I’ve added a higher resolution/larger copy to Geoff’s comment to make it easier for my eyes to make out the graphic.

Aynsley

I should think that they really mean change in annual precipitation per decade. Still even 500 mm out of 42,000 or 50 out of 2,000 doesn’t sound calamitous.

http://ms.media1.converdge.com/uploads/da/e0/fd/36/31/9e/12/d8/47/23/b0/db/e6/05/66/dae0fd36319e12d84723b0dbe605661e@t10.jpg

The image is 4b of the Drought Exceptional Circumstances Report by K Hennessy et al July 2008.

That spoiled my evening.

Look in the upper dark brown area on the east coast. This is supposed to have a lower rainfall, at least 50 mm per decade. It includes the small town of Tully, whose rainfall for the decade is some 42000 mm. Do you suppose that 50 mm in 42000 mm is easily derived from the detail of a GCM?

At the other extreme, look near the NE border of South Australia, where sits the town on Moomba on the course to SW Queensland, rainfall 2,000 mm per decade. It is supposed to suffer a loss of about 5 mm per decade. As if that would make much difference.

Unless I am misinterpreting the map, which is conveniently labelled as annual rainfall, with the contours expressed as rainfall change per decade, I wonder what it all means?

The key finding of the report, as presented in the opening words of Minister’s Burke’s media release of 6 July, was that ‘Australia could experience drought twice as often and the events will be twice as severe within 20 to 30 years.’ At his press conference on the same day, the Minister said that “What we’ve done is taken the best climate scientists in Australia and asked them to come up with their best information … They’ve worked through that scientifically … [T]he bottom line of all this is that this [report is] entirely written from beginning-to-end by scientists.’ On the following day Mr. Burke reiterated his total confidence in the scientific excellence of the report: ‘My priority on all of this is to make sure that we go with the best available science and I do believe that’s what the CSIRO and the Bureau of Meteorology gave us yesterday.’

What the Minister didn’t mention was that the report hadn’t been reviewed by outsiders. It was authored by 11 scientists from two government agencies. The only review comments had come from scientists from the same agencies and from the Publications Unit staff at BoM. There doesn’t seem to have been any involvement of statisticians – even from the 100-strong CSIRO Division of Mathematical and Information Sciences.

As it happens, a peer-reviewed study of the prospective incidence of droughts in Australia HAS just been published. The paper ‘Comparison of suitable drought indices for climate change impacts assessment over Australia towards resource management’ appears in the current (August 2008) edition of the International Journal of Climatology – the journal of the UK Royal Meteorological Society. It is authored by four CSIRO scientists, three of whom were also authors of the BoM/CSIRO report (Freddie Mpelasoka, Kevin Hennessy and Bryson Bates) and two of whom were Coordinating Lead Authors of the IPCC’s 2007 Assessment Report (Kevin Hennessy and Roger Jones).

The International Journal of Climatology paper (hereafter Mpelasoka et al., 2008) presents two drought indices for Australia – the Rainfall Deciles-based Drought Index (RDDI) and the Soil-Moisture Deciles Based Drought Index (SMDDI) – which are stated to be measures of, respectively, rainfall deficiency and soil-moisture deficiency attributed to drought and potential evaporation. The results are reported for two climate models (CSIRO Mk 2 and the CCCmaI model developed by the Canadian Climate Centre), two periods (the 30-year periods centred on 2030 and 2070) and two emissions scenarios (the IPCC SRES scenarios B1 and A1FI).

The most suitable basis for comparison with the BoM/CSIRO report finding that ‘Australia could experience drought twice as often … within 20 to 30 years’ is the Mpelasoka et al (2008) results for the 2030-centred period using the high (A1FI) emissions scenario assumptions. This basis is used in the discussion that follows: changes in drought frequency represent increases or decreases in the 2030-centred period by comparison with the 1975-2004 baseline period.

Consider first the RDDI results. The ‘2030 high’ outcome for the CSIRO model is ‘A general increase of 0-20% over Australia, except for scattered patches with DECREASES [in drought frequency] of 0-20% over the southern parts of the Murray-Darling Basin, the south-western parts of the Western Plateau, and Tasmania’ (EMPHASIS added). And for the CCC model the ‘2030 high’ outcome is ‘A 0-20% increase over most areas of Australia except for a DECREASE [in drought frequency] of 0-20% over the southern parts of the Murray-Darling Basin, the South Australian gulf areas, and Tasmania.’ (EMPHASIS added).

