TheHolocene 2,3 (19921pp.205-277

Climatic change over the last millennium in reconstructed from ttee-rings E. Cook*,T.Birdt, M. Petersono,M. Barbettif,B. Buckleyn, R. D'Arrigo*and R. Francey** (*Lamont-DohertyGeological Observatory, Palisades, New York 10964,USA; fTasmania Tradesand Labor Council, New Town, Tasmania7008, ; "TasmaniaForestry Commission,Hobart, Tasmania7001, Australia; iUniversity of Sydney,Sydney, New South Wales2006, Australia; x*CSlRO Division of AtmosphericResearch, Mordialloc, Victoria 3195,Australia)

ReceivedL4 January1.992; revised manuscript accepted 26 May 1992

Abstract Tree-ring widths from millennium-old Huon pine (Lagarostrobosfranklinii) trees have been used to reconstructwann season(November-April) temperaturesback to AD 900 for Tasmania.The reconstructionindicates that the most recent 100 years of climate have been highly unusual,with the coldest and warmest 25-year periods occurring from 1890 to l9L4 and 1965 to 1989, respectively. Although the most recent 25-year period is warmer than any comparable period over the past 1090 years,it is not yet statisticallyunprecedented. Some evidencefor the Medieval Warm Period and the 'Little Ice Age' can be found in the reconstruction, especiallyduring the twelfth and seventeenth centuries respectively.However, the latter is weakly expressedcompared to many records from the northern hemisphere,suggesting that the southernoceans may havesignificantly moderated its effect on Tasmania.Regular oscillationson the order of 30, 56, 80 and 180years in length have been identified in A the reconstructionusing spectralanalysis. These oscillationscannot be easilylinked to climatic forcing HOLOCENE functions related to solar or volcanic activity. Thus, they are best regardedas arisingfrom the internal RESEARCH PAPER dynamicsof the ocean-atmosphere-cryospheresystem.

Key words: climatic variations,tree-rings, Lagarostrobos franklinii,Tasmania, Medieval Warm Period, Little Ice Age, spectral analysis.

IntrodUCtion One of the few regions of the Southern Hemisphere that has the potential for millennia-long, annual tree-ring chron- Comparedto the availableinstrumental and historical climate ologies is Tasmania,with its temperate rainforests of long- records of the Northern Hemisphere (e.g., Lamb, 1977),the lived conifers.This palaeoclimaticresource was evaluatedin Southern Hemisphere has relatively few high-resolution cli- the 1970s(Ogden, 1978a,btDunwiddie and LaMarche, 1980), matic time series that extend back more than 100 years but never fully exploited. Recently, Cook et al. (199I) re- (Barry, 1-978).Yet, to properly evaluaterecent climatic trends ported on the development of a climatically-sensitiveHuon and fluctuations (e.g., Hansen and Lebedeff, 1987), much pine (LagarostrobosfrankliniiC.J. Quinn) tree-ring chronol- longer recordsare needed(Bryson, 1974).To fulfill this need, ogy from Tasmania that extends back to AD 900. They long paleoclimaticrecords with annual to decadaltime scale demonstratedthat the yearly variations in ring width corre- resolution must be developedfor the Southern Hemisphere. lated well with changes in warm-season(i.e., November- Tree rings (e.g.,LaMarche,1975; Norton et a1.,1989;Villalba, April) temperature. They then went on to draw inferences 1990) and ice cores from high snow accumulationrate areas about the signiflcance of the present-day warming trend (e.g., Morgan, 1985;Thompson el al.,1985; Aristarain et al., apparentin the seriesand speculatedabout the occurrenceof 1990) have demonstratedthe capacity to produce such re- the 'Little Ice Age' (LIA) and Medieval Warm Period cords.However, exceptfor Antarctica and the high Andes bf (MWP) in Tasmania.However, they did not derive quantita- South America, the availability of suitable ice cores is ex- tive estimatesof past temperature from that record. In this tremely low. This essentiallyleaves tree-rings as the primary paper,we describemore fully the developmentand character- source of high-resolution paleoclimatic information in the istics of this tree-ring chronology and extend the analysesof temperatesouthern latitudes. Cook et al, (199t) to the actual reconstructionof Novernber-

@ Edward Amold 7992 206 The Holocene 2 (1992)

