The browning of Australia

Reader Proust points me to this helpful BOM site showing rainfall trends in Australia. You can choose your own region, season and time period.

Here’s the most relevant to consideration of the effects of global warming, the trend since 1970, which demonstrates how much drier the climate has become over the period in which warming has been observed. As various people have pointed out, is was even drier during the famous Federation drought at the beginning of C20, so the role of global warming isn’t conclusively established, but it would certainly seem unwise to bet on a rapid return to the average observed in the historical record

Rainfall trend

82 thoughts on “The browning of Australia

  1. Heh. Nice try. But dishonest (or just dumb, either way pretty sad coming from soneone in your position).

    Here’s the 1900-present trend map. Professor Q, how about you post both side-by-side and then explain why your map has nothing to say about AGW whatsoever?

    (hint – it’s a first-year statistics exercise).

    Proust – please note policy on civilised discussion. Your comments so far have been uniformly rude, and (the two usually go together) uninformed. The point about 1900 being drier is noted in the post, and had been discussed at length before you raised it. Also, if you want to make claims implying some degree of expertise in statistics, either demonstrate it, or use a real name and state your qualifications. JQ

  2. Great tool BOM.

    Of course, as they point out “because rainfall is such a variable element, trend values are highly dependent on the start and end dates of the analysis” and the fact they are simply using linear interpolation it is very difficult to derive anything meaningful in terms of climate change from just one map.

    Analysing each plot in sequence from 1910 to 1970, however, and the trend towards drying in the east and wetting in the northwest seems fairly consistent.

  3. Actually, the 1970-2005 and 1900-2005 maps have a lot in common. Drying in coastal WA, Queensland and Tasmania, increases in rainfall in Northern WA (which doesn’t need it) and inland WA (where it is of no economic use). The bid difference is in NSW and Vic. (And in inland SA where rainfall is so low to start off with that it won’t make much difference).

    What has happened is that from 1900-1970 trends were in different directions in different parts of the country and generally not particularly strong. Since 1970 we have seen exactly what global warming models predict – more rainfall in the North-West and some desert areas and less in the major agricultural regions. In some cases this has either combined with, or overcome, pre-1970 trends. In other places it might take a bit longer. Nevertheless the signature of global warming looks pretty clear.

  4. Well, my concern is that the scale stops at -50 mm/10 years. It could be a lot higher in parts of the brown zone: -60 or -70 or even -100mm/10 years. (Aka, one less cm of rain for each year.) Seriously, if you have roughly half of Queensland falling into that category, I think we need BOM to provide more colors to show the full picture.

    I’m not so worried about places like Cairns – but global warming could ruin inland farming regions like the Darling Downs.

  5. Its an interesting map. Why has rainfall increased in the West? Roughly rainfall seems to get worst as you go from East to west. StephenL suggests this is consistent with global warming models. Why?

    Would not global warming be expected to produce a uniform average decrease?

  6. Your comments so far have been uniformly rude, and (the two usually go together) uninformed.

    How so? Put up the 1900-present map alongside the above, and I’ll believe that you’re not trying to dishonestly distort the debate.

    Also, if you want to make claims implying some degree of expertise in statistics, either demonstrate it

    Hmm, ok, start simple.

    Bayes rule: P(A|B) P(B) = P(B|A) P(A)

    [I’ve always preferred writing it like that because then it is manifestly symmetric beyween events A and B]
    Bayes rule is the fundamental building block of probabilistic reasoning. However, an infrequently mentioned caveat: it doesn’t work if either A or B are events of zero probability. “Conditioning on events of zero probability” is naughty. Remember those sneaky proofs of “1=0” you discovered in high-school? They always relied on dividing by zero somewhere. Well, that’s the equivalent of using Bayes rule to reason about events of probability zero – you can establish anything you like.

    Bit more complicated, but no less fundamental is the

    Strong Law of Large Numbers: The sample mean of any infinite sequence of independent and identically distributed (iid) random variables with mean x converges almost surely to x.

    The “Strong Law” is why we can represent samples by averages.

