cc: Chris Miller , mhughes@ltrr.arizona.edu, dverardo@nsf.gov, broecker@ldeo.columbia.edu, rfweiss@ucsd.edu, k.briffa@uea.ac.uk, drdendro@ldeo.columbia.edu date: Mon, 03 Feb 2003 14:16:20 -0500 from: "Michael E. Mann" subject: Re: [Fwd: tree rings and late 20th century warming] to: Jeff Severinghaus , Phil Jones , Thomas R Karl , Ray Bradley Jeff, Choice of aligning has no influence on the slope of the curve, it simply changes the mean baseline for comparison. The Mann et al reconstruction has the same amplitude increase as the full Northern Hemisphere annual mean instrumental record over the calibration interval (1900-1980). On this simple point, there is no debate. And this seems to be the origin of your misunderstanding of the issues involved. Briffa & Osborn use a slightly different convention from that used elsewhere (e.g. IPCC and in the attached Science piece which I've re-sent for the benefit of your expanded recipient list), and by their convention the instrumental record is observed to lie ever-so-slightly above the MBH reconstruction over the entire interval available for comparison (mid 19th century-> 1980). This difference is actually quite small, so I'm not sure why we're even discussing it in the first place. It, however, does not in any case impact a comparison of the trends in the two series, which match remarkably well over that same interval. This is despite the fact that the MBH reconstruction represents the entire Northern Hemisphere (which gets half of its contribution from the tropics i.e., latitudes < 30N) while the instrumental series shown by Briffa & Osborn is only the extratropics north of 20N. This is old stuff, and I would guess that the others cc'd in on this message (Ray, Malcolm, Keith, Phil) are not interested in re-hashing these old discussions. The state of the the science here has moved well beyond these semantic and/or conventional arguments, focusing instead on detailed intercomparisons of methods and data (employing rigorous diagnostics of reconstructive fidelity (collaborative between Bradley/Briffa/Hughes/Jones/Mann/Osborn/Rutherford). There is little disagreement between us on the broad trends when seasonal and spatial sampling issues, and differing conventions for e.g. defining reference periods, have been taken appropriately into account. I hope you find that the above information clarifying Jeff. Due to other demands on my time, I have to sign out now on this series of exchanges. best regards, Mike Mann At 09:55 AM 2/3/03 -0800, Jeff Severinghaus wrote: Gentlemen: Please accept my apologies if I have gotten the story wrong. I am not a specialist in the tree-ring field, and was simply reporting what I saw in the Briffa and Osborne paper, several other papers, and what several tree-ring people have told me in conversations. I agree, we need to keep the level of misinformation out there down to a minimum! I regret adding to it. I am still confused, however, about Mike's explanation for the Briffa and Osborne paper's curve appearing flat after 1950 AD. Can you try explaining this again, Mike, please? I don't understand how aligning could change the slope of a curve. The curves appear to continue to 1990 AD or so, and the Esper et al. curve continues to 1993. So the explanation that the records only go up to 1980 doesn't seem to hold in this case. The dashed black line is the instrumental record for warm-season >20 N latitudes and it does indeed diverge from the tree-ring records in the 1980s. Can you help me out here? Sincerely, Jeff At 4:36 PM +0000 2/3/03, Phil Jones wrote: Tom, Mike's answer is a fair response. Jeff has mixed some facts up and this is maybe because we've never explained them clearly enough. There are two facts: 1. There are few tree-core series that extend beyond the early 1980s. This is because many of the sites we're using were cored before the early 1980s. So most tree-ring records just don't exist post 1980. 2. The majority of the recent warming is post-1980, so no proxy would pick this up. This warming has been large and it would be good to go back and see if the trees have picked it up. It would give more faith in tree-ring reconstructions, but any reconstruction method is being pushed to the limit by the rate of temperature rise over the late 20th century. Applies to other proxies but you have to note the following: It is important to remember that locally few regions exhibit statistically significant warming. Highly significant at the hemispheric level, but not great at the local level due to high level's of variability. The spatial scales are important and this is difficult to get across. Cheers Phil At 09:15 03/02/03 -0500, Michael E. Mann wrote: Dear Tom, Have no fear, Jeff has still got his facts wrong, even after going back and checking once... First off, I never made any such comment to Jeff--he clearly misunderstood comments that I made at EGS a year ago in response to a question he asked. Of course, it is well know that there are a number of competing explanations [this is what I said--to quote this as offering "no explanation" is a bit unfair Jeff, don't you think? As I recall, I even invited Tim Osborn in the audience to add his own comments--but he had little to say] for the fact that *high latitude*, primarily *summer responsive*, tree-ring *density* data have exhibited a noteable decline in the past few decades in the amplitude of their response to temperature variability. We have discussed this issue time and again in our own work, and Keith Briffa, Malcolm Hughes, and many others have published on this, w/ competing possible explanations (stratospheric ozone changes, incidentally, is the least plausible to me of multiple competing, more plausible explanations that have been published). See e.g.: Vaganov, E.A., M.K. Hughes, A.V. Kirdyanov, F.H. Schweingruber, and P.P. Silkin, Influence