Mann et al. [2007a] (hereafter M07a) test the climate field reconstruction (CFR) method known as regularized expectation maximization (RegEM) using pseudoproxies derived from millennial simulations of past climate. These simulations were derived from two General Circulation Models (GCMs) driven with natural and anthropogenic forcings: the National Center for Atmospheric Research Climate System Model (CSM) [Boville et al., 2001] and the Hamburg Atmosphere-Ocean Coupled Circulation Model (ECHO-g) [Legutke and Voss, 1999]. There has been some discussion about the amplitude of millennial changes in simulations from these two GCMs [Goosse et al., 2005; Mann et al., 2005; Osborn et al., 2006; González-Rouco et al., 2006], particularly with regard to how it may impact the assessment of CFR methods in pseudoproxy experiments [Mann et al., 2005; Mann, 2007; Zorita et al., 2007; Mann et al., 2007a, 2007b]

González-Rouco, Jesús Fidel

Smerdon, Jason E.

Zorita, Eduardo

Columbia University Academic Commons

Comment on ‘‘Robustness of proxy-based climate field reconstructionmethods’’ by Michael E. Mann et al.Jason E. Smerdon,1 J. Fidel Gonza´lez-Rouco,2 and Eduardo Zorita3Received 27 October 2007; revised 8 February 2008; accepted 17 July 2008; published 19 September 2008.Citation: Smerdon, J. E., J. F. Gonza´lez-Rouco, and E. Zorita (2008), Comment on ‘‘Robustness of proxy-based climate fieldreconstruction methods’’ by Michael E. Mann et al., J. Geophys. Res., 113, D18106, doi:10.1029/2007JD009542.[1] Mann et al. [2007a] (hereafter M07a) test the climatefield reconstruction (CFR) method known as regularizedexpectation maximization (RegEM) using pseudoproxiesderived from millennial simulations of past climate. Thesesimulations were derived from two General CirculationModels (GCMs) driven with natural and anthropogenicforcings: the National Center for Atmospheric ResearchClimate System Model (CSM) [Boville et al., 2001] andthe Hamburg Atmosphere-Ocean Coupled CirculationModel (ECHO-g) [Legutke and Voss, 1999]. There has beensome discussion about the amplitude of millennial changesin simulations from these two GCMs [Goosse et al., 2005;Mann et al., 2005; Osborn et al., 2006; Gonza´lez-Rouco etal., 2006], particularly with regard to how it may impact theassessment of CFR methods in pseudoproxy experiments[Mann et al., 2005;Mann, 2007; Zorita et al., 2007;Mann etal., 2007a, 2007b].[2] Mann et al. [2005] argued that pseudoproxy testsperformed with the ECHO-g ERIK1 millennial integration[von Storch et al., 2004] were jeopardized by the fact thatthe simulation suffered from an initialization drift in itsearly centuries and that it did not include aerosol forcingsduring the 20th century; by contrast, the CSM integrationused by Mann et al. [2005] was drift corrected and included20th-century aerosol forcings [Ammann et al., 2007]. Sub-sequent studies and discussions [Zorita et al., 2007; Mannet al., 2007a, 2007b] have clearly demonstrated, however,that differences between the ERIK1 and CSM modelintegrations have little impact on reported pseudoproxytests. Nevertheless, there is an obvious importance to thesediscussions that goes beyond the context of pseudoproxyexperiments and it is essential to accurately represent thesimilarities and differences between the two sets of modelresults within the public record. The purpose of thiscomment is to point out that M07a have misrepresentedthe mean Northern Hemisphere (NH) time series of theERIK1 simulation and an accurate comparison between theCSM and ECHO-g simulations is therefore compromised.Furthermore, this mistake affects the verification statisticsthat have been reported for the RegEM CFR method, aswell as exaggerate the impact of detrending on RegEMreconstructions that use the ERIK1 simulation.[3] In Figure 1a we plot our calculation of the NH timeseries for the ECHO-g ERIK1 millennial integration, as wellas the ERIK1 time series plotted by M07a in their Figures 4and 5 and auxiliary material Figure S9. As in M07a, ourcalculated time series in Figure 1a has been referenced to theperiod 1900–1980 C.E. and has been filtered with the samedecadal low-pass filter used by M07a. Also consistent withM07a, we calculate the area-weighted result for the restrictedsubset of the 5 grid in the NH over which Mann andRutherford [2002] report there to be sufficient coverage ofinstrumental observations since the mid-19th century. Thestraightforward interpretation of the comparison in Figure 1ais that the ERIK1 time series from M07a depicts consider-ably more variability in the simulation than exists in the timeseries that we have calculated.