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VOLCAN
IC ERUPTIONS
AN D CLIMATE
Alan Robock
Departmentof Environmental
Sciences
RutgersUniversity
New Brunswick,New Jersey
Abstract. Volcaniceruptionsare an importantnatural
cause of climate change on many timescales.A new
capability to predict the climatic responseto a large
tropical eruption for the succeeding2 yearswill prove
valuableto society.In addition, to detect and attribute
anthropogenicinfluenceson climate,includingeffectsof
greenhousegases,aerosols,and ozone-depleting
chemicals,it is crucial to quantify the natural fluctuationsso
as to separatethem from anthropogenicfluctuationsin
the climate record. Studyingthe responsesof climate to
volcanic eruptions also helps us to better understand
important radiative and dynamical processesthat respondin the climate systemto both natural and anthropogenicforcings.Furthermore, modeling the effectsof
volcaniceruptionshelps us to improve climate models
that are neededto studyanthropogeniceffects.Large
volcanic eruptions inject sulfur gasesinto the stratosphere,which convertto sulfateaerosolswith an e-folding residencetime of about 1 year. Large ash particles
fall out muchquicker.The radiativeandchemicaleffects
of this aerosolcloud produce responsesin the climate
system.By scatteringsomesolarradiationbackto space,
the aerosolscoolthe surface,but by absorbingboth solar
and terrestrial radiation, the aerosol layer heats the
stratosphere.For a tropical eruption this heating is
largerin the tropicsthan in the high latitudes,producing
1.
INTRODUCTION
Volcanism has long been implicated as a possible
causeof weatherand climatevariations.Even 2000years
ago,Plutarchand others[Forsyth,1988]pointedout that
the eruption of Mount Etna in 44 B.C. dimmed the Sun
and suggested
that the resultingcoolingcausedcropsto
shrivel and producedfamine in Rome and Egypt. No
other publicationson this subjectappeareduntil Benjamin Franklin suggestedthat the Lakagigareruptionin
Iceland in 1783 might have been responsiblefor the
abnormallycold summerof 1783 in Europe and the cold
winter of 1783-1784 [Franklin,1784].Humphreys[1913,
1940]associated
coolingeventsafter largevolcaniceruptionswith the radiativeeffectsof the stratosphericaerosolsbut did not have a sufficientlylong or horizontally
extensivetemperature databaseto quantify the effects.
(Termsin italicare definedin the glossary,
whichfollows
an enhancedpole-to-equatortemperature gradient, especially in winter. In the Northern Hemisphere winter
this enhancedgradientproducesa strongerpolar vortex,
and this strongerjet stream producesa characteristic
stationarywave pattern of troposphericcirculation,resultingin winter warmingof Northern Hemispherecontinents.This indirect advectiveeffect on temperatureis
strongerthan the radiativecoolingeffect that dominates
at lower
latitudes
and in the summer.
The volcanic
aerosolsalso serveas surfacesfor heterogeneouschemical reactions that destroy stratosphericozone, which
lowers ultraviolet absorptionand reducesthe radiative
heating in the lower stratosphere,but the net effect is
still heating.Becausethischemicaleffectdependson the
presenceof anthropogenicchlorine,it has only become
important in recent decades.For a few days after an
eruptionthe amplitudeof the diurnal cycleof surfaceair
temperature is reduced under the cloud. On a much
longer timescale,volcaniceffectsplayed a large role in
interdecadalclimatechangeof the Little Ice Age. There
is no perfect index of pastvolcanism,but more ice cores
fromGreenland
andAntarctica
willimprove
therecord.