of Snowfall and Melt Timing on Tree Growth in Subarctic Eurasia, Nature, 400 (July 8), 149-151, 1999. It should *also* be noted that we used essentially none of these data in the multiproxy Mann/Bradley/Hughes (MBH) reconstruction, and that the MBH reconstruction tracks the instrumental record quite well through the very end of our calibration interval (1980--it stops then because there are far fewer paleo records available after 1980). This was shown in our 1998 Nature article quite clearly, and of course remains true today. Jeff made the mistake of only looking at the Briffa & Osborn paper, which doesn't properly align the 20th century means of the various reconstructions and instrumental record. An appropriate alignment of all the records is provided in IPCC, and in the attached Science perspective from last year. This shows how well the Mann et al reconstruction (and several model-based estimates) track the entire instrumental record. There are some good reasons that some of the other purely tree-ring based reconstructions differ in their details, in addition to the greater influence of the recent high-latitude density decline issue, and these are discussed in IPCC and the Science piece. Of course, we have in, our own work provided detailed calibration and verification statistics that establish the skill in our reconsruction in capturing the details of both the modern instrumental record, and independent, withheld earlier instrumental data (19th century and, more sparsely, 18th century), and we publish uncertainties that are based on rigorous analysis of the calibration and cross-validation residuals. I know that Jeff has seen me talk on this many times, and probably has read our work (I would hope), so I'm frankly a bit disappointed at the comments. I would have liked to think that he would have approached us first, before broadcasting a message full of factual errors. Please let me, or any of the others know, if we can provide any further information that would help to clarify (rather than obscure!) the facts, cheers, mike At 07:49 AM 2/3/2003 -0500, Thomas R Karl wrote: Colleagues, Correct me if I am wrong, but I always thought the failure was a lack of tree cores subsequent to the 1980s. Please correct me if I am wrong, and if Jeff is correct, then indeed we have a significant implication. Tom -------- Original Message -------- Subject: tree rings and late 20th century warming Date: Sun, 2 Feb 2003 16:15:04 -0800 From: Jeff Severinghaus <[1]mailto:jseveringhaus@ucsd.edu> To: <[2]mailto:Thomas.R.Karl@noaa.gov>Thomas.R.Karl@noaa.gov Dear Dr. Karl, I enjoyed your presentation yesterday at the MIT Global Change forum. You may recall that I asked about the failure of tree rings to record the 20th century warming. Now that I look at my records, I realize that I remembered this wrongly: it is the LATE 20th century warming that the tree rings fail to record, and indeed, they do record the early 20th century warming. If you look at the figure in the attached article in Science by Briffa and Osborn, you will note that tree-ring temperature reconstructions are flat from 1950 onward. I asked Mike Mann about this discrepancy at a meeting recently, and he said he didn't have an explanation. It sounded like it is an embarrassment to the tree ring community that their indicator does not seem to be responding to the pronounced warming of the past 50 years. Ed Cook of the Lamont Tree-Ring Lab tells me that there is some speculation that stratospheric ozone depletion may have affected the trees, in which case the pre-1950 record is OK. But alternatively, he says it is possible that the trees have exceeded the linear part of their temperature-sensitive range, and they no longer are stimulated by temperature. In this case there is trouble for the paleo record. Kieth Briffa first documented this late 20th century loss of response. Personally, I think that the tree ring records should be able to reproduce the instrumental record, as a first test of the validity of this proxy. To me it casts doubt on the integrity of this proxy that it fails this test. Sincerely, Jeff copies to Ray Weiss, Wally Broecker Jeff Severinghaus Associate Professor of Geosciences Scripps Institution of Oceanography University of California, San Diego 92093-0244 (858) 822-2483 voice (858) 822-3310 fax Address for Fedex deliveries: Rm 211 Vaughan Hall 8675 Discovery Way La Jolla, CA 92037 ______________________________________________________________ Professor Michael E. Mann Department of Environmental Sciences, Clark Hall University of Virginia Charlottesville, VA 22903 _______________________________________________________________________ e-mail: mann@virginia.edu Phone: (434) 924-7770 FAX: (434) 982-2137 <[3]http://www.evsc.virginia.edu/faculty/people/mann.shtml>[4]http://www.evsc.virginia.e du/faculty/people/mann.shtml Prof. Phil Jones Climatic Research Unit Telephone +44 (0) 1603 592090 School of Environmental Sciences Fax +44 (0) 1603 507784 University of East Anglia Norwich Email p.jones@uea.ac.uk NR4 7TJ UK ----------------------------------------------------------------------------

Jeff Severinghaus Associate Professor of Geosciences Scripps Institution of Oceanography University of California, San Diego 92093-0244 (858) 822-2483 voice (858) 822-3310 fax Address for Fedex deliveries: Rm 211 Vaughan Hall 8675 Discovery Way La Jolla, CA 92037
_______________________________________________________________________ Professor Michael E. Mann Department of Environmental Sciences, Clark Hall University of Virginia Charlottesville, VA 22903 _______________________________________________________________________ e-mail: mann@virginia.edu Phone: (434) 924-7770 FAX: (434) 982-2137 [5]http://www.evsc.virginia.edu/faculty/people/mann.[6]shtml Attachment Converted: "c:\eudora\attach\MannPersp2002.pdf"