Thursday, March 15, 2007

The weather again

There is an interesting post over at Steve Edney's site Criticality on trends in temperatures and rainfall in South Eastern Australia since around 1910.

Temperatures show a secular increase that is of the same order as that predicted by climate change theorists. But just eyeballing the rainfall data doesn't suggest much of a trend at all.

As a somewhat naive observer this suggests to me that the current drought can be best understood as an errant event rather than a consequence of climate change. It also helps to explain to me the vast range in forecast rainfall effects of climate change.

The Bureau of Meteorology website has much useful information. There are no doubts here about the realities of climate change:
Australia and the globe are experiencing rapid climate change. Since the middle of the 20th century, Australian temperatures have, on average, risen by about 1°C with an increase in the frequency of heatwaves and a decrease in the numbers of frosts and cold days. Rainfall patterns have also changed - the northwest has seen an increase in rainfall over the last 50 years while much of eastern Australia and the far southwest have experienced a decline.
Steve's comment challenges this last sentence - at least for Eastern Australia. I'd be very interested in getting commentary on this one.


Steve Edney said...

Harry, I agree with the statement that Southwestern Australia has had declining rainfall, its pretty clear if you look at the rainfall graphs and time series. I just don't see it in terms of South eastern Australia, except if you compare it to the unusually wet 50's.

There is a high variability in Eastern and South Eastern Aus rainfall driven by the El nino, la nina cycle, there were a couple of intense la nina events in the 50's which gave plenty of rain. Its not clear to me why this seems to be assumed as usual rather than the situation of the 1920-1940 when it was typically as dry or drier than now. Or indeed earlier in the century.

Of course my analysis is pretty limited, there could be a trend in say a shift from winter to summer rainfall which doesn't appear in annual averages but would be a climatic shift, and may result in less usable rainfall. If so it would be good to have this explained and made clear because the BOM statement as written above while literally accurate, rainfall was higher in the 50's, is not really a fair comparison as rainfall in the 50's was the highest the entire century.

derrida derider said...

I think the lesson for policymakers and engineeers to take away from any study of rainfall variability in Australia is that it's not just a matter of dry years and wet years but dry decades and wet decades. For example, if you're planning water supplies you have to build the dams accordingly.

I've always argued that its a gross tactical error on the part of greenies to link the current dry to global warming even if it was true. When wetter years arrive you'll have given the remaining denialists (if any still exist by then) ammunition and, far worse, you'll leave average Australians with the incorrect impression that AGW is not such a big deal for them.

hc said...

DD, Even averaging over decades I don't think will reveal much in the south-east. A good exercise would be to get the raionfall data and test for trends etc using time series analysis - someone may have done it.

Steve, The SW of WA is interesting and an area I am studying at present. The seasonal dispersal of rain has changed and, yes, average rainfall has fallen. This is important since a major wheatbelt area, an important biodiversity resource and, in my memory serves me rightly, the home town of Yobbo (Sam Ward).