There is no evidencethat volcaniceruptionsproduceE1
Nifio events, but the climatic effects of E1 Nifio and
volcaniceruptionsmust be separatedto understandthe
climatic responseto each.
the main text.) Mitchell [1961]wasthe first to conducta
superposedepoch analysis,averagingthe effectsof several eruptionsto isolate the volcanic effect from other
presumablyrandom fluctuations. He only looked at
5-yearaverageperiods,however,and did not havea very
long temperaturerecord.Severalpreviousreviewsof the
effects of volcanoeson climate include Lamb [1970],
Toonand Pollack [1980], Toon [1982],Ellsaesser[1983],
Asaturovet al. [1986],Kondratyev[1988],Robock[1989,
1991],andKondratyevand Galindo [1997].Pasttheoretical studies of the radiative
effects include Pollack
et al.
[1976],Harshvardhan[1979], Hansenet al. [1992], and
Stenchikov
et al. [1998].The work of H. H. Lamb, in fact,
was extremely influential in the modern study of the
impact of volcanic eruptions on climate [Kelly et al.,
1998].Sincethesereviews,a deeperand more complex
understanding
of the impactsof volcaniceruptionson
weather and climate has resulted, driven by the many
Copyright2000 by the AmericanGeophysicalUnion.
Reviewsof Geophysics,
38, 2 / May'2000
pages191-219
8755-1209/00/1998
RG000054 $15.00
Papernumber1998RG000054
ß 191
ß
192 ß Robock: VOLCANIC ERUPTIONS AND CLIMATE
TABLE 1.
38, 2 / REVIEWS OF GEOPHYSICS
Major Volcanic Eruptions of the Past 250 Years
VolcaFlo
Grimsvotn[Lakagigar],Iceland
Tambora, Sumbawa, Indonesia
Cosiguina,Nicaragua
Askja, Iceland
Krakatau, Indonesia
Okataina [Tarawera],North Island, New Zealand
Santa Maria, Guatemala
Ksudach, Kamchatka, Russia
Novarupta[Katmai], Alaska, United States
Agung,Bali, Indonesia
Mount St. Helens, Washington,United States
E1 Chich6n, Chiapas,Mexico
Mount Pinatubo,Luzon, Philippines
Year of
Eruption
VEI
D VI/Emax
IVI
1783
1815
1835
1875
1883
1886
1902
1907
1912
1963
1980
1982
1991
4
7
5
5
6
5
6
5
6
4
5
5
6
2300
3000
4000
1000
1000
800
600
500
500
800
500
800
1000
0.19
0.50
0.11
0.01'
0.12
0.04
0.05
0.02
0.15
0.06
0.00
0.06
...
The officialnamesof the volcanoesandthe volcanicexplosivityindex(VEI) [Newhalland Self,1982]are from Simkinand Siebert[1994].The
dustveil index(DVI/Emax)comesfrom Lamb [1970,1977,1983],updatedby Robockand Free [1995].The ice corevolcanicindex(IVI) is the
averageof Northern and SouthernHemispherevaluesand is representedas opticaldepth at X - 0.55 pom[fromRobockand Free, 1995,1996].
*SouthernHemispheresignalonly; probablynot Askja.
studiesof the impactof the 1991Pinatuboeruptionand London [Symons,1888; Simkin and Fiske, 1983]. This
continuinganalysesof the 1982 E1 Chich6n eruption in wasprobablythe loudestexplosionof historictimes,and
Mexico.
the book includescolor figuresof the resultingpressure
This paper reviewsthese new results,includingthe wave'sfour circuitsof the globe as measuredby microindirecteffect on atmosphericcirculationthat produces barographs.The 1963 Agung eruption produced the
winterwarmingof the NorthernHemisphere(NH) con- largeststratosphericdustveil in more than 50 yearsand
tinentsand the new impactson ozonedue to the strato- inspired many modern scientific studies. While the
sphericpresenceof anthropogenicchlorine. A better Mount St. Helenseruptionof 1980wasvery explosive,it
understandingof the impactsof volcaniceruptionshas did not inject much sulfurinto the stratosphere.Thereimportantapplicationsin a numberof areas.Attribution fore it had very smallglobaleffects[Robock,1981a].Its
of the warming of the past century to anthropogenic troposphericeffectslastedonly a few days[Robockand
greenhousegasesrequiresassessment
of other causesof Mass, 1982;Mass and Robock, 1982], but it occurredin
climate changeduring the past severalhundredyears, the United States and so received much attention.