April averagetemperatures for Tasmaniaback to AD 900.In especiallyin subalpine environments.However, it is reason- so doing, we will provide the first well-verified estimatesof able in this climatically-temperateregion to expect radial past temperature changein the Australia-New Zealand sec- growth to commence sometime in late spring (i.e., No- tor of the SouthernHemisphere that extend back through the vember-December) and end in mid- to late-summer (i.e., LIA and MWP as broadly defined by Lamb (1965), Lamb February-March), with final cell wall thickening and lignifica- (1977), and Grove (1988).This researchbuilds upon earlier tion probably occurring in the early autumn months. tree-ring reconstructionsof climate for Tasmania(Campbell, A total of 56 radii from 23 Huon pine trees were used in 1982;LaMarche and Pittock, 1982) and New Zealand (Nor- developing the tree-ring chronology. These included incre- ton, 1987; Norton et al., 7989), but none of these records ment cores from 17 living trees and samplesfrom six dead extend back before 1730. trees killed by a fire in 196L.The oldest tree dated back to AD779. However, for purposes of dendroclimatic analysis and reconstruction,the final chronologywas truncatedbefore The Huon pine tree-ring data AD 900where the samplesize dropped below three trees and sevenradii. The Huon pine ring widths were rigorously cross- The tree-ring data used here were collected from a disjunct dated and measured to a precision of 10.01 mm following stand of subalpineHuon pine growing at an elevation of 950 established methods (Fritts, 1976; Pilcher, 1989). By the metres near Lake Johnston on Mount Read in western convention used to date Southern Hemisphere tree-rings Tasmania (Figure L). Other tree speciesassociated with this (Schulman,1956), the calendaryear assignedto each annual stand are deciduous beech (Nothofagusgunni), pencil pine ring was the year in which radial growth began,e.9., the ring (Athrotaxus cuppressoides),King Billy pine (A. selaginoides), formed in the 1988/89growing seasonwas dated as L988. the putative hybrid A. laxifulia and Diselma archeri, a Among the key yearsused for cross-datingwere a number of 'light shrubby conifer that achievessmall tree status on Mount significantlynarrow rings, numerous rings' (Filion er al., Read. AII of these associatedspecies are natural endemicsin 1986)characterized by poorly lignified, low-densitylatewood 'frost the subalpine forests of western Tasmania. However, until bands,and several rings' (LaMarche and Hirsghboeck, recently, Huon pine was thought tq grow only in more L984) characterized by cell wall damage caused by the temperate rainforestsbelow 700 m elevation (Gibson, 1986). formation of ice crystalsin the newly'formed tracheids.These This belief was shatteredby the discoveryof high-elevation, key yearsare listed in Table L. They do not indicate any clear subalpine standsnear FrenchmansCap (Hickey and Felton, associationwith known explosive volcanic eruptions as has 1988) and Lake Johnston (Peterson, 1990). Although in a been suggestedin some Northern Hemisphere tree-ring re- subalpinelocation, the Lake Johnstonstand has a fully closed cords (LaMarche and Hirschboeck, 1984;Filion et aL.,t986). canopy and extremely dense understory vegetation. Huon Nor is there much evidence for the joint occurrence of pine is extremely shade-tolerantand can exist as a suppressed narrow, light, and frost rings exceptby chancealone. understory tree for decades,if not centuries.Its reproduction Figure 2 showsa plot of the average,smoothed ring widths is principally vegetative although seedlingscan occasionally from AD 900 to 1988 with nonparametric 95% confidence be found. Little is known about the phenologyof Huon pine, intervals estimated using the bootstrap (Cook, 1990). Be- neath it is a plot showingthe changein samplesize per year. important propertiesof the ring- I 45' t46' t47" 148' Theseplots illustrate several width data used in developing the standardized tree-ring chronologyfor climatic reconstruction.With regardsto non- 100km climatic growth trends related to tree size, age,and canopy status, there is little evidence for the classical negative (Fritts 41" exponential trend in ring width et a1.,1969), except perhaps after 1400. The overall growth trend is broadly convex with a peak in growth occurring around 1300.This pattern of growth justifies the use of more data-adaptive methods of tree-ring standardizationas advocatedby Cook (1987), specifically the use of the cubic smoothing spline (Cook and Peters, 1981). In this case,it is apparent that a smoothingspline with a fixed 50% frequencyresponse cutoff 410 of 500 years will remove little else than the trend in growth attributable to changing tree size, age, and canopy status. With regard to sample size,the maximum replication occurs in the middle of the data and is fairly constantfrom 1300to 1900.The decreasein sample size after 1900 is due to two factors: the inclusion of dead trees in the compositethat all end no later than 1958,and severegrowth suppressionin the 1898-1913period in some living trees that made their ring widths, in that interval, undatableand unmeasurable.Prior to 1300,the decline in samplesize is simply due to the decreas- ing number of very old trees in the sample. For the purpose of later climatic interpretations, it is apparent that the Huon pine trees rarely experiencedpro- I45" 146" 147' l4U' longed periods of below- or above-averagegrowth. Most growth fluctuationswere relatively small and on the order of Figure 1 Map of Tasmaniashowing the locationsof the tree-ring site The most notable exceptionis in by Lake Johnstonon Mount Read (star) and the climate stationsused 10 to 40 years in duration. for dendroclimatic modelling and reconstruction (LHL: the 940-1000period when averageradial growth achievedits ). maximum.However, this is also an interval of low replication E. Cook et ql.:Tree-rings and climaticreconstruction in Tasmania 207

* Table 1 Key yearsused in cross-datingthe Huon pine tree-ring seriesback to AD 900,The indicatesespecially notable narrow and light rings. Prior to AD 900 the number of tree-ring specimensis too small to provide a reliable list of key years.

Narrow rings: 919 94L 1006 1010* 1032 1054* 1081* 1086* Lrr2 1130 L1,43 tl44 115s 1158* 1184 It93* 1203 1301 1318 r32t 1344* 1351 L379 1403 1,4t1 L444* L448 L457 L474 t542 1606* L61,4 l6t9 1,626 L646 1650 1663* r69L 1700 r7r7 t728* 1745* I /ff L183 1786x 181.4 1849 1880 1890 1898* 1908* 1911 1928* 1951*

Light rings: 9L8 955 964 1007 1015 1017 1050 1086 1088 1104* 1110 7120 1131 L139 1183 L193 1,199 1207 1270 732'l 1367 1390x L41,5* L419* 1,432 L448 1449* 7450* 1453* t460 t4't4* 1505 1509* 151'1* 1529* 1584 1.606* 1.609* 1641 1653* 1,662* 1674 1709* 1723* 7728 1744* 1749* rt tJ 1836* t843 1867 1918

Frost rings: 1080 1155 1187 r277 1379 1361, r737 1785 1848

and quite young trees. So, while the rapid growth in the trend, the variance in each ring-width serieswas independ- 940-1000period may be reflecting unusually good climatic ently stabilized (i.e., made homogeneousin time) using a conditions, it may also be reflecting other conditions not data-adaptivepower transformation before detrending.This directly associatedwith climate. In fact, the 95o/oconfldence was accomplishedwith an automatic,one-step procedure that intervals in that period are wider than in any other period usesthe relationshipbetween spread and level to estimatethe since AD 900, making it the least'reliable for interpretation. best power transformation (Emerson and Strenio, 1983).It is Perhapsthe most striking fluctuation in Figure 2 is the growth basedon the premise that the local spread (in this casethe surge that has been in progress since 1965. The average standard deviation, S) is proportional to the power of the growth rate of the past25years is approximatelyequal to that local level (in this casethe mean,M) of the ring widths. This when the treeswere 300 to 400 yearsyounger. This is unusual model can be expressedas years is clearly for trees that are typically over 700 old and S: cW (1) inconsistentwith the long-term negative growth trend since 1400. logS:logc*blogM (2) Tree-ring standardization or, lettingk:logc, logS: k+blogM (3) The Huon pine ring-width series were standardized for dendroclimatic analysisin a novel way. Instead of allowing which is a simple linear regressionin logarithmic space.If b is the tree-ring indexingprocedure (i.e., the division of eachring the slope of the spread-versus-levelrelationship, then width by its estimated growth trend value) to stabilize the p --l-b is the appropriatevalue of tn-eexponent used for variance simultaneously with the removal of the growth the power transformation (Emerson and Strenio, 1983).Be-

