cc: Bradley Ray date: Sat, 14 Mar 2009 14:47:39 -0700 from: Darrell Kaufman subject: Re: citations and comments on draft to: Keith Briffa Hi Keith (cc: Ray): I'm working through your comments on the Arctic 2k synthesis paper and wanted to point out some comments that I'm having trouble addressing. I don't think they require any action on your part, but I wanted to make sure you were comfortable with the way I've dealt with them. The good news is that I have now expanded the calibration/scaling period back to 1860 and subdivided the record into 10-year averages. With n = 14, the calibration is statistically robust, and it's comforting that the result is essentially identical to the 20-year averages based on a smaller calibration set. ------------- Regarding the sentence: "In contrast, temperature changes during the penultimate millennium are only sparsely documented, especially in the Arctic" Your comment: "The wrong impression is given here, because what little evidence we do have for this period IS ALL from the Arctic". I agree that a major share of ice cores come from the Arctic, but I don't think that high-latitudes are especially well covered by proxy data. For example, only a couple of the long-term records in the recent Mann et al. (2008) compilation are from >60N. ------------ Regarding the sentence: "The forcing was most pronounced at high latitude where it was amplified by positive feedbacks" Your comment: "Need to be careful here because you are implying content that is not found in this manuscript – i.e. evidence for the amplification , and by implication lack thereof in non-Arctic regions". I agree that the paper does not prove Arctic amplification. Instead, we discuss previous studies that show an amplified response in the Arctic to help explain our record. --------- Regarding the last sentence of the abstract, which I have now revised to: "The long-term cooling trend was reversed during the 20th century, and the late 20th century was the warmest half-century of the last two millennia." Your comment: "you need to look at the distributuion/frequency of all equivalent warming trends over the last 1950 years to be able to place some probabilistic estimate of the likelihood that this recent warming is significant" I understand the rationale for the quantitative approach, but am not sure we have space for this analysis in this short paper, unless we add it to the Supporting Online Materials. I think that the plot of the temperature history speaks for itself. Also, the single greatest shift of 200 decades was between 1980-90, which seems significant in itself. ------ Regarding the sentence: "Because climate change is amplified in the Arctic (Serreze & Francis, 2006), warming during these historical intervals may be more easily detected in Arctic paleoclimate records than in global datasets" Your comment: "Many – including me – would argue against this. It is a question of signal and noise – and other authors have discussed that tropical areas are better region for detecting change because low natural variability outweighs magnitude of expected signal in artic – that is at least until major (non-linear) effects of ice melt kick in perhaps- but then timescale critical and could not be realistically invoked up until present day". I agree that it is a question of signal:noise (detection). The fact that the arctic has warmed much faster than the rest of the globe is good evidence for amplification in the arctic, and I believe that we can defend this position. ------- Regarding the sentence: "A simple forward projection of the linear trend based on the proxy data from 1-1900 AD (Fig. 3C) indicates that the summer temperature should have been about -0.5°C by mid 20th century relative to the 960-1800 reference period." Your comment: "Why do this ? More logical to project a forward temperature curve based on a simple energy balance model tuned to some GCM driven by projected irradiance change from Milankovitch forcing?" I brought this up with our NCAR collaborators, and with Ray. They think that it will be difficult to isolate the regional impact of changes in insolation forcing. We discussed possible strategies, but I'm afraid that they will have to wait for the next publication. -------- Regarding the sentence: "Temperatures were 1.2C higher than the projected value based on the linear cooling trend, and even more anomalous than has been previously documented" Your comment: "This also begs the question about Mann et 1999 – where they postulated a long-term cooling (over 1000 years) for the globe - that they said was orbitally driven." I'm not sure how to respond to this. Others have pointed out the overall cooling trend since the Holocene thermal maximum and I have discussed this for the Arctic in the paper. -------- Let me know if you have any strong reactions to any of this, otherwise I'm hoping to submit the paper in the next couple of days. Thanks. Darrell On Mar 9, 2009, at 10:26 AM, Keith Briffa wrote: > Darrell > am only trying to play devil's advocate. As for the df problem (and I > agree it is) , I think some direct reference to it in