[4] Upon the original drafting of this comment, we werenot able to identify the source of the discrepancy reportedherein. In their reply, however, Rutherford et al. [2008]associate the differences with the interpolation method usedto transform the T30 Gaussian grid for the ECHO-gsimulation (3.75 latitude-longitude resolution) to a 5latitude-longitude grid; this discrepancy does not affect theCSM data, which were interpolated by a different means[Rutherford et al., 2008]. We note here that our results inFigure 1 are calculated for a 5  5 interpolation of theoriginal ECHO-g data using a bilinear interpolationscheme. While there are many interpolation methods fromwhich to choose, it is important to note that the chosenmethod should yield an interpolated grid that closelycompares to the originally resolved data. In Figure 1bwe compare the area-weighted NH mean time seriescalculated from the originally resolved ERIK1 data andour interpolated 5  5 grid. The two time series areessentially identical, as expected for a viable interpolationmethod and a final grid resolution that is not significantlydifferent from the original. A similar result would presum-ably not be achieved for the interpolated grid used byM07a. The simple demonstration in Figure 1b thereforesuggests that our interpolation is a least better than thatused by M07a, if not implicating that the latter method andresulting grid are plainly incorrect.[5] Perhaps the most important implication of the com-parison shown in Figure 1a is the fact that the M07a ERIK1JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, D18106, doi:10.1029/2007JD009542, 20081Lamont-Doherty Earth Observatory, Columbia University, Palisades,New York, USA.2Departamento de Fı´sica de la Tierra, Astronomı´a y Astrofı´sica II,Universidad Complutense, Madrid, Spain.3GKSS Research Center, Geesthacht, Germany.Copyright 2008 by the American Geophysical Union.0148-0227/08/2007JD009542D18106 1 of 3time series displays a larger amount of variability thanactually exists in the simulation (for the purpose of com-parison, Figure 1a is plotted over the temperature rangeused for the CSM data in M07a and therefore does not showa few of the largest temperature anomalies in the M07aERIK1 series). This misrepresentation of the ERIK1 timeseries therefore depicts larger differences between the CSMand ECHO-g simulations. In Figures 1c and 1d we plot thearea-weighted NH time series for the CSM and ERIK1millennial integrations, using the restricted (669 grid cells)and full (1296) NH domains, respectively. The plots indi-cate that there are indeed differences between the twointegrations, particularly in the early centuries when ERIK1is warmer. The overall ranges of the mean time series arenot that different, however, and they can be easily plottedwith the same vertical scaling. By contrast, Figures 4 and 5of M07a plot the CSM IK1 time series over differentranges because their calculated ERIK1 time series rangesfrom 1.51 K to 0.97 K (far exceeding the range of thecorrect time series from 0.99 to 0.55 K).[6] In conclusion, we note that the technical issue dis-cussed herein does not impact the basic conclusions ofM07a regarding the performance of the RegEM CFRmethod. Nevertheless, several important implicationsshould be noted. First, the verification statistics in Table 1of M07a are weakened when an appropriate interpolationscheme is adopted; these numbers should be corrected toavoid hindering efforts to reproduce the M07a results and toprovide consistent evaluations of the RegEM CFR methodacross different model integrations. Second, the effect ofdetrending the target data, while clearly significant, isvisually amplified in Figure 5b of M07a because the targetNH time series will not range as far from zero as is presentlyshown. We expect that a corrected figure for the ECHO-gFigure 1. Comparison of: (a) the actual time series for the ECHO-g ERIK1 simulation for the restricted669 grid-cell region of the NH and the time series shown in M07a; (b) the full area-weighted NH meantime series using the original ECHO-g grid (3.75 grid point distance) and the interpolated 5 latitude-longitude grid used in all other representations within this text; (c) the correct time series for the ECHO-gERIK1 and CSM simulations for the restricted 669 grid-cell region in the NH; and (d) the correct timeseries for the ECHO-g ERIK1 and CSM simulations for all grid cells in the NH. All time series have beensmoothed with the same decadal low-pass filter used in M07a and referenced to the 1900–1980 C.E.interval. Figure 1 should be compared with M07a Figures 4, 5, and S9.D18106 SMERDON ET AL.: COMMENTARY2 of 3D18106data would look more similar to the result shown in theM07a Figure 5a for the CSM result. Finally, we reiteratethat the misrepresentation noted herein depicts a notablylarger variability