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CLIMAT RESEARCH Published January 31
Vol238-1,20 Clim Ries
Proxy climatic and environmental changes of the
past 1 00 years
Willie Soon'"2 *, Sallie Baliunas1' 2
Ii~arvard..Smithsonian Center for Astrophysics, 60 Garden Street, MS 16, Cambridge, Massachusetts 02138, USA
records is reviewed. As indicators, the proxi s duly represent local climate. Because each is of a
different nature, the results from the proxy nrdicators cannot be combined into a hemispheric or
global quantitative composite. However, con idered as an ensemble of individual expert opinions.
the assemblage of local representations of climate establishes both the Little Ice Age and Medieval
warm Period as climatic anomalies with worl dwide imprints, extending earlier results by Bryson et
al. (1963), Lamb (1965), and numerous intewening research efforts. Furthermore, the individual
proxes an e ued o adres th qustin 3f whether the 20th century is the warmest of the 2nd
millennium locally. Across the world, manyr cords reveal that the2tcnurispoalntth
warmest nor a uniquely extreme climatic per od of the last millennium.
KEY WORDS: Paleoclimate proxies - immte change Environmental change Little Ice Age
Medieval Warm Period
_______________________Resale orrpublication not Per attend without writtefl consent Of the publisher
1. INTRODUCTION be found to support or reject a medieval warming. But
the updated composite tree-ring summer temperature
Are the Little Ice Age and Medieval Warm Pei iod curve in Fig. 1 of Briffa (2000) shows evidence for an
widespread climatic anomalies? Lamb (1965) write, anomalously warm interval from about 950 to 1100 in
'IMlultifarious evidence of a meteorological na ure the northern high-latitude zone, which coincides with
from historical records, as well as archaeological, Lamb's Medieval Warm Period. Also, a similar early
botanical and glaciological evidence in various parts warm period appears prominently in the averaged
of the world from the Arctic to New Zealand . .. has tree ring chronologies carefully selected and processed
been found to suggest a warmer epoch lasting several from 14 sites spreading over 30 to 70 0 N (Esper et al.
centuries between about A.D. 900 or 1000 and a' out 2002).
1200 or 1300. . . . Both the "Little Optinmnm" in the Those results are but a few of many that have be-
early Middle Ages and the cold epochs li~e. "Little Ice come available since Lamb's analysis. Given advance-
Age"], now known to have reached its culminating ments in retrieval of information from and extension of
stages between 1550 and 1700, can today be substanti- surface coverage for the proxies, we review the accu-
ated by enough data to repay meteorological investi- mulated evidence on regional climatic anomalies over
gation. ... .It is high time therefore to marshal the cli- the last 1000 yr. We also recommend Ogilvie & JMnsson
matic evidence and attempt a quantitative evi~d nce' (2001), who recently provided the most authoritative
(p. 14-15). Research on large-scale patterns of cli ate discussion on the historical development of the long-
change continued with vigor, standing debates on the climatic nature of the Medi-
Jones et al. (1998) tentatively concluded that hule eval Warm Penod and Little Ice Age, especially con-
a Little Ice Age cooling existed, little evidence could cerning the North Atlantic, including Iceland.
'Email: wsoon~cfa.harvard.edu © Inter-Research 2003 www.int-res.corn
90 ClimRa~s 3: 89-110, 2003
2. WORKING DEFINITIONS Question (3) differs from Questions (1) and (2). Ques-
tion (3) looks f or a 50 yr anomaly within the 20th cen-
What are the regional patterns of climatic change tury compared to any other anomaly throughout the
ovrte last 1000 yr? Accurate results could serve as period of a proxy record. Questions (1) and (2) look for
bvenchmak o 0hcnuylblaeaewrinj a 50 yr anomaly within the previously suggested 500
seen in the surface thermometer records and as phys - an 00y ineraspfcthvely M oedea Wharmi Pheriodeand
Cal constraints for theories or mechanisms of clina e Little Ice Agwe, trespectively Noef thatio in the cas more
change on timescales of decades to centuries. Question (3), wetettedfntio ofeat50yr nor more
The proxies used to study climatic change over tl e period of sustained anomaly in the 20tceurnod-
lat100 yr are addressed individually and theretor e ferent from that of any prior century. Thus, if a sus-
locally because they differ in nature too greatly to be tamned warm anomlc eeidniidadapndt
quantitatively averaged or compared. To make prog- 'reside in the Medieval Warm Period and appeared
ress, we consider 3 questions of many individua1 warmer than an anomaly found in the 20th century,
then we would assign 'No' to Question (3). Similarly, a
climate proxies: te2t etr
(1) Is there an objectively discernible climatic anom- proxy record may show both that te2t etr
aly dringthe ittl IceAge interval, defined as anomaly is the most extreme (warmest) and that the
1300-1900? This broad peniod in our definition derives Medieval Warm eideit.I nwrn us
from historical sea-ice, glaciological and geomorpl O- tion (3), the existence of the Medieval Warm Period or
logical studies synthesized in Grove (2001a,b) a d Little Ice Age is not considered, because they are
Ogilvie & Munsson (2001). assessed independently, in Questions (1) and (2).
(2) Is there an objectively discernible climatic ano T- We started with the framework of past researchers,
aly duning the Medieval Warm Period, defined as naey h ugse xs ene a of the Meudiea
800-'1300? This definition is motivated by e.g. Pfistei et Warm Period and Little Ice Age. Thre golof clmthe study
al. (1998) and Broecker (2001), and is a slight modifica- is to deduce the geographical naturerioflimatic nd
tion ofLamb's rigina study 1965).environmental conditions during these peros itn
(3) Is there an objectively discernible climatic an i- guihigite a0t centurase af sheparaters peno ise
aly within the 20th century that is the most extreme largely a practicalba eas fth neeti h
(the warmest, if such information is retrievable) period role of human activity on Earth's climate and environ-
in the record? An important consideration in ans er- ment.
ing this question is to distinguish the cases for w icentetmotntcnieaini tatdr temorary
the20t cetur wamig bganeary i th cntur regional cooling (or shift between wetaddycni
versus after the 1970s, as recorded by surface ther- tion) may ocu0 ndcdl u o utdcdf
omter. Tisis ecesay i tepeatue ca s timescales during the Medieval Warm Period, and
are tobe rlTeds to anthssrypgeic fomprcingeinputske similarly, occasional, short-lived regional warming (or
inreasoed reatmdospei cantrbogndioxieconteinpt, shift between wet and dry condition) may occur during
Anomaly is simply defined as a peniod of more t acn the Little Ice Age, as indicated byGoe201b)
50yr of sustained warmth, wetness or dryness. wi hin Thus the terms Medieval Warm Peniod and Little Ice
the stpltditrafte Medieval Warm Period ,or Age should indicate persistent but not necessarily
a 50 yr Or longer period of cold, dryness or wet ess constant wann ormoolng repetiel , oe broad
within the stipulated Little Ice Age. We define anoi aly areas (see also Stine 1998, Lucma20,Grv
in the 20th century within each proxy in the same vay. 2001a,b, Ogilvie & Jdnsson 2001, Esper et al. 2002).
The urfce nstumetalrecrd f te 2th erury Stine (1998) suggests that more appropriate terms may
contains 3 distinct, multidecadal trends: early-cer tury be 'Little ICe-g lmtcAoay n Mdea
warmngmidcenurycooingandlat-cetur w in- Climatic Anomaly'. it is also noteworthy that the defin-
ing. But that knowledge comes from instrumental her- itions of discrilpsstncimeaoaisfr
momery ithitshig tie resolution and other bni ses, the Little Ice Age and Medieval Warm Period may
whichr prelud at directi coprsnt h r xies include not only distinct changes in the mean but also
(whih hae thir on bases). our goal here is to om- changes in the multidecadal variance(gli ~s
parec thae 2thcentry ownbjetvi thmoeexeded son 2001). Through the microscope of daily and re-
past changes than is available from thermo etry. gional spatial scale variability, it is importn orcg
Give th bisesof achproy, uesion(3) was nize that even the relation between multidecadal mean
aivnsw here byiaskin ifwti each proxy rcr, hereiontemperature and its daily vaniability may undergo sig-
was an earlier (pre-2Oth century) 50 yr interval w~ rmer nificant non-stationary changes(egKnpebrert
(or more extreme, in the case of precipitation) tha iany al. 2001, who document those time-dependent changes
50 yr average within the 20th century. in temperature variability across the United States).
