Monday, October 06, 2008

Did climate change cause the drought?

I've said my piece on this issue before. I am happy to concede that the current drought might prove to be a manifestation of climate change but my claim has been that this is not implied by the evidence. My impression is based in part on the following rainfall graph:

which seems to me trendless at least until the mid-1990s. Other graphs for particular parts of Australia - not the far South West of Western Australia - show a similar absence of trends.

The CSIRO have recently completed a study of the case for worsening drought frequency and intensity and for tightening the 'exceptional circumstances' motivation for paying 'drought relief' to farmers. They provide evidence of increasing temperature - we knew that anyway - but argue that evidence of decreased rainfall depends on what period you choose. Selecting data post 1950 it looks like drought frequency has increased provided that the current drought is included. This study has been criticised by David Stockwell who finds that the climate models used wrongly predicted increased drought in areas where it had not occurred. This is of interest becauyse it is these models which predict iuncreasing drought frequencies and intensities. But ignoring this point this study does not provide historical evidence of increasing drought frequency.

The heading in today's The Age - 'What Victorians are living through now is not just a drought, it's climate change' suggests what it says. Moreover, it is written by the head of climate analysis - Dr David Jones - at the Bureau of Meterology so I must stop and carefully read the analysis. But again this analysis does not quite make the claim of increased droughts being associated with climate change. What it says is that rainfall has been below normal for 11 years and this is unlikely to be something generated by chance - it is double the previous record run of dry spells from 1979-1984 and the data base drawn on goes back to 1855. It would be nice to have this data. In the final paragraphs Jones writes:

'Climate change caused by humans is now acting to make droughts more severe and ncreasingly likely, while substantial warming of our climate is inevitable as a result of greenhouse gases already emitted and those that will be emitted in the decades to come.

Regardless of the underlying cause, the drought provides Victorians with a snapshot of a hot and dry future that we all will collectively face'. (my bold).
My inference from the bolded bit is that he has taken a guess that, on the balance of probabilities, the current drought is likely to be caused by climate change. To emphasise the point, Jones may well be right. He writes:
'A notable feature of this drought is the long series of failed autumn rains. Victoria, and indeed most of southern Australia, has experienced a substantial decline in its autumn rainfall since the 1970s. This is important because autumn rainfall "wets up" the soil in catchments, allowing winter rain and snow to flow into rivers. Decent autumn rain also gives crops and pastures a start after the heat and dry of summer. Victoria has now experienced eight dry autumns in a row, and indeed 16 of the past 19 autumns have received below-average rainfall.

The autumn drying trend was first noticed in south-west Western Australia in the mid-1970s, and has subsequently spread to the south-eastern states. This rainfall decline is driven by a rise in atmospheric pressures, and a weakening of cold fronts and low-pressure systems that once reliably brought rainfall to southern Australia. This shift in weather systems and rainfall has been linked by scientists to human-induced climate change, be it through greenhouse gases or changes in the ozone layer over Antarctica.

While we will continue to see the occasional wet autumn in the future, the chances are that we will not see a return to the wet autumns that were once commonplace. Over the past few years we have also seen a repeated failure of spring rains, which is a projected response to global warming across southern Australia'.
Thus Jones is also confirming the causal claim by drawing a link between predictions of climate change effects and what has happened. I am not well up enough on specific predictions to confirm this claimed coincidence. The IPPC forecasts I have seen suggest different rainfall trends in different parts of the country.

It seems plausible to suggest that longer-term temperature increases will be associated with increased drought frequency. Eventually there will then be the ex post judgement that climate change has driven drought. I remain unconvinced that this judgement can be drawn now but I will pursue this matter and if I find convincing evidence to the contrary will publicly retract my views.

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