Climatic Change (2011) 107:593–613DOI 10.1007/s10584-010-0016-2

Extensive glaciers in northwest North Americaduring Medieval time

Johannes Koch · John J. Clague

Received: 22 July 2009 / Accepted: 5 October 2010 / Published online: 22 February 2011© Springer Science+Business Media B.V. 2011

Abstract The Medieval Warm Period is an interval of purportedly warm climateduring the early part of the past millennium. The duration, areal extent, and evenexistence of the Medieval Warm Period have been debated; in some areas the climateof this interval appears to have been affected more by changes in precipitation than intemperature. Here, we provide new evidence showing that several glaciers in westernNorth America advanced during Medieval time and that some glaciers achievedextents similar to those at the peak of the Little Ice Age, many hundred years later.The advances cannot be reconciled with a climate similar to that of the twentieth cen-tury, which has been argued to be an analog, and likely were the result of increasedwinter precipitation due to prolonged La Niña-like conditions that, in turn, may belinked to elevated solar activity. Changes in solar output may initiate a responsein the tropical Pacific that directly impacts the El Niño/Southern Oscillation andassociated North Pacific teleconnections.

1 Introduction

The climatic history of the past millennium has traditionally been divided intothe Medieval Warm Epoch (or Medieval Warm Period), the Little Ice Age, andtwentieth century warming (Lamb 1995). However, research completed over the pasttwo decades points to highly complex changes in climate on decadal and centennialtimescales, raising questions about this simple tripartite classification (Bradley andJones 1993; Crowley and Lowery 2000; Luckman 2000; Nesje and Dahl 2003; Kochet al. 2007a; Clague et al. 2009).

J. Koch (B) · J. J. ClagueDepartment of Earth Sciences, Simon Fraser University, Burnaby, BC,V5A 1S6, Canadae-mail: jkoch@sfu.ca

594 Climatic Change (2011) 107:593–613

The term “Medieval Warm Epoch” was first used by Lamb (1965) for a period ofwarm climate centred on AD 1100–1200. Other researchers subsequently proposeddifferent times for the Medieval Warm Period: ninth through fourteenth centuries(Hughes and Diaz 1994), AD 900–1250 (Grove and Switsur 1994), AD 1000–1300(Crowley and Lowery 2000), AD 800–1200 (Broecker 2001), AD 1100–1200 (Bradleyet al. 2003), AD 960–1050 (Cook et al. 2004a), and AD 1000–1450 (Herweijer et al.2007). Given these different definitions, we adopt the more general term “Medievaltime” for the interval of interest in this paper—AD 800–1400. However, we use theterms “Medieval Warm Period” and “Medieval Climatic Anomaly” when referringspecifically to the work of other researchers.

The most compelling evidence for a warmer or drier interval about one thousandyears ago comes from western Europe, and especially Iceland, initially settled byVikings in AD 874 and Greenland, first settled in AD 982 (Fitzhugh and Ward 2000).Agriculture reached farther north and to higher elevations in England, Scandinavia,Germany, and the European Alps at this time (Lamb 1995). Anastazi occupationof the northern Colorado Plateau and human numbers at Cahokia, an archaeologicalsite near what is now St. Louis, Missouri, peaked during Medieval time (Emerson andLewis 1991; Petersen 1994). Some cultures in the Americas declined and collapsedduring Medieval time, due in part to prolonged drought; examples include the Nazcaand Moche in coastal Peru in the eighth and ninth centuries (Shimada et al. 1991;DeMenocal 2001; Quilter 2002; Dillehay et al. 2004; Castillo and Uceda 2008; Proulx2008), the Tiwanaku culture in the Bolivian–Peruvian altiplano around Lake Titicacain the eleventh and twelfth centuries (DeMenocal 2001; Binford et al. 1997), and,most famously, the Maya in southern Mexico and northern Central America in theninth and tenth centuries (Hodell et al. 1995, 2001; DeMenocal 2001; Haug et al.2003).

The Medieval Warm Period has assumed much importance in the current dis-cussion about climate change and its impacts in the twenty-first century. The Inter-governmental Panel on Climate Change (2007) has forecast a rise in global averagesurface temperature of 1.8 to 4◦C by the end of the century, and some scientists havesuggested that the Medieval Warm Period could serve as an appropriate analogue forconditions created by initial human-induced warming of Earth’s atmosphere (U.S.Department of Energy 1989; Lamb 1995; Karlén 1998; Karlén et al. 1999). However,the datum for ‘warmer-than-present’ climate, when referring to Medieval time, is theearly twentieth century (Hughes and Diaz 1994; Crowley and Lowery 2000; Bradleyet al. 2003), and climate has warmed significantly since then (Jones et al. 1998; Mannet al. 1999, 2008; Crowley and Lowery 2000). The warmest part of the MedievalWarm Period in the extra-tropical Northern Hemisphere may have resembled that ofthe last decade of the twentieth century (Esper et al. 2002; Moberg et al. 2005; Losoet al. 2006; Mann et al. 2008).