includingvolcaniceruptionsand solarvariations.After Quantificationof the size of theseeruptionsis difficult,
the next major eruption,new knowledgeof the indirect as different measures reveal different information. For
effectson atmosphericcirculationwill allow better sea- example,one could examinethe total massejected,the
or the sulfur input to the stratosphere.
sonalforecasts,especiallyfor the NH in the winter. The explosiveness,
impactsof volcaniceruptionsserveas analogs,although The limitationsof data for each of thesepotentialmeaimperfectones,for the effectsof other massiveaerosol sures,and a descriptionof indicesthat have been proloadings of the atmosphere,including meteorite or duced, are discussedlater.
Volcaniceruptionscan inject into the stratosphere
comet impactsor nuclearwinter.
acThe largesteruptionsof the past250 years(Table 1) tens of teragramsof chemicallyand microphysically
have each drawn attention to the atmosphericand po- tive gasesand solid aerosolparticles,which affect the
tential climatic effects becauseof their large effects in Earth's radiative balance and climate, and disturb the
the English-speaking
world. (Simkin et al. [1981] and stratosphericchemicalequilibrium.The volcaniccloud
Simkinand Siebert[1994]providea comprehensive
list of forms in several weeks by SO2 conversionto sulfate
all known volcanoesand their eruptions.) The 1783 aerosol and its subsequentmicrophysicaltransformaeruptionin Icelandproducedlarge effectsall that sum- tions [Pintoet al., 1989;Zhao et al., 1995].The resulting
mer in Europe [Franklin,1784;Grattanet al., 1998].The cloudof sulfateaerosolparticles,with an e-foldingdecay
1815 Tambora eruption producedthe "year without a time of approximately1 year [e.g.,Barnesand Hoffman,
summer"in 1816 [Stommeland Stommel,1983;Stothers, 1997], has important impacts on both shortwaveand
1984; Robock, 1984a, 1994; Harington, 1992] and in- longwave radiation. The resulting disturbanceto the
spiredthe book Frankenstein[Shelley,1818]. The most Earth's radiation balance affectssurfacetemperatures
extensivestudyof the impactsof a singlevolcanicerup- throughdirect radiative effectsas well as throughindition wascarriedout by the Royal Society,examiningthe rect effectson the atmosphericcirculation.In cold re1883 Krakatau eruption,in a beautifullyproducedvol- gions of the stratospherethese aerosolparticles also
chemical reactions
ume includingwatercolorsof the volcanicsunsetsnear serve as surfacesfor heterogeneous
38, 2 / REVIEWSOF GEOPHYSICS
that liberate chlorineto destroyozone in the sameway
that water and nitric acid aerosolsin polar stratospheric
cloudsproducethe seasonalAntarcticozonehole.
In thispaper I firstbrieflysummarizevolcanicinputs
to the atmosphereand reviewour newunderstanding
of
the radiativeforcingof the climatesystemproducedby
volcanic aerosols.Next, I briefly review the results of
new analysesof ice cores,sincethey give information
about the record of pastvolcanism,and comparethese
new recordsto pastanalyses.
The effectsof eruptionson
the local diurnal cycle are reviewed.Summer cooling
and winter warmingfrom large explosiveeruptionsare
then explained.The impactsof volcaniceruptionson
decadal- and century-scaleclimate changes,and their
contributionsto the Little Ice Age and their relative
contributionto the warmingof the pastcentury,are next
Robock:VOLCANIC ERUPTIONSAND CLIMATE ß 193
H20 , N2, and CO2 being the most abundant.Over the
lifetime of the Earth these gaseshave been the main
sourceof the planet's atmosphereand ocean, after the
primitive atmospherewas lost to space.The water has
condensedinto the oceans,the CO2 has been changed
by plants into 02, with someof the C turned into fossil
fuels. Of course,we eat the plants and the animalsthat
eat the plants,we drink the water, and we breathe the
oxygen,so eachof us is made of volcanicemissions.