LAKEJOHNSTON HUON PINE MEANRING WIDTHS AND SAMPLE SIZE 0.5

E 0.4 E I 0.3 F o

(5 = 50 E 0.1 25N 0 900 1100 1300 1500 1700 1900 YEAR

Figure 2 Lake JohnstonHuon pine mean ring-width seriesin millimetres and samplesize (N). The annualdata havebeen smoothed with a 10-yearsmoothing spline to highlight the multiyear fluctuations.The dashedlines are 95% confidencelimits estiniated using the bootstrap method. The heavy solid line describesthe general shapeof the growth trend believedto be largely nonclimaticin origin. 208 The Holocene 2 (19921

causetree rings are time series,the estimation of the local growth, the yearly growth variations about that level tend to mean and standard deviation requires a windowing proce- be symmetricallydistributed. This meansthat with a defined dure. In order to make the windowing aslocal aspossible and, growth minimum of zero for any given level, a tree growing at therefore, useful for short as well as long series,estimates of a fasterrate hasa wider'corridor'within whichit canrespond the l,o local mean and standarddeviation were computed as to climatic effectsthan does a slower growingtree. Thus, the variance in ring width increasesin proportion to the mean. M,: (X,+X,-1)12 i = 2... N (4) This property of tree growth does not directly causethe bias describedby Eriksson (1989).Rather, the effect of division on (s) the resultant index is highly nonlinear especiallywhen the estimatedgrowth curve approacheszero. Figure 34, showsa where X; is the ring width for year i. Thus, for eachring-width schematicrepresentation of this functional relationship.For a seriesof length N there are N - 1 estimatesof the local mean constant, hypothetical ring-width value of 1 mm, this plot and standarddeviation. Equations (4) and (5) were motivated shows the change in the ratio (or tree-ring index) as the by the fact that S, is a first-difference, which removes the hypotheticalcurve-flt value variesfrom 3 mm to 0.1 mm. Note effect of local trend on the estimation of the standard that even for ratios below 2, a range that includesmost tree- deviation. S, and M, are also the numerator and denominator ring indices,there is a discernablenonlinear effect.This effect terms usedto estimatethe coefficientof mean sensitivityused can be completely eliminated if the ring widths are flrst in tree-ring research (Fritts, L976). Emerson and Strenio transformedto stabilizethe variance and then residualsused (1983)stressed the use of robust and resistantprocedures for insteadof ratios. estimating level and spread. However, extensive tests on Figure 3B showsthe effect of standardizingthe Huon pine hundreds of tree-ring series indicated that the estimators ring widths in the traditional way (RATIOS) versusthe two- describedhere work very well in practice. stage procedure (RESIDUALS). All other aspectsof the The power transformation of each Huon pine ring-width chronology developmentprocedure were kept constant (i.e., series was estimated in the manner just described. The the 500-yearspline and robust mean were usedin eachcase to optimal power transformation fell in the 0.4-0.8 range for create the averagechronology). However, to keep this com- most series.Then, the transformed series were individually parisonsimple and interpretable,no autoregressivemodelling detrended in program ARSTAN (Cook, 1985) using a was applied in the creation of either series. For ease of smoothing spline with a flxed 50% frequencyresponse cutoff comparisonthe residual-basedchronology was also scaledto of 500 years. Since the variance was already stabilized, the residualsfrom the spline were usedto develop the final mean tree-ring chronology for dendroclimatic purposes. As de- 10 scribedin Cook (1985,1987), this processalso involved the = use of autoregressivemodeling to reduce the autocorrelated =2 \. 8 noise component in the detrended seriesdue to endogenous lrl - VALUE disturbancesor out-of-phasefluctuations. A robust mean was D -- - cuRvE 60 also used to reduce the influenceof outliers in the computa- tion of the mean chronology.The final chronologydeveloped here for dendroclimatic analysis and reconstruction covers 4E the period AD 900-1988. At this point it is worth discussingthe motivation behind 2 using power transformationsto stabilizethe variancein tree- rings prior to detrending.Eriksson (L989)evaluated the linear 0 aggregatemodel of tree-ring seriesproposed by Cook (1985, 80 100 1987) and showed that regressionmodels developed within N that model formulation were theoretically biased.The proof of this bias is beyond the scope of this paper. However, Eriksson (L989)showed that the sourceof bias was in the division processused to compute the tree-ring indices during standardization. If simple residuals from the growth curve a were used instead of ratios or indices,regressions estimated E from the linear aggregatemodel were unbiased. Eriksson = ..1 (1989) performed tests on actual tree-ring and climate data x UJ using both the traditional dendrochronologicalapproach and o more complicated unbiasedregression methods, Her results z indicatedthe potential for substantialbias in regression-based dendroclimatic models that use tree-ring chronologiesstan- dardized in the traditional way. 1900 1920 1940 1960 1980 Eriksson(1989) did not describethe actualsource ofbias in YEAR the tree-ring indexing procedure.However, it is apparentthat one sourceof bias could be related to the inherent asymmetry Figure 3 Potential bias in the tree-ring indexing procedure.The top of radial growth. Ring widths have a defined lower bound of plot is a schematicshowing the nonlineareffect that division can have resultanttree-ring index (RATIO) when the estimatedgrowth zero and an ill-defined, highly variable maximum. This asym- on the curve approacheszero. The lower plot showsthe most recent89 years metry in achievablemaximum and growth minimum servesto of tree-ring chronology estimatedby the traditional indexing proce- create the dependencebetween the local mean and standard dure (RATIO) and the two stage procedure used here (RESID- deviation in the ring widths by roughly defining the overall UAL). Note the systematicdeviation between the RATIOS and range within which a tree can respond to environmental RESIDUALS after 1965, which probably reflects a bias in the effects