Precipitation is also an important component of climate variability. Stine (1994)presented evidence that drought characterized most of the Medieval Warm Periodin California and Patagonia. He suggested that a more appropriate term for thisperiod is “Medieval Climatic Anomaly”, given the importance of precipitation inclimate variability during Medieval time. Since Stine published his work, additionalevidence for widespread, prolonged drought during Medieval time has been foundin the western United States (Woodhouse and Overpeck 1998; Stahle et al. 2000;

Climatic Change (2011) 107:593–613 595

Fig. 1 Map of northwestern North America showing sites with radiocarbon ages used in this study.A Garibaldi PP; B Juneau Icefield; C Lillooet Icefield; D Waddington area; E Bella Coola area;F Klinaklini; G Kluane NP; H Bugaboo; I Peyto; J Athabasca; K Robson; L Kiwa; M Hubbard;N Icy Bay; O Wrangell Mountains; P western Prince William Sound; Q Kenai Mountains

Cook et al. 2004b, 2007; Yuan et al. 2006; Graham and Hughes 2007; Graham et al.2007; Herweijer et al. 2007; Meko et al. 2007).

In this paper, we consider climate in northwest North America during Medievaltime, based on the activity of alpine glaciers in British Columbia and Alaska (Fig. 1).We show that climate in this region and in other mountain ranges throughout theworld, although variable, allowed glaciers to advance to positions near Little Ice Agelimits during Medieval time. We attempt to explain these advances by consideringpossible forcing mechanisms.

2 Glacier advances during Medieval time

Alpine glaciers respond rapidly to changes in temperature and precipitation, andthus have long been used to investigate climate variability (e.g., Denton and Karlén1973; Röthlisberger 1986; Glasser et al. 2004; Oerlemans 2005). However, ourknowledge of Holocene glacier fluctuations is incomplete because, in the NorthernHemisphere, glacier advances during the past millennium (“Little Ice Age” sensuGrove 1988; Luckman 2000) were generally the most extensive of the past10,000 years (Luckman 2004) and obliterated landforms and sediments producedduring earlier advances. Evidence for pre-Little Ice Age glacier advances, however,has been found in forefields that were deglaciated during the past century. Detritaland in situ plant fossils, including stumps, stems, branches, and forest soils, have beenreported from recently deglacierized forefields in front of many glaciers in western

596 Climatic Change (2011) 107:593–613

Canada, Alaska, the European Alps, New Zealand, and Patagonia (Figs. 2 and 3).In situ stumps and soils provide unequivocal evidence of previous glacier overridingand of subsequent exposure during the twentieth century. Much of the detrital woodshows evidence of glacial transport, including missing bark, abrasion, embeddedsediment, and flattening. Such wood is interpreted to have been entrained from treeswhen or shortly after they were overridden and to then have been transported at thebase of and within the glacier.

Some of the in situ and detrital wood recovered from recently deglacierizedforefields in northwest North America dates to Medieval time and constrains glaciermargins immediately before the Little Ice Age (Table 1). We summarize this evi-dence here and discuss it in the context of similar evidence found in other parts ofthe world.

2.1 Western Canada

Two glaciers on the east side of the Juneau Icefield in northwest British Columbia(Fig. 1) advanced one or more times between AD 900 and 1400, and possiblycontinued to do so until reaching their maximum Little Ice Age extents (Clagueet al. 2010). Tulsequah Glacier advanced into a forest 3 km downvalley of its presentterminus sometime between AD 660 and 880 (Table 1; Clague et al. 2010). The southlobe of Llewellyn Glacier overrode a forest 1,600 m downvalley from its presentterminus sometime between AD 1030 and 1150 (Fig. 2a). At about the same time(between AD 995 and 1175), the east lobe advanced to within 400 m of its LittleIce Age maximum limit (Fig. 2b). Llewellyn Glacier continued to advance, killingtrees between about AD 1200 and 1390, indicating that both lobes were advancingfor most of the eleventh to fourteenth century (Clague et al. 2010).

Several glaciers in Garibaldi Provincial Park in southwest British Columbia (Fig. 1)were advancing between AD 1000 and 1300 (Table 1; Koch et al. 2007a, 2009).

Fig. 2 Examples of subfossil wood in the Llewellyn Glacier forefield. a Overridden in-situ stumpradiocarbon dated to AD 1030–1150 (UCIAMS-45009). b Mats of wood plastered against a rockslope by Llewellyn Glacier when it advanced between AD 1015 and AD 1150 (Beta-200740)

Climatic Change (2011) 107:593–613 597

Fig. 3 The north lateral moraine of Weart Glacier. Three wood layers are delineated by stippledlines. The lowest layer (bottom right) dates to AD 985–1015 (UCIAMS-45010); the middle layer (topright) to AD 1045–1160 (UCIAMS-45011); and the uppermost layer to AD1450–1660

A tributary of Warren Glacier overrode a tree sometime between AD 1030 and1170, and at least four other glaciers in the Park advanced between the twelfthand fourteenth centuries. Lava Glacier advanced to a position beyond its earlytwentieth century terminus between AD 1150 and 1250, and reached to within 200 mof its Little Ice Age maximum extent before the end of the thirteenth century.This advance was followed by a second, more extensive advance sometime betweenAD 1290 and 1430. Stave and Helm glaciers were more extensive than in the mid-twentieth century sometime between AD 1170 and 1260, and AD 1260 and 1310,respectively; and Garibaldi Glacier was at least as extensive as in the 1920s sometimebetween AD 1270 and 1390.

