Drying up or typical climatic variation?

I noticed this article Big dry: no one knows why in the SMH this week, and in particular the claim that eastern Australia’s rainfall had dried up since the 1950’s. The report seems to be related to this press release announcing a new centre at UNSW studying climate change.

A priority for the new centre is to better understand the mystery of why Australia’s most populated region, the continent’s east coast, has suffered such major declines in rainfall in recent decades.

“We recently had a round-table of Australia’s leading climate-change researchers and this emerged as the biggest unknown issue and, of course, it seriously affects the largest concentration of people stretching right down the coast from Cairns to Melbourne,” Professor England said.

So I had a look again at the rainfall graphs and time series provided by the Bureau of Meteorology for eastern Australia and there is no doubt that we have been receiving less rainfall in recent decades than the 1950’s. However it also seems that we are receiving more rainfall than in the first half of the 20th century, so the question seems to me to be as much why did rainfall rise in the 50’s as why has in fallen in the 80’s and 90’s.

EasternAustraliaAnnualrainfall

While it would be great it we could get more rainfall it seems that it is weak to link this to climate change, when it is well within the variation that we have seen over the observed history. Indeed if we look at averages they are close to the highest point they reached pre-1940. If we look at years in the bottom and highest deciles of rainfall they are right where we expect for the last 20 years (2 in the highest and 2 in the lowest deciles). If we look at percent of regions with rainfall in the lowest decile then we also see that while 2002 was high it was not the worst on record.

It seems curious to me that we should be studying climate change induced drying when we appear to be returning to the norms of relatively recent history. Now contrast this to say the below graph of mean temperature over the same region.

easternausmeantemp.gif
Now here it is clear that you may want to ask why has it become warmer, as there appears to be a clear warming trend in the last 50 years, and the recent mean values are the highest on record. I can even understand wanting to study the likely effect of warmer weather on rainfall, but the question of why has rainfall decreased seems to be a badly flawed one, as nothing we’ve seen recently in terms of rainfall seems to be beyond the normal climatic variation. Is there any reason for considering the 50’s to be more climatically typical than the 20’s, 30’s or 40’s?

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10 Responses to Drying up or typical climatic variation?

  1. […] Steve Edney examines rainfall figures in south-eastern Australia, and discovers that while they’ve fallen […]

  2. jemima says:

    Good question, well put. I think climate modeling, predicting warmer and drier conditions for the Aus eastern seaboard under increasing greenhouse, may be having an unreported effect on the scientists’ thinking.

  3. hc says:

    Interesting observation Steve. Eyeballing the thing I cannot see any trend in rainfall at all. Temperatures yep, an increase.

    I’ve been studying climate change forecasts. Your temperature results consistent with them. The rainfall forecasts very uncertain for many areas including SE Australia. After looking at your graph I understand why.

  4. Steve says:

    If you look at the South western Australia one there is a fairly clear down trend, although I wouldn’t be convinced that it is a break from previous behaviour, but certainly worth looking at. I just don’t see it though in Eastern Australia.

  5. SimonC says:

    I don’t think that you can draw too many conclusions one way or the other with the data you’ve provided. Changes in rainfall would be easier to see if you used rainfall anomaly (removing the average) like the second graph showing the temperature increases. Also the area of ‘Eastern Australian’ is very large and includes all of Qld, NSW, Vic and Tas. If you want to answer the question of whether or not the east coast is significantly drier now than pre 1950 it would be better to use data from the east coast only, as the climate of tropical North Qld, in-land Australia, Tasmania and the Australian East Coast are different. Also it would be worth looking at evaporation trends – if evaporation increased due to increased temperatures, etc then, even if you had average rainfall, things would be drier.

  6. Steve says:

    I was responding to the particular claim in these articles that eastern australia had decreasing rainfall. It specifically mentions the tropical north (ie. Cairns). Also if you look at the rainfall patern of tasmania it is quite similar to the above pattern.

    I don’t dispute that usable rain may decrease. with rising temperature or seasonal variation, but that wasn’t the claim that was being made, rather they were claiming the average rainfall had decreased.

  7. Brian Bahnisch says:

    Steve, I’ve been personally keeping rainfall records since 1995 and referencing them to the averages for the BOM office about 7km to the east in Brisbane. From 1995 to 1999 we got 113% of average. From 2000 to 2006 we’ve had 71%. This accords with the graph you’ve supplied only a bit more so. 1999 was particularly wet and 2000 very dry.

    Right now it looks like there has been a distinct break in the pattern from 2000+, But the next 10-15 years will start to tell. I say ‘start’ because they say you need about 30 years before you can talk climate rather than weather.

    There have been distinct differences in the weather patterns and the kind of rain we get, which seem to be linked with the story of the temperate lows moving south and the east coast coming under the influence of large highs, and that story does have a basis in climate change. This gives us more wind in summer, for instance, and less humidity.

    But as they say, time will tell.

  8. SimonC says:

    The articles you link to mention ‘east coast’, ‘eastern seaboard’ and the ‘coast’ which is different to the Eastern Australia in the graph from the BOM. I’m not saying that the ‘dry’ isn’t due to natural variation, I’m just pointing out that the data you’ve provided covers an area much larger than the east coast and that any trends (or lack of) would be easier to observe if you removed the mean.

    To get an idea of the ‘dry out’ experienced over the last 50 years and where it is being felt it is better to look at the trend maps:

    http://www.bom.gov.au/cgi-bin/silo/reg/cli_chg/trendmaps.cgi

    As you can see the thin east coast has experienced a large drop in rainfall while inland has a small drop and the area north and west of Cairns has increased. Again I’m not making a call on why this is and if it’s natural variation, due to global warming, localised factors or combinations of the three but the only way to determine this is to dedicate more resources to study the climate.

  9. Steve says:

    True, the quote I provided douse say east coast, and the drop of 400 mm per year they discuss is the eastern seaboard, but they also say:

    Andy Pitman, a climate change expert, said changing rainfall patterns in the west and south of the country had been well studied. “But no one has yet made a systematic attack on why NSW climate has changed in the way it has,” he said.

    Anyhow I’m not disputing the drop over the last 50 years. I’m noting that the 50 year comparison is biased by starting with what appears to be an unusually wet period.

    If you look at those trend maps for 100 years, 90 years or 80 years the drop on the east coast, at least NSW and Victorian coast largely disappears (actually it appears to have increased over much of it) , although there is some drying along the queensland coast.

  10. BillC says:

    The only changes to rainfall patterns that look convincing are the decline in South West Western Australia, and an increase over wide areas of North West Australia – unfortunately an area with little population or agricultural production.
    I believe there has been a substantial increase in the number of cyclones hitting the North West coast in recent decades, and they can dump large amounts of rain as far south as Kalgoorlie. Conceivably this could be related to the dramatic rise in Asian aerosol pollution over this period.
    The decline in Eastern Australia just suits media and environmentalist agendas, and only exists if you use wet decades (1950’s and 1970’s) as the starting point.

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