in the ERIK1 simulation. Given that therange of variability in this ECHO-g integration has been thesubject of much criticism in previous publications by Mannand coauthors [e.g., Mann et al., 2005; Mann, 2007; Mannet al., 2007b], it is important that accurate representations ofthese simulations are maintained. Contrary to what isimplied by the time series shown in M07a, Figure 1 andseveral previous publications [e.g., Goosse et al., 2005;Zorita et al., 2007] clearly demonstrate that the ECHO-gand CSM simulations exhibit a comparable mean NHtemperature range. Furthermore, we would be remiss ifwe did not point out that this same misrepresentation ofthe ERIK1 mean NH time series appears in several otherpublications of which we are aware [Mann, 2007; Mann etal., 2007b]. It is essential that these misrepresentations aredocumented in order to avoid equivocal interpretations ofthe ECHO-g data and related results.[7] Acknowledgments. This research was supported by the U.S.National Oceanic and Atmospheric Administration by grantNA07OAR4310060 (J.E.S.), by project SPECT (CGL2005-06097/CLI)and the RyC Program, by the Spanish MEC (J.F.G.-R), and by theEuropean project Millennium European Climate (contract 017008 GOCE;E.Z.). LDEO contribution 7194.ReferencesAmmann, C. M., F. Joos, D. S. Schimel, B. L. Otto-Bliesner, and R. A.Tomas (2007), Solar influence on climate during the past millennium:Results from transient simulations with the NCAR Climate SystemModel, Proc. Natl. Acad. Sci. U. S. A., 104, 3713–3718, doi:10.1073/pnas.0605064103.Boville, B. A., J. T. Kiehl, P. J. Rasch, and F. O. Bryan (2001), Improve-ments to the NCAR CSM-1 for transient climate simulations, J. Clim.,14, 164 – 179, doi:10.1175/1520-0442(2001)014<0164:ITTNCF>2.0.CO;2.Gonza´lez-Rouco, J. F., H. Beltrami, E. Zorita, and H. von Storch (2006),Simulation and inversion of borehole temperature profiles in surrogateclimates: Spatial distribution and surface coupling, Geophys. Res. Lett.,33, L01703, doi:10.1029/2005GL024693.Goosse, H., T. J. Crowley, E. Zorita, C. M. Ammann, H. Renssen, andE. Driesschaert (2005), Modelling the climate of the last millennium:What causes the differences between simulations?, Geophys. Res.Lett., 32, L06710, doi:10.1029/2005GL022368.Legutke, S., and R. Voss (1999), The Hamburg Atmosphere-OceanCoupled Circulation Model ECHO-g, Tech. Rep. 18, Dtch. Klimarechen-zentrum, Hamburg.Mann, M. E. (2007), Climate over the past two millennia, Annu. Rev. EarthPlanet. Sci., 35, 111–136, doi:10.1146/annurev.earth.35.031306.140042.Mann, M. E., and S. Rutherford (2002), Climate reconstructions using‘‘Pseudoproxies,’’ Geophys. Res. Lett., 29(10), 1501, doi:10.1029/2001GL014554.Mann, M. E., S. Rutherford, E. Wahl, and C. Amman (2005), Testing thefidelity of methods used in proxy-based reconstructions of past climate,J. Clim., 18, 4097–4107, doi:10.1175/JCLI3564.1.Mann, M. E., S. Rutherford, E. Wahl, and C. Ammann (2007a), Robustnessof proxy-based climate field reconstruction methods, J. Geophys. Res.,112, D12109, doi:10.1029/2006JD008272.Mann, M. E., S. Rutherford, E. Wahl, and C. Ammann (2007b), Reply tocomments on ‘Testing the fidelity of methods used in proxy-based re-constructions of past climate’ by Zorita et al., J. Clim., 20, 3699–3703,doi:10.1175/JCLI4172.1.Osborn, T., S. C. B. Raper, and K. R. Briffa (2006), Simulated climatechange during the last 1000 years: Comparing the ECHO-g general cir-culation model with the MAGICC simple climate model, Clim. Dyn., 27,185–197, doi:10.1007/s00382-006-0129-5.Rutherford, R., M. E. Mann, E. Wahl, and C. Ammann (2008), Reply toComment on ‘Testing the fidelity of methods used in proxy-based recon-structions of past climate’ by Mann et al., J. Clim., 20, 3699–3703,doi:10.115/JCLI4172.1.von Storch, H., E. Zorita, Y. Dimitriev, F. Gonza´lez-Rouco, and S. Tett(2004), Reconstructing past climate from noisy data, Science, 306,679–682, doi:10.1126/science.1096109.Zorita, E., F. Gonza´lez-Rouco, and H. von Storch (2007), Comments to‘Testing the fidelity of methods used in proxy-based reconstructions ofpast climate’ by Mann et al., J. Clim., 20, 3693–3698, doi:10.1175/JCLI4171.1.J. F. Gonza´lez-Rouco, Departamento de Fı´sica de la Tierra, Astronomı´a yAstrofı´sica II, Universidad Complutense, Madrid E-28040, Spain.(fidelgr@fis.ucm.es)J. E. Smerdon, Lamont-Doherty Earth Observatory, Columbia University,Palisades, NY 10964, USA. (jsmerdon@ldeo.columbia.edu)E. Zorita, GKSS Research Center, Max Planck Strasse 1, D-21502Geesthacht, Germany. (Eduardo.Zorita@gkss.de)D18106 SMERDON ET AL.: COMMENTARY3 of 3D18106

Comment on "Robustness of proxy-based climate field reconstruction methods" by Michael E. Mann et al.

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