Suffers Its Worst Recorded Drought
Melbourne has suffered its worst six year drought in recorded history. A record low rainfall over the past six years has combined with a record high temperature, and hence evaporation, to magnify the severity of this drought.
There is a significant upward trend in temperature, making it likely that future droughts will be even more severe. This trend is good news for vendors of cold drinks, ice creams, and air conditioners. But the need to act on more efficient use of water is clearly urgent.
Chart 1 shows the annual rainfall since 1856, along with
the deviation from average over a moving six year period.
Over the six years 1997 to 2002 inclusive, total rainfall was 741 millimetres below average. This is 18.9% below the average for a six year period of 3,928 millimetres.
Previous severe deficiencies of over 400 millimetres occurred in the six years ending 1986, 1969, 1930, 1910, 1898, and 1869. The previous worst six year drought was in 1898, when the deficiency was 538 millimetres.
There is no statistically significant trend in annual rainfall, but there is a statistically significant cycle of 18 years. That is, nine years after drought there tends to be a period of higher than average rainfall, followed by drought in another nine years. It is unclear what drives this cycle.
There is a correlation between Melbourneís rainfall and El Nino, but it is not reliable. The lowest recorded rainfall in 1967 coincided with a La Nina (reverse of El Nino) event. The low rainfall years of 1994, 1997, and 2002 coincided with El Nino. But in the La Nina years of 1998 to 2000, Melbourneís rainfall remained stubbornly below average.
Chart 2 shows the annual mean maximum temperature since 1856 and the six year average.
The hottest year on record is 1961 (21.4 degrees), but the next three hottest years have occurred in the past four years.
Until 1999, the highest six year average was 20.2 degrees in 1962. In 1999, the six year average was 20.3, 2000 was 20.4, 2001 was 20.6, and 2002 was 20.8.
There is a statistically significant upward trend in maximum temperature and the rate of increase is increasing. Over the period 1856 to 2002, the rate of increase was 0.4 degrees per century. Since 1930, the rate of increase has been 0.8 degrees per century. But since 1976, the rate of increase has been 3.5 degrees per century.
There are several driving factors and these may include nature forces such as the sunís output, and variations in the earthís orbit and reflectivity. Increased greenhouse gasses is also a likely factor. The more rapid rate of increase since the mid 1970ís has been observed elsewhere and is probably due to reduced concentrations of sulphur dioxide in the atmosphere. Sulphur dioxide is emitted by coal fired power stations and has a cooling effect. Clean air initiatives resulted in reduced emissions since the 1970ís.
While it is unlikely that Melbourneís temperature will continue to rise at the rate of 3.5 degrees per century, it is very likely to continue on the upward trend. This means that future droughts will be more severe than past droughts because of the increased evaporation rate.
The next severe drought is expected around 2015 to 2020.
Based on historical patterns, the predicted rainfall in 2003 is 659 millimetres (marginally above average), with a 90% confidence band of 448 to 871 millimetres indicating the high degree of variability. The predicted mean maximum temperature is 20.6 degrees, with a 90% confidence band of 19.8 to 21.4 degrees. This means that the temperature is almost certain to be warmer than average and has a five percent chance of exceeding the 1961 record.
Higher temperatures are not limited to Melbourne (for more information on global warming, see www.healingforests.com/globalwarming.htm). But the citizens of Melbourne should urgently consider how they can act locally to adapt to droughts of increasing severity and also contribute to the global fight against rising concentrations of greenhouse gasses.