such as climate. However, for any given local level of traditional tree-ring indexing procedure. E. Cook et ql.:Tree-rings and climaticreconstruction in Tasmania 209 the same overall mean and variance of the ratio-based Figure 4 shows the distribution of monthly mean tem- chronology.Although differencescan be found in other time peraturesand their standarddeviations based on the average periods, only the chronologiessince 1900are comparedhere recordsof Hobart, Launceston,and Low Head. The average to illustrate the bias that is most likely due to the traditional record covers the period 1883-1989and is the same as that standardizationprocedure. Up to about t969 the two series used previously by Cook et al. (1991). Interestingly, the are extremely similar. Thereafter, a clear divergenceoccurs standard deviation tracks the annual cycle such that yearly with the RATIOS chronology systematicallyoverestimating variability increaseswith temperature.This behaviour is the the RESIDUALS chronology by an averageof 0.064 index oppositeof that seenfrequently in mcirecontinental Northern units. This differencemay not soundlike much,but compared Hemispheretemperature records, where winter temperatures to the overall post-1969level of 0.227 index units above the are more variable than summer temperatures.This means mean in the RESIDUALS chronology, the level in the that the November-April temperaturereconstruction we will RATIOS chronologyis 0.291index units above the mean, an describehere reflectschanges in the largestfraction (-60yo) increaseof 28.20/". Since all other proceduresused in creating of the total annual temperaturevariation over Tasmania. thesechronologies were identical, it is reasonableto conclude that the ratio method hasbiased the mean chronologyby that 18 1.3 amount in the post-L969period. The final Lake Johnston Huon pine tree-ring chronology 16 + MEAN 1.2 was estimated using power transformations and residualsin .T- STDDEV O 14 1.1 the ARSTAN methodology.Since it is extremely similar to S- ul (see o the temperature reconstructionestimated from it Figure z 12 1 7), the chronologyitself is not shown.Ffowever, a comparison o ul t-l of Figures 2 and7 indicatesthat the 500-yearspline detrend- = 10 0.9 o ing has not removed any signiflcantlong term variance from the raw data other than the broadly convex growth tiend 0.8 \-r describedearlier. o.7 J FMAMJJASOND Modern climate data MONTH Figure 4 Monthly temperature means and standard deviations for The monthly mean temperature record used for correlation Tasmaniabased on an averageof the Hobart, Launceston,and Low and regression analysis was a regional average of three Head Lighthouse temperaturerecords since 1885. individual station records in Tasmania:Hobart (1883-1989), Launceston (1885-1989), and Low Head Lighthouse (1895-1989).These records were obtained from the Labo- ratory of Tree-Ring Researcharchive of Australian climatic Statistical calibration and veriftcation data developed by V.C. LaMarche and A.B. Pittock in the 1970s(see its use in LaMarche and Pittock, 1982),with more Basedon previouscorrelation analyses (Cook et al.,1991,),a recent data obtained from the Meteorological Office in single 6-month warm seasonaverage, comprising the months Hobart. These stations are centrally located in Tasmania of November through April, was created from the average along a north-south transect (Figure 1) approximately temperaturerecord for regressionmodelling and veriflcation 150km east of the Huon pine site on Mount Read. As such purposes.However, unlike Cook et al. (I99I) who only used they are low-elevation(maximum: 171m) stationssituated on the L895-1989 period common to all three temperature the leeward side of the high mountains of westernTasmania. records, we used the average record back to 1885, which Long temperatureand precipitation recordsfrom more prox- includes at least two stationsin the average.The creation of imal western interior and coastal locations are extremely the November-April seasonresulted in the lossof one year of scarceand discontinuous(e.g., the Waratah, Cape Sorell, and data. Autoregressive prewhitening of the data (see below) Stanleystations ceased operating in the 1970s).Thus, in terms resulted in the loss of an additional year of data. Hence, the of proximity to Mount Read, the Hobart, Launceston, and regressionmodelling and model verification testswere based Low Head .station records are the longest and most con- on data covering the 1887-1989period. tinuous. Even so, a number of missingmonthly valueshad to Simple linear regressionanalysis was usedto transform the be estimated for each station and the Launceston record is Lake JohnstonHuon pine tree-ring chronologyinto estimates actually a suitably adjusted composite of two overlapping of November-April seasonaltemperatures. Prior to regres- records:Launceston Pump Station (1885-1956)and Launces- sion, both the tree-ring and climate serieswere prewhitened ton Airport (1939-1989).These station recordshave not been as order-p autoregressive(AR) processesof the general testedfor homogeneity;the presentlack of other long climate form records of known homogeneityin Tasmaniawould make the interpretation of suchtests problematical anyway. p (6) To determine how well the Hobart-Launceston-Low Head x,: 2 QtX,-ite, monthly mean tbmperaturesreflect changing thermal condi- tions at high elevationsin western Tasmania,we correlated where X, is the time seriesbeing modelled, the $, are the p each month of that record with the respectivemonthly mean autoregressivecoefficients, and e, is tlre series of random temperatures recorded at Butlers Gorge and Lake St Clair shocks or white noiSe residuals unexplained by the AR(p) (1945-1988).Ten of the L2 months and all of the warm season model (Box and Jenkins,1970). It is the e, of tree-ringsand (November-April) monthshad correlationsof 0.90or greater. climate that will be used in developingthe regressionmodel This result indicatesthat the long, low-elevation temperature and subsequenttemperature reconstruction. record can be legitimately used to model the climatic signal The tree-rings were modelled and prewhitened as an in the high-elevationHuon pine tree-ring chronology. AR(3) process,with the AR order determined by the mini- 270 The Holocene 2 (1992)