Two wood layers in the north lateral moraine of Weart Glacier (Fig. 3) in northernGaribaldi Park provide evidence for extensive and advancing ice at the end of thefirst millennium AD and beginning of the second millennium AD. Four branchesfrom the lower layer crossdate with one another (r = 0.647); one of the four yieldeda radiocarbon age with a calibrated range of AD 985–1015 (Table 1). Five piecesof wood collected from the upper layer and ranging from small branches to logs alsocrossdate (r = 0.589); one of them returned a radiocarbon age with a calibrated range

598 Climatic Change (2011) 107:593–613

Tab

le1

Rad

ioca

rbon

ages

onfo

ssil

plan

tsre

cove

red

from

glac

ier

fore

fiel

dsin

Bri

tish

Col

umbi

a,A

lber

ta,Y

ukon

Ter

rito

ry,a

ndA

lask

ath

atda

teto

Med

ieva

ltim

e

Mou

ntai

nra

nge

Gla

cier

aL

ab.n

o.(1

)14

Cag

eSt

dC

alA

Db

Insi

tu?

Not

esR

efer

ence

Roc

kyM

ount

ains

Rob

son

(K)

B94

5811

4080

780–

980

nL

ogin

outw

ash

chan

nel;

cros

sdat

edL

uckm

an(1

986,

1994

)to

AD

1253

Roc

kyM

ount

ains

Pey

to(I

)SR

C29

9011

4075

780–

980

nL

ogon

till

surf

ace

Luc

kman

etal

.(19

93)

Wra

ngel

lMou

ntai

nsN

izin

a(O

)B

eta–

1229

7411

4060

815–

980

nD

etri

talw

ood

betw

een

tills

Wile

set

al.(

2002

)St

.Elia

sM

ount

ains

Klu

tlan

(G)

GSC

–222

711

2090

810–

1015

nW

ood

belo

wti

llR

ampt

onet

al.(

1978

)K

enai

Mou

ntai

nsM

cCar

ty(Q

)B

eta–

3962

911

2060

865–

995

ySt

ump

Wile

san

dC

alki

n(1

994)

Coa

stM

ount

ains

Pur

gato

ry(E

)S–

2976

1110

7087

0–10

15n

Det

rita

lwoo

d;lo

wer

pale

osol

Ryd

eran

dT

hom

son

(198

6)in

late

ralm

orai

neC

oast

Mou

ntai

nsL

illoe

et(C

)W

k–12

308

1093

4589

5–99

0n

Woo

dfr

agem

entf

rom

pale

osol

Rey

esan

dC

lagu

e(2

004)

(upp

er3

cm)

inla

tera

lmor

aine

Coa

stM

ount

ains

Hub

bard

(M)

Mus

keg

#110

9080

865–

1025

yO

rgan

icse

dim

entf

rom

base

Bar

clay

etal

.(20

01)

ofm

uske

gco

reK

enai

Mou

ntai

nsM

cCar

ty(Q

)B

eta–

3592

710

9070

885–

1020

ySt

ump

inou

twas

hW

iles

and

Cal

kin

(199

4)C

oast

Mou

ntai

nsM

ende

nhal

l(B

)Y

–132

–84

1090

6089

0–10

00y

Stum

pin

outw

ash

Pre

ston

etal

.(19

55)

Coa

stM

ount

ains

Bea

re(N

)B

eta–

8492

310

9060

890–

1000

yW

ood

info

rest

bed

Bar

clay

etal

.(20

06)

Coa

stM

ount

ains

Lill

oeet

(C)

GSC

–660

610

9050

895–

995

nB

ranc

hin

peat

inla

tera

lmor

aine

Rey

esan

dC

lagu

e(2

004)

Ken

aiM

ount

ains

McC

arty

(Q)

Bet

a–39

630

1090

5089

5–99

5y

Stum

pin

outw

ash

Wile

san

dC

alki

n(1

994)

Pur

cell

Mou

ntai

nsB

ugab

oo(H

)B

eta–

7572

1080

7089

0–10

20n

Woo

din

late

ralm

orai

neO

sbor

n(1

986)

Coa

stM

ount

ains

WSt

ave

(A)

Bet

a–17

0668

1080

6089

5–10

15n

Smal

llog

inla

tera

lmor

aine

Koc

het

al.(

2007

b)C

oast

Mou

ntai

nsW

eart

(A)

UC

IAM

S–45

010

1055

2098

5–10

15n

Low

estw

ood

laye

rin

late

ralm

orai

ne;

this

stud

yF

ig.3

,bot

tom

righ

tK

enai

Mou

ntai

nsN

orth

wes

tern

(Q)

Bet

a–47

836

1030

7089

5–11

20n

Det

rita

llog

Wile

san

dC

alki

n(1

994)

Coa

stM

ount

ains

Gilk

ey/B

attl

e(B

)H

v–11

308

1020

6596

5–11

25n

Det

rita

llog

inla

tera

lmor

aine

Röt

hlis

berg

er(1

986)

Coa

stM

ount

ains

inne

rIc

yB

ay(N

)I–

1221

410

0090

970–

1160

yB

ase

ofpe

atla

yer

Por

ter

(198

9)P

urce

llM

ount

ains

Bug

aboo

(H)

Bet

a–10

730

1000

8097

5–11

55n

Woo

din

late

ralm

orai

neO

sbor

n(1

986)

