Presentation on theme: "1 Presentation to the National Academy of Sciences Expert Panel, Surface Temperature Reconstructions for the Past 1,000-2,000 Years. Stephen McIntyre Toronto."— Presentation transcript:
1 Presentation to the National Academy of Sciences Expert Panel, Surface Temperature Reconstructions for the Past 1,000-2,000 Years. Stephen McIntyre Toronto Ontario Ross McKitrick Guelph Ontario Washington DC, March 2 2006.
2 Guiding Questions (Boehlert) 2) (b) What are the principal scientific criticisms of their [MBH] work and how significant are they? (c) Has the information needed to replicate their work been available? (d) Have other scientists been able to replicate their work? 3) (b) How central is the work of Drs. Mann, Bradley and Hughes to the consensus on the temperature record?
3 Our Answers 2b) Principal Criticisms new statistical methods mined for hockey stick shaped series. These methods were not accurately described Reconstruction failed an important verification test said to have used in the study and this failure was unreported and the statistical skill was misrepresented both in the original article and by the IPCC. Dominant weight was placed on proxies known to be inappropriate temperature proxies and a false claim of robustness was made; The method of confidence interval calculation leads to unrealistically narrow confidence intervals.
4 Our Answers 2c) Information available for replication? No. Many obstructions encountered in replication attempts. The underlying data as used has not always been available. The methodology was not accurately described in the paper and requests for source code were refused.
5 Our Answers 2d) Have others replicated MBH? No. Ammann and Wahl have not confirmed MBH claims of statistical skill and robustness In fact, their code actually confirms our claims that MBH verification statistics are insignificant. Their emulation of MBH is almost identical to ours. Differences between us pertain entirely to interpretation
6 Our Answers 3b) How central is hockey stick? Origin of claims of warmest decade of millennium Relied upon by IPCC and governments Became standard for subsequent studies Results and methods continue to affect papers published today
8 Why was the hockey stick so influential? New Statistical Approach Skill Robustness Confidence Intervals
9 New statistical method PC methods Climate fields PC methods on tree ring networks Novel multivariate method for regression
10 Changes the picture Figure 2: Top – Average of 450 series in MBH98 dataall dataset archived in July 2004. Bottom – MBH98 reconstruction.
11 Bias in Tree Ring PC method Data decentered against post-1902 mean Preferentially adds weight to hockey stick- shaped series in PC1
12 Questions about multivariate methodology Sui generis method with unknown statistical properties No literature on estimating confidence intervals
13 Skill claims The reconstruction failed an important verification test (r 2 ) said to have been used in the study. This failure was not reported The statistical skill was misrepresented both in the original article and by the IPCC.
14 Claim in MBH98 β [or RE] is a quite rigorous measure of the similarity between two variables, measuring their correspondence not only in terms of the relative departures from mean values (as does the correlation coefficient r) but also in terms of the means and absolute variance of the two series. For comparison, correlation (r) and squared- correlation (r2) statistics are also determined.
15 Claim in IPCC IPCC TAR WG1, Pages 133, 136 …they [MBH] estimated the Northern Hemisphere mean temperature back to AD 1400, a reconstruction which had significant skill in independent cross-validation tests. Self- consistent estimates were also made of the uncertainties…. Taking into account these substantial uncertainties, Mann et al. (1999) concluded that the 1990s were likely to have been the warmest decade, and 1998 the warmest year, of the past millennium for at least the Northern Hemisphere.
17 Robustness – warnings about bristlecones IPCC 1996 the extent to which multidecadal, century and longer time-scale variability is expressed can vary, depending on the length of individual …series that make up the chronologies and the way in which these series have been processed to remove non-climatic trends. In addition, the possible confounding effects of carbon dioxide fertilization needs to be taken into account when calibrating tree ring data against climate variations Biondi et al. (1999) [Bristlecone] are not a reliable temperature proxy for the last 150 years as it shows an increasing trend in about 1850 that has been attributed to atmospheric CO2 fertilization (Graybill and Idso 1993)
18 Claims about robustness MBH98: the long-term trend in NH is relatively robust to the inclusion of dendroclimatic indicators in the network, suggesting that potential tree growth trend biases are not influential in the multiproxy climate reconstructions. (p. 783, emphasis added.) Mann et al. 2000: possible low-frequency bias due to non-climatic influences on dendroclimatic (tree-ring) indicators is not problematic in our temperature reconstructions…Whether we use all data, exclude tree rings, or base a reconstruction only on tree rings, has no significant effect on the form of the reconstruction for the period in question. (http://www.ngdc.noaa.gov/paleo/ei/ei_nodendro.html, emphasis added.)
19 Dominant PC weight on Bristlecones 70 series in the North American AD1400 network 15 are Graybill-Idso bristlecone/foxtail pines In MBH method they account for 93% of PC1 weights; PC1 assigned 38% explained variance Under conventional (covariance) PC method they fall to PC4 and get < 8% explained variance
20 Role of Bristlecones Removing them eliminates the hockey stick shape from PCs and final reconstruction Contradicts robustness claim
21 Role of bristlecones From Manns FTP site (CENSORED folder)
23 Confidence Intervals Confidence intervals based on calibration-period residuals not verification-period residuals, thus understating actual uncertainty
24 Red Zone autocorrelation properties … Modeled as ARMA(1,1), AR1 ρ > 0.9 in all cases and sometimes >0.97. Above figure: left- AR1 coefficient ρ; right – MA1 coefficient.
25 Durbin-Watson Statistic All multiproxy reconstructions as archived, except MBH99, fail Durbin-Watson statistic (minimum 1.5). Passing a DW test is a necessary but not sufficient test of model validity.
26 Cross-Validation R 2 Statistic Multiproxy reconstructions show decreased and insignificant cross-validation R 2 statistics. BJ01 does not have MWP results and is reported not to verify after 1960. Left – calibration; right – verification R 2.
27 Replication MBH results are not limited to a squiggly line: they include claims of skill and robustness Ammann and Wahl claim to have replicated MBH results but they have only confirmed ours
28 Replication: Statistical Skill Ammann and Wahl code yields the same conclusion as ours There is no support for MBH skill claims
30 Replication: Robustness Ammann and Wahl code shows results hinge on bristlecones There is no support for MBH claims of robustness
31 Problems with other studies Replication Robustness Proxy selection Proxy validity Lack of independence of authorship and proxies
32 MBH outputs used in other studies PC methods used in Mann and Jones (2003), Jones and Mann (2004) PC series used in Rutherford et al. (2005) NOAMER PC1 used a few weeks ago in Osborne and Briffa (2006, Science)
34 Why do VZ differ? Did PC on decentered correlation matrix and not on decentered data matrix: only the same if centered Assumed >0.3 proxy correlation to temperature
35 The Divergence Problem: large population of 387 temperature- sensitive sites
36 The Explanation Briffa et al. (1998b) discuss various causes for this decline in tree growth parameters, and Vaganov et al. (1999) suggest a role for increasing winter snowfall.… In the absence of a substantiated explanation for the decline, we make the assumption that it is likely to be a response to some kind of recent anthropogenic forcing. On the basis of this assumption, the pre-twentieth century part of the reconstructions can be considered to be free from similar events and thus accurately represent past temperature variability. [Briffa et al. 2002]