Dec 1, 2000 - Solar cycle lengths and climate: A reference revisited. Peter Laut. Department of Applied Chemistry, Technical University of Denmark, Lyngby.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. A12, PAGES 27,489-27,492,DECEMBER 1, 2000
Solar cyclelengthsand climate: A referencerevisited Peter Laut
Department of AppliedChemistry, Technical University of Denmark,Lyngby
JesperGundermann DanishEnergyAgency,Copenhagen
Abstract.Anarticle published byFriis-Christensen andLassen [1991]appeared toindicate an association betweensolarcyclelengths(SCLs)andclimate.It attracted worldwideattentionand
hassincebeenextensively referred to.Weherepresent anupdated analysis usinga recent temperature reconstruction withthetimeperiodof comparison considerably expanded. The correlation isfoundto beweak.In thelightof thisnewresultweanalyze thequestion howthe articlebyFriis-Christensen andLassen wasabletocreate theimpression of a 'strikingly good agreement', astheauthors described it. We showthatthemainreason isanunacceptable mixingof filteredandnonfiltered datain thegraphical representation. Hereby,anartificialagreement of the solardatawiththeglobalwarming since1970wasestablished. ThearticlebyFriis-Christensen andLassen hascreatedandstillcreates confusion bothin scientific andpublicdiscussions on climatechange. We havetherefore foundit relevant to deliverthepresent analysis.
1.
Introduction
An articlepublishedby Friis-Christensen and Lassen[ 1991] (hereinafterreferred to as FCL91) appearedto indicate an association betweenthe lengthof the solarcycleand climate.It attractedworldwideattentionand has since been extensively referredto. Searchesperformedin the ScienceCitation Index Expandedof the Institutefor ScientificInformationshowthat FCL91 hasbeencited 152 timesfrom 1991 to January2000 and at presentis citedonceeverysecondweek.In particular, Figure2 is often presentedas strongevidencethat solaractivityplaysa dominantrole in determiningterrestrialtemperatures. However,a carefulanalysisof Figure2 of FCL91 showsit is misleading. The correlation between solar cycle lengths (SCLs) and terrestrialtemperatureswas again investigatedby Lassenand Friis-Christensen [1995] (hereinafter referred to as LFC95). Figure 6 in this paperagain presentsa comparisoncoveringthe time period 1579-1980. Figure 6 (like Figure 2 of FCL91) conveysthe impressionof a 'strikingly good agreement'.The agreementappearsto be so goodasto rule out any anthropogenic contributionto the global temperatureincreaseof the last 140 years.However,a carefulanalysisleadsto the oppositeresultand revealsthat also Figure 6 of LFC95 is misleading.As shownby Laut and Gundermann[1998], this is mainly becausethe two temperatureseries which are joined together in the graph [Grovemanand Landsberg,1979a, 1979b, 1979c;Joneset al., 1986;dones,1988] are displayedusingdifferentscales.They are definedrelative to differentreferenceperiods,and the necessary zero-point adjustmenthas not been undertaken.If the two temperatureseriesare properly.joinedtogether,the sameanalysis shows that the agreementis much improved if a man-made contributionto the global warmingis assumed,which roughly concurswith the assessment of the Intergovernmental Panal on Copyright2000 by theAmericanGeophysical Union. Papernumber2000JA900068. 0148-0227/00/2000JA900068
$09.00
Climate Change. The article by Lassen and Friis-Christensen [1995] also containsan updatedversionof Figure 2 of FCL91, whereanothertemperaturecurvehasbeenemployedandthe two nonfiltered SCLs have been removed [Lassen and FriisChristensen1995]. This givessomeimprovement.
The mainreasonfor deliveringthe presentanalysisis thatthe
originalarticle[Friis-Christensen andLassen,1991]hascreated, and still creates,much confusionboth in scientificand public discussions on globalclimatechange.We havethereforeseenit asan obligationto clarifythe matter.