    What’s this weasely “almost surely” you ask? Well, it really means what it says. It means that the measure (or probability) of the samples whose mean does not converge to x is zero. Or if you’re into tossing coins, it means that the probability of the average number of heads in a long sequence of tosses differing significantly from one half gets smaller and smaller (towards zero) as the length of the sequence of tosses increases [strictly, that last statement was closer to a statement of the weak law of large numbers, but we can leave the distinction between the two laws for another day]

    Since we’re talking trends here, how about a bit of Stochastic Calculus:

    Ito’s Equation: If x(t) is a generalized Wiener process

    dx = a dt + b dWt

    then f(x(t), t) is also a generalized Wiener process with:

    df = (a f_x + f_t + 1/2 b^2 f_xx) dt + b f_x dWt

    [f_x means the partial derivative of f wrt x. f_xx is the scond partial derivative]

    Suprisingly, if you’re cavalier with interchanging limits and integrals, Ito’s formula is trivial to prove.

    But, what is a Wiener process I hear you ask? No, it is not a sausage machine. It is the equivalent of a normal distribution for processes. Wt – Ws behaves like a normal distribution of (t-s). I won’t go into more details, except to say that the financial markets (particularly hedge funds) run on Ito’s formula and extensions (eg Black-Scholes option-pricing is just Ito’s formula applied to options).

  7. Harry – I was thinking the same thing about WA. I wonder could it be the influence of an increasing number of cyclones (caused by global warming???) hitting the north west coast and moving inland in a south easterly direction, providing brief but heavy rain to an otherwise dry area? Just speculation, I have no data.

  8. OK, Proust, you can talk the talk. That makes it easy.

    In the previous thread, I gave an informal Bayesian argument for the proposition that AGW is leading to generally hotter and drier weather in SE Australia. You should have no trouble filling in the details, and pointing out where you think I’ve gone wrong. Alternatively, set out your own priors and show why you think the proposition should be rejected.

  9. John, You say that the BoM map of the trend in rainfall in southeastern Australia since 1970 “demonstrates how much drier the climate has become over the period in which warming has been observed.” Even if that were true, you would only be demonstrating a correlation between rising temperatures and declining rainfall, not that one had caused the other.

    But “the period in which warming has been observed” did not begin in 1970. In fact, over half of the increase in the 11-year moving average of temperature in the BoM’s “Southeastern Australia” between 1950 and 2000 occurred BEFORE 1970. (I haven’t checked my arithmetic, but this looks to be right from the BoM chart). Yet the early 1970s were uncommonly wet, and the BoM’s very helpful sorted data shows that 1973 was the second wettest and 1974 was the third wettest year in the 106-year instrumental record of rainfall in southeastern Australia.

    So an obvious question presents itself: if the warming of about 0.4 degrees C in temperatures in southeastern Australia in the 1970s, 1980s and 1990s caused the lower rainfall of the recent past, why was the warming of 0.4 degrees C in the 1950s and 1960s followed by a period of above-average rainfall?

    But the cause-and-effect argument that you are mounting must confront a more serious problem: according to my calculations, the seasonal pattern of the recent change in rainfall in south-eastern Australia is completely at odds with CSIRO’s projections.

    The paper prepared by CSIRO and published by the Australian Greenhouse Office that I’ve already cited (“Climate change scenarios for initial assessment of risk in accordance with risk management guidance”, May 2006) provides projections FOR EACH SEASON of the change in rainfall between 1990 and 2030 for ten Australian regions, two of which (NSW and Victoria) make up “southeastern Australia.”

    Take NSW first. The CSIRO projections are for an INCREASE of 1.5% (low global warming scenario) and 3.5% (high warming scenario) in rainfall in summer and autumn; and for a DECREASE of 3% (low global warming scenario) and 7.5% (high global warming scenario) in winter and spring. I’ve checked the BoM data for rainfall for the most recent 11-year period for which the information is available (Sept. 1995-August 2006) with the immediately preceding 11-year period (Sept. 1984-Aug. 1995), and I find exactly the opposite. In summer and autumn, when rainfall should have increased with global warming according to the CSIRO’s projections, there was a decrease of 16% in rainfall in the more recent period. But in winter and spring, when CSIRO projected decreases in rainfall, my calculations show an increase in rainfall in 1995 to 2006 compared with 1984 to 1995.

