. Associated reductions in modelled southern Australian rainfall can be as much as twice those seen at the end of the 20th century."

CSIRO has already contributed important studies into climate trends and water availability especially regarding Securing Melbourne’s water supply and Water for the future of Perth and South-West Western Australia.

The CSIRO has recently launched the Cape Grim Greenhouse gas website which monitors greenhouse gases, ozone depleting CFCs and other elements in the atmosphere and makes this data directly available to the public for the first time.

Sources:
* CSIRO, July 4, 2011 - Fewer rain storms across southern Australia
* International Journal of Climate Change: Impacts and Responses - Changes and Projections in Australian Winter Rainfall and Circulation: Anthropogenic Forcing and Internal Variability (abstract)