Table 2 Autoregressivemodelling resultsfor the Huon pine tree-ring chronologyand November-April averagetemperatures for Tasmania.

Series Time span AR(p) Order R2 AR cofficients: 1- p

Huon Pine 880-1988 J 0.2r8 0.397 0.086 0.059 Temperature 188G1989 I 0.105 U.5ZJ

mum AIC procedure (Akaike, 1974). The AR coefficients temperaturedata, a highly significant(p < 0.001)result. Addi- and explained variance of this model are given in Table 2. tional parametric and nonparametric tests also used for Note that the majority of the persistenceis contained in the verification purposes (see below) were highly significant first coefficient(fi:0.397), which basicallyreflects an ex- (p<0.05), with only the inefficient sign test indicating some ponentially-dampedresponse to environmentalinputs suchas weakness.Neither the DW nor the RUNS test indicated any climate. In total, this AR(3) model explained 21,8% of the significantautocorrelation in the residuals.Given thesehighly tree-ring variance.In contrast,the November-April average significantmodel calibration statistics,tree-ring estimatesof temperatures were modelled as an AR(L) processthat ex- temperature were obtained for the L887-7937verification plained 70.50/"of the variance (Table 2). Assuming that the period and comparedto the actual temperaturedata. seasonal temperatures are causally linked to Huon pine The verification results are also shown in Table 3. The I growth, the larger explainedvariance due to autoregressionin verification tests used here include the Pearson product- the tree-rings is probably related to internal physiological moment correlation coefficient (Ro), the sign test (ST), the processesthat preconditionthe trees'responseto climatein product means (PM) test, and the reduction of error (RE) any given year (Fritts, t976).In this particular case,the tree- test, as described in Fritts (1976) and Fritts er al. (1990). ring AR coefficients indicate a positive feedback between Additional tests performed here were the coefficient of climate and tree growth that has resulted in an amplification efficiency (CE), the Spearman rank correlation coefficient and smoothing of the original cimatic signal. However, it is (R5), and a robust correlation coefflcient (Rp) basedon the also clear from the temperatureAR modelling results that a biweight influence curve. The RE and CE can not be tested significant portion of the persistencein the tree rings prob- for statisticalsignificance. However, any positive RE or CE ably originated in the climate systemover Tasmania.There- usually indicates some skill in the regressionmodel. Except forb, the regression of prewhitened temperatures on for the RE and CE, the verification testswere also applied to prewhitened tree rings can not be used alone to reconstruit the calibration period data for purposeof comparison.In the past temperatures.To do so would result in a reconstruction calibration period, the RE and CE are mathematicallyequiv- without sufficient persistencedue to climate. To correct for alent to the R2 statistic,making them redundant. this deficiency, the autoregressionmodelled in the actual Except for the CE,'the verificationperiod testsindicate that temperature data was added back into the climatic re- the regressionmodel is valid. That is, the Ro, Rs, R*, ST, and constructionafter it was estimatedfrom the prewhitenedtree PM statisticspassed with significancelevels well below the rings(Meko, 1981). nominal 5o/oacceptance limit and the RE was positive. Only A split calibration-verificationscheme was used to develop the CE was negative,indicating a lack of skill in the regres- and validate the regression model. For this purpose, the sion model. However, CE is unquestionablythe most difficult prewhitened temperature data were split into two nearly verification statistic to pass on a routine basis.So given the equal 51 and 52 year periods, 1887-7937and 1938-1989,with other strongly positive verification results,a slightly negative the former reservedfor model verification and the latter used CE is not sufficientto reject the regressionmodel. for model calibration (sensu Fritts, 1976). The regression A comparison of the statistics computed for both the results are shown in Table 3. These include the usual coeffi- calibration and verification periods indicatesthat the regres- cient of determination (R2), the degrees-of-freedomadjusted sion model is nearly as good in both time periods.The biggest R2 (AR'z), and two testsfor autocorrelationin the residuals, differencelies in the Ro coefflcientwhich drops from 0.607to the Durbin-Watson (DW) and runs (RUNS) tests. The 0.471.However, the more robust measuresof association,R, regressionmodel R2 explained 36.90/oof the Variancein the and Rp, are both larger in the veriflcation period. This result

Table 3 Calibration and verification resultsof prewhitened November-Apfil averagetemperatures regressed on prewhitenedHuon pine tfee- rings. The statisticsare identified and describedin the text. Probability levelsfor the statisticsare shown in parentheseswhen available.No tests 'not exist for AR3, RE, or CE, but an RE or CE > 0 indicatessome model skill. For DW, ns means significant'(cr = 0.05).

A Calibratianperiod: 1938-1989 RP RS RR ST PM R2 AR2 DW RUNS 0.607 0.515 0.569 31+ 2t- 3.60 0.369 0.356 2.09 -0.410 (<0.001) (<0.001) (<0.001) (0.106) (<0.001) (<0.001) ns (0.341)

B Virificationperiod: 1887-1937 &RSRR ST PM RE (E 0.471, 0.553 0.592 39+ 12- 2.57 0.101 -0.038 (<0.001) (<0.001) (<0.001) (<0.001) (0.006) E. Cook et al.:Tree-nngs and climatic reconstructionin Tasmania 277 indicates that one or more highly anomalous estimates of .20 temperature (i.e., outliers) are responsiblefor the degrada- UI tion of Ro. This interpretation is strongly supportedin Figure o 2.15 5, which shows both the scatterplot and time series plot of .

16 o uJ E 15 f

tr 14 o. = ul F 13

900 1100 1300 1500 1700 1900 YEAR

Figure 7 November-April averagetemperatures reconstructed from Tasmaniantree-rings (AD 900-1989).The heavy solid line is the seriessmoothed with a 25-yearlowpass filter.