    Similarly for Victoria. The CSIRO projections are for no change in rainfall in the summer, both in the low and the high global warming scenarios. My calculations for Victoria from the BoM data show a larger reduction in rainfall in summer (14%) than in average annual rainfall (13%) in the more recent period. For spring, it is the opposite. The CSIRO projections show larger reductions in spring than in any of the other seasons (5% for the low global warming scenario, 11% for the high global warming scenario), But the BoM data of actual rainfall show that the smallest reduction in rainfall in the more recent period was in the spring (down 4% compared with reductions of of 14% in summer, 16% in winter and 21% in autumn).

    These figures are E&OE – I’d be happy to have them checked, perhaps by one of the water supply planners who are using climate model results that predict permanently lower levels of rainfall in southeastern Australia.

    It’s not my argument that such an assumption should not be made: that raises other issues. My point is simply that observations of the actual rainfall trend in recent times are not consistent with CSIRO model projections (I will of course withdraw this comment if my figuring is shown to be wrong). PLease note that the CSIRO projections I am citing were published only four months ago.

  10. Ian, I am well aware of the distinction between correlation and causation, and I didn’t claim that causality could be inferred from the trend alone.

    As regards your first point, I don’t think it’s appropriate to try and correlate rainfall in SE Australia with temperature in the same area. It’s my understanding (not expert) that sea-surface temperatures in the Pacific are more important for rainfall, and the Indian Ocean may also be relevant. The sustained rise in global temperatures dates from the 1970s, so I don’t see a big problem here.

    Your second point is, as you say, more serious. On the face of it, the seasonal projections aren’t supported by the trends observed so far, although the aggregate trends seem to be about right. I’ll look into this.

    On the final point, if we return to your original point, my article made the claim that it was appropriate to assume permanently lower levels, and proceeded to consider the implications. It appears you don’t dispute this.

  11. This doesn’t correlate with the map of predicted rainfall that Al Gore showed in his movie. He verbally noted the drying around the Sahara, but the map showed an increase in rainfall on the east coast of Australia and a reduction on the west coast.

    This weather predicting thing looks like a dangerous game to be in. A bit like playing the money markets. But I guess that you’re not going to be sent to jail for getting your weather predictions wrong (though the farmers might not be happy).

  12. John, If it’s the increase in global temperatures that’s relevant to southeastern Australian rainfall (which I’m happy to accept), the same problem arises. The increase in global temperatures between 1910 and 1945 was comparable with that since 1970: see Table 2.1 of the IPCC Third Assessment Report at .

    So I’ll reformulate my question: “if the increase of about 0.5 degrees C in global temperatures since 1970 caused the recent downward trend in rainfall in southeastern Australia, why was the increase of this order in global temperatures between 1910 and 1945 followed by the strong increase in rainfall in southeastern Australia which occurred in the 1950s?

    On the final point, I’m merely trying to distinguish the issues. Your article made the claim that it was appropriate to assume permanently lower levels of rainfall because of global warming. If the modelled effects of global warming are not being borne out by the observations, as it appears may be the case, I wouldn’t see any reason arising from the grounds that you cited to assume permanently lower levels of rainfall.

  13. I hadn’t noticed Roger Jones’ informative posting on the “Water again” thread until now. I probably would have expressed some of my points differently if I’d read Roger’s comments, but I think most of my arguments stand. I’d be interested in Roger’s reaction to them.

  14. Ian,

    I have a number of problems with your revised question, and before I can respond, I need a better idea of where we agree and disagree. As I’ve made pretty clear, I think the “consensus” view on climate change is broadly correct. In particular,

    1. There is a clear global warming trend, both at the surface and in the troposphere
    2. This warming is due, to a substantial extent to human activity, particularly emissions of greenhouse gases
    3. The cooling observed in the mid-20th century was in part due to emissions of particulates, and reductions in these emissions will tend to exacerbate warming
    4. At the global level, climate models represent these processes reasonably accurately. Analyses at the regional and seasonal level are less reliable, but still represent the best available basis for future planning (in particular, better than planning on the basis of distributions derived from C20 experience)
    5. Under any plausible interpretation of “business as usual”, there is a high probability of warming of 2 degrees or more, relative to baseline, by 2100
    6. Such warming would have severe environmental consequences

    For the purposes of our current discussion concerning experience from 1900 to the present, only 1-4 are really important. If you broadly agree with these, I think I can respond usefully to your question.