Coa

stM

ount

ains

Bea

re(N

)B

eta–

9598

910

0060

980–

1155

yW

ood

info

rest

bed

Bar

clay

etal

.(20

06)

Coa

stM

ount

ains

Lle

wel

lyn

(B)

Bet

a–20

0741

1000

5098

5–11

20n

Bra

nch

orro

otin

till

near

end

mor

aine

Cla

gue

etal

.(20

10)

Pur

cell

Mou

ntai

nsB

ugab

oo(H

)B

eta–

7575

990

7098

5–11

55n

Woo

din

late

ralm

orai

neO

sbor

n(1

986)

Wra

ngel

lMou

ntai

nsK

enni

cott

(O)

Bet

a–13

3808

990

6099

0–11

55y

Stum

pW

iles

etal

.(20

02)

Coa

stM

ount

ains

Bea

re(N

)B

eta–

9598

798

060

1010

–115

5y

Woo

din

fore

stbe

dB

arcl

ayet

al.(

2006

)

Climatic Change (2011) 107:593–613 599

Coa

stM

ount

ains

Lle

wel

lyn

(B)

Bet

a–20

0740

980

4010

15–1

150

yW

ood

inti

llne

aren

dm

orai

ne;F

ig.2

bC

lagu

eet

al.(

2010

)C

oast

Mou

ntai

nsL

lew

elly

n(B

)B

eta–

2007

4297

060

1015

–115

5n

Det

rita

ltw

igin

till

near

end

mor

aine

Cla

gue

etal

.(20

10)

Wra

ngel

lMou

ntai

nsK

enni

cott

(O)

Bet

a–13

3810

970

6010

15–1

155

yR

oots

inso

ilbe

twee

nti

llsW

iles

etal

.(20

02)

Coa

stM

ount

ains

Lle

wel

lyn

(B)

UC

IAM

S–45

009

950

1510

30–1

150

ySt

ump

onhi

ll;16

00m

infr

ont

Cla

gue

etal

.(20

10)

of20

04sn

out;

Fig

.2a

Coa

stM

ount

ains

Her

bert

(B)

HE

RG

3.2

945

2410

30–1

125

nT

ree

trun

kL

ache

r(1

999)

;Web

er(2

006)

Roc

kyM

ount

ains

Stut

fiel

d(J

)B

eta–

6242

294

060

1030

–115

5y

Roo

ted

stem

;lat

eral

mor

aine

Osb

orn

etal

.(20

01)

Coa

stM

ount

ains

Gilk

ey/B

attl

e(B

)H

v–11

299

930

8510

20–1

180

yW

ood

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Röt

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berg

er(1

986)

Wra

ngel

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ntai

nsK

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cott

(O)

Bet

a–13

3809

930

6010

35–1

160

ySt

ump

betw

een

tills

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set

al.(

2002

)St

.Elia

sM

ount

ains

Bar

nard

(O)

Bet

a–13

3803

930

6010

35–1

160

nD

etri

talw

ood

inla

tera

lmor

aine

Wile

set

al.(

2002

)C

oast

Mou

ntai

nsL

lew

elly

n(B

)G

SC–6

632

930

4010

40–1

155

nB

ranc

hin

till;

near

end

mor

aine

Cla

gue

etal

.(20

10)

Coa

stM

ount

ains

Hub

bard

(M)

Bet

a–54

384

920

110

1020

–121

5n

Woo

din

basa

lpea

tB

arcl

ayet

al.(

2001

)C

oast

Mou

ntai

nsW

arre

n(A

)B

eta–

1487

9192

070

1030

–117

0y

Roo

tsto

ck;c

irqu

egl

acie

r;K

och

etal

.(20

07a)

alm

osta

tmax

imum

Roc

kyM

ount

ains

Pey

to(I

)SR

C31

1192

060

1035

–116

5y

(?)

Roo

tsto

ck;o

uter

ring

sL

uckm

anet

al.(

1993

)P

urce

llM

ount

ains

Bug

aboo

(H)

Bet

a–10

734

910

7010

35–1

180

nW

ood

inla

tera

lmor

aine

Osb

orn

(198

6)K

enai

Mou

ntai

nsA

ialik

(Q)

Bet

a–39

634

910

6010

40–1

175

nD

etri

tals

tum

pin

lake

sedi

men

tsW

iles

and

Cal

kin

(199

4)C

oast

Mou

ntai

nsW

eart

(A)

UC

IAM

S–45

011

910

2010

45–1

160

nM

iddl

ew

ood

laye

rin

late

ralm

orai

ne;

this

stud

yF

ig.3

,top

righ

tP

urce

llM

ount

ains

Bug

aboo

(H)

Bet

a–75

7690

060

1045

–119

0n

Woo

din

late

ralm

orai

neO

sbor

n(1

986)

Roc

kyM

ount

ains

Rob

son

(K)

B37

3190

060

1045

–119

0y

Shea

red

stum

p;cr

ossd

ated

Luc

kman

(198

6,19

94)

toA

D12

53C

oast

Mou

ntai

nsK

linak

lini(

F)

S–15

6790

040

1045

–118

5y

Stum

pR

yder

and

Tho

mso

n(1

986)

Coa

stM

ount

ains

Lill

oeet

(C)

Bet

a–18

0885

890

4010

50–1

210

nW

ood

frag

men

tfro

mpa

leos

olR

eyes

and

Cla

gue

(200

4)(u

pper

3cm

)in

late

ralm

orai

neC

oast

Mou

ntai

nsM

ende

nhal

l(B

)M

EN

R2.