both in degreesCelsius and in departuresfrom the mean of latitude (40" to 50"S) sea level pressure associatedwith the calibration period (15.11"C)and the full reconstruction oscillations in wave-number-zero zonal flow (Enomoto, period (14.99'C). There are some clear differences in the 199L).Thus, there appearsto have been a significantreorgan- magnitudesof the anomaliesbecause of the 0.12"Cdifference ization of the southern hemisphere ocean-atmosphgre-cryo- in the means.With this difference,the cold-period anomalies spheresystem in the L890-1914interval. appear magnified and the warm-period anomalies appear The resultsin Table 4 also suggestthe existenceof climate diminishedwhen basedon the 1938-1989period mean.Given regimes(Lorenz,1965) in the reconstruction,i.e., subperiods that the calibration period is biasedby the recent warming, it with distinctly different means. Such behaviour would sup- seemspreferable to use the mean of the full 1090-yearperiod port the conceptof almost-intransitivity(Lorenz 1965,L976), for comparing intervals of below- and above-average in which persistent changesor vacillations about the long- warmth. term mean arise internally from the nonlinear dynamicsof With this considerationin mind. the coldest and warmest the ocean-atmosphere-cryospheresystem. Trenberth (1976) 25-year periods are 189G-1914and 1965-1989,with average has also proposed this possibility in his examination of departuresof -0.33"C and *0.35oC,respectively. Each repre- Southern Hemisphere sea level pressureindices. To charac- sents a departure of at least three standard errors from the long-term mean, which itself has a standard error of only -+0.012'C. Even so, neither 25-year period is statistically Table4 List of theeight coldest and warmest 25-year periods in the unprecedentedbecause each has a standard error that over- temperaturereconstruction. The periods are ranked from most laps with that of other warm or cold periods in Table 4. In this extremeto leastextreme. DEP1 are departuresfrom the 1938-1989 sense,the recent warming is still within the natural back- calibrationperiod mean of 15.11"C;DEP2 arethe departuresfrom ground variability of climate over Tasmania. Regardless,it the AD 900-1989reconstructed mean of L4.99"C.The * indicates clearly reflectsa signiflcanttemperature increase that extends O"nuta"resthat are at least two standarderrors from the over much of the Southern Hemisphere as documented by H:;:. both land (Joneset aL.,1986)and marine (Folland et a1.,1984) instrumental temperature records. The past L00 years of climate over Tasmania also appear to have been unusually Cold periods variable and extreme compared to most earlier 100-year Interval Mean SD STD DEP1 DEY} periods. However, the 120-year period spanning AD 944- ERR -0.45* 1065contains two of the coldest(1011-1035, 1041-1065) and 1890-1914 1.4.66 0.534 o.lw -0.33* -0.M* -0.32* warmest (94+968, 975-g9g) 25-yearperiods in the record, 1194-121,8 t4.67 0.575 0.115 -0.35* -0.23* making it the most concentrated period of unusual tem- 1278-1302 L4.76 0.307 0.061 1011-1035 t4.77 0.407 0.081 -0.34* -0.22* perature fluctuationsin the reconstruction. 1,604-1,628 1,4.79 0.376 0.075 -0.32* -0.20* Like the recent warming, the 1890-1914 period, cold which 1,M5-1,469, 14.79 0.374 0.075 -0.32* -0.20* was especiallysevere from L898to L908,apparently reflects a 1041-1065 14.81 0.331 0.66 -0.30* -0.18* much broader pattern of unusual cold in the mid- to high- 1,664-'1,688 14.85 0.406 0.081 -0.26* -0.L4 southern latitudes, It includesfour of the nine poorest years of accumulatedgrowing degree-dayssince 1870 in Canter- Warm periods bury, New Zealand (Salinger, 1979),'which was part of the lnterval Mean SD STD DEP1 DEP2 coldest period (1900-1935)in recorded New Zealand history ERR (Salinger and Gunn, 7975).lt is also coincidental with unu- 1965-t989 75.34 0.329 0.066 +0.23* +0.35* +0.13* sually cold winters at Punta Arenas, Chile (Lamb, 1977),afi 975-999 L5.24 0.252 0.050 +0.25x 147Gt500 75.23 0.323 0.065 +0.I2 +0.24x was apparently preceded by a circumpolar expansion of 1L69-1.L93 1.5.22 0.414 0.083 +0.11 +0.23* Antarctic pack ice and the irruption of numerous icebergs 1855-1879 15.18 0.290 0.058 +0.07 +0.19x into normally ice-free low latitudes (Burrows, 1976). Other 94+968 L5.17 0.269 0.054 +0.06 +0.18* characteristicsof this anomalousperiod were a decreasein 1115-1139 15.16 0.298 0.060 +0.05 +0.1.7* scalar wind speed in the Tasmania-New Zealand sector 1808-1832 15.16 0.346 0.069 +0.05 +0.17* (Fletcher et al., 1982) and ciroumpolar changes in middle E. Cook et al.:Tree-rings and climatic reconstructionin Tasmania 213 terize this kind of behaviour in the temperature reconstruc- 1990),but only to the degreethat the overall 900-1300period tion, a step-functionmodel was fitted to the series,in which experiencedintervals of unusual warmth in each case.The stepswere addedsequentially until a minimum AIC (Akaike, specific timing of the warm episodes are highly variable 1974) was achieved.In so doing, the duration of each fitted betweenregions, however, with the warmestperiod being 900 step was allowed to vary betweenL0 and 100years long. The to 1100in Fennoscandia,1150 to 1400in the PolarUrals, L100 result is shownin Figure 8, which showsthe sequenceof fitted to 1300in the White Mountains,and 1040to 1150 in the steps and, for comparison, the Z1-year low-pass filtered Andes. This variation in timing suggeststhat no simple reconstructionfrom Figure 7. The number of stepsis 35, with forcing function or model will easily explain the MWP as a a mean duration of 31 yearsand a range of 1,0to 99 years.This global phenomenon. model explains19.8olo of the variance,a level quite compara- Estimates of when the LIA occurred are highly variable, ble to that explainedby the low-passfiltered curve. The step- with beginningdates as early as 1250and ending datesas late function model reveals some putative, regimes that lend as 1850 (Grove, 1988).A scrutiny of Figure 7 indicates that support to the concept of almost-intransitivityin the climate only two time periods are reasonablecandidates within that system.The most striking examplesare found prior to 1300 interval: the seventeenthcentury and the mid-flfteenth cen- (e.g., 90G-998,999-L066, 1,t65-1193,1194-7236) and after tur!. Three of the eight coldest 25-year periods fall in the 1850( 18ss-1896, 1897-L974, t9L5-7966,1967 -1989). Most of LIA, with a mean anomaly of -0.18'C. These events repre- these regimes,or level changes,have t-statistic probabilities sent only 75 years out of about 50G-600candidate years. of 1.0-3or less.Other regimesappear to be less well defined Consequently,the LIA, as reflected by warm seasontem- (e.g., 1087-11 74, 1255-1303, 147 7 -1494), but most again have peratures over Tasmania,appears to have been a relatively relatively small (<10-2) Type 1 error probabilities. These minor event in that part of the Southern Hemisphere. This results do not prove that the Southern Hemisphere climate finding may reflect the moderating influence of the southern systemis almost-intransitive.However, they do indicate that oceanson climate over that small island. Even so, there is temperature changes over Tasmania can be abrupt and some interesting correspondencebetween the generally cool sustained. seventeenthcentury in Tasmania and other regions of the The length of this temperaturereconstruction allows for an world. In Fennoscandia,the coldest period in the record is assessmentof two notable climatic events of the past 1000 found in the t570-7750 interval (Briffa et a1.,1992); in the years:the Medieval Warm Period (MWP) and the 'Little Ice Polar Urals. unusual cold is indicated from 1600 to 1650 Age' (LIA). These events have been largely described in (Graybill and Shiyatov, 1989); in the White Mountains, terms of the Northern Hemisphereexperience (Lamb, 1965; below-averagetemperatures is inferred for the seventeenth Grove, 1988).However, it is unclearjust how globally perva- century (LaMarche, 1974); in the Patagonian Andes, pro- sive they were. Figure 7 providessome clues as to the possible longed below-averagetemperatures are indicated over the occurrencesof thesewarm and cold eventsin Tasmania. 1530-1660period (Villalba, 1990);and in Alaska, unusual The MWP, with a nominal time coverage of AD 900 to summer cold is reconstructedfor the 162U1710period (Ja- 1300 according to mostly European records (Lamb, 1965, coby et a1.,7985).Compared to the MWP, the timing of the 7977), may be reflected in the reconstruction by above- seventeenthcentury LIA effect is more consistentbetween averagetemperatures in the 94G-1000and 1100-1200inter- regions,although there is still some significantvariation. An vals. Indeed, four of the eight warmest Z1-year periods in exhaustivereview of northern hemisphereinstrumental, his- Table 4 fall in the 400 candidateyears of the MWP, with a torical, and proxy climatic records by Williams and Wigley -r0.21"C. mean anomaly of However, the warm tenth and (1983) likewise found that summer temperaturesduring the twelfth centuries were apparently split by an anomalously seventeenthcentury were unusually cold. While these find- cold period in the eleventhcentury. The generaltiming of the ings do not necessarilyimply the existence of a common MWP in Tasmania agrees reasonably well with summer forcing function, they do make an inquiry into this possibility temperature restructions from Fennoscandia(Biffa et al., more tenable. 7992),thenorth Polar Urals of Russia(Graybill and Shiyatov, The cool seventeenthcentury is broadly coincidental with 1989),the White Mountains in California (LaMarche, 1974), the Maunder Minimum period of reduced sunspot activity and the northern PatagonianAndes in Argentina (Villalba, around L64U1710(Eddy, 7977) and comparativelyhigh vol-