  15. The map may need revising at end 2006 because the greenish WA wheatbelt is much drier this year. Whatever statistical hypothesis testing reveals, I believe most Australians over 30 now believe rainfall patterns are different. This can’t be all due to biased memory. On the other hand they don’t want to pay for it in terms of food prices, water restrictions, carbon taxes and so on. I’d call it begrudging acceptance.

  16. Thanks John. I didn’t put my “final point” correctly – what I should have said was that, if the current understanding of the causes of changes in regional rainfall is as low as it appears to be, it would not be appropriate to count on a recovery to levels that have prevailed in the past – and that would be true irrespective of the view taken on the implications of global warming.

    My question as reformulated was the one that I would have asked in the first place if I had realised that your reference to “warming [which] has been observed” related to changes in mean temperature at the global level. As the title of the thread is “The Browning of Australia” and you’d said in your introductory post that “the role of global warming isn’t conclusively established”, I didn’t think that my revised question would give you problems. But it is not a matter of great importance.

  17. We went over this at Prodeo.

    And I discovered (as it seems did proust) that the only significant thing it showed was that the 1970’s were likely an excellent year for rainfall in Australia.

    The other thing is they cut the time-series off earlier then they needed to. Which was tendentious since it is implied that having made a diagram with the later information would have shown almost nothing untoward.

  18. I think we need a new word here..
    To be castled.. To be shot down in flames statistically

  19. Or, alternatively


    To be christled… to receive support of negative value, or criticism that increases your credibility.

  20. In the previous thread, I gave an informal Bayesian argument for the proposition that AGW is leading to generally hotter and drier weather in SE Australia

    I searched the thread for the string “Bayes” – I could not find it. Could you please reproduce the argument here? Feel free to put it in mathematical terms – the more precise the better – that way there can be no confusion.

    Sure. My propositions are:

    A: Global warming implies drier weather in SE Australia

    (Here ~A encompasses everything else “Global warming is not taking place or does not imply drier weather in SE Australia”)

    B: Hot, dry weather as observed in the period 1995-2006 occurs

    My prior probabilities are Pr(A)=0.5, Pr(B|~A)=0.25, Pr(B|A)=0.75. Applying Bayes rule, the posterior probability for A|B is 0.75, as claimed.

  21. Suppose we take 1900 as the base, admittedly an exceptionally dry period. The Queensland coastal areas still present a problem, as does southwest WA.

    What if you statistiians were to plot the rate of rise/decline in rainfall against the rate of increase in water consumption in the east?

  22. John, You said on the “Water, againâ€? thread that you thought the CSIRO projections were for lower rainfall for SE Australia generally, but that you’d look at the sources I’d cited. Have you had a chance to look at the AGO publication “Climate change scenarios for initial assessment of risk …â€?? For NSW the CSIRO central projections are for NO CHANGE both for the low and high global warming scenarios, and for Victoria their projections are for fractional decreases of 1.5% and 3.5% respectively over the 40-year period 1990 to 2030. During this period CSIRO’s projections of global temperature (based on the flawed SRES scenarios) are for increases of 0.54 deg. C and 1.24 deg. C.

    In short, CSIRO’s expectation of change in annual rainfall in SE Australia, on a decadal timescale, ranges from nil to imperceptible. Moreover, as I’ve shown, seasonal variations have generally been in the opposite direction to those projected on the assumption of global warming. These differences between projected and observed trends in rainfall seem to raise serious questions about the ability of the models to predict changes in rainfall – though I’d be interested in CSIRO views, especially on whether it is appropriate to use successive 11-year averages as measures of outcome and, if it is not, how the relationship between projections and outcome should be monitored.