689

023

1050

–121

0n

Woo

din

rive

rL

ache

r(1

999)

;Web

er(2

006)

Coa

stM

ount

ains

Dav

idso

n(B

)M

–192

488

010

010

40–1

225

nW

ood

unde

rti

llan

dou

twas

hC

rane

and

Gri

ffin

(196

8)C

oast

Mou

ntai

nsH

ubba

rd(M

)U

SGS–

923

880

6510

45–1

220

yB

asal

peat

Bar

clay

etal

.(20

01)

Coa

stM

ount

ains

Inne

rIc

yB

ay(N

)I–

1228

186

580

1045

–125

5n

Log

indr

ift

Por

ter

(198

9)C

oast

Mou

ntai

nsIn

ner

Icy

Bay

(N)

I–12

303

860

8010

50–1

260

yP

eatl

ayer

Por

ter

(198

9)P

rinc

eW

illia

mP

rinc

eton

(P)

UW

–519

860

7510

50–1

255

nD

etri

tall

ogW

iles

etal

.(19

99)

Soun

d

600 Climatic Change (2011) 107:593–613

Tab

le1

(con

tinu

ed)

Mou

ntai

nra

nge

Gla

cier

aL

ab.n

o.(1

)14

Cag

eSt

dC

alA

Db

Insi

tu?

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Climatic Change (2011) 107:593–613 601

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602 Climatic Change (2011) 107:593–613

of AD 1045–1160. The lateral moraine stratigraphy indicates that Weart Glacierthickened from the late tenth century, through much of the eleventh century, andreached beyond the position of its 2006 terminus to within about 200 m of its LittleIce Age limit.

Glacier advances in western Canada during Medieval time were not limited tothe areas described above (Fig. 1 and Table 1). Robson, Kiwa, and Peyto glaciersin the Canadian Rockies advanced between the mid-twelfth and mid-fourteenthcenturies (Luckman 1986, 1996). Warmer conditions between AD 950 and 1100are implied by ring widths and snags at tree line near Athabasca Glacier and byfossil remains of a larch tree, which grew about 90 km farther northwest duringMedieval time than any larch did in the late 1980s (Luckman 1986, 1994). BugabooGlacier in southeast British Columbia was larger than today, and advancing, betweenAD 880 and 1190, based on radiocarbon ages on wood from a lateral moraine(Osborn 1986). A short-lived glacier advance sometime between AD 710 and 1030 isdocumented in the White River valley in Yukon Territory (Denton and Karlén 1977).Glaciers in the Mt. Waddington area in the central Coast Mountains built morainesthat have been dated by lichenometry to AD 925–933 and about AD 1031 (OvalGlacier), AD 1120 (Tiedemann Glacier), and AD 1146 (Escape Glacier); severalother glaciers constructed moraines in the early thirteenth century (Larocque andSmith 2003). Farther west, in the Bella Coola area, Purgatory and Fyles glacierswere more extensive in the early to mid-thirteenth century than in the late 1980s(Desloges and Ryder 1990). Klinaklini and Franklin glaciers in the southern CoastMountains advanced sometime between AD 1040 and 1260 (Ryder and Thomson1986), and Bridge Glacier was advancing between AD 1270 and 1390 (Ryder andThomson 1986). Lateral moraine stratigraphy at Lillooet Glacier in the southernCoast Mountains provides evidence for an advance between AD 890 and 1210 (Reyesand Clague 2004).

2.2 Alaska

Evidence from Alaska is summarized in Table 1; study sites are shown in Fig. 1.Glaciers on the west side of the Juneau Icefield in southeast Alaska were in advancedpositions through most of the period AD 800–1400 (Motyka and Beget 1996).At about AD 1000, they retreated synchronously but remained at more advancedpositions than today. One of two lobes of Hubbard Glacier achieved its maximumHolocene extent around AD 1000 and was retreating by AD 1308, but the otherlobe was in an advanced position throughout that period (Barclay et al. 2001).Glaciers in Icy Bay were near their maxima until AD 1075, after which they rapidlyretreated (Barclay et al. 2006). Tree remains in glacier forefields in the WrangellMountains provide evidence for an advance that was underway by about AD 1100;the locations of the tree remains indicate that the glaciers were small prior to theadvance (Wiles et al. 2002). Several glaciers in the Kenai Mountains advancedin the eleventh and early twelfth centuries, and McCarthy Glacier advanced asearly as AD 900 (Wiles and Calkin 1994). Princeton Glacier in western PrinceWilliam Sound began to advance around AD 1180, and several glaciers, includingPrinceton, continued to advance well into the thirteenth century (Wiles et al. 1999).A recent review of glacier activity in southern Alaska, however, shows that severalglaciers were less extensive than at present from the tenth to thirteenth centuries,

Climatic Change (2011) 107:593–613 603

coincident with warming inferred from Alaskan tree-ring chronologies (Wiles et al.2008).