Soon & laliunas: Climatic and eni oninental changes of the past 1000 years 91
Also, from a combination of field evidence and model noinena. could be multi-phased events acting under
ing based on the understanding from synoptic cluna- distinct local and regional constraints and Modes.
toloy, rysn &Bryon 199) dmontraed ow Ca Bradley & Jones (1993) and Hughes & Diaz (1994) initi-
and regional factors (for horizontal spatial distances a ated and strongly hedteve httesphenomena.
small as 100 kin) can produce significantly differe t were not global, but Grove (1996; see espeitoally p.tai
precipitation histories for 2 Near East station to 54) disagrees. However, in the tradtoal aa
(Jersale andIcamshli Syra) and for 2 stations i rich areas of Western Europe or the Northern Atlantic
therCascadem Rangaeof Oyreo (mutiosvrsi s including Iceland and Greenland, both the Little ice
teCoastal-lik ngemic olmt sites).monanosv Age and Medieval Warm Period do exist as distinct
courtaclassificatonofat widespra) aoay ae climate anomalies (Pfister et al. 1998, Grove 200ia,
on multidecadal persistence at many locales rests on Ogilvie & JMnsson 2001). No objective profdsrdt
goo prcednt.Forexaple th moerngloall y the existence of those phenomena in other regions.
averaged surface warming inferred from thermc- Thus, hike other resetarcer (Lamb 1965 Poarterit1986,i
meter readings includes large-scale cooling tren s Grove 1996, rEut eta19) e sart w eithpeva-
over oth he Grenlad/Larador Sea area and t e ously indicated periods of Little Ice AgeadM ivl
easern regont o the Urelnitedb Sae(3to4N80 o Warm Period, and ask whether they are widespread,
1100 W; see Hansen et al. 1999, Robinson et al. 2001) teleconnectedevnstanedotecsriylt
or te Atarticconinet (~g.Dorn e. a. 202) throughout the defined periods. The terms Medieval
the last 30 to 50 Yr. Another example is relati e Warm Period antitec g emanpatical axtndio
warmth duning the Little Ice Age and relative ccol viable, especially considening the Poenta xeso
during the Medieval Warm Period seen in t e of the concept to past and future climatic events that
borehole record of reconstructed temperature at Tay- are 'similar or equivalent' in physical scope (e.g. Bond
lor Dome, Antarctica (77.80S, 158,720E, elevati n et al. 1997, 1999).serneolca
2374 in), compared to results from Greenland's bo e- Current knowledge on the divers raned ofeloalWr
hole (see Glow & Waddington 1999). which do lot climatic behavior suggests that the Mxeediea Warme
show those features. These vaniations are of sh rt Period and the Little Ice Age are not epce ob
duration compared to the anomaly, and of limi ed homogeneous and sustained. To define the beginning
regional extent, and end dates of these climate anomalies requires bet-
ter understanding (for the Little ice Age see Porter
1981, 1986, Kaser 1999, Grove 2001a,b, Luckman 2000.
3. APPROACH Schuster et al. 2000, Winkler 2000, Ogilvie & JMnsson
2001, Henrdy et al. 2002). Imprecise data on the begin-
Table 1 and Figs. i to 3 summarize the answers to he ning and end of both events contributes in part to con-
quetions posed here about local climatic anornales. fusion about the phenomena. For example, Ogilvie &
FrQuestos 1 Ind() we answered 'Yes' if the Farmer (1997) have commented that Lamb's sugges-
proxy record showed a period longer than 50 ylof tion of a Medieval Warm Period mayntbsupre
cooling, wetness or dryness duning the Little Ice Age, by documentary data even for England because theft
and similarly for a peniod of 50 yr or longer for wa in- extensive studies based on a historical dataset showed
ing, wetness or dryness during the Medieval W rmn that England suffered relatively cold winters from 1260
Period. A dash indicates that either there is no ex ,ert to 1360. However, as that period is near the transition
opiion ortha te poxyrecrddoe no coerthe between the Medieval Warm Period and Little Ice Age
perid i qustin. A'Ye?' r 'o?'answer means :hat defined in this study, this fact does not srnl ota
theorigdina qexpert. opiniorn wsYsor o bttti dict our results. Evidence based primarily on glacier
doe no machourcriera; or xamle iftheine rval activity points to both a poorly defined beginning and
of warmth duning the Medieval Warm Period werE too end of the Medieval Warm Period, wieteLtl c
shot b or dfiitin o b 'es' i woldmert V s Age interval seems to have had a gradual beginning
In sverl csesin te 2th entrya 'Yes-' designa- but more abrupt end, Althoughthnoinfte
tIon weeas casesignted fortheanswery toQetin ) in Medieval Warm Period or Little Ice Age with sharply
order to highlight the fact that the 20th century warin- defined transitiosmyb conenin onearit iseprob-l
ing first occurred early in the century, ca. 1920-1950, ably a non-physical construct becausoflreegna
when the aft's content of anthropogenic CO 2 was still differences in the timing of both phenomena. As sug-
cumulatively small, gested by Grove (2001a). an inhomogeneous climate
A global association for the Little Ice Age or pattern (though not necessarily an analog) can already
Medieval Warm Period is premature because proxy be identified in the 20th century warm intervals as
data are geographically sparse and either or both phe- defined by instrumental records.
92 Chlin Res 2 :89-110, 2003
Table 1. A full list Of paleoclimatic proxies that have sufficient length of continuous records to entertain the 3 specifc questions:
(1) Is there an objectively discernible climatic anomaly duin n the Little Ice Age interval (A.D. 1300-1900) in this proxy record?
(2)Is her anobjctielydisernblecliati anmal dnn the Medieval Warm Period (A.D. 800-1300) in this proxy record?
(3) Is there an objectively discernible cliMatic anomaly wurithi the 20th century that is the most extreme (the warmest, if such in-
formation is available) period in the record? A question mark after an answer indicatesucranyoideso.Smblfrth
type of climate proxy used include B: borehole; Cl: cultural; E : documentary; 0: glacier advance or retreat; Gm: geomorphology;
In: instrumental; IS: isotopic analysis from lake sedimentary or ice cores, tree or peat celluloses, corals, stalagmite or biological
fossils; Ic: net ice accumulation rate, including dust or che nial counts; iA: lake fossils and sediments; river sediments; NE: melt
layers in ice cores; Mp: multiproxy (any combination of roxies listed here); Pf: phenological and paleontological fossils;
Worldwide - Mp Mann et al. (1999) Yes No Yes0
Arctic wide - MP Overpeck et al. (1997) Yes - Yes8
Worldwide - MP Crowley & Lowery (2000) Yes No Yes8
Worldwide - MP Jones et al. (1998) Yes No Yes8
Worldwide - T Briffa (2000) Yes No Yes8
Worldwide - T Briffa et al. (2001) Yes - Yes'
Worldwide - T Jones et al. (2001) Yes - Yes8
NHj mid-latitude - T Esper et al. (2002) Yes Yes No
Worldwide - MP Lamb (1977, 1982) Yes Yes -
Worldwide - G + Is Porter (1986) Yes Yes -
Worldwide - G Grove & Switsur (1994) - Yes -
Worldwide - T+G+D Hughes & Diaz (1994) Yes No?b No?b
Worldwide - MP Grove (1996) - Yes -
Worldwide - B Huang et al. (1997) Yes Yes No
Worldwide - D Perry & Hsu (2000c) Yes Yes No
Worldwide - D deMenocal (2001) Yes Yes -
Americas - Ts+Gm+Mp Shine (1998) - Yes -
N. Atlantic (Iceland) 63-660 N 14-24- MP Ogilvie et al. (2000)
Ogilvie & Jdnsson (2001) Yes Yes No
N. Atlantic (S. Greenland) 60-700 N 20-550 Mp Ogilvie et al. (2000) Yes Yes No
W. Europe 45-540 N 0-15- MP Pfister et al. (1998) Yes Yes No
N. Atlantic (Europe) 35-700 N 250 W-30 8 E In,-D Luterbacher et al. (2000) Yes - -
Central England 520 N 20 In+D Lamb 65, Manley (1974) Yes Yes No
S. Spain 37 .300 N 4.3g0 In+D Rodrigo et al. (2000) Yes - No
Crete Is. 35.150 N 25.000E D Grove & Conterio (1995) Yes.- No
Mid-RusSIa 50-608 N 30-SO E Ini-D Borisenkov (1995) Yes - -
Czech Republic 48.5-51.20 N 12-19 B lni-B Bodri & Cermdlk (1999) Yes Yes Yes?
S. USA 37-380 N 107.5-10 .50 W Pf+Cl+D Petersen (1994) Yes Yes -
E. China (Guang Dong Prov.) 22-250 N 112-114 30 E D Chan & Shi (2000) Yes - -
E. China-wide 20-400 N 90-12 0E D Song (2000) Yes - No?
Japan 30-400 N 125-14 0'E D Tagarni,(1
9 9 3 , 1996) Yes Yes No
S. Africa 22.20 5 28.38' E ci Huffmnan (1996) Yes Yes -
E. Greenland (Nansen Fjord) 68.30 N 29.70 Is Jennings & Weiner (1996) Yes Yes No
C. Greenland (Cr~te) 71.120 N 37.32 0 W Is Dansgaard et al. (1975) Yes Yes No
C. Greenland (GRIP) 72.60 N 37.60 B Dahl-Jensen et al. (1998) Yes Yes No
S. Greenland (Dye 3) 65.2 0 N 43.80 B Dahi-Jensein et al. (1998) Yes Yes No
C. Greenland (GISP2) 72.580 N 38.50 Ic+Ml Meese et al. (1994) Yes Yes No
C. Greenland (GISP2) 72.580 N 38550W Is Stuiver et al, (1995) Yes Yes No
Svalsbard 79 0 N 15'1 ml Tarussov (1995) Yes Yes No
Devon Island 750 N 870W ml Koerner (1977) Yes - Yes0
Ellesmere Island 80.70 N 73101 NE Koerner & Fisher (1990) Yes Yes No
Ellesmere Island 80.70 N 73.1 0 W B-Us Beltramii & Taylor (1995) Yes Yes No
Gulf of Alaska 60-610 N 1490 G+T Calkin et al. (2001) Yes Yes -
Swiss Alps (Corner Glacier) 45.8-46.50 N 7.75-8. 60 E G+Gm Holzhauser (1997) Yes Yes No
(Grosser Aletsch Glacier) 45.8-46.50 N 7.75-8.16'EB Is+T Holzhauser (1997) Yes Yes No
South Georgia Island 54-55O 5 36-3 0W G+Gm Clapperton et al. (1989) Yes Yes -
Southern Alps (Mueller Glacier) 43,440 S 170.0 0lE CGiGm Winkler (2000) Yes - -
Antarctica (James Ross Island) 64.220 S 57,68,W is Aristarain et al. (1990) Yes? - No
Antarctica (Law Dome) 66.730 S 112.8 0'E Is Morgan (1985) Yes Yes No
Antarctica (Victoria Land) 74.330 S 165.130 B G+Gnvils Baronl& Orombelli (19 9 4 ) Yes Yes -
Soon & Baliunan: Climatic and envi onmental changes of the past 1000 years 93
Table 1(continued)
Location Latitude Longitudc Type Source Answer(1) (2) (3)
Antarctica (Dome C) 74.650 S 124.170 B Is Benoist et al. (1982) Yes Yes No
Prince William Sound, Alaska 60 0 N 149 0 W T+G Barclay et al. (1999) Yes Yes? -
Alberta, CanadaColumbia Icefield 52.20 N 117.8 0 W T+In+C Luckman et al, (1997) Yes Yes Yes0
N. Qu6bec 57,730 N 76.170 W T Arsenedult&Payette(19 97 ) - Yes -
Central US 33-490 N 91-109 I T+Mp Woodhouse & Yes Yes Yes4Overpeck (1998)
E. Idaho 44.1 0 N 1140 W T Biondi et al. (1999) Yes - No
N. Carolina 34,50 N 78.30 W T Stahle et al. (1988) Yes Yes No?
California (SN) 36.5-37.50 N 118.5-120 ow T Graunilich (1993) Yes Yes No
California (SN) 36.5-37.50 N 118.5-120.
