Early sowing the symptom of a new climate

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Key points

  • A new Australian climate is developing
  • Seasonal rainfall zones have shifted across Australia since 2000
  • There are clear implications for farming systems and management 

Early sowing has become the norm in many grain-growing areas, but only now are some people openly saying why: our climate has changed

Portrait of Dr David Stephens

A study of rainfall patterns over the past 115 years has shown that a new climate is emerging in Australia, with obvious implications for agriculture.

The Australian Export Grains Innovation Centre (AEGIC) has analysed data from more than 8000 Bureau of Meteorology stations around the country, going back to 1900, and found that traditional rainfall zones have changed significantly in the past 16 years.

The findings were delivered as part of a presentation, ‘South-west Western Australia is losing its Mediterranean Climate’, by AEGIC agro-meteorologist Dr David Stephens at the GRDC Grains Research Update in Perth at the end of February.

While the Perth presentation concentrated on the WA wheatbelt, the study itself was national.

Dr Stephens said the new analysis revealed striking changes to the Australian climate since 2000: “There has been a general increase in summer rainfall across Australia and a corresponding decrease in winter rainfall,” he said.

“This has led to shifts in rainfall zones extending for hundreds of kilometres.”

Dr Stephens showed how rainfall between May to October over much of the more heavily populated regions of southern Australia has decreased by 10 to 30 per cent, while summer rain has increased by up to 40 per cent in some areas.

“This change in climate has major implications for farming and pastoral systems as the profitability of different crop types change, disease risk changes and the composition of rangeland grasses changes with stocking rates,” he said.

Map of Australia showing seasonal rainfall zones

Figure 1 Australian seasonal rainfall zones 1900–99

AEGIC has released detailed maps showing the rainfall zone shifts (Figures 1 and 2, right). They indicate:

  • for regions with a Mediterranean climate, winter (and winter-dominant) rainfall zones are contracting in a south-westerly direction;
  • in northern and eastern areas, summer (and summer-dominant) rainfall zones are expanding southward;
  • between these regions, there is a uniform rainfall zone where summer and winter rainfall are similar. The southern boundary of this zone has shifted from southern/central New South Wales down into central Victoria and the Mallee region of south-east South Australia; and
  • in the south-west of Western Australia, a uniform rainfall zone has appeared along the eastern edge of the wheatbelt from Beacon to Southern Cross to Grass Patch.
Map of Australia showing seasonal rainfall zones

Figure 2 Australia seasonal rainfall zones 2000–15

The study shows most rainfall zone boundaries have typically shifted 100 to 400km over the past 16 years. The only expansion of the winter rainfall zone has occurred in south-east Tasmania, where winter rainfall has become more reliable.

Dr Stephens said the analysis highlighted that the shift to earlier sowing of winter crops should continue because early sown crops take advantage of any additional summer soil moisture.

“They also experience a lower evaporative demand through the growing season and are less affected by declining rain in October and rising spring temperatures,” he said.

“In pastoral regions in much of WA, increasing summer rain with a reduction in rainfall variability has assisted perennial C4 (tropical) plants at the expense of C3 (temperate) grasses (especially in southern areas), while in central and northern Queensland, an increase in rainfall variability has been detrimental to pasture production and stocking rates.”

Advanced agriculture

Dr Stephens said Australia was going to need some of the most water-efficient farming systems in the world to mitigate the effects of a drier and warmer climate in southern Australia.

He said research in this area was vital because Australian crop yields have been among the most affected by climate change, compared with other grain-exporting nations.

Dr Stephens said the changes appeared to be connected to changes in barometric pressure, sea surface temperatures and upper-level westerly winds.

In the mid-1970s there was a weakening of the Indian Ocean Trough to the west of Perth, which appears to be related to a decline since then in winter rainfall. In the 2000s, this trough weakened further, in conjunction with strengthening high pressures over Australia.

In addition, sea surface temperatures have warmed in all seasons, which benefits summer rainfall. However, a more marked warming in oceans west of Perth in winter has an inverse relationship to rainfall and has contributed to weaker cloud-band activity in recent years.

“At a hemispheric scale, one of the drivers of weather is the temperature gradient between the equator and the South Pole. This gradient dropped at the beginning of the 2000s as westerly winds in May–July weakened over Australia,” Dr Stephens said.

Consequently, the variability in annual rainfall across Australia has changed since 2000. Reduced variability in some regions is due to the loss of wet years (as in south-west Australia) or more consistent average to above average rain (as in the north-western parts of the country and southern SA).

In contrast, variability in annual rainfall has increased in inland Victoria, southern NSW and much of central Queensland.

More information:

Dr David Stephens,
08 6168 9990,
david.stephens@aegic.org.au

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