2.3 Europe

Here, we broaden our review of glacier activity during Medieval time to Europe(Fig. 4). Many records of glacier fluctuations in the European Alps are of high qualityand provide more detail than those from other mountainous areas. Hormes et al.(2001) reported eight phases of reduced glacier extent in the Swiss Alps during theHolocene; Medieval time is not one of them. Joerin et al. (2006), in a follow-up study,do not identify Medieval time as a period of retracted glaciers. More specifically,Great Aletsch, Gorner, and Lower Grindelwald glaciers in the Swiss Alps advancedbetween AD 800 and 900, retreated between AD 900 and 1100, and advanced againbetween AD 1100 and 1200 (Holzhauser et al. 2005). Gepatschferner in the AustrianAlps advanced about AD 809 and again between AD 1140 and 1170, at which time itwas about as extensive as in the late nineteenth century and early twentieth century(Nicolussi and Patzelt 2000). The glacier must have advanced during most of the

Fig. 4 Times of glacier advances during Medieval time and early Little Ice Age in western NorthAmerica, Europe (Alps and Scandinavia), southern South America, New Zealand, and the Hi-malayas. a Periods of advance in each area are indicated by black bars. b Comparison of generalizedMedieval and early Little Ice Age history of: (1) glaciers in western Canada (Koch and Clague 2006).(2) Three glaciers in the European Alps (Holzhauser et al. 2005). (3) Two glaciers in northwestBritish Columbia (Clague et al. 2010). (4) Weart Glacier. Present-day refers to glacier extent in2004–2007; LIA max (Little Ice Age maximum) indicates glacier limits reached in the seventeenth tonineteenth centuries

604 Climatic Change (2011) 107:593–613

twelfth century, and probably earlier, to reach such an advanced position. It thusappears that glaciers in the European Alps advanced early and late in Medieval time,but retreated during most of the tenth and eleventh centuries.

Some glaciers in Breheimen, Norway, advanced sometime between AD 655 and963 and again between AD 981 and 1399 (Winkler et al. 2003). Glaciers in northernFolgefonna, Norway, also were advancing during Medieval time; Bakke et al. (2005)suggested that they were larger at that time than today. In their review of Holoceneglacier activity in Norway, Nesje et al. (2008) show that many glaciers advancedbetween AD 800 and 1400, although some underwent short-lived, but significantretreat.

2.4 Southern Hemisphere

Evidence from the Southern Hemisphere comes from Patagonia and New Zealand(Fig. 4). Glaciers in the Cordillera Darwin in Chile advanced sometime between AD1020 and 1170 and again between AD 1270 and 1390 (Kuylenstierna et al. 1996).Ema Glacier, 75 km west of the Cordillera Darwin, advanced between AD 1240and 1390 (Strelin et al. 2008). Some glaciers in Patagonia built moraines aroundAD 1040, before AD 1236, and about AD 1330 (Koch and Kilian 2005), and threeglaciers advanced into forests between AD 1000 and 1200 (Luckman and Villalba2001). Glasser et al. (2004) concluded that many Patagonian glaciers advanced latein Medieval time. Glaciers in the Southern Alps in New Zealand advanced threeseparate times between AD 880 and 1410; the first and third advances are majorregional events (Gellatly et al. 1988).

Temperate glaciers influenced by a monsoonal climate advanced between AD 800and 1150 in the southern Himalayas and around AD 1000 in the central Himalayas;temperatures are inferred to have been about 0.7◦C lower than today (Yang et al.2008). Owen (2009) reports advances of similar age elsewhere in the Himalayas andin Tibet.

3 Discussion and conclusion

Here, we discuss the cause, extent, and character of the Medieval ClimaticAnomaly—What caused this event? Was it a global event? How warm was it duringMedieval times? Was it dry or wet?

The cause of the Medieval Climatic Anomaly is not well understood, but severalhypotheses have been proposed, including changes in solar activity and the El Niño-Southern Oscillation system (ENSO) (Jirikowic and Damon 1994; Hodell et al. 2001;Cobb et al. 2003; Mann et al. 2005; Graham et al. 2007). Atmospheric productionof 14C, 10Be, and other cosmogenic isotopes is controlled by the rate of cosmic raybombardment of nitrogen and oxygen in the upper atmosphere. A less active Sunresults in a weak solar wind, increased cosmic ray bombardment of Earth’s upperatmosphere, greater 14C and 10Be production, and, all other things being equal,atmospheric cooling (Eddy 1977). Numerous researchers have investigated the roleof solar forcing on global or Northern Hemisphere temperatures over the past severalcenturies and have concluded that solar radiation is an important forcing mechanism,at least until the middle of the twentieth century (Lean et al. 1995; Crowley and

Climatic Change (2011) 107:593–613 605

Kim 1996; Mann et al. 1998; Crowley 2000; Mauquoy et al. 2004; Reimer 2004;Solanki et al. 2004; Ammann et al. 2007). Solar minima during the historic periodcoincide with glacier advances (Lawrence 1950; Wiles et al. 2004; Luckman andWilson 2005; Koch et al. 2007a, b), and reconstructed solar irradiance and NorthernHemisphere surface temperature between AD 1610 and 1800 are strongly positivelycorrelated (Lean et al. 1995; Crowley and Kim 1996). Three solar indices and twoNorthern Hemisphere surface temperature reconstructions for the past millennium,prior to AD 1850, also are positively correlated (Crowley 2000). 14C and 10Be pro-duction values were low during Medieval time, suggesting that solar irradiance washigh (Jirikowic and Damon 1994). This period of high solar irradiance correspondsto a time of warm dry climate in temperate regions of the world and in the Mayanlowlands (Hodell et al. 2001), and with warm wet climate in areas influenced bymonsoonal precipitation (Davis 1994).