2.
Comparing Solar Cycle Lengthsand Terrestrial Temperatures
Figure l a showsa comparisonbetweenSCLs and Northern Hemisphere landtemperatures. It showsthe samedataasFigure 2 of FCL91.The SCL curveconsists of (1,2,2,2,1)-filtered values exceptfor the lastfourpoints(1-4) whichcorrespond to partially filtered (1-2) and nonfiltered(3-4) values.These four values shouldnot havebeenincludedin Figure2 of FCL91, sincethey conveythe false impressionthat the valuesof SCL reflect the global warmingwhich has occurredsince 1970 and which has attractedconsiderable public attention.Figure lb showshow a proper presentationof the availablephysicaldata would have looked. Figure l c showsthat the nonfilteredSCLs oscillateso violently that it doesnot make senseto includethem in the same curve as the (1,2,2,2,1)-filteredSCLs. The mixing of data is
actuallybrieflymentionedin FCL91 butwithoutindicatingthat a spectacular trend therebyis addedartificiallyto the SCL curve. Points a-d show the fictitious
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SCLs which had to
materializein the futurein orderto justifythe inclusionof points 1-4 in the curveof (1,2,2,2,1)-filteredpoints.Figure l d is an updatedproperpresentation of the physicaldata.We noticethat the two new SCLs (pointsa' and b') differ considerablyfrom their implicitlyanticipatedvalues(pointsa andb in Figure1c). In orderto determinethe last SCL in Figure 1d (point b') we have used the predicted epoch for the next sunspotmaximum (July/August 2000) offered by the International Space
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Figure 1 (a) Showsthe samedataas Figure2 of FCL91. The thin line is thetemperature curve,andthe bold line is the solar cycle length (SCL) curve, consistingof (l,2,2,2,1)-filtered valuesexceptfor the last four points (1-4) which correspond to partiallyfiltered(1-2) and nonfiltered(3-4) values.Thesefour valuesshouldnot have been includedin Figure 2 of FCL91, sincethey conveythe false impressionthat the SCLs reflectthe global warming whichhasoccurredsince1970 andwhichhasattractedconsiderable publicattention.(b) How a properpresentation of the physicaldata would have looked.(c) Showsthe samedataas Figure2 of FCL91, but with the nonfiltered SCLs added(dottedline), both the valueswhich had been observedat the time of publicationof FCL91 and the fictitiousvalues(a-d) which were implicitly assumed,i.e., the SCLs which shouldhave occurredin the future in order to justify the inclusionof points 1-4 in the (l,2,2,2,1)-filtered curve.We noticethat the nonfilteredSCLs oscillateso violentlythat it doesnot makesenseto includethem in the samecurveas the (1,2,2,2,l)-filtered SCLs. We alsonoticethatthe fictitiousfutureSCLsshouldhavebeenextraordinarily shortin orderto justify the predicted filteredvalues1-4. (d) Updatedproperpresentation of the physicaldata.The SCLs corresponding to the minimum epochwhich was observedin 1996 (a') and the maximumepochwhich is predictedto occurin mid-2000 (b') are included.We noticethat the two new SCLs(a' and b') diftbrconsiderably from their implicitlyanticipatedvalues (a andb). Points1' and2' arethe properly(1,2,2,2,1)-filteredSCLswhichreplacethe partiallyfilteredpoints1 and 2 in Figure l a.
Environment Service (at http://www.ips.gov.au/mail-lists)and the National Oceanic and AtmosphericAdministration(ftp:/ftp. ngdc.noaa.gov/STP/SOLA R_DATA/S UN SPOT_NUMB ERS /sunspot.predict.November 1999). These predicted sunspot numbersare computedusingthe methodof McNish and Lincoln [1949] andmodified usingregressioncoefficientsand meancycle values computed for earlier cycles starting with Cycle 8. Accordingto the implicit assumptions in Figure2 of LFC91 this sun spot maximumshouldalready have occurredin April/May 1998 in orderto justify point2 in Figure 1a.