    You also mentioned that there was an expectation of drier weather in SW Australia, and that this had influenced your thinking in terms of rejecting the hypothesis of no change. I’ve now looked at the observed rainfall in SW Australia, using the same successive 11-year periods as I used above for NSW and Vic. Again I find a uniformly negative correlation between the CSIRO projections of rainfall and the observed outcomes. In SW Australia there was virtually no change in the observed annual rainfall between the running average for 1984-95 and that for 1995-2006 (a decrease of 1%). But here too there was a marked change in seasonal patterns, and here too it was in precisely the opposite direction to the CSIRO projections.

    These assumed that the largest decreases in rainfall would be in winter and spring (decreases of 5% and 11% between 1990 and 2030 on the low and high global warming scenarios respectively), but rainfall was fractionally HIGHER in winter/spring in the more recent period (1995-2006) than in the previous 11-year period. In summer and autumn the CSIRO projections were for smaller decreases in rainfall than in winter and spring, but the observed change was a substantial decrease: in fact, as large a decrease between the successive 11-year periods as CSIRO projected on the high global warming scenario over the 40-year period from 1990 to 2030. This too raises seems to raise questions about the CSIRO’s projections.

    I’d be interested in your views.

  23. From the BOM the average rainfall in Australia, over all the sites, was 400mm and in 2000 it was 500mm (and in the ’70s it shot up to 800mm.)

    In 25 words or less, is this a drier or wetter climate?

  24. Rog, you’re making less sense than usual (which is not much). At a minimum, there’s a date missing somewhere, and you appear not to have followed the discussion of regional variations. And please avoid peremptory demands for responses – if there’s one thing I can’t stand it’s a bossy troll.

  25. The sea surface maps are interesting. If you look at the 1900-2005 map, there is a slight warming all around. Now look at 1970-present. Who wants to tell me this doesn’t have anything to do with the rainfall pattern over the same period?

  26. Ian, looking at the projections it certainly appears that increased evaporation plays a bigger role than decreases in rainfall, but of course the two interact, and the reduction in streamflow is larger again (commonly by a factor of three) than the reduction in rainfall net of evaporation. Since reductions in streamflow are the variable of interest for most of my work, I’ve tended to focus on this, and I’ll be more careful to say so in future.

    As regards the issue of seasonality, while one would expect less reliability at the seasonal level than the annual level, there certainly appears to be a problem here. I’ll look into this.

    As I mentioned above, in responding to other points you’ve raised it would be very helpful if you could clarify your own position, so that I can see how much common ground we share.

  27. It’s a pity the Bureau doesn’t have an evaporation graph along with the rainfall graph because the expectation with rising temperatures is that evaporation rises. So if rainfall doesn’t also rise at the same time then you can look forward to a drier future. Regional decreases in rainfall just make the problem worse.

    In general, nearly every continental area is warming faster than the oceans and since continental evaporation is tied to continental temperatures but continental rainfall also depends on surrounding oceanic temperatures, most of the world’s landmasses can look forward to a drier future (drier meaning evaporation going up faster than rainfall).

  28. IMO it’s rash to be dogmatic on such a small sample – the current long dry may or may not be a product of ACC, and thus may or may not be a harbingerof things to come.

    But in any case our water planning has got to be better anyway, because what the current long dry tends to confirm is what people have said in the past, using the Federation drought as an example – that drought in Australia can manifest as dry *decades*, not just dry years.

  29. Chris O’Neill, I’ve recently posted on another thread on this site the conclusions of a Workshop supported by the Australian Academy of Science and the Australian Greenhouse Office on “Pan Evaporation: an example of the detection and attribution of trends in climate variables” (22-23 November 2004). Among these was the following:

    “It is affirmed that global atmospheric warming does not necessarily mean a more drying atmosphere or a drier land surface.”

    This suggests that this issue may not be as clear cut as your posting implies. Are you able to provide some references to support your view that most of the world’s landmasses can look forward to a drier future?

    John, On the “Presentation: Precautionary Principle …” thread you told me that you think it’s “unhelpful to conflate discussion of climate-science issues like the modelling of SO2, about which none of us here know very much, with discussion of economic projections, where we can have a useful discussion.”