16

o S- ul E f k 1E E lr| o. = UJ

14 900 1100 1300 1500 1700 1900 YEAR

Figure E A step-functionmodel fitted to the 1090-yeartemperature reconstruction.For comparison,the 25-yearlowpass filtered chronologyis superimposed.This result indicates the tendencyfor temperature'regimesl in the reconstruction. 274 The Holocene2 (1992\ canic activity (Hammer et al.,1980). Either of theseforcings, The BT spectrum was estimated from 363 lags of the alone or in combination, could be at least partly responsible autocorrelation function and smoothed with the Hamming for below-averagetemperatures during that time. There is window. With this number of lags(-1i3 the serieslength), the also an approximate correspondencebetween the 1445-t469 spectrumhas high resolution,but somewhatlow stability.The cool period and the Spdrer Minimum around 140G-L510;and number of degreesof freedom per spectralestimate is 10.The earlier yet, the twelfth century warm period and the Medieval 95oloconfidence level of the spectrumwas estimatedfrom a 'red Maximum around 7I2U1280 (Eddy, 1977). Solar forcing is noise' first-order Markov null continuum based on the the easiestto test for in the Tasmaniantemperature record Iag-1autocorrelation of the time series(Mitchell et al.,1966). becausesunspot activity is highly periodic. If solar variability Strictly speaking, this is an a priori test that can only be is influencing climate over Tasmania,then there should be applied to spectralfrequencies selected prior to the analysis. some evidencefor this effect in the power spectrum(Jenkins In this sense,the 11.,22, and 90-yearsunspot periods qualify and Watts, 1968) related to the 11-year sunspot cycle, the for testing becauseof their a priori expectation.However, as Z2-yearHale solar magneticcycle, and/or the 90-yearGleiss- will be seen,'significant'spectralpeaks at other frequencies berg cycle. were discovered,which probably reveal some real features Spectrat analysiswas applied to the Tasmaniantempera- about the climate system over Tasmania. Thus, the 95o/" tuie reconstruction using two different methods:Blackman- confidencelevel will also be used as an exploratory tool for Tukey (BT; Jenkins and Watts, 1968)and maximum entropy discoveringunexpected phenomena in the spectrum. (ME; Marple, 1987). The BT method is useful becauseit The ME spectrumwas estimatedfrom a 200-termpredic- offers a statisticalsignificance test for spectralpeaks (Mitchell tion error filter (PEF) using the modified covariancealgo- et al., 1966). However, its resolution is somewhat limited rithm of Marple (1987). The length of the PEF (-1l5 the especially in the lower frequencies. In contrast, the ME serieslength) is somewhatconservative in order to reducethe method offers excellent resolution, but no simple means of number of potentially spurious peaks at medium and high assessingthe significance of the spectral peaks. Since no frequenciesin the spectrum. spectral analysismethod is ideal, the BT and ME methods Figure 9 shows the BT and ME spectra estimated as just compliment each other when used together and produce a described.In the BT spectrum,peaks that exceed the 95o/o more accurate appraisal of the spectrum (Berger et al., confidencelimit are identified by period; the sameis done in Le91_). the ME spectrumfor peaks that appear to stand out locally.

an BLACKMAN.TUKEYSPECTRUM H .02 55.8 AUTOCORRELAT]ONS= 363 E o ul f '01 o IJJ - 5.3 o. - s-.78 r.17 ,.P, o i n - "'t"

0.1 0.2 0.3 0.4 FREOUENCY]N CYCLESPER YEAR

20

o O MAXIMUMENTROPY SPECTRUM 16 PREDICTIONERROR FILTER = 200 H 5.6 = 6 12 30.2 IJJ s8 o H4 att 3.16 2.75

0.1 0.2 0.3 0.4 0.5 FREOUENCYIN CYCLESPER YEAR

Figure 9 Blackman-Tukey (BT) and maximum entropy power spectra of the full reconstruction.The a priori 95oloconfidence level of the BT spectrum(dashed curve) is derived from the red noise null continuum (smooth solid curve) estimatedfrom the lag-1 autocorrelationcoefficient. Each estimateindicates the presenceof multi-decadaloscilla- tions with averageperiods of about 30, 56, 80, and 175-180years. E. Cook et ql,:Tree-nngs and climatic reconstructionin Tasmania 275