We concludethat the impressiveagreementbetweenthe solar curve and the steeprise in terrestrialtemperatures experienced since 1970 is solely due to thesefour points 1-4 in Figure l a. Therefore the agreementmust be regardedas an artefact. Nevertheless,in FCL91 this artificialupgoingtrendsince1970 is explicitlymentionedasan importantpartof the 'closeassociation between the two curves'.
Figure2 compares ( 1,2,2,2,1)-filteredSCLs[LassenandFriisChristensen,1995] with a smoothedNorthern Hemisphere temperatureseries for the period 1400-1990, obtained by
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Figure2. Mann/Hadley Northern Hemisphere (NH) teJnperatures compared to solarcyclelengths, (1,2,2,2,1)filteredandnonfiltered.The solid line showssmoothed NH temperatures for the period1400-1990,obtainedby
combining thereconstruction ofMannetal. [1998]witha modern series issued bytheHadleyCentre (Combined land air and sea surface temperatureanomaliesfor the Northern Hemisphere1951-1998, at
http://www.meto.gov.uk/sec5/CR_div/Tempertr/lsst_vals_nh.html). The boldline with solidcirclesshowsthe (1,2,2,2,1)-filtered minimum-minimum SCLsasgivenbyLassen andFriis-Christensen [1995]withthemostrecent SCLadded. Thescaling of thesecyclelengths is obtained by linearregression, i.e.,byfittingthefilteredSCLsto thetemperature curvesoasto obtainthelowestpossible meansquare deviation. Thedottedlinewithsolid diamondsshowsthe individualnonfilteredminimum-minimumSCLs with the last two pointsmarked3 and a'
corresponding to Figures l a andl d. Thenonfiltered maximum-maximum cyclelength whichis alsoshown in Figure 1a aspoint4 isheremarked byaplussign.Therange ofFigure 2 ofFCL91ismarked bytherectangle. The Sshapes ofthetwocurves canberecognized. It wastheaddition ofthetwononfiltered SCLs(points 3 and4) and thesubsequent scaling upof oneS shape intotheotherwhichcreated thegoodagreement of thetwocurves in Figure2 of FCL91.
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Figure3. The(1,2,2,2,1)-filtered minimum-minimum solarcyclelengths plotted against theGaussian-smoothed Mann/Hadley temperatures ofthesame years. Thestartandtheendoftheseries aredenoted bythecorresponding years. Thesolidstraight lineshows thelinearregression e•nployed inFigure2.
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LAUTAND GUNDERMANN. COMMENTARY
combiningthe reconstruction for the period 1400-1980 of Mann disagreement with the findingsof Friis-Christensen and Lassen et al. [1998] with an updatefor the period 1851-1998 issuedby [1991]. The centralfigure(Figure2 of FCL91)in theirpaper the Hadley Centre (Combinedland air and sea surfacetempera- conveys theimpression of a strongcorrelation betweenSCLsand ture anomalies for the Northern Hemisphere 1951-1998, at NorthernHemisphere land temperatures. The impression is, http://www.meto.gov.uk/sec5/CR_div/Tempertr/lsst_vals_nh.htmhowever, misleading andmainlydueto anunacceptable mixing 1). The Mann et al. temperaturesare chosento coverthe period of (1,2,2,2,1 )-filteredandnonfiltered dataintoa single curve. 1400-1850 and the zero-point-adjusted Hadley Centre temperatures the period 1851-1990.The curvesare fitted by linear Acknowledgments. MichelBlancthanksJoanna Haighandanother referee fortheirassistance in evaluating thispaper. regression.