    Against that background, it’s not clear to me why, in responding to other points I’ve raised, you’d find it very helpful if I were to clarify my position on issues such as points 1-4 of your posting on this thread at 3.19 pm on 1 October. I’m not a climate scientist, and I don’t understand why you’re not able to answer my questions unless I state my position on climate-science issues about which I don’t know very much. To put it bluntly, why do my views matter?

  30. Ian,

    from a climatological view, there’s no point in trying to contrast 11-year averages with climate model projections. The projections are calculated as a function of global warming over the entire period of a model run e.g. from nominal year 1881-2100, or 1901-2100 depending on the model. Therefore, the projections contain little of decadal or shorter signals of variability – we’re quite confident they have the global warming signal as represented by the model (except in desert regions, where one event can define the signal). Whether we have confidence in that signal depends on convergence in the agreement of seasonal signals from the models, their representation of current climate etc.

    This suggests that yes, as both you and John have concluded, its best not to read the attribution of a rainfall signal from acceleration of warming from the 1960s or 70s on the basis of correlation. There are too many variables affecting rainfall. One would want some supporting physics in terms of circulation changes that are reflected in models and obs or similar.

    As to your comments on the CSIRO projections of change. Yes, the rainfall projections are not large. This is a good thing. We should be grateful the numbers are not larger. However, what we found when we looked at central NSW, was that climate variability could produce swings in decadal mean rainfall of +/- 20% over the long-term mean. This means that we have benign combinations of variability and change, and really nasty combinations. Which will eventuate? (and over the next century, decadal variability will give us wetter and drier periods imposed on, and maybe interacting with, a greenhouse signal) We don’t know for sure, and so we must try and assess the risks with the info we have at hand.

    Let’s say for example, that we have a scenario in SE Aust where summer and autumn rainfall increase by +5% and winter-spring decreases by -5%. The summer rainfall will have little effect on streamflow and the winter spring-rainfall will lead (with increases in evaporative demand) to a decrease in streamflow of >10%. This is what John was referring to as the amplification effect. And why in the report CSIRO did for the Vic govt, we found few signs of streamflow increase anywhere in Victoria (link given in earlier comment above). Record extended periods of low flow in a number of rivers (including the Murray) must give us pause.

    As to the comments about temperatures in 2030 being based on the “flawed” SRES scenarios. Projections of climate change are dependent on changes in radiative forcing. For regional climate projections it does not matter overly much what assumptions underlie that forcing as long as the magnitudes of increase in radiative forcing themselves are plausible. Projected changes for rainfall and temperature in 2030 are mainly due to climate sensitivity, a scientific uncertainty. The highest of that range you quoted, the 1.24°C in 2030 is attached to the A1T scenario that has more agressive sulphate emission reduction than the other SRES scenarios. Interestingly, the recent ABARE AP6 reference emission scenario gives an upper temperature almost as high in 2030 (0.05°C lower and it is largely based on IEA projections). If more aggressive sulphate reductions were to occur, warming would be as high by that time. To suggest (by implication or otherwise) that projected emissions will somehow deliver a lower range, means showing that the entire data-set of available emission scenarios (not just SRES) are over-estimated.

    It would be interesting to try a Bayesian assessment of the different cases of rainfall reductions in the obs and how the current run of dry years affect the probabilities. Given a reduction of 10 years in rainfall what is the posterior probability of it being a short-term departure, long-term but temporary departure and/or change. The longer that period of lower rainfall extends, the higher the posterior probabilities for the latter combinations become. Given the number of possible alternatives, this may not be simple. Posteriors of 11, 12, 13 years etc will change the numbers quite dramatically, I would expect.

  31. I was steered this way by a posting in another blog, and I am afraid that I may not understand Australian geography well enough to pass judgment. I would make one comment on the relationship between SSTs and precipitation, however. I would concur with Ian Castles discussion of the pan evap study that conditions will not be uniformly drier in a warmer world, nor would they be uniformly wetter. Physical theory would lead one to predict a faster evaporation rate, depending upon atmospheric pressure and moisture content, wind speed, etc…

    While I agree with all of you that statistical criteria are important, it is also important to identify physically meaninful relationships in deterministic equations.