0W T Scuderi (1993) Yes Yes No
Califormia (SN) 36-380 N 118-120' I T Swetnamn (1993) Yes Yes No
New Mexico 34.5 0 N 1080 W T Grissino-Mayer (1996) Yes Yes No
Central Eq. Pacific (NINO 3.4) 50 N-50 S 1600 E-150 W T+ln Evans et al. (2000) Yes Yes? No
C. Siberia (Taymiri + Putoran) 72.470 N 1020 E T Naurzbaev & Vaganov (2000)Yes Yes No
Kola Peninsula 67-680 N 33-3401 T+ls 1-iller et al. (2001) - Yes -
N. Fennoscandia 680 N 220 E T Briff a et al. (1992) Yes Yes No
NE, Italy 450 N 100 E T Serre-Bachet (1994) Yes Yes No?
Morocco 28-360 N 2-120A T Till & Cuiot (1990) Yes Yes? No
Mongolia (Tarvagatay Mts.) 48,30N 98.930. T Jacoby et al, (1996) Yes - Yes
Mongolia (Tarvagatay MtS.) 48.31 N 98.930E T+D D'Arrigo et al. (2001) Yes Yes Yes
N. Patagonia (Rio Alerce,
Argentina) 41.170 S 71.770W T Villalba (1990) Yes Yes No
S. Chile (Lenca) 41.55 0 S 72.60W T Lara & Villalba (1993) Yes No No
S. South America 33-5505 60-750W T±G Villalba (1994) Yes Yes No
W. Tasmania 420 S 146.501 T Cook et al. (2000) No Yes Yes?
New Zealand 35-48' S 167-177 B T D'Arrigo et al. (1998) Yes - No
N. Scandinavia 68 0 N 200 E T+G Karl6n (1998) Yes Yes No
California (SN) 38 0 N 1100W TS Stine (1994) -, Yes No
California (SN) 37.50N 119.450V TS Stine (1994) - Yes No
California (SN) 38.380 N 119.450W Ts Stinle (1994) - Yes No
California (SN) 38.850 N 120.470. TS Stine (1994) - Yes' No
Patagonia 48.950 5 71.430W TS Stinle (1994) - Yes -
Patagorna 50.470S 72.970W TS Stine (1994) - Yes -
NW Michigan (L. Marion) 45 0 N 85'W Po Bernabo (1981) Yes Yes Yes?
Qinghai-Tibetan Piat.ad(Dunde Ice Cap) 38.10 N 96.401 Po Liu et al. (1998) Yes Yes Yes?0.
NE China (Maili) 42.870 N 122,870 E Po Ren (1998) - Yes -
NE China (Hangzhou) 30-330 N 105-122 E Pf Zhang (1994) - Yes -
China (Taibai Mt.) 33.97 0 N 107.730 E Pf+PO Tong et al. (1996) Yes Yes No
Himalaya 28,380 N 85.720 Is Thompson et al. (2000) Yes No Yes
Himalaya (Dasuopu Glacier) 28.380 N 85.720 IC Thompson et al. (2000) Yes - Yes
Guliya Ice Cap 35.20 N 81.501 Ic+Is Thompson et al. (1995) Yes Yes No
E. China 30-400 N 100-12(0E lc+D Shi et al. (1999) Yes Yes Yes?
W. China (Guliya Cap) 35.20 N 81.50E Ic+D Shid et al, (1999) Yes Yes Yes?
Quelccaya Ice Cap 13.930 S 70.830 Is+lc Thompson et al. (1986) Yes Yes? No
Antarctica (Siple Station) 75.920 S 84.250 A Ic+Is Mosley-Thompsofl (1995) No - No
Antarctica (Dyer Plateau) 70.67 0 S 64.880 Ic+Is Thompson et al. (1994) Yes? - Yes
Antarctica(Dronning Maud Land) 7605 8.050W Ic+Is Karlbf et al, (2000) Yes Yes? No?
South Pole 900 S IC Mosley-Thompson & Yes Yes? No
Thompson (1982) Ye YsNo
N. Atlantic 54.270 N 16.780 Sd Bond et al, (1997)Ye Ys No
N. Atlantic 44.50 N 46.330 Sdi Bond et al. (1999) Yes Yes No
N. Atlantic 56.370 N 27.810 Sd Bianchi & McCave (1999) Yes Yes NO?
N. Ellesmere Island 810 N 800W Sd+Lf Lamoureux & Bradley (1996)Yes Yes -
SW Baltic Sea
(Bornholm Basin) 55.380 N 15.40 Sd+Is Andrdn et al. (2000) Yes Yes No
N. Fennoscandia(L., Tsuolbmajavri) 68.680 N 22.08'E Lf Korhola et al. (2000) Yes Yes No
94 Clim Res 2 :89-110, 2003
Table I(continued)
Location Latitude Longitud( Type Source Answer(1) (2) (3)
Switzerland (L. Neuchatel) 47 0 N 6.55VW LfI +s Filippi et al. (1999) yes Yes Yes?
NW Scotland (Assynt) 58.110 N 5.060 W Sp Proctor et al. (2000) Yes Yes No
W. Ireland 53.530 N 9.930 W Is Bladdford &Chambers (1
9 9 5 ) Yes Yes -
SW Ireland 52.50 N 9.25 0 W Sp McDermott et al. (2001) Yes Yes -
Bermuda Rise 32.170 N 64.50 W Is IKeigwmn (1996) Yes Yes No
Chesapeake Bay 37-38.40 N 76.I 0 W Sd Verardo et al. (1998) Yes Yes -
NW Alaska (Farewell L.) 62.550 N 153.630 Lf 4Is Hu et al. (2001) Yes Yes No
S. Dakota (Pickerel L.) 45.5 10 N 97.270W If Dean & Schwalb (2000) Yes Yes No
N. Dakota (Moon L.) 46.850 N 98.160W L Laird et al. (1996) Yes Yes No
N. Dakota (Rice L.) 48,010 N 101.530W Lf Yu & Ito (1999) Yes Yes No
Yellowstone P. (Lamar Cave) 44.560 N 110.240W Pf i-s H-adly (1996) Yes Yes No
Colorado Plateau (L. Canyon) 37.420 N 110.67'V Lf4-Gm+Is Pederson (2000) Yes Yes -
NE Colorado 40-411 N 102-1050 Gmv-Is+D Madole (1994) - Yes No
SW US (Colorado + Arizona) 34-37.5O N 105-112' Lf+1s Davis (1994) - Yes No?