ENSO may also play a role in the response of global climate to changes in radiativeforcing. Model experiments reproduce the tendency for an El Niño-like state to existin the tropical Pacific during the Little Ice Age due to decreased radiative forcing,and a La Niña-like state to exist in the tropical Pacific during the Medieval WarmPeriod due to increased radiative forcing (Mann et al. 2005). The El Niño-like stateis associated with widespread tropical warming and weaker warming or cooling atmiddle latitudes and a higher temperature gradient between the two regions. Astrong latitudinal temperature gradient and intensified circulation result in advectionand wind-induced evaporation in the tropical Pacific, which leads to greater polewardtransport of water vapour (Hendy et al. 2002). In contrast, the La Niña-like stateis associated with a decreased latitudinal temperature gradient (Seager et al. 2003;Mann et al. 2005). During Medieval time, the tropical Pacific experienced extendedLa Niña-like conditions (Cobb et al. 2003; Graham et al. 2007), consistent with tree-ring data from the western Americas (Villalba 1994, Cook et al. 2004b, 2007; Grahamet al. 2007; Herweijer et al. 2007).

Tree-ring records for the past 400 years from the extra-tropical west coast of theAmericas indicate synchronous decadal-scale climate oscillations in North and SouthAmerica, likely forced by the Pacific Ocean (Villalba et al. 2001). In recent times, thissymmetric hemispheric climate response is associated with ENSO (Seager et al. 2003,2005a, b). Recent glacier mass balance studies in North and South America havefound significant correlations between glacier regime and ENSO (Bitz and Battisti1999; Depetris and Pasquini 2000; Moore and Demuth 2001; Watson et al. 2006),suggesting that the glaciers are sensitive to ENSO.

Turning to questions about the extent and character of the Medieval ClimaticAnomaly, little evidence has been reported for the event in the Southern Hemisphere(Villalba 1994; Jones et al. 1998). The scarcity of evidence, however, is mostly dueto the lack of high-resolution climate proxies spanning Medieval time. Most high-resolution temperature reconstructions for the past millennium in the NorthernHemisphere indicate generally warmer conditions during Medieval time than atother times during the past millennium (Jones et al. 1998, 2001; Mann et al. 1998,1999, 2008; Crowley and Lowery 2000; Esper et al. 2002; Moberg et al. 2005). Thegreatest warmth is centred on the periods AD 950–1000 and 1040–1100 (Fig. 5a;Esper et al. 2002; Mann et al. 2008). These warm periods may be similar to the earlytwentieth century (Esper et al. 2002; Moberg et al. 2005; Loso et al. 2006; Mann et al.2008).

606 Climatic Change (2011) 107:593–613

Fig. 5 Comparison ofreconstructed temperaturesduring Medieval time andearly Little Ice Age.a Northern Hemisphere basedon extratropical tree ringwidths; thin black lines arebootstrap 95% confidencelimits (Cook et al. 2004a).b Canadian Rocky Mountainsbased on a maximum latewooddensity record (Luckman andWilson 2005). c Glacieradvances in western NorthAmerica; light grey verticalbars denote intervals of glacieradvance

High-resolution reconstructions of precipitation indicate widespread drought inwestern North America during Medieval time (Woodhouse and Overpeck 1998;Stahle et al. 2000; Cook et al. 2004b, 2007; Yuan et al. 2006; Graham and Hughes2007; Graham et al. 2007; Herweijer et al. 2007; Meko et al. 2007). The dry inter-vals have been referred to as “megadroughts” due to their duration and severity(Woodhouse and Overpeck 1998; Stahle et al. 2000). Two low-stands of MonoLake in California probably correspond to the most severe of the late Holocenemegadroughts—around AD 910–1110 and 1210–1350 (Stine 1994) or, alternatively,AD 870–1030 and 1130–1300 (Graham and Hughes 2007); the differences reflectdating uncertainties. Reconstructions of the Palmer Drought Severity Index with treerings indicate megadrought conditions through much of the western United States atAD 900–1050, 1130–1170, 1240–1270, and 1360–1380 (Cook et al. 2007; Herweijeret al. 2007).

Summer temperature reconstructions in the Canadian Rocky Mountains basedon tree rings indicate periods of warmer-than-average climate during Medieval time(Fig. 5b; Luckman 1994; Luckman and Wilson 2005), as do reconstructions fromcoastal Alaska (Barclay et al. 1999; D’Arrigo et al. 2005; Loso et al. 2006). It wasrelatively warm in western Canada at AD 1010–1050, 1090–1100, 1160–1180, and1360–1410, and relatively cool around AD 1220–1330 (Luckman and Wilson 2005).Similar climate trends have been reported from coastal Alaska by Loso et al. (2006).Wildfire frequency in southeast British Columbia was higher than average at AD980–1060 and 1100–1240 (Hallett et al. 2003), and wildfires were particularly frequentin Yukon Territory at AD 1240–1410 (Yalcin et al. 2006).