For the SMDDI, the RDDI is extended ‘to include evapotranspiration in order to produce a measure of soil-moisture deficit.’ The SMDDI for the ‘2030 high’ outcome for the CSIRO model shows ‘A 0-20% DECREASE over most of the western half of Australia, and increases of 0-20% in the eastern one-third and parts of coastal WA’ (EMPHASIS added). The SMDDI ‘2030 high’ outcome for the CCC model is ‘A general increase of 0-20% over most areas; 20-40% increase over south-western coast catchment. Patches of DECREASE of 0-20% over Timor Sea areas and northeast coast catchment’ (EMPHASIS added).

In short, the simulations from the two models suggest that in the medium-term (20-30 years) there will be a tendency towards a small increase in drought frequency in most of Australia, but with decreases in drought frequency in some areas. Although the peer-reviewed study comes from several of the same scientists, it gives no support to the cries of alarm and disaster which have accompanied the release of the BoM/CSIRO report. (A CSIRO media release of 15 July provides a link to a podcast in which Kevin Hennessy ‘explains why farmers and the Government have reacted with alarm to a joint report from CSIRO and the Bureau of Meteorology’).

The impression given by the peer-reviewed paper is vastly different from that given in the report to the Australian Government. This may be partly explained by the difference in the definition of ‘drought’; the authors of the report to government were obliged to adopt the definition specified in the terms of reference. This however have prevented them from giving an explanation of why the two studies yield such different results.

The BoM/CSIRO report states that ‘it is likely that there will be changes in the nature and frequency of exceptionally hot years, low rainfall years and low soil moisture years’, and claims that ‘previous studies have not adequately done this analysis’ (p. 13). In that connection, it identifies Mpelasoka et al (2007) as one of six such analyses. But, surprisingly, the BoM/CSIRO report gives the wrong citation: the Mpelosaka et al paper was published in Int.J.Climatol. in 2008, not 2007; it appears on pps. 1283-1292 of vol. 28 of the journal, not pps. 1673-1690 of vol. 27; and its DOI reference number is ‘joc.1649′, not ‘joc.1508′.

The volume, page details, year of publication and DOI reference number given in the BoM/CSIRO report for Mpelasoka et al. relate not to a paper by Mpelasoka et al but to a different paper by four Australian scientists which is entitled ‘Effect of GCM bias on downscaled precipitation and runoff projections for the Serpentine catchment, Western Australia.’

It’s surprising that this error was not picked up by any of the 11 authors or 3 reviewers of the paper – not to mention the officials and staffs of the departments and ministerial offices serving the Prime Minister and several other Ministers with interests in the science and policy of climate change and the agency for which we’re now told the report was prepared: the Bureau of Rural Sciences. This raises a question about how closely the report was scrutinised before publication.

Finally, it should be noted that a Mpelasoka et al (2007) paper on the future frequency of Australian droughts was cited in the ‘Australia and New Zealand’ chapter of the 2007 IPCC report. Here’s the statement and the full recommended citation:

‘Up to 20% more droughts (defined as the 1-in-10 year soil moisture deficit from 1974 to 2003) are simulated over most of Australia by 2030 and up to 80% more droughts by 2070 in south-western Australia (Mpelasoka et al., 2007)’ (Hennessy, K., B. Fitzharris, B.C. Bates, N. Harvey, S.M. Howden, L. Hughes, J. Salinger and R. Warrick, 2007: Australia and New Zealand. Climate Change 2007: Impacts Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson, Eds., Cambridge University Press, Cambridge, UK, p. 515).

There’s a big difference between ‘up to 20% more droughts … over most of Australia by 2030′ and ‘Australia could experience droughts twice as often [i.e., 100% more frequently] within 20 to 30 years.’ The citation to the ‘Mpelasoka et al., 2007′ paper which supported the former statement was given in the list of References as follows:

‘Mpelasoka, F., K.J. Hennessy, R. Jones and J. Bathols, 2007. Comparison of suitable drought indices for climate change impacts assessment over Australia towards resource management. Int.J.Climatol., accepted.’ (‘Climate Science 2007: Impacts, Adaptation and Vulnerability’, p. 537).

These bibliographical details were also wrong. The fourth-named author of the paper as published in Int.J.Climatol was B.C. Bates (one of the Lead Authors of the IPCC Chapter), not J. Bathols. The year of publication was 2008, not 2007. And the paper could not have been ‘accepted’ by the journal at the time that the IPCC report was finalised: according to the details on the cover page, Mpelasoka et al. (2008) was revised on 18 September 2007 and accepted on 2 October 2007 – six months after the WGII Contribution to the IPCC Report had been approved in Brussels by 130 governments.