CLIMATE - is it changing or just varying?
This is the question the Birchip Cropping Group asked Crop Facts consultant Harm van Rees to unravel.
- AT A GLANCE
- Who: Harm van Rees
- What: climate
- Why: is it changing or varying?
- Where: Birchip
- Report: JOHN PARRY
Birchip, in the heart of northwest Victoria's grain belt, has been dry since 2001 and has not experienced a wet year since 1996.
"We are not sure if this current pattern of dry years is permanent and due to changing climate, or if it's just due to variability," Harm said.
"If it is due to climate change we have to adapt and cope with the shift in rain and temperatures, otherwise we might not be viable.
"But if the current weather patterns are due to normal variability, we can still learn from what has happened so we are better prepared the next time we have a run of dry years."
Harm said the so-called millennium drought (1997 onwards) was the longest and driest period experienced in southeast Australia, at least according to the limited data available.
It was also warmer compared to previous droughts and the evaporation rates were higher, making it more difficult to grow crops and pastures.
The probability of such a long dry spell could be worked out using rainfall records for the past 120 years, as far back as reliable records go.
The figures for Birchip show that growing-season rainfall for the past nine years has been less than 215mm, regarded as the minimum amount of rain to be profitable. Since 1891, there had been 41 years with growing-season rainfall of 215mm, roughly one in three years.
The probability of two dry years in a row was one in nine years and this has occurred one in 14 years.
The probability of three dry years in a row was one in 27 years and this had occurred once in 83 years.
The probability of nine dry years in a row was one in every 20,000 years.
Harm said it was not possible that the recent long dry spell without any wet years was purely due to chance, but it was highly improbable.
"Does that answer the question of whether it is due to climate change or normal variability?" Harm asked. "No, it does not, but it does point the finger to a changing climate."
Harm said there had always been a large variability in summer and winter rainfalls, but had the pattern changed?
The Birchip trend is for more summer rain at the expense of winter rain.
The trend was the same at Swan Hill but less so at Longerenong.
There has also been a 1-1.5C increase in average daily temperature in September and October (critical months for flowering and grain filling) compared with long-term averages.
"These small differences in temperature may not seem like much but it will mean crops grow and mature faster," Harm said.
He said the frost risk appeared not to have changed significantly over the past 30 or so years which meant crops were still susceptible to the same risk.
The BCG is working on management options to alleviate the effect of the current climate.
The project is funded by the Grains Research and Development Corporation, the Federal Government's Climate Change Research Program and the CSIRO's Climate Adaption Flagship.