  32. I am not so sure if anything of value can be extracted from the BOM images, as they do warn “.. analysis periods starting after 1970 are considered too short to calculate meaningful trend values” and “..trend values calculated here using past observations should not be used to imply future rates of change”

    An opportune moment to apply the Precautionary Principle?

  33. Thanks Roger, especially for the advice on the rainfall projections. I was interested to know whether it was appropriate to use successive 11-year averages to test observations against projections, and I accept your view that it’s not. I also take the point in your last paragraph that it is not simple to test how the current run of dry years affects the probability of a permanent change. Nonetheless it would seem important that this be done.

    I know that temperature projections in the period to 2030 are not highly dependent on projections of GHG emissions, and it is certainly significant that the highest projection for that year is attached to the scenario with lowest sulphate emissions. Do you have any comments on my series of postings on this subject on the “presentation: precautionary principle …” thread of this blog, beginning at 11.08 am on 21 September? The best estimates I have been able to find of sulphur emissions – those by Stern (2003), cited in van Vuuren and O’Neill, “The Consistency of IPCC’s SRES Scenarios to Recent Literature and Recent Projections”, “Climatic Change”, March 2006: Table VI on p. 38 – show a steep drop in the first half of the 1990s to a level comparable to that projected in the A1T scenario for 2030.

    I’d be particularly interested in your view on the question whether, if sulphate concentrations have been as low as A1T projects for 2030 during the past decade, you would have expected global mean temperatures to have risen more than they have.

    You say that the ABARE AP6 reference emission scenario is largely based on IEA projections.The only IEA emissions scenarios to which I give credence are the Reference and Alternative Scenarios published in the Agency’s excellent World Energy Outlook. These are fully articulated scenarios for CO2 that can be tested against realistic projections of power generating capacity, vehicle usage etc.

    The latest IEA Reference Scenario (that in WEO 2005) shows the same increase in CO2 emissions between 2000 and 2030 – the end-point of the projection – as the IPCC B1 scenario. This scenario has the LOWEST emissions in 2100 of the six IPCC illustrative scenarios, whereas the ABARE AP6 scenario projected HIGHER emissions in 2100 than any of the IPCC scenarios. So I do have difficulty understanding how it can be claimed that ABARE’s scenario is based on IEA projections. Are there IEA projections that I don’t know about?

    Of course I realise that projections of climate change are dependent on changes in radiative forcing, but projections of radiative forcing are in turn depend on projections of emissions. The latest relevant ABARE publication (“Economic impact of climate change policy”, ABARE Research Report 06-7) says that global CO2 emissions in its reference case closely follow those under the IPCC’s A2 scenario to 2030 and that the latter scenario assumes a decline in economic growth after that year (pps. 22-23). In fact, projected decadal economic growth under A2 between 2030 and 2040 exceeds 40%, a higher rate than in any of the four preceding decades (SRES, p. 471).

    Your statement that “it does not matter overly much what assumptions underlie that forcing as long as the magnitudes of increase in radiative forcing themselves are plausible” seems to me to be entirely circular. If the underlying assumptions about population, energy use etc. are implausible, the radiative forcing assumptions that depend on them can’t be plausible. As Erwin Diewert says in his comment on John Quiggin’s critique of Castles and Henderson, the SRES projections should be reestimated. I have no doubt that he would take the same view of the ABARE reference scenario and most (if not all) of “the entire data-set of available emission scenarios” to which you refer.

  34. I meant to say that the ABARE reference scenario projects higher fossil CO2 emissions in 2100 than any of the six IPCC illustrative scenarios – some of the other 29 IPCC scenarios used in the 1.4-5.8°C are higher than ABARE’s.

  35. Ian, I’d appreciate a statement of your position for several reasons

    (1) My responses to several points you’ve raised depend on my understanding of the climate science. If you have a different view, I’d prefer to know about it first.

    (2) In any scientific debate, it’s always possible to make lots of (not necessarily consistent) criticisms of a model, but this isn’t of much value unless there is an alternative, better model.