Definitions of the Medieval Climatic Anomaly have been complicated by findingsthat Medieval time was interrupted by short-lived intervals of cool or wet conditions(Fig. 5a and b; Leavitt 1994; Luckman 1994; Stine 1994; Hallett et al. 2003; Cook et al.2004b, 2007; Luckman and Wilson 2005; Loso et al. 2006; Yalcin et al. 2006; Grahamand Hughes 2007; Graham et al. 2007; Meko et al. 2007). Proxy reconstructions ofNorthern Hemisphere temperature commonly show two or three relatively short

Climatic Change (2011) 107:593–613 607

warming intervals rather than a single continuous warm period (Jones et al. 1998,2001; Mann et al. 1998, 1999, 2008; Crowley and Lowery 2000; Esper et al. 2002;Moberg et al. 2005). Short-lived glacier advances between AD 1050 and 1150have been reported from the European Alps, Norway, Alaska, extratropical SouthAmerica, and New Zealand (Grove and Switsur 1994). Glacier advances duringMedieval time in western Canada and Alaska occurred around AD 950–1100 and1200–1300 (Fig. 5c). The later advance occurred at a time of cooler temperatures(Esper et al. 2002; Luckman and Wilson 2005; Mann et al. 2008), but the former iscoincident with severe drought in the western U.S. and high fire frequency in theYukon (Stine 1994; Yalcin et al. 2006; Cook et al. 2007; Graham and Hughes 2007;Herweijer et al. 2007).

If it was relatively warm and dry in western Canada and Alaska during Medievaltime, why did glaciers in those areas advance? Today, the mass balance of glaciersin northwest North America is sensitive to winter precipitation and summer temper-atures (Bitz and Battisti 1999; Moore and Demuth 2001). If summers were warmerduring Medieval time, as temperature reconstructions suggest, glaciers should haveretreated significantly. The answer may lie in the fact that climate was warmer onlyduring part of this interval.

Several authors have argued that Holocene glacier advances are driven bychanges in solar radiation, primarily sunspots (Denton and Karlén 1973; Karlén andKuylenstierna 1996; Koch and Clague 2006). A correlation between sunspot numbersand glacier activity has been noted in Alaska (Wiles et al. 2004), the CanadianRockies (Luckman and Wilson 2005), and the southern Coast Mountains (Koch et al.2007a, b). Reconstructions of solar irradiance and sunspot numbers for Medievaltime, based on 14C activity, show relatively high irradiance at AD 920–970 and 1100–1200 and low irradiance at AD 970–1100 and after AD 1200 (Fig. 6a; Bard et al. 2000;Solanki et al. 2004). Times of low irradiance correspond well with glacier advancesin coastal British Columbia (AD 950–1100 and 1200–1300), lending support to thesignificance of solar irradiance as a factor affecting glacier behaviour (Fig. 6).

Several authors have proposed that prolonged La Niña-like conditions may beresponsible for the Medieval megadroughts in the western United States (Cook et al.2004b, 2007; Herweijer et al. 2006, 2007; Graham et al. 2007). During La Niña eventstoday, much of the western United States is dry and warm, but the coasts of BritishColumbia and Alaska experience average or cooler-than-average temperatures andwetter winters (Ropelewski and Halpert 1986; Kiladis and Diaz 1989; Shabbar andKhandekar 1996; Shabbar et al. 1997; Wang and Ting 2000; Hoerling and Kumar2003; Seager et al. 2003, 2005a, b). Pacific storms track towards more northerly

Fig. 6 Comparison of glacieradvances in western NorthAmerica during Medieval timeand early Little Ice Age, andinferred solar irradiance(Solanki et al. 2004). Lightgrey vertical bars denoteintervals of sunspot minima(Stuiver 1961; Bond et al.2001)

608 Climatic Change (2011) 107:593–613

locations during La Niña events, bringing more precipitation to coastal BritishColumbia and Alaska and less to the western U.S. It thus appears that a significantincrease in winter precipitation during times dominated by La Niña conditions mayhave counteracted warmer summers, leading to overall positive mass balances forglaciers in the region.

In summary, glacier advances in western Canada and Alaska during Medievaltime were most likely the result of increased winter precipitation due to prolongedLa Niña-like conditions that were caused by increased solar activity. We argue thatchanges in solar output initiate a response in the tropical Pacific that impacts the ElNiño/Southern Oscillation and associated North Pacific teleconnections.

We also argue that climate during Medieval time was much more variable thanthe term “Medieval Warm Period” implies, and that climatic variability in manyareas was driven more by precipitation than temperature. In this sense, the term“Medieval Warm Period” is inappropriate and should be replaced by Medieval Cli-matic Anomaly, as suggested by others. Our study also indicates that Medieval timeis not a good analogue for current warming. Glaciers in western Canada and Alaskaare presently retreating rapidly and are significantly smaller than they were duringMedieval time.

Acknowledgements The research was supported by a Natural Sciences and Engineering ResearchCouncil of Canada Discovery Grant to Clague and funding from The College of Wooster to Koch.We thank BC Parks for permission to work in Garibaldi and Atlin provincial parks. We thank B.H.Luckman and two anonymous reviewers for helpful reviews of drafts of the paper.

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