the text is > preferable to letting referee use it as a stick to beat you. Yes, do > try higher resolution regression , obviously for the proxies that > allow this. To be honest, when I look at Figure 2 , I can see that > decadally-smoothed data (or band-passed data) would show some inverse > correlation between temp. and proxy average though - so you may be > accused of cherry picking your time scale . Nevertheless, the logic > of justifying how you are building from local high-resolution > association, to regional lower-resolution interpretation should be > attempted. We do not expect high-resolution association on Arctic > average scale to be strong without very dense coverage of proxies, as > is shown by instrumental analyses ( Briffa and Jones , 1993 ) - > while at decadal scales coherence is greater (Jones and Briffa , > 1996) so we can get away with less records - and presumably even less > for long-term trend , provided it is captured in records . > I find that I do not have pdfs of the papers I have mentioned - so > will need to get them scanned and sent to you . I am not in the > office tomorrow but will leave instructions to this effect > cheers > Keith > > > Briffa, K.R. and Jones, P.D., 1993 > "Global surface air temperature variations during the twentieth > century: Part 2, Implications for large-scale high-frequency > palaeoclimatic studies." > The Holocene 3, 77-88 > > Jones, P.D. and Briffa, K.R., 1996 > "What can the instrumental record tell us about longer timescale > paleoclimatic reconstructions?" > pp.625-644 in: Climatic Variations and Forcing Mechanisms of the Last > 2000 Years (Eds. P.D. Jones, R.S. Bradley and J. Jouzel), NATO ASI > Series Vol. 141. Springer Verlag, Berlin, Heidelberg. (R) > > > At 16:49 09/03/2009, you wrote: >> Keith: >> Thank you for your insights. I think I can deal with nearly all of >> your suggestions. You picked up on the plug for proxies from lakes >> at high latitudes. I'll try to tone that down, but do want to retain >> the message. >> >> I'll see whether our NCAR co-authors can address your suggestion >> about using an energy balance model to relate Milankovitch forcing >> to temperature. That will take some time, but I'd rather submit a >> manuscript with a high probability of success than to have to go >> back to the drawing board. >> >> Most importantly: You commented about the significance of the >> regression used to scale the proxy values to temperature. This is >> the weakest point of the paper, in my view. Because the focus is on >> the 20-year intervals, we only have 5 points for the regression, >> which is a serious limitation, and I doubt would hold up to >> statistical scrutiny. On the other hand, I am confident that the >> proxy values do correlate with temperature and the correlation is >> significant at even the annual scale (see Fig 2), and here I mean >> 'significant' even after accounting for autocorrelation effects on >> the df. Would it be valid to present the statistics for the annual >> correlation, then use this to support the scaling for the 20-year >> means, even though the n (= 5) for the 20-year means is too small to >> derive statistical significance? Or can you see another way around >> this issue? It's important to scale the proxy data to temperature, >> and I believe that our data can support this, I'm just not sure how >> best to make the case. I'll append the values for the five, 20-year >> intervals that I used to calculate the scaling in case you have some >> ideas to try. >> >> Also, could you please forward a pdf of your 1990 Nature paper? Our >> electronic subscription picks up at 2000 and I wanted to follow up >> on your suggestion that we place some probabilistic estimate on the >> significance of the recent warming. >> >> Thanks again; the paper will be much stronger with your input. >> Darrell >> >> >> JJA(C) Proxy >> 0.26 1.66 >> -0.05 1.35 >> 0.09 1.54 >> 0.11 1.32 >> -0.31 0.29 >> >> >> On Mar 9, 2009, at 8:44 AM, Keith Briffa wrote: >> >>> Darell >>> perhaps a short piece in the Supp. Inf. could give a little more >>> detail on RCS , cite the original refs and show a Figure? but CAN >>> NOT >>> do for now , so let's go with it as is to make your deadline. This >>> could be done later if referees say so. Attached is a tracked >>> version >>> with my comments (forwarded only to Ray also for hard-nosed >>> comment!). >>> >>> Cheers >>> Keith >>> >>> >>> >>> >>> At 16:54 08/03/2009, you wrote: >>>> Also: >>>> Would it be important to cite the original sources for the tree- >>>> ring >>>> data, or do you think citing your new compilation would suffice? >>>> >>>> thanks. >>>> Darrell >>> >>> -- >>> Professor Keith Briffa, >>> Climatic Research Unit >>> University of East Anglia >>> Norwich, NR4 7TJ, U.K. >>> >>> Phone: +44-1603-593909 >>> Fax: +44-1603-507784 >>> >>> http:// >>> www.cru.uea.ac.uk/cru/people/briffa/ >>> <2k synthesis v6-KRB.doc> > > -- > Professor Keith Briffa, > Climatic Research Unit > University of East Anglia > Norwich, NR4 7TJ, U.K. > > Phone: +44-1603-593909 > Fax: +44-1603-507784 > > http://www.cru.uea.ac.uk/cru/people/briffa/ >