Figure3 showsthe (1,2,2,2,1)-filtered SCLsplottedagainst thecorresponding Mann/Hadley temperatures shownin Figure2. In order to avoid the effect of year-to-yearvariationsthe temperatures havebeensmoothed by applyinga Gaussianfilter with a width of 15 years,whichgivesa smoothing roughly comparable to the (l,2,2,2,1)-filteringof the SCLs.In this case, wherethe periodof comparisonis 439 years,the considerable width of the (1,2,2,2,1)-filteris moreappropriate thanthat in
References
Friis-Christensen, E., and K. Lassen, Lengthof the solarcycle:An indicatorof solaractivitycloselyassociated with climate,Science, 254, 698-700, 1991.
Groveman, B. S., andH. E. Landsberg, Simulated Northern Hemisphere temperature departures 1579-1880,Geophys. Res.Lett., 6, 767-769, 1979a.
Groveman,B. S., and H. E. Landsberg, Reconstruction of Northern
Figure2 of FCL91. From both Figures2 and3 it is evidentthat Hemisphere temperature: 1579-1880,Publ. 79-181,Univ. of M., CollegePark,Md., 1979b. the correlationbetweentemperature andSCL is weak. B. S., andH. E. Landsberg, Dataappendices to publication One may now ask the questionhow the 'strikinglygood Groveman, 79-181,Pubi.79-182,Univ.of M., College Park,Md., 1979c. agreement'of Figure 2 of FCL91 was obtained.It is due to the surfaceair temperature variations: Recent combined effectof threefactors:(1) The misleading additionof Jones,P. D., Hemispheric trendsandanupdateto 1987,d. Clim.,1,654-660,1988. thefourpointsmarked1-4 in Figurela. (2) The smallnumberof Jones, P.D., S.C. B. Raper,R. S.Bradley, H. F. Diaz,P.M. Kelly,andT. characteristic detailsin the two heavilysmoothed curves.They M. L. WigIcy, NorthernHemisphere surfaceair temperature variations: 1851-1984, d. Clim.Appl.Meteorol.,25, 161-179,1986. had actuallyonly a certainresemblance of an S shapein K., andE. Friis-Christensen, Variability of thesolarcyclelength common.This pitfall is avoidedin Figure2 of the presentwork Lassen, duringthe pastfive centuries and the apparent association with by choosing a longertimeperiod.(3) The strongpotentialof the terrestrial climate, d. Atrnos. Sol.Terr.Phys.,57(8),835-845,1995. appliedmethod( linearregression) to makedifferentcurveslook Laut,P., andJ. Gundermann, Solarcyclelengthhypothesis appears to alike. This is due to the employment of arbitraryscalingand support the IPCC on globalwarming, d. Atrnos. Sol. Terr. Phys., displacement of thecurvesin orderto makethemagree. 60(18) 1719-1728,1998. In Figure 2 the S shapesof the two curves still can be Mann,M. E.,R. S.Bradley, andM. K. Hughes, Global-scale temperature recognized, but now the scalinghasto takeaccountof the large patterns andclimateforcingoverthepastsixcenturies, Nature,392, number of characteristic details which are contained in the time
779-787, 1998.
Prediction of sunspot numbers, Trans. periodchosenhere,whichis muchlongerthantheoneemployed McNish,A. G., andJ. V. Lincoln, Am. Geophys.Umon,30, 673-685, 1949. in Figure2 of FCL91. The obtainable agreement is seento be poor.
3.
Conclusion
J. Gundermann, DanishEnergyAgency,Amaliegade 44, DK-1256 Copenhagen, Denmark. (jgu@ens.dk)
P. Laut,Department of AppliedChemistry, Technical University of Denmark, Building 377, DK-2800 Lyngby, Denmark.
The comparisonof a smoothedNorthern Hemisphere (ifakpl@pop.dtu.dk)
temperatureseries for the period 1400-1990 with (1,2,2,2,1)filteredsolarcyclelengthsindicatesthat the correlationbetween (Received July5, 1999;revisedMay 1, 2000; SCLsandNorthernHemisphere temperatures is weak.This is in accepted May 1, 2000.)