The
atmosphereis now mainly composedof N 2 (78%) and
02 (21%), both of whichhad sourcesin volcanicemissions.
Of these abundant gases,both H20 and CO2 are
importantgreenhousegases,but their atmosphericconcentrationsare solarge(evenfor CO2 at onlyabout370
ppm but growing)that individualeruptionshave a negdiscussed.Then, I show that the simultaneous occur- ligible effect on their concentrations
and do not directly
rence of the 1982 E1 Nifio and the E1 Chich6n eruption impact the greenhouseeffect. Rather, the most imporwasjust a coincidenceand that it wasnot evidenceof a tant climatic effect of explosivevolcanic eruptions is
cause and effect relationship.Finally, the impacts of through their emissionof sulfur speciesto the stratovolcanic eruptions on stratosphericozone are briefly sphere,mainly in the form of SO2 [Pollacket al., 1976;
reviewed.
Newhall and Self, 1982; Rampino and Self, 1984] but
possiblysometimes
asH2S [Luhret al., 1984;Ahn, 1997].
These sulfur speciesreact with OH and H20 to form
2.
VOLCANIC
INPUTS TO THE ATMOSPHERE
H2SO4 on a timescaleof weeks,and the resultingH2SO4
aerosolsproduce the dominant radiative effect from
Volcanic eruptionsinject severaldifferent types of volcaniceruptions.Bluth et al. [1992], from satellite
particlesandgasesinto the atmosphere(Plate 1). In the measurements, estimated that the 1982 E1 Chich6n
past,it wasonlypossibleto estimatethesevolatileinputs eruptioninjected7 Mt of SO2 into the atmosphere,and
basedon measurementsfrom active,but not explosive, the 1991 Pinatuboeruption injected 20 Mt.
Once injectedinto the stratosphere,the large aerosol
eruptionsand remote sensingof the resultingaerosol
clouds from lidar, radiometers, and satellites. The ser- particlesand smallonesbeingformedby the sulfurgases
endipitousdiscoveryof the ability of the Total Ozone are rapidly advectedaround the globe. Observations
Mapping Spectrometer
(TOMS) instrumentto monitor after the 1883 Krakatau eruption showedthat the aeroSO2[e.g.,Bluthet al., 1992],however,hasgivenus a new sol cloud circledthe globe in 2 weeks [Symons,1888].
tool to directlymeasurestratosphericinjectionof gases Both the 1982 E1 Chich6n cloud [Robockand Matson,
1983] and the 1991 Pinatubocloud [Bluthet al., 1992]
from eruptions.
The major componentof volcaniceruptionsis mag- circled the globe in 3 weeks. Although E1 Chich6n
matic material,which emergesas solid,lithic material or (17øN)and Pinatubo(15øN)are separatedby only 2ø of
solidifiesinto largeparticles,whichare referredto asash latitude, their clouds,after only one circuitof the globe,
or tephra.Theseparticlesfall out of the atmospherevery endedup separatedby 15øof latitude,with the Pinatubo
rapidly,on timescalesof minutesto a few weeksin the cloudstraddlingthe equator[Stoweet al., 1992]and the
troposphere.Smallamountscanlastfor a few monthsin E1 Chich6n cloud extending approximatelyfrom the
dispersionof
the stratospherebut have very small climatic impacts. equatorto 30øN[Strong,1984].Subsequent
Symons[1888], after the 1883 Krakatau eruption, and a stratosphericvolcanic cloud dependsheavily on the
Robock and Mass [1982], after the 1980 Mount St. particulardistributionof winds at the time of eruption,
Helenseruption,showedthat thistemporarylargeat- althoughhigh-latitudeeruptioncloudsare seldomtransmosphericloadingreducedthe amplitudeof the diurnal portedbeyondthe midlatitudesof the samehemisphere.
cycle of surfaceair temperature in the region of the For trying to reconstructthe effectsof older eruptions,
troposphericcloud.Theseeffects,however,disappearas this factor addsa further complication,asthe latitude of
soon as the particles settle to the ground. When an the volcano is not sufficient information.