    (3) On this specific issue, I’ve found little to be gained in arguing with those who are sceptics/denialists regarding the basic science. Even though the main arguments that gave their position some credibility (urban heat islands, discrepancies with the satellite data, the Medieval Warm period and so on) have collapsed, they don’t change their views. Rather they just keep on finding new arguments to support a predetermined position. I’ve only got a finite amount of time to allocate, and I’d rather use it productively.

  36. John, I also have only a finite amount of time to allocate (in all probability a good deal less than you have, given our respective ages), and I’m not interested in devoting any more time to explaining my views to someone whose predetermined position is that “sceptics/denialists” keep on finding new arguments to support a predetermined position.

  37. That was a overt display of petulance JQ, using BOM images to prove a point was always a risky enterprise.

  38. jquiggin:

    Sure. My propositions are:

    A: Global warming implies drier weather in SE Australia

    (Here ~A encompasses everything else “Global warming is not taking place or does not imply drier weather in SE Australia�)

    B: Hot, dry weather as observed in the period 1995-2006 occurs

    My prior probabilities are Pr(A)=0.5, Pr(B|~A)=0.25, Pr(B|A)=0.75. Applying Bayes rule, the posterior probability for A|B is 0.75, as claimed.

    Hmm – and I thought you were being serious for a moment there. I thought we were going to get something sophisticated like an ARM analysis of the rainfall timeseries for SE Australia, and from that a demonstration that the current drying has a residual anomaly that cannot easily be explained by the historical variation in the data.

    Instead, we get the Milly-Molly-Mandy analysis.

    Of course you can get any number you like for Pr(A|B) if you’re allowed to plug in arbitrary numbers for Pr(A), Pr(B) and Pr(B|A). But that just begs the question: are your assumptions justified by the data?

    Specifically, do the climate models and existing timeseries data support your assumption that the current dry period is three times more likely to be due to AGW (Pr(B|A) = 0.75), than otherwise (Pr(B|~A) = 0.25)?

    I think not.

  39. Well, Proust, I understood you had a “first-year stats” demonstration of error. Instead, all we get is blather. If you disagree with my priors, state and justify yours.

    Just as a hint, I’ll start with (Pr(B|~A) = 0.25). You can get this one straight from the historical data. Look at the most recent data (including 2006 so far), take whatever smoothing period you like to get the shortfall from the average, and then look at the frequency distribution. Or, you could try an ARMA (I assume that’s what you meant) analysis to get the same thing if you’re feeling energetic.

  40. “on my understanding of the climate science”

    No doubt the weather keeps changing and a lot of hot air is expired air discussing that fact.

  41. Bob has the answer to predicting the weather.

    How many roads must a man walk down
    Before you call him a man?
    Yes, ‘n’ how many seas must a white dove soil
    Before she sleeps in the sand?
    Yes, ‘n’ how many times must the carbon dioxide fly
    Before they’re forever banned?
    The answer, my friend, is blowin’ in the wind,
    The answer is blowin’ in the wind.

    How many times must a man look up
    Before he can’t see the sky?
    Yes, ‘n’ how many ears must one man have
    Before he can hear people fry?
    Yes, ‘n’ how many droughts will it take till he knows
    That too many people have dried?
    The answer, my friend, is blowin’ in the wind,
    The answer is blowin’ in the wind.

    How many years can a mountain exist
    Before it’s rained to the sea?
    Yes, ‘n’ how many years can methane exist
    Before they’re allowed it to be green?
    Yes, ‘n’ how many times can a man turn his head,
    Pretending he just doesn’t see?
    The answer, my friend, is blowin’ in the wind,
    The answer is blowin’ in the wind.

  42. re the observation that skeptics denialists keep on finding new explanations.

    My reading of the data linked to below is that the rate of increase in glogal temperature has markedly decreased for the last three years.


    Am I interpreting the data correctly. If my intepretation is corrrect do any of the computer simulations give such a signal.

    What is the explanation for the signal?

    Before I get monstered (or put on JQ’s list) I am not denier nor a mitigator but for Australia an adaptationer.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s