SW us 32-390 N 109-1140 Lf+iGm Ely et al. (1993) Yes Yes No
California (White Mts.) 37.43 0 N 118.170 Is Feng & Epstein (1994) Yes Yes -
California (L. Owen) 36 0 N 118.170W Is Liet al. (2000) Yes Yes No
Yucatan Peninsula
(L. Chichanicanab) 200 N 88.40W Lf iIs Hodell et al. (2001) Yes Yes -
Cariaco Basin 11 0 N 650 W Sd Black et al. (1999) Yes Yes No
Caniaco Basin 10.71 0 N 65.170 Sd4-Is Haug et al. (2001) Yes Yes
S. Florida 24.950 N 80osso Is Druffel (1982) Yes - -
SW Puerto Rico 18.12 0 N 67.090W is Winter et al. (2000) Yes - -
NE China (Jinchuan) 42.30 N 126.370 Is IHong et al. (2000) Yes Yes No
S. Japan (Yakcushima Is.) 30.330 N 130.50 Is Kitagawa & Yes Yes NoMatsumoto (1995)
N. India (Pahalgarn) 34.02' N 75.2001 Is Ramesh (1993) Yes - -
S. India (Nilgiris) 10-10.50 N 770 E is Ramesh (1993) - Yes -
E. Africa (L. Malawi) 10 0 S 350 E U Johnson et al. (2001) Yes - No
E. Africa (L. Naivasha) 0.460 S 36.210] 1f Verschuren et al. (2000) Yes Yes No
W. Africa (Cap Blanc) 20.750 N 18.580W Is deMenocal et al. (2000) Yes Yes No
S. Africa 19-35' S 10-33' MP Tyson & Lindesay (1992) Yes Yes No
S, Africa (Nelson Bay Cave) 3403 230 E Is Cohen & Tyson (1995) Yes - No
S. Africa (Makapansgat) 24.540 S 29.250 Sp Tyson et al. (2000) Yes Yes No
N. New Zealand (Waitomo) 38.270 S 1750 Sp Williams et al. (1999) Yes - -
S. New Zealand (Nelson) 40.670 S 172.430 E Sp Wilson et al. (1979) Yes Yes No
S. America (several regions) 33-38'S5 59.3-67 W MP Iniondo (1999) Yes Yes -
C. Argentina 29.5-350 S 61.75-65. 50 W Gm+D Carignano (1999) Yes Yes No
C. Argentina 28-360 S 61-670 G+Mp Cioccale (1999) Yes Yes No
NW Argentina 26.50S 68,090V Sd-iIs Valero-Garc6s et al, (2000) Yes - -
W. Antarctica (Palmer Deep) 64.860 S 64.210M Sd Domack et al. (2001) Yes Yes No
W, Antarctica (Siple Dome) 81.650 5 148.810 W Is Kreutz et al. (1997) Yes - No
0 Warming or extreme excursion peaked around 192 -1950 before any sigmifcant anthropogenic CO, release to air
bHughes & Diaz concluded that tourl review indicates th it for some areas of the globe (for example, Scandinavia, China, the
Sierra Nevada in California, the Canadian Rockies, and Tasmania), temperatures, particularly in summer, appear to have
been higher during some parts of this period than those that were to prevail until the most recent decades of the twentieth
century. These regional episodes were not strongly syn -hronous. Evidence from other regions (for example, the Southeast
United States, southern Europe along the Mediterranean, and parts of South America) indicates that the clnnate duning that
time was little different to that of later times, or that wa rming, if it occurred, was recorded at a later time than has been as-
sumed. ..,. To the extent that glacial retreat is associated with warm summers, the glacial geology evidence would be consis-
tent with a warmer period in A.D. 900-1250 than imnme iately before or for most of the following seven hundred years.' The
main conclusion of Hughes & Diaz (1994) may be in actual agreement with the qualitative Classifcation in our paper
cOnly documentary, histonical and archaeological reseai ch results, rather than the solar-output model results, of this paper
were referred to
dpor the Dounde ice cap, Thompson et al. (1989) noted tha , according to the 8"O climate proxy, the 1940s, 1950s and 1980s are
at least as warm as the H-olocene maximum 6000 to 8000 BP. To confrmn Thompson et al. (1989) cf. Fig. 6 in Thompson (2000),
because the claim that 1930s-1980s are the warmest of the last 6000-8000 yr is not clear from any figure in Thompson et
al. (1989). But the iman warming of the 1940s-1950s occurred before a significant rise of anthropogemc CO, in the air
Soon & Baliunas: Climatic and envi onniental changes of the past 1000 years 95
EYes
~&Yes? or No? *.- *: 7
Fig. 1. Geographical distribution of local answers to the foil wing question: Is there an objectively discermible climatic anomaly
during the Little Ice Age interval (AD. 1300-1900) in this pr sy record? Yes'lis indicated by red filled squares or unfilled boxes,
'No' is indicated by green filled circles and 'Yes! or No?' (undecided) is shown with blue filled triangles
*No.&Yes9 or No?9 .
Fig. 2. Geographical distribution of local answers to the for owing question: Is there an objectively discernible climatic anomaly
during the Medieval Warm Period (A.D. 800 1300) in this p oxy record? Yes' is indicated by red filled squares or unfilled boxes,
'No' is indicated by green filled circles and 'Yes? or NoV (undecided) is shown with blue filled triangles or unfi med boxes
96 Chin Res 2 3:89-110, 2003
* Yes*Yes'
... .
.aYes? or No? -. . -
Fig. 3. Geographical distribution of local answers to the foil rwing question: is there an objectively discernible climatic anomaly
within the 20th century that is the most extreme (the warmest, if such information is available) period in the record? 'Yes' is
indicated by red filled squares, 'No' is indicated by green fi led circles or unfilled boxes and 'Yes? or No?' (undecided) is shown
with blue filledtriangles or unfilled boxes. Answer of Vesa i; indicated by yellow filled diamonds to mark an early to nmddle-20th
century warning rath, r than the post- 19 7O5 warming
The climate indicators considered here include info r- ture changes inferred for the Medieval Warm and
mation from documentary and cultural sources, i e Little Ice Age Chimatic Anomalies are generally
cores, glaciers, boreholes, speleothems, tree-growth accepted to be no more than 1 to 20G when averaged
limits, lake fossils, mammalian fauna, coral and tre - over hemispheric or global spatial scale and over
ring growth, peat cellulose; and pollen, phenological decades to a century. Broecker (2001) deduced that
and seafloor sediments. In a rather inhomogefleous only the results from mountain snowline and borehole
way, each proxy is influenced by both climatic and thermomietry are precise to within 0.50C in revealing
non-climartic factors, We rely on individual research rs changes on a centennial timescale. But the quantifica-
for theft best judgments in interpreting climatic si - tion of errors is complex, and both Bradley et dl. (2001)
nals. The 3 questions are addressed in the context of and Esper et al. (2002) have challenged Broecker's
local or regional sensitivity of the proxies to relevant statement. Earlier, Jones et al. (1998) provided an
climatic vanrables, including air temperature, sea sur- enlightening review of the quantitative and qualitative
face temperature, precipitation, and any combinati n limitations of paleodlimatology. Others like Ingram et
of large-scale patterns of pressure, wind and oceanic al. (1978) and Ogilvie & Farmer (1997) had cautioned
circulation. against the use of quantitative interpretations of
climatic results that are based on historical documenta-
tion.
4. UNCERTAINTIES IN INFERRING CLIMATE In our survey of the literature we have observed 3
FROM PROXIES distinct types of warnings (Bryson 1985, Glow 1992,
Graybill & Idso 1993, Huang et al. 1996, Briffa et al.
The accuracy of climate reconstruction from prod s, 1998, Cowling & Skyes 1999, Schleser et al. 1999,
including the awareness of anthropogenic interven- Evans et al. 2000, Schmuitz et al. 2000, Aykroyd et al.
tions that could pose serious problems for a qualitat ye 2001, Ogilvie & JMnsson 2001):
and quantitative paleoclimatology, has been discus! ed (1) the lack of timescale resolution for the longest-
by Bryson (1985), Idso (1989) and others. The tempera- term component of climate signals, e.g. in tree ring and
Soon & Baliuinas: Climatic and envir rnmental changes of the past 1000 years 9
coral records, or the loss of short-term climate iniforma- records, it requires 'considerable faith' to compare, for
tion in borehole temperature reconstructions; example, the climate of the 12th and 20th centuries
(2) the nonlineanties (related to age, threshold, dis- from tree-ring proxies. To date, the practical goal of
continuous or insufficient sampling, saturated response, combining information from borehole and tree-ring
limited dynamic range of proxy, etc.) of biological, proxies, or even comparing borehole and thermometer
chemical and physical transfer functions necessary for data, to yield an accurate proxy record that suriultane-
temperature reconstruction; ously resolves thimescales of years to centuries, remains
(3) the time dependence or nonstatioriarity of thE unfulfilled.
climate-proxy calibration relations. Despite complicating factors such as the mismatch of
Estimates of ground temperature trends from bore climate sensitivities among proxies, a first step has
hole data can be complicated by non-climatic factor, been taken by Beltrami et al. (1995) and Harris &
associated with changes in pattern of landuse and lanc Chapman (2001). Also, Beltrami & Taylor (1995) suc-
cover over time (Lewis & Wang 1998, Skinner cessfully calibrated a 2000 yr oxygen isotope record
majorowicz 1999). In general, climate proxies froi from an ice core (near Agassiz) with the help of bore-
floral and faunal fossils in lake and bog sediments ar( hole temperature-depth data (near Neil) for the
only sensitive enough to resolve change to within ± 1.' Canadian Arctic region. Such careful research may
to 1.80C (e.g. Lotter et al. 2000). isotope-coral proxie help resolve the difficulty of interpreting climate sig-
lack the climate-sensitivity resolution and the con nals that degrade with borehole depth or time. This
tinuous length of record to address millennial climatic depth-dependent, increasing degradation has led to
change. Jones et al. (1998) showed that both coral- an the false impression that reconstructed temperatures
ice-core-based reconstructions performed more poorl I from geothermal heat flows contained a significantly
than tree-ring records when calibrated again t smaller variability in the distant past than at present.
thermometer data since A.D. 1880. On the other hanc, The approach used here relies on local representa-
tree ring proxies, which usually have annual tim tions of climate change, which is an advantage
resolution, suffer from the loss of information on mnult - because understanding local proxies is the prerequi-
decadal to centennial and longer components f site for constructing regional and global patterns of
climate change. change. Another advantage is that by working with a
The amplitude of large-scale surface temperatu local or regional perspective, we avoid the difficult
change derived from tree-ring proxies can be substar - questions concerning the spatio-teinporal coupling of
tially underestimated-by a factor of 2 to 3 compared observed changes among various regions and any
to results from borehole thermometry (Huang et al. specific large-scale pattern responsible for those
2000, Harris & Chapman 2001). It is surprising that the climatic anomalies. Our study has the disadvantage
amplitude of climate variability broadly resolved ty of being non-quantitative. Thus, our assessment falls
borehole reconstruction on timescales of at least 50 o short of Lamb's (1965) original call for quantitative
100 yr is larger, rather than smaller, than the high tir e answers.
resolution results from tree-ring proxies, becau e An early attempt to study the interlinkage of geo-
short-term climate fluctuations are smoothed out y graphically separated and different proxies, e.g. be-
the geothermal heat-flow that acts as a low-pass filte .l tween marine sediments at Palmer Deep, Antarctica,
The different amplitudes found from borehole a d and atmospheric signals in Greenland ice cores, has
tree-ring climate proxies suggest that longer timesc le been reported by Domack & Mayewski (1999). But
(multi-decades and century) variability is more fait a- many chronologies depend on radiocarbon dating and
fully captured by borehole results, while the sa e are too limited in accuracy to allow for reliable inter-
information can be irretrievably lost from tree-ring pretation of the timing of events from different areas
recrds(se e~. Cllis t a, 202)becuseoft re (e~g. Stine 1998, Domack & Mayewski 1999). The diffi-
standardization procedure (to remove bias due to cult task of areal weighting of different rx eod
aging of trees). This is why Jones et al. (1998) co - has been attempted; for the Arctic region by Over-
merited that although one may be confident of int r- peck et al. (1997), the Northern Hemisphere by Crow-
comparing year-to-year and decade-to-decade (limit ~d ley & Lowery (2000), Northern Hemisphere extratrop-
to periods shorter than 20 to 30 yr) variability, which ics by Esper et al. (2002) and both Northern
should be more sensitively imprinted in tree-riag Hemisphere and global domains by Mann et al. (1998,
1999, 2000). However, Briffa et al. (2001) criticized the
lack of consideration of uncertainties in some of these
'There are exceptions in careful tree-ring results like th )se reosucin Frxapthcmoiesresn
of Esper et al, (20021 that are optimiZed to capture Ion jer Overpeck et al,'s (1997) reconstruction is not even cal-
tiinescale variability ibrated with instrumental data.