In most cases,the identifi.edpeaks agree between spectra. reconstructedwarm-season temperatures over Tasmania.The With regards to the a priori ll, 22, and 90-year solar precise cause of these oscillations and why they persist periodicities,none are evident.This result indicatesthat solar through time remainsunclear. forcing is not a significantfactor in determining warm season 'sig- temperaturesover Tasmania.However, there are other nificant' peaks in the spectrathat may reflect a tendencyfor Conclusions oscillations in the climate system.There are several peaks that fall within the preferred frequency band (periods of21 Old-growth Huon pines from a subalpine site in western years) of the El Niflo/Southern Oscillation (ENSO). Un- Tasmaniahave produced one of the longestannual records of fortunately, the ENSO appearsto influence Tasmanian cli- past temperature for the Southern Hemisphere. The re- mate only weakly (Ropelewski and Halpert, 1987),although construction, based on tree-ring width variations, extends there may be a strongereffect in mid- and upper troposphere back to AD 900 and reflects changesin warm season(No- levels (Drosdowsky and Williams, 1991).The more provoca- vember-April) averagetemperatures. The reconstructionin- tive peaksare found in the lower frequencies.Each spectrum dicatesthat the most recent 100 years of climate have been indicates the presenceof periodicities or oscillations in the higlily unusual,with the coldest and warmest25-year periods series at periods of approximately 30, 56, 80, and L80 years. occurring from 1890 to 191,4and 1965to 1989,respectively. None of these periods were anticipated,yet all four exceed Although the most recentZ'-year period is warmer than any the a priori 95% confidencelevel in the BT spectrum and comparable period over the past 1090 years, it is not yet clearly stand out in the ME spectrum. statistically unprecedented. Therefore, it is premature to Oscillations on the order of 20-30 and 55-65 years have claim that the recent warming over Tasmaniais a responseto been documented in much shorter hemispheric and global the greenhouseeffect. It is evidencein support of that theory, temperaturerecords (c.1851-present), both from land and but it is not yet proof of its existence. marine sources(Folland et a\.,1984;Newell et a\.,1989;Kane Some evidence for the Medieval Warm Period and the and Teixeira, 1990;Ghil and Vautard, 1991).In somecas'es, a 'Little Ice Age'can be found in the reconstruction,especially 20-22 year cycle has been described and attributed to the during the twelfth and seventeenthcenturies, respectively. Hale solar magnetic cycle (Newell et al.,1989) or changesin However, the latter is weakly expressedcompared to many extratropical ocan circulation (Ghil and Vautard, L991). records from the Northern Hemisphere, suggestingthat the However, the very existenceof such bidecadaloscillations in southern oceansmay have significantly moderated its effect hemispheric and global temperature records has recently on Tasmania. been questioned (Elsner and Tsonis, 1991). Our spectral Regular oscillations on the order of 30, 56, 80, and 180 analysesdo not indicate an obvious solar cycle influence on years in length have been identifled in the reconstruction climate over Tasmania,but they do suggestthe existenceof using spectralanalysis. Similar 20-30 and 4040 year oscilla- other, presumably internal, climatic oscillations.This differ- tions have also been identifled in temperature and sea level ence may relate to the much smaller climatic region repre- pressurerecords covering the past 130 years. These oscilla- sentedby this study. Alternately, the -2Z-year cycle found in tions can not be easily linked to climatic forcing functions the relatively short hemisphericand global temperature re- related to solar or volcanic activity. Thus, they are best cords may be fortuitous or reflect an internal oscillation that, regardedas arising from the internal dynamicsof the ocean- over the long-term, has an averageperiod closer to 30 years. atmosphere-cryospheresystem. How suchregular oscillations In fact, a spectralanalysis of the most recent 130 yearsof the can persist over long periods of time is a mystery. Yet, they reconstruction indicates an oscillation of 26.1 years,which is appear.tobe significantcontributors to the overall variability closer to the 20-22 year cycles described by Newell er a/. of climate over Tasmania. These multidecadal sources of (1989), Kane and Teixeira (1990), and Ghil and Vautard variance need to be better understood if the role of green- (7eer). housegases on climate is to be properly understood. A recent study of long-term sea level pfessure (SLP) data in the 40-50"5zone lendsfurther credenceto the existenceof the 20-30 and 4G{0 year oscillationsin the southern hemi- sphere.Enomoto (1991) found that distinct changesin SLP Acknowledgements occurred in the 1890s.1920s, and 1950s.These he attributed to fluctuations in the overall strength of the zonal westerlies This researchwas possible through support by the Climate associatedwith the symmetric, wave-number-zeroflow re- Center of Lamont-Doherty Geological Observatory, the gime around Antarctica. The timing of theseSLP fluctuations Global ChangeProgram of the National Oceanicand Atmo- agreeswell with temperature changesover Tasmania,espe- sphericAdministration, Grant NA 98AA-D-AC199, and the cially in the L890-L910interval. In addition, he found evi- Geologic Record of Global ChangeProgram of the National dence for a 4040 year SLP oscillation associatedwith the ScienceFoundation, Grant EAR-91-04922.We alsothank the development of asymmetric, wave-number-onecirculation TasmaniaForestry Commission and the C.S.I.R.O. Division around Antarctica.Thus, Enomoto's (1991)study provides of Forestry and Forest Products in Hobart, Tasmania for circumstantialevidence for a physicallink betweenthe South- logistical support. Lamont-Doherty Geological Observatory ern Hemisphere climate system and oscillations found in ContributionNo.4977.

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Editon in Ghief Announcinga new ioutnal... Paul J. Valdet Departmentof Meteorology University of Reading 2 EarleyGate Whiteknights,Reading BerkshireRG6 2AU, LIK Judith Totman Parrish Da,ta, a,nd Modelling D@artnentofhciences Goud-SimpsonBuilding The pdmary aim of Palaeoclinatesis to provide a focus for all researchersworking in the interdisciplinary Tte Univasitycf Arizona subjectarea of palaeoclimatologryThe field of palaeoclimatologyhas e>rpandedrapidly over the past Tbcson,Adzona 85721, LJSA decade,as attentionhas becomeincreasingly focused on environmentalproblems. Fundamental AssociateEditors questionson such topics as the natural variabilityof our environment,which may have strong A.K. Behrensmeyet(USA) past M.A. Chandler(USA) implicationsfor the futureof our planet,can only be answeledby the studyof ciimates. T.J. Crowley (USA) publish high guality papers on both the geologicaland climatologdcal R. Crineo(Argentina) Palaeoclimateswill a balance of K B. Follni (Switzerland) aspects of palaeoclimateresearch. It wil be unique in bringing together palaeoclimatemodelling and (Australia) L. Frakes publication geologistsand modellerseach aware of the details, A Hallam (IlK) data studies in a single and by making C.M.Janis ruSA) strengthsand limitations of the others' approacheswill enhanceand fr:rther ioint research. K, Kelts (USA) R.J. Oglesby(USA) Palaeoclimateswill include original researchpapers on all aspeclsof the climate and envtonment of the (USA) B. Otto-Bliesner Ouaternaryand earlier times. Discussionand review paperswill also be included, concerningnew W. Preu(USA) R.A. Spicer((X) aspectsof researchand specialtopics of cuuent and widespreadinterest to the palaeoclimate F.A. Street-Penott(UK) C. Summerhaym(UK) E. Ttuswell (Australia) A cau for papers - the Editorsin Chiefwelcome contributions to the iournal Pleasecontact either the (Australia) P. Vickers-Rich Editors or the publisherfor Notes for Contributors J. wiedmann (Germany) v.P. wright (uK) No page chargesa Free colour illustrationswhen necessarya 25 Frce Ieprints of each article

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