The normal residualstratosphericmeridional circueruption columnstill laden with thesehot particlesdescendsdownthe slopesof a volcano,thispyroclastic
flow lation lifts the aerosolsin the tropics,transportsthem
can be deadlyto thoseunluckyenoughto be at the base polewardin the midlatitudes,and bringsthem backinto
of the volcano.The destructionof Pompeii and Hercu- the troposphereat higherlatitudeson a timescaleof 1-2
laneum after the 79 A.D. Vesuviuseruption is the most years [Trepteand Hitchman, 1992; Trepteet al., 1993;
Holton et al., 1995].
famousexample.
Quiescent continuousvolcanic emissions,including
Volcanic eruptions typically also emit gases,with
194 ß Robock:VOLCANIC ERUPTIONSAND CLIMATE
38, 2 / REVIEWSOF GEOPHYSICS
0.94
0.92
0.9
0.88
0.86
0.84
0.82
0.8
0.78
1960
1965
19•0
1975
1980
1985
1990
1995
Figure 1. Broadbandspectrally
integratedatmospheric
transmission
factor,measured
with thepyrheliometer
shownin Plate2.Duttonetal. [1985]andDutton[1992]describe
the detailsof the calculations,
whicheliminate
instrumentcalibrationand solarconstantvariationdependence,and showmainlythe effectsof aerosols.
Effectsof the 1963Agung,1982E1 Chich6n,and 1991Pinatuboeruptionscan clearlybe seen.Years on
abscissa
indicateJanuaryof that year. Data courtesyof E. Dutton.
fumarolesand small episodiceruptions,add sulfatesto
the troposphere, but their lifetimes there are much
shorterthan thoseof stratospheric
aerosols.Therefore
they are not importantfor climate change,but they
couldbe if there is a suddenchangeor a long-termtrend
in them develops.Global sulfuremissionof volcanoesto
the troposphereis about 14% of the total natural and
anthropogenicemission[Graf et al., 1997] but has a
much larger relative contribution to radiative effects.
Many volcanic emissionsare from the sides of mountains, abovethe atmosphericboundarylayer, and thus
theyhavelongerlifetimesthan anthropogenic
aerosols.
Radiativeforcing(measuredat the surface)from such
radiationby scattering.Some of the light is backscattered, reflectingsunlightback to space,increasingthe
net planetaryalbedo and reducingthe amountof solar
energythat reachesthe Earth's surface.This backscattering is the dominantradiative effect at the surfaceand
resultsin a net coolingthere.Much of the solarradiation
is forward scattered,resultingin enhanceddownward
diffuseradiationthat somewhatcompensates
for a large
reductionin the directsolarbeam.The longestcontinuousrecord of the effectsof volcaniceruptionson atmospherictransmissionof radiation is the apparent
transmis.
sion record [Duttonet al., 1985;Dutton, 1992]
from the Mauna Loa Observatory(Plate 2) shownin
emissionsis estimated to be about -0.2 W m -2 for the Figure 1. The effectsof the 1963 Agung, 1982 E1 Chiglobeand-0.3 W m-2 fortheNH, onlya littlelessthan ch6n,and 1991Pinatuboeruptionscanbe clearlyseen.
anthropogeniceffects.
Although the Pinatubo eruption producedthe largest
stratosphericinput of the three, the center of the E1
Chich6ncloudwentdirectlyoverHawaii,whileonlythe
3.