98 ClimnRes 23:89-110,2003
5. RESULTS warmest or most extreme climatic anoinahes in the
proxy indicators occurred in the early to mid-2Oth
Table 1 lists the worldwide proxy climate records century (Yesa'), rather 'than sustaining throughout
we have collected and studied. We restnicted the list the century.
to records that contain either direct information 5.1. Glaciers
about the 3 specific questions we posed or at leas
a continuous time series for 400 to 500 yr. F r Broadly, glaciers retreated all over the world during
the majority of cases we followed what individu 1 the Medieval Warm Period, with a notable, but minor,
researchers stated according to their paleoclimati re-advance between 1050 and 1150 (Grove & Switsur
reconstructions. In a few cases we elaborated or 1994). Large portions of the world's glaciers, both in
their results in order to remain consistent to ot r the Northern and Southern Hemispheres, advanced
framework. during the 1300 to 1900s (Grove 2001b, see also Win-
The figures show the results from Table 1 for th kler 2000). The world's small glaciers and tropical glac-
Little Ice Age (Fig. 1), Medieval Warm Period (Fig, 2) iers have simultaneously retreated since the 19th cen-
and the nature of the 20th century's change (Fig. 3). tury, but some glaciers have advanced (Kaser 1999,
The figures graphically emphasize the shortage of Dyurgerov & Meier 2000, D. Evans 2000). Kaser (1999)
climatic information extending back to the Mediev I reemphasized the key role played by atmospheric
Warm Peniod for at least 7 geographical zones: t e humidity in controlling the net accumulation and abla-
Australian and Indian continents, the SE Asian arc i- tion of glaciers by modulating the sublimation and
pelago. large parts of Eastern Europe/Russia, tI e long-wave radiative forcing-feedback budgets in both
Middle Eastern deserts, the tropical African ar d dry and humid areas, So far, the proposition of the
South American lowlands (although the large nur - 20th century warming as a natural recovery since
ber of available borehole-heat flow measurements n the Little Ice Age, together with an amplification by
Australia seems adequate for the reconstruction f anthropogenic CO2. is plausible but not definitive
ground temperatures back to medieval times; S e (Bradley & Jones 1993, Kreutz et al. 1997, Kaser 1999,
Huang et al. 2000). Therefore, our conclusions aie Beltrami et al. 2000, Dyurgerov & Meier 2000). On the
provisional, other hand, D. Evans (2000) discussed the possibility of
Fig. 1 indicates that Little Ice Age exists as a disfin- recent widespread recession of glaciers as a glacio-
guishable climate anomaly from all regions of t e chinatic response to the termination of the Little Ice
world that have been assessed. Only 2 records-tree Age and commented that significant warming phases
ring growth from western Tasmania and isotopic me I- during interglacials, especially those accompanied by
surements from ice cores at Siple Dome, Antarctica - relatively warm winters and cool summers, may lead to
do not exhibit any persistent climatic change over tl is the onset of another global glaciation.
period (although the western Tasmania reconstruction Additional proxy records used here reveal that the
contains an exceptionally cold decade centered aromid climatic anomaly patterns known as the Medieval
1900; Cook et al. 2000). Warm Period (ca. A.D. 800-1300) as well as the Little
Fig. 2 shows the Medieval Warm Period with orly Ice Age interval (A.D. 1300-1900) occurred across the
2 negative results. The Himalayan ice core res lt world. The next 2 subsections describe detailed local
(Thompson et al. 2000) seems unambiguous, but the changes in the Northern and Southern Hemispheres.
tree-ning proxy data from Lenca, southern Chile (Le ra
& Villalba 1993) is countered by nearby evidence oftI e
Medieval Warm Period (Villalba 1990, 1994). 5.2. Northern Hemisphere
Fig. 3 shows that most of the proxy records do iot
suggest the 20th century to be the warmest or t ie A composite reconstruction of summer temperature
most extreme in their local representations. There ere anomaly assuming a simple, uniform weighting of
only 3 unambiguous findings favoring the 20th cE n- proxy records by Bradley & Jones (1993) showed that
tury as the warmest anomaly of the last 1000 Y- the 1530-1730 interval was the coldest period for the
the records from the Dyer Plateau, Antarctica, te Northern Hemisphere, and the 19th century was the
Himalayas and Mongolia (Thompson et al. 1994, second coldest interval in the last 500 yr.
2000, D'Arngo et al. 2001). An important, seemin ly
counter-intuitive, feature of Fig. 3 is the large numn er
of uncertain answers compared to the 2 prior ques- 5.2.1. Western Europe
tions, perhaps partly owing to inaccurate calibration
between proxy and instrumental data. Also, anot er Cold winters and wet summers prevailed during the
feature of the result is the many cases in which he Little Ice Age in Switzerland, a location with detailed
Soon & Baliunas: Climatic and envir )nnental changes of the past 1000 years 99
and reliable information (Pfister 1995). A careful corn- 5.2.2. North Atlantic and other oceans
panson of the Swiss and central England temperaturesieaasron
records (Manley 1974) from 1659-1960 reveals a During the Little Ice Age, extensieaesaon
general correspondence of climatic conditions between Mediterranean Europe and the North Atlantic, includ-
the 2 regions. In the Andalusia region of southern ing Western and Northern Europe, Greenland and Ice-
Spain, rainfall appears to have alternated between dr land, were expeniencing unusually cold and wet con-
and wet century-long spells (wet periods persisted ditions as well as many extreme weather events,
1590-1649 and 1776-1937; dry periods 1501-1589 and including deluges, landslides and avalanches (Grove
1650-1775) throughout the Little Ice Age, and with nc 1996, Ogilvie et al. 2000, A. T. Grove 2001). Climate
significant difference to the modern dry period ol over Iceland was found, based on various proxies, to be
1938-1997 (Rodrigo et al. 2000). Enhanced fluvia mild from 870 to 1 170, with cold periods setting in after
activity was documented in river basins of North 1200. Instead of being a period of unrelenting cold,
Westen an Cenral urop for1250-1550 and Ogilvie (1984) emphasized that the most notable
1750-1900 (A. T, Grove 2001). Over Western Europe, aspect of climate over Iceland duning te1t o1t
Pfister et al. (1998) concluded that severe winters were centuries, with its very cold decades during the 1690s,
less frequent and less extreme during 900-1300 tha 170,1 an Oglvies w assits lageyer-o-eavn
those during 1300-1900. The mild-winter condition ability (see alsogive&Jnsn20)
was hypothesized by Pfister et al. (1998) to haye The viking colonization of Greenland's coastal area
caused the northward migration of Mediterranean starting in 986 is well documented, and the gener-
subtropical plants, where St. Albertus Magnus note ally mild and benign climatic conditions from about
the abundance of pomegranates and fig trees in thn 800 to 1200 that helped to sustain the settlement, are
13th century around Cologne and parts of the Rhine also well supported by ice core and borehole proxy
valey.Olie tees whchlik fi tresareals sns: - information (Dansgaard et al. 1975, Dahl-Jensen et al.
tive to prolonged periods of air temperature belo 19) h osee 'etr Setlemn (bardouned
freezing, must have grown in Italy (Po valley), Franc the Godthab district) was mysterioulabnoe
and Germany because a chronicler documented the sometime between 1341 and 1362, while the 'Eastern
damage to the olive trees by the bitter frost in Janua y Settlement', actually near the southernmost tip of west
1234. Lamb (1965) noted generally wet winters b t Greenland, atound the Narssaq and Julianehab dis-
drier summers for the lowlands in England, Ireland, tricts, died out between 1450 and 1500 (Grove 1996,
the Netherlands, Denmark, Sweden and NWM Germa y Ogilvie et al. 2000). It also seems that both cultural and
from about 1200 to 1400. Those conditions are sup- political factors contributed in making the None
portd b doumenaryrecrds that describe freque it Greenlanders at the Western Settlement more vulner-
flooding and storms around those regions during t e able to the harsh climatic conditions(alwe l
transitional period between Medieval Warm Peri d 1997). The timing for the abandonment of these settle-
and Little Ice Age. ments coincided with the general cooling over Green-
Is the warmth of the 20th century for western Euro e land, a~s established by both ice-core isotopic and bore-
exceptional or unusual? Weather reconstruction resu ts hole thermometry (Dansgaard et al. 1975, Stuiver et
for the Low Countnies, the present-day Benelux regi n, al. 1995, Dahl-Jensen et al. 1998). From sediment
suggest that in order to compare the 20th century to cores near Nansen Fjord. East Greenland, Jennings &
previous centuries, seasonal information in a pro y- Weiner (1996) confirmed ant initial cooling between
cliatereltio wil b reuired (van Engelen et al. 1270 and 1370, together with the most severe and vani-
2001). For example, van Engelen et al. (2001) sho d able climatic COniosarudteEtGenld
that when the historical reconstructed series from region from 1630 to 1900. The results of Ogilvie et al.
