While I accept and indeed am concerned about the prospect of global warming, I’ve always been a little bit sceptical of the claim that it will make Australia drier. Naively I expect that if it gets more tropical we’ll get more rain. I also accept that there are many other things happening which may confound this trend. Anyhow I notice via John Quiggin this BOM site, giving maps and time series of annual, monthly etc rainfall for Australia and various other regional breakdowns. John seems to have picked out a graph which makes the browning, appear particularly severe, but is it really so?
Too often we are just presented with just the trend, and without knowledge of the variability over time, how any particular result fits in is difficult to understand. For example if we plot the South Eastern Australia average rainfall for the last 100 years we get this
The obvious things to note is that the average for the first 50 years was well below the later part of the graph. Indeed by the standards of the first half of last century recent drought years with the exception of 2002 haven’t been particularly severe. Instead most have been above the average rainfall experienced in the first half of last century.
If we take the years to 1950, we find an average rainfall of 575mm, for the remaining 55 years the average is 625, with the five highest rainfall measurements all falling in the same half of the graph.
Plotting as a histogram with the rain fall amounts along the bottom axis we get this:
Which can be seen to be fairly skewed. A few high results boosting the average in later years.
What exactly it shows is difficult to assess. Certainly we can see that the recent drought years haven’t been particularly severe given the entire history, with only one year outside 1 Standard devation (100mm) from the mean of 603mm. In the first half of last century, such rainfall would have been typical or even good.
I find it somewhat comforting to see that unlike the temperature charts, which do show a clear upward trend the rainfall at least hasn’t become abnormal, at least yet.