RADIATIVE FORCING
side of the Pinatubocloudwas observed.The Agung
cloudwas mostlyin the SouthernHemisphere,so only
Plate 1 indicatesthe major radiativeprocesses
result- the edgewas seenin Hawaii. Figure 2 showsseparate
ing from the stratospheric
aerosolcloudfrom a major direct and diffuse radiation measurements,also from
volcanic eruption. The most obvious and well-known Mauna Loa, whichshownot onlythe strongreductionof
effect is on solar radiation. Since the sulfate aerosol
direct radiation by the 1982 E1 Chich6n and 1991 Pinaparticlesare aboutthe samesizeas visiblelight, with a tubo eruptionsbut alsothe compensating
increase(of
typical effectiveradius of 0.5 •xm, but have a single- slightlysmalleramplitude)in the diffuseradiation.
scatter albedo of 1, they strongly interact with solar
The effect on solarradiationis so strongthat it can
'*'"
X
lit
196 ß Robock:VOLCANIC ERUPTIONSAND CLIMATE
38, 2 / REVIEWSOF GEOPHYSICS
Plate 2. Photographof radiationinstrumentsat the Mauna Loa Observatoryon the Island of Hawaii,
United States,lookingnorth towardHualalai, on March 27, 1992. Observations
in Figures1 and 2 and
photograph
in Plate3 are takenfrom theseand similarinstruments.
Photograph
by A. Robock.
Plate 3. Photographof skysurrounding
the Sun on March 27, 1992,lessthan 1 year after the Pinatubo
eruption,takenat MaunaLoa Observatory
bylayingthe cameraon the supportshownin Plate2 andhaving
a portionof that supportblockout the directsolarradiation.The milkyappearance
is the enhancedforward
scattering
(Figure2), clearlyvisibleto the nakedeye.Whenthe stratosphere
is clear,the normalappearance
is a deepblue producedby molecularRayleighscattering.
Photograph
by A. Robock.
38 2 / REVIEWS OF GEOPHYSICS
Robock: VOLCANIC
ERUPTIONS
AND CLIMATE
ß 197
Plate 4. Sunsetover Lake Mendota in Madison, Wisconsin,in May 1983, one year after the E1 Chich6n
eruption. Photographby A. Robock.
easily be seen by the naked eye, making the normally
blueskymilkywhite(forwardscatteringeffect)(Plate3).
Volcanic aerosolcloudsare clearlyvisiblefrom spacein
solarwavelengthimages[e.g.,Robockand Matson, 1983]
and in spaceshuttlephotographs(backscattering).
The
reflectionof the settingSun from the bottom of stratosphericvolcanicaerosollayers (called "dust veils" by
Lamb [1970]) producesthe characteristicred sunset
(Plate 4) usedby Lamb as one meansof detectingpast
eruptions. The famous 1893 Edvard Munch painting,
"The Scream," showsa red volcanic sunsetover the Oslo
harbor producedby the 1892 Awu eruption. The timing
of the reportsof red sunsetswasusedby Symons[1888]
and Lamb [1970] to calculatethe height of the aerosol
layer, taking into accountthe geometryof the setting
Sun. Robock [1983a] also providesa diagram showing
how these red sunsets can be observed after the Sun has
set.
[Houghtonet al., 1996, p. 109]. For aerosolswith a
nonuniform
vertical
and
horizontal
distribution,
Stenchikovet al. [1998] showedthat a completeformulation of radiative forcing must include not only the
changesof net fluxes at the tropopause,but also the
verticaldistributionof atmosphericheatingratesand the
change of downward thermal and net solar radiative
fluxes at the surface. Using a detailed data set they
developedfrom satelliteand ground-basedobservations,
they calculated the aerosol radiative forcing with the
ECHAM4 (European Center/Hamburg)general circulation model (GCM) [Roeckneret al., 1996].