abou 80 to 000weie calibrated to the instrumental (2000) and Ogilvie & JMnsson (2001) suggest that the
temperature records at De Bilt, the 20th century winter overall climatic conditions in teNrhAlni
temperatures may have been slightly higher (ab ut (50-80o N, 0-600 W), especially near Iceland during
0.5CC-the quantitative information on the 20th cen- the 20th century, including the 1970s to 1990s, were
tury warmth is certainly within the margin of an- neither unusual nor extreme.
certainties) than the high winter temperatures of In the Mediterranean basin, the island of Crete ex-
1000110 bu tha reentwarming began in the 1 )th perienced many severe winters and prolonged droughts
century. in contrast, the 20th century summer tern er- during the wne n pigsaosbten14
atures are neither unusual nor extraordinarily wc rmn and 1648 (Grove & Conterio 1995). In Morocco, the cli-
when compared to summer temperature variabilfi es mate during the 16th, 17th and 18th centunies was gen-
during other times of the 2nd millennium (see Figs. 1 erally more variable, with frequently drier conditions,
& 2 of van Engelen et al, 2001). than in the early to mid-2Oth century (Till & Guiot
100 Clin Res 23: 89-110, 2003
1990). But no distinctive precipitation anomaly was 5.2.3. Asia and Eastern Europe
observed for Morocco during the Medieval Warm
Period, although just like conditions during the Little From 49 radiocarbon-dated subfossil wood samples,
Ice Age, an episode of notable drought occurred from Hiller et al. (2001) found that the alpine tree-limit on
1186 to 1234. Thus, precipitation anomahies for t e the Khibiny low mountains of the Kola Peninsula was
Little Ice Age and Medieval Warm Period do not diff r located at least 100 to 140 mn above current tree-limit
from each other in this region-both intervals suffere elevation during the relatively warmer time between
from persistent drought conditions. For this reason, Ie 1000 and 1300. The summer temperatures correspond-
gave an uncertain designation for the occurrence of a ing to this tree-line shift during this warm time are esti-
distinct climatic anomaly associated with Medley l mated to have been at least o.80 C warmer than today.
Warm Period for Morocco in Table 1 and Fig. 2. Based mostly on documentary evidence, Borisenkov
Distinctly cooler conditions prevailed over the ocean (1995) noted that Little Ice Age conditions began as
surface-in the Canibbean Sea by about 2 to 30 C(Wint r early as the 13th century in Russia with the character-
et al, 2000) and in the Sargasso Sea by about 1PC (Keig- istic of frequent climate extremes both in terms of
win 1996), especially during the 17th and 18th centuri 5 severe winters, rainy and cool summers, and sustained
as opposed to the present. DeMenocal et al. (200 ) droughts (up to a decade long). Middle Russia (around
showed that the subtropical North Atlantic's sea surfa e 50-600 N and 30-500 E) seems to have experienced the
temperature off Cap Blanc of Mauritania (west Afric i) coolest winters around 1620-1680, the coolest sum-
was also cooler by 3to 40C between 1300 and lOS0 thE n mers-springs around 1B60-1900, as well as distinc-
at present. During the Medieval Warm Peniod, the Sar- tively warm conditions during the first half of the 16th
gasso sea surface temperature was about 1 0C warm ~r century, similar to conditions for Western Europe
than the present-day value (Keigwin 1996), while the s a described above, The ground surface temperature his-
surface temperature off the coast of Maunitania was only tones deduced from boreholes around the Czech
marginally wanner than at present (DeMenocal et I. Republic suggest that winters duning 1600 and 1700
2000). H-igh-resolution coral skeletal 818 and Sr/Ca r a- were the most severe compared to any other winters
tio records from Bermuda indicate sea-surface tempera- since at least 1100 (Bodri & Cermdk 1999). The temn-
ture standard deviations of about ± 0.50 C on interannual perature-depth borehole records also yield a clear
and ±0,30C on dlecadal timescales during the 16th signature of an anomalously warm period for central
century, and those ranges of vaniability are comparable Bohemia, especially around 1100-1300.
to estimates from modern 20th century's instrumen al Bradley and Jones (1993) showed that the mnid-l7th
data (Kuhnert et al. 2002). But those sub-annually century was the coldest in China. In NE China, fre-
resolved coral proxy data also show that although the e quent occurrences of extremely dry conditions pre-
may be large-scale climate signals like the No th vailed during the 16th and 17th centuries (Song 2000).
Atlantic Oscillation detectable at Bermuda, no cone- The dry conditions returned again in the 20th century,
lation can be found with other northern-hemnispher c- and now cover a wider area (with indications including
wide proxy reconstructions because of large spat al the increasing number of days with no discharge from
differences in climate variability, the Yellow River; but these 20th-century events are
From sedimentary concentrations of titanium a id likely to be confused with other man-made factors).
iron, Haug et al. (200 1) inferred a very dry climate Jor Chan & Shi (2000) further documented the notably
the Cariaco Basin during the Little Ice Age and re a- larger number of land-falling typhoons over Guandong
tively wetter conditions in the Medieval Warm Penod. Province in the early-to-mid-l9th century based on a
Over the equatorial Central Pacific, around the homogeneous set of typhoon records from 1470 to
NINO3.4 (50 N-5 0 S, 1600 E-1500 W) region, Evans et 1931. Using a 8180 proxy record from peat cellulose
al. (2000), in thenr skillful reconstruction of the ENSD- with 20 yr resolution and various Chinese historical
like decadal variability of the NINO3.4 sea surface records, Hong et al. (2000) showed the general cooling
temperature (SST), showed that there appeared to be a trend in the surface air temperature during the Little
sustained cool phase of the proxy NINO3.4 SST vi- Ice Age interval in NE China, Hong et al. found 3 of
ability from about 1550 to about 1895, hence extend' ng the coolest minima in the record centered around
the geographical area covered by the Little Ice Age 1550, 1650 and 1750. An obvious warm period peaked
Climate Anomaly. Evans et al. (2000) also added t at around 1100-1200, coinciding with the Medieval
the reconstructed NINO3 .4 decadal-scale SST vdri- Warm Peniod. The study of documented cultivation of
ability prior to the 17th centr ssiia othat of ihe Citrus reticulata Blanco (a citrus tree) and Boehmnera
20th century, thus suggesting that the recent 20th c n- nivea (a perennial herb), both subtropical and temper-
tury decadal-scale changes in the equato lal Pactic ature-sensitive plants, duning the last 1300 yr showed
Ocean are neither unusual nor unpreceden ed. that northern boundaries for these plants had
Soon & Baliunas: Cilmati( and env onmental changes of the past 1000 years 101
shifted and expanded; their northernmost loc tion was 1930s-1950s drought, Additionally, both Yu & Ito
reached around 1264 (Zhang 1994). Zhang thea (1999) and Dean & Schwalb (2000) pointed to the
deduced that temperature conditions in the 13th cer- cycles of aridity lasting about 400 yr from lake records
tury around central China must have been about l'C of the Northern Great Plains, where the last dry con-
warmer than present. Ren (1998) found further ev.- dition peaked around 1550-1700.
dence from a fossil pollen record at Maihi Bog, N2 From an extensive collection of multiproxy evi-
China, that summer monsoon rainfall from 950 to 1270 dence, Stine (1998) concluded that duning the Medi-
must have been generally more vigorous in order eval Warm Peniod prolonged intervals of extreme
explain the high deposition of several pollen taxa drought visited California, the NW Great Basin,
which are otherwise unexplainable by human activit the northern Rocky Mountains/Great Plains, while
at those times, markedly wetter regimes persisted in the upper Mid-
Based on the less precise climate proxies like west/sub-Arctic Canada and Southern Alaska./Bntish
cherry blossom viewing dates, lake freezing datE s Columbia areas. There was also a significant but
and histonical documentation of climate hazards an brief interval around 1110-1140 when moisture con-
unusual weather, Tagami (1993, 1996) found that a ditions switched from dry to wet in California, the
warm period prevailed between the 10th and 14t NW Great Basin, the northern Rocky Mountains/
centuries, and a cold period between the late 15t Great Plains, and from wet to dry in the upper Mid-
and 19th centuries over large parts of souther west/sub-Arctic Canada and southern Alaska/British
Japan. From the study of number of days with sno - Columbia, The most likely explanation for this rapid
fall relative to days with rainfall, Tagamni (1996) and dramatic switch from wet to dry conditions in
concluded that the 11th and 12th centuries we e the upper Midwestern U.S. around 1100 is the con-
unusually warm in Japan. During the Little Ice Ag , traction and subsequent expansion of the circumpo-
summers were relatively cool from the 1730s to lar vortex. Summer polar fronts shifted significantly
1750s, in the 1780s, from the iSS0s to the 1840s a d southward, and stopped the penetration of moisture-
in the i1fi0s, and winters were cold through t e laden air from the Gull of Mexico (Bryson et al.
