cc: "Tett, Simon" , philip.brohan@metoffice.gov.uk,"Keith Briffa" date: Mon, 20 Jun 2005 10:58:19 +0100 from: Tim Osborn subject: RE: Palaeo reconstructions to: "Jenkins, Geoff" Dear Geoff, well of course Defra can do as they like, but the 3 alternative figures all have problems: (1) they all show results from Moberg et al. (2005) which we have criticised in a response to Nature (which Nature have declined to publish, on the grounds that it was too technical) and which Philip Brohan at the Hadley Centre has also criticised. I have cc'd this to Philip in case he wishes to share his criticisms with you. The basic complaint is that the low resolution proxies, which entirely determine the long-term variations shown by the Moberg series, have not been properly calibrated and thus the long-term curve cannot be accepted as representing "degrees C". (2) As the blue uncertainty ranges clearly show, this also has narrower uncertainties near 1000 AD than in the "Little Ice Age". So if they are unhappy that the figure you drew from my advice shows this same behaviour, why would they be happy with the Moberg figures. (3) The uncertainty ranges given for the MBH reconstruction look wrong to me - compare with Fig 2.21 of IPCC TAR. Maybe it can be explained by different smoothing/filtering or different reference period, or maybe it can't. Finally I would add that, on reflection, there are actually four reasons why the envelope narrowed near 1000 AD: (1) The reason I gave before (fewer reconstructions from which to define the envelope). (2) There is some proxy data that is common to almost all the reconstructions (thus they aren't all independent) and the proportion of this common data is greater in the earlier parts of the series (thus there is even less independence near 1000 AD). (3) The envelope of reconstructions, nor any of the uncertainty ranges associated with individual reconstructions, represent the full uncertainty - in particular, very few include uncertainty due to the deterioration of proxy data back in time (e.g., some tree-ring chronologies comprise of fewer actual tree cores further back). (4) Some aspects of the uncertainty are actually reduced in the 1000-1100 period compared with the "Little Ice Age". It seems that many of the uncertainties and potential biases are compounded when reconstructing values that are further from the calibration period mean (usually 20th century mean). Thus the "Little Ice Age" values are perhaps truly more uncertain than the 1000-1100 values, because the latter are nearer to the 20th century level. Therefore perhaps the narrowing of the uncertainty range early on in these figures is correct (unless this point (4) is dominated by points (1-3)). Hope this helps. Perhaps Defra would like to fund us to give them some advice on these issues (or even a small, critical study), instead of getting the advice for free? :-) Cheers Tim At 14:50 16/06/2005, Jenkins, Geoff wrote: >Tim > >Sorry to did all this up again, but Defra arent happy with the figure we >came up with on your advice, showing the envalope of all >reconstructions. The reason is the point (3) you made below, ie it looks >like uncertainties are smaller earlier on. Despite making this point in >the text, Defra would prefer to use suomething else and suggest one of >the figs from Moburg et al. Hence I have included a couple of >alternatives with text attached (plus the old one for comparison). I >would be really grateful if you could let me have your advice on which >of these you think is the more acceptable. > >Thanks > >Geoff > >(3) It might also be worth saying that the apparent narrowing of the >range >in the first 2 centuries isn't because of smaller uncertainties, but >rather >because of few reconstructions from which to define the "envelope". Dr Timothy J Osborn Climatic Research Unit School of Environmental Sciences, University of East Anglia Norwich NR4 7TJ, UK e-mail: t.osborn@uea.ac.uk phone: +44 1603 592089 fax: +44 1603 507784 web: http://www.cru.uea.ac.uk/~timo/ sunclock: http://www.cru.uea.ac.uk/~timo/sunclock.htm