An exampleof the radiativeheatingfrom Pinatubois
shownin Plate 5. At the top of the aerosol cloud, the
atmosphereis warmed by absorptionof solar radiation
in the near infrared (near-IR). This effect dominates
over the enhanced IR cooling due to the enhanced
emissivitybecause of the presence of aerosols.Andronovaet al. [1999]recentlyrepeatedthesecalculations
To evaluate the effects of a volcanic eruption on
climate, the radiativeforcingfrom the aerosolsmust be with a more detailed radiation model and confirmed the
calculated.Stenchikovet al. [1998] presenteda detailed importanceof near-IR abosorption.In the lower stratostudy of the radiative forcing from the 1991 Mount sphere the atmosphereis heated by absorptionof upPinatuboeruption.Althoughthiswas the mostcompre- ward longwaveradiation from the troposphereand surhensivelyobservedlarge eruption ever [e.g., Russellet face. Hence this warming would be affected by the
al., 1996],they still neededto make severalassumptions distributionof cloudsin the troposphere,but Stenchikov
to compensatefor gapsin the observations.For globally et al. [1998] found that this effect (on changedupward
smoothradiativeperturbations,suchas changinggreen- longwaveflux) was random and an order of magnitude
housegasconcentrations,
radiativeforcingis definedas smallerthan the amplitudeof the warming.In the trothe changein the net radiative flux at the tropopause posphere, there are small radiative effects, since the
198 ß Robock: VOLCANIC ERUPTIONS AND CLIMATE
38, 2 / REVIEWSOF GEOPHYSICS
August, 1991
January• 1992
e)
IO
30.
5O
50
lOO
O0
300
O0
500
O0
700
900'
O0
O0
505
305
EQ
3ON
5ON
6O5
30S
EQ
30N
60N
EQ
30N
60N
EQ
3ON
60N
Visible
b)
5o
5o
100
100-
300
300
500
500
700900
700
go0
605
30S
EO
30N
!
60S
60•
Near
30S
IR
g)
c)
-,3
-
1
30
•o
5o
•oo
ioo
ß
300'
0'05
.•
,
]oo
5o0
500
7OO
9O0
7oo
90o
.,
6OS
3OS
EO
30N
60S
6ON
30S
IR
h)
10 ' '
d)
J
•o
3O
'
•o, Z' "•
I -n'n•
1•
1
' ,X•• ,,•u•
•"
-
-0.01•
'1I'
5o
I00
3o0
500
500
50O
700
700
900
6OS
30S
EO
3ON
60N
.01
I
c•.y.,
•
•-of•
....
6OS
3OS
EQ
30N
60N
Total
Plate 5. Radiativeheatingfrom Pinatubofor three differentwavelengthbands,from Figure 10 of Stenchikov
etal. [1998].Shownaremonthlyaverage,
zonalaverage,
perturbations
of theradiativeheatingrates(K d-•)
causedby the Pinatuboaerosolsfor (a) visible(X -< 0.68 txm),(b) near-IR (0.68 txm< X < 4 txm),(c) IR (X ->
4 txm),and (d) total, for August1991,and (e) visible,(f) near-IR, (g) IR, and (h) total, for January1992.
38, 2 / REVIEWSOF GEOPHYSICS
Robock' VOLCANIC ERUPTIONS AND CLIMATE ß 199
540
520
500
480
460
440
420
400
38O
Direct
,360
54O
22O
Diffuse
2OO
Radiation
180
160
140
120
1 O0
8O
6O
4O
2O
0
1978
1980
1982
1984
1986
1988
1990
'1992
1994
1996
1998
Figure 2. Direct and diffuse broadbandradiation measurementsfrom the Mauna Loa observatory,measuredwith a trackingpyrheliometerand shadediskpyranometeron morningswith clear skiesat solarzenith
angleof 60ø, equivalentto two relativeair masses[Duttonand Christy,1992].The reductionof direct radiation
and enhancementof diffuseradiationafter