1680s to 1690s, and in the 1730s and 1810s. From t e 1965). Stinle (1998) added the requirement of a con-
tree-cellulose 8'3C record of a giant Japanese cedar comitant jet-stream -change, from zonal to azonal, in
Cryptomneha japonica grown on Yakushima Island f order to explain the distinct observed differences of
southern Japan, Kitagawa & Matsumoto (1995) the moisture patterns between the upper Midwest
inferred a cool temperature of 20C below avera e and southern Alaska/British Columbia. Graumlich
from 1600 to 1700 and a warm period of about 1 C (1993)'s reconstruction of summer temperature and
above average between 800 and 1200. winter precipitation from trees in the Sierra Nevadaconfirmed the overall warm and dry conditions
for California during medieval times, when the 2
5.2.4. North America warmest and the 2 dniest 50 yr intervals occurred,
at 1118-1167, 1245-1294 and 1250-1299, 1315-1364,
Overall, the composite summer temperature anlo - respectively.
aly from Bradley & Jones (1993) showed that over H-u et al, (2001), based on their high-resolution (mul-
North America the temperature during the 15th to 17 ~h tidecadal) geochemnical analysis of sediments from
centuries was 1PC cooler than the average of the rf f- Farewell Lake by the NW foothills of the Alaska
erence period 1860-1959. over the southern Sierra Range, also found pronounced signatures of the
Nevada, California, Graumlich (1993) found that t e Medieval Warm Period around 850-1200. Duning the
coolest 50 yr interval in the 1000 yr tree-ring procy Little Ice Age the surface water temperature of
record was around 1595-1644, while the wettest 50 yr Farewell Lake suffered a low in 1700 calibrated to be
period was 1712-1761. These periods are consistent about 1.750 C cooler than present. They also noted
with our definition of a discernible climatic anomaly es- that colder periods were in general wetter (in contrast
sociated with the Little Ice Age interval of 1300-19 0. to drier conditions duning Little Ice Age in the Central
Ely et al. (1993) noted from river records in Arizoia U.S. region described above) than the warm peniods
and Utah that the most extreme flooding events in this part of NW Alaska. On the Yucatan Penin-
occurred during transitions from cool to warm dlim te sula, prolonged drought episodes recur approximately
conditions, especially during the late 1800s to ea l1y every 208 yr. with the 2 most significant recent peaks
1900s, For the Central U.S.A. (33-49O N, 91-1090 ), centered around 800 and 1020 (Hodell et al. 2001). The
drought episodes were noted for the 13th to 16th CE n- timing df severe droughts also seems to fit several
tunies (Woodhouse & Overpeck 1998). These droug ts known discontinuities in the evolution of the Mayan
were of longer duration and greater extent than te culture.
102 Clim Res 2 3:89-110, 2003
5.3. Southern Hemisphere about 3-4 0C warmer than present around 1200-1300
(Tyson et al. 2000). The multiproxy review by Tyson &
Figs. 1-3 highlight the sparse coverage of the South- Lindesay (1992) showed evidence for a wetter South
ern Hemisphere by proxy climatic information through Afnica after 1000, when forest and wetland become
the 2nd millennium, more extensive, including the development of a river-
ime forest in the northern Namib desert along the
Hoanib river during the 1 lth-l3th centuries.
5.3.1I. New Zealand
In New Zealand, the 8180 concentration in a stalag 5.3.3. South America
mite record from a cave in NW Nelson showed thE
coldest times during the Little Ice Age to be arounc over southern South America's Patagonia, the Little
1600-1700, while exceptionally warm temperature! Ice Age's climatic anomalies, as deduced from tree ring
occurred around 1200-1400, in association with th( records, were manifest as cold and moist summers with
general phenomenology of the Medieval Warm Perio( the most notable, persistent century-long wet intervals
(Wilson et al. 1979). The cooling anomaly aroun( centered around 1340 and 1610 (Villalba 1994). From
1600-1700 apparent in the 58'0 stalagmite recor, a multiproxy study of lacustrine sediments at Lake
coincides with the smallest growth ring (i.e. coole t Aculeo (about 340S; 50 kin southeast of Santiago,
period) for the silver pine Lagarostro bus colensoi fror Chile), Jenny et al. (2002) found a period of greatly
Mangawhero of the North Island. However, at Ahauri increased flood events centered around 1400-1600
in the South island, the smallest ring width index of th 2 (and in 3 other intervals: 200-400, 500-700 and
600 yr record occurred about 1500-1550 (DArngo et 1850-1998), which could be interpreted as increased
al. 1998). Williams et al. (1999) advise caution in intei - winter rains from enhanced mid-latitude westerhies
preting stable isotope data from New Zealand, espe- that ushered in more frontal system activities. In con-
cially the correctional functional relations amon trast, during the Medieval Warm Period, the southern
temperature, precipitation and 8180 data (which a e Patagonia region at latitudes between 47 and 5105 be-
strongly influenced by oceans surrounding New came abnormally dry for several centuries before 1130
Zealand) from Waitomo, North Island's speleothems. when water levels in several lakes (Lake Argentino,
The mean annual temperatures at Waitomno from Lake Cardiel and Lake Ghio) dropped significantly.
1430-1670 were deduced, based on the analysis Of Also, trees like the southern beech Nothofagus sP.
8180 data from Max's cave stalagmite, to be abo it grew as old as 100 yr in the basin of these lakes before
0.8 0C cooler than today. being killed by reflooding of the lakes (Stine 1994).
Slightly north toward the central region of Argentina
(around C6rdoba Province), Carignano (1999), Cioccale
5.3.2. South Africa (1999) and Iriondo (1999) noted the prevailing conditions
for the advancement of the Andean glaciers during the
Tyson et al. (2000) showed through isotopic measu . Little Ice Age, with 2 distinct cold and dry intervals
ments in speleothem that the interior region of South around the 15th to 16th, and the 18th to the early 19th
Africa, near the Makapansgat Valley (eastern part of centuries. The significant climate aridification and de-
South Africa), had a maximum cooling Of about 1 IC terioration in central Argentina (in contrast to the more
around 1700 compared to the present. This cooling fE a- humid conditions and increased flood frequency in cen-
ture corresponds well with the maximum cooling sig Lai tral Chile near Lake Aculeo) during the Little Ice Age in-
contained in a coral record from SW Madagas ar terval is supported by the formation of large, parabolic
(Tyson et al. 2000). Tyson & Lindesay (1992) show d sand dunes 150-200 in long, 60-80 mn wide, and 2-3mi
that the Little Ice Age in South Africa had 2 major coDI- high in the Salinas Grandes basin (Carignano 1999).
ing phases, around 1300-1500 and 1675-1850, with a Menhl h a hqia aewstasomdit
sudden warming interval between 1500 and 1675. In a swamp surrounded by dunes in the 18th century. Today
addition, Tyson & Lindesay suggested a weakening of Mar Chiquita is the largest lake in Argentina, covering a
the tropical easterlies that increased the incidence of surface area of 6000 km2 and with a depth of 13 mn (Iri-
drought during the Little Ice Age in South Africa-'V ith ondo 1999). The climatic conditions during the Medieval
a relatively drier condition for the summer rainfall re- Warm Peniod around Central Argentina were generally
gion in the northeast, but a wetter condition for he warmer and more humid than at other times in the 2nd
winter rainfall region near the coastal Mediterran an millennium, when the dune fields were conquered by
zone in the southwest, At Makapansgat Valley, hfe lakes and the Mar Chiquita. Lake expanded beyond its
Medieval Warm Period peaked with a teinperat ire present dimensions. Precipitation exceeded current vatl-
Soon & Ballunas: Climatic and environmental changes of the past 1000 years 103
uies, and the mean local temperature may have bee a sea-ice conditions, coinciding with the Medieval Warm
about 2.50C warmer, perhaps because of the southiward Period. In the same record, Domnack and colleagues
shift of the tropical climate belt into this area (Iriond found a decrease in bio-productivity, hence an mn-
1999). The northern part of C6rdoba Province was in- crease mn magnetic susceptibility owing to less dilution
vaded by the eastern boundary of the Chaco Forest, of the magnetic minerals by biogenic materials, from
which is located hundreds of krns to the northwest toda y about 700 to 100 yr BP. This time period corresponds to
(Carignano 1999). Cioccale (1999) noted evidence of hu- the Little Ice Age of ca. l4th-l9th centunies and is
man cultivation of hillside areas in Central Andes, Per , likely to have been accompanied by cool and windy
at places as high as 4300 mn above sea level around 100 ., conditions. Abundance analyses of Na± sea salt in the
ice core from Siple Dome (81.650 S, 148.810 W) also
confirm the Little Ice Age anomaly charactenized by
5.3.4. Antarctica substantial variability in the strength of mneridional cir-
culation that extended between 1400 and 1900 (Kreutz
The last important source of geographical informa- et al. 1997).
tion for conditions during the Medieval Warm Peric d But there are also indications for significant regional
and the Little Ice Age in the southern hemisphere s differences in climatic anomalies associated with the 2
obtained from glaciers, ice cores and sea sedimems phenomena at Antarctica. The temperature at Siple
studies on and around Antarctica. Although mai y Station (75.92'S, 84.250 W, elevation 1054 mn) was
notable physical, biological and environmental chang !s relatively warm from the 15th to early 19th centunies
have recently occurred there, especially around t e (although there were also noticeable decade-long
Antarctic Peninsula durnug the last 50 yr (Mercer 197 3, cooling dips centered around 1525, 1600 and 1750;
Thomas et al. 1979, Rott et al. 1996, Vaughan & Doal e Mosley-Thompson 1995). The 400 yr isotopic tern-
1996, Smith et al. 1999, Doran et al. 2002, Marshall t perature inferred from a core at the Dalinger Dome
al. 2002), most of the 20th-century changes contain d (64,220 S, 57.680 W, elevation. 1640 mn) on James Ross -
in the proxy records discussed here cannot be consil- Island, off the Antarctic Peninsula, also showed
ered extreme or unusual (see Fig. 3, also Vaughan & 1750-1850 to be the warmest interval, followed by a
Doake 1996, D. Evans 2000). cooling of about 20C since 1850 and continuing to 1980
For the Little Ice Age, advances of glaciers in South (Aristarain et al. 1990). A recent borehole tempera-
Georgia Island, which is currently hall-covered )y lure reconstruction from Taylor Dome, east Antarctica
glaciers, began after the late 13th century, with a Pe ik (77.80 S, 158.720 E, elevation 2374 in), also reported the
advancement around the l8th-2Oth centuries (Clap- same inverted temperature anomalies, during which
perton et al. 1989). Glacier retreats occurred afe the Little Ice Age interval was about 20C warmer, while
about 1000, which corresponds to the tuning for t ie the coldest temperature of the past 4000 yr was reached
Medieval Warm Period. Baroni & Orombelli (19! 4) around 1000 (Clow & Waddington 1999); note that we
described a similar sequence for glacier advances a id have omitted these discussions in our Table 1 or Figs. 1
retreats during the Little Ice Age and Medieval Wa mn to 3 because the results are claimed as preliminary and
Period for the Edmonson Point glacier at the Te ra they were only presented in a conference abstract.
Nova Bay area of Victoria Land on the Antarctic con ti-
nent (East Antarctica). The Edmonson Point glac er
retreated in 2 distinct phases, around 920-1020 ad 6. DISCUSSION
1270-1400, and then advanced at least 150 mn after hedecasmbd
15th century. isotopic thermometry from ice cores at The widespread geographical evidec sebe
Dome C (74.650 S, 124.170EF, elevation 3240 in) ad here supports the existence of both the Little Ice Age
Law Dome (66.73' S, 112.830 E, elevation 1390 mn) both and the Medieval Warm Period, and should serve as
indicate cooler and warmer anomalies for the Little ce useful validation targets for any reconstruction of
Age and Medieval Warm Period respectively (Ben ist global climate history of the last 1000 yr. our results
et al. 1982, Morgan 1985). High-resolution records of suggest a different interpretation of the multiproxy cli-
magnetic susceptibility from deep sea cores (Domack mates compared to recent conclusions of Mann et al.
& Mayewski 1999, Domack et al. 2001) drilied n ar (1998, 1999, 2000). Because the calibration of proxy
Palmer Deep site (64.860 S, 64.210 W) off the At- indicators to instrumental data is stiil a matter of open-
arctic Peninsula also show a marked increased, in io- ended research (with differing sensitivities not only for
productivity, hence a decrease in magnetic suscepti- the same proxy at different locations but also for differ-
bility because of dilution of the magnetite, with a p ak ent proxies at the same location), it is premature to
centered around 1000-1100 yr HP. This observation select a year or decade as the warmest or coldest in a
probably implied warm temperatures and minmal multiproxy-based record.
104 Chin Res 23: 89-110, 2003
Barnett et al. (1999) has pointed out that it is impo, - An important aspect of both the Briffa et al. (2001)
sible to use available instrumental records to provid and Esper et al. (2002) studies is the new derivation of
estimates for the multi-decadal and century-long typ formal, time-dependent standard errors for their tern-
of natural climatic variations, owing to the specifi perature reconstructions, amounting to about ± 0.1I to
period and short duration of instrumental records. 0.3 0C from 1000 through 1960 (see also Jones et al.
Thus, paleo-proxies remain the only hope for assessin 1999, 2001). This assignment of standard errors con-
the amplitude and pattern of climatic and environmer - trasts with those assigned in Mann et al.'s (1999) an-
tal change in the pre-anthropogenic era. We agre nually-resolved series, where the uncertainties were
with Barnett et al. (1999) that each proxy should be assigned only for pre-instrumental data points in their
studied first in terms of local change before sever I original publication (that assumption of 'error-free' in-
records can be combined for regional and larger Spa- strumental thermometer data is incorrect-see Jones
tial-scale analyses and interpretations. The conclusio et al. 1999, Folland et al. 2001). over the full 2nd mill-
derives mainly from the real possibility of non-station- lennium, Esper et al. (2002) deduced a slightly larger
arity in the proxy-climate calibration to instrumenta1 range in their confidence limits after 1950 (compared
records, the lack of adequate superposition rules given to the pre-1950 interval extending back to 800) and
vaniability in each type of proxy, as well as the lack f attributed those higher uncertainties to the accounting
clear physical understanding on the multidlecadal c i- for the anomalous modern ring-growth problem (Gray-
mate variability from theoretical or empinical studiE . bill & Idso 1993, Jacoby & DArrigo 1995, Briff a et al.
AlU current calibration of proxies to large-scale instr r- 1998, Feng 1999, Barber et al. 2000, Jacoby et al. 2000,
mental measurements have been mainly valid ov ~r Kniapp et al. 2001).
phases of rising temperature (Ogilvie & JMnsson 200 ).
The concern is that a different calibration response aris ~swhen the procedure is extended to an untested china 7. CONCLUSION
regime associated with a persistent cooling phase. Evai istegorpia
et al. (2002) worried about the reality of spuriois Many interesting questions on tegorpia
frequency evolution that may contaminate a multiproly nature and physical factors of surface temperature
reconstruction, in which the type of proxy data chang ~s or precipitation changes over the last 1000 yr cannot
over time and no sufficient overlap of proxy data exif ts be quantitatively and conclusively answered by cur-
for a proper inter-proxy calibration/validation proce- rent knowledge. The adopted period of 1000 yr is
dure. In other words, each proxy may have its distic ct strictly a convenience that merits little scientific
frequency response function, which could confuse t ie weight.
interpretation of climate variability. Finally, anot er Climate proxy research provides an aggregate, broad
concern is the lack of understanding of the air-s a perspective on questions regarding the reality of Little
relationship at the multidecadal timescale, even in t re Ice Age, Medieval Warm Period and the 20th century
reasonably well observed region of the North Atlan ic surface thermometer global warming. The picture
(H-akkinen 2000, Seager et al. 2000, Marshall et al. 2001, emerges from many localities that both the Little Ice
Slonosky &Yiou 2001,von Storch et al. 2001). Age and Medieval Warm epoch are widespread and
Briffa (2000) concluded that dendroclilnatological near-synchronous phenomena, as conceived by Bryson
records may support 'the notion that the last 100 ye rs et al. (1963), Lamb (1965) and numerous researchers
have been unusually warm, at least within the cont xt since. Overall, the 20th century does not contain the
of the last two millennia' Slightly later, Briff a et al. warmest anomaly of the past millennium in most of the
(2001), by adopting a new analysis procedure t at proxy records, which have been sampled world-wide.
seeks to preserve greater, long timescale variability Past researchers implied that unusual 20th century
(which shows a notable increase in variance at the warming means a global human impact. However, the
24-37 yr timescale compared with a previous Stan- proxies show that the 20th century is not unusually
dlardlization procedure) in their tree-ring density d ta warm or extreme.
than previously possible, stated that the 20th centurv is The lack of unusual warmth in the 20th century does
the globally warmest century of the last 600 yr. Tis not argue against human impacts on local and regional
conclusion is consistent with the borehole reconstr Ic- scales (perhaps on scales as small as 10 to 1000 kin 2 for
tion results of Huang et al. (2000). However, Ion er precipitation and io4 to io5 kmn2 for temperature).
and more carefully reconstructed tree-ring chronclo- Recently, Lawton et al. (2001) demonstrated how the
gies from Esper et al. (2002) showed that the Medieval deforested areas of tropical lowlands can, in coin-
Warm Period was as warm as the 20th century foi at bination with favorable topographical conditions and
least a region covering the Northern Hemisphere altered atmospheric air flow across the landscape, sig-
extratropics from about 30 to 700 N. nificantly raise the bases of convective and orographic
Soon & BaliunaS: Climatic and eny; onnientall changes of the past 1000 years 105
clouds around the Monteverde mnontane cloud forest; change: a status report. Bull Am Meteor Soc 80:2631-2659
of Costa Rica during the dry season, and thus drasti Baron C, Orombelli G (1994) Holocene glacier vaniations inthe Terra Nova Bay area (Victoria Land, Antarctica).
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Acknowledgements. This work was supported by funds fro Bianchi CC, McCave IN (1999) Holocene periodicity in North
the American Petroleum Institute (01-0000-4579). the Ar Atlantic climate and deep-ocean flow south of Iceland.
Force Office of Scientific Research (Grant AF49620-02-1- Nature 397: 515-517
0194) and the National Aeronautics and Space Admninistrati n Biondli F, Perkins DL, Cayan DR, Hughes MK (1999) July tem-
(Grant NACS-7635). The niews expressed herein are those f perature during the second millennium reconstructed
the authors and are independent of the sponsoring agencic S. from Idaho tree nings. Geophys Res Lett 26: 1445-1448
We have benefitted greatly from the true and kind spirit 3f Black DE, Peterson LC, Overpeck JT, Kaplan A, Evans MN,
research communications (including a preview of th ir Kashgarian M (1999) Eight centunies of North Atlantic
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Yoshio Tagami and Referee #3. We thank John Daly, Dia e the last 1000 years from Irish blanket peat and a possible
Douglas-Dalziel, Craig and Keith Idso for their unselfish co link to solar variability. Earth Planet Sdt Left 133:145-150
tributions to the references. We also thank the Editor, Ch is Bor L~, CermAk (1999) Climate change of the last millennium
de Freitas, for very helpful editorial changes that improv d inferred from borehole temperatures: regional pafterns of
the manuscript. We are very grateful to Maria McEachemn, climatic changes in the Czech Republic. Part mI. Glob
Melissa Hilbert, Barbara Palmer and Will Craves for invalu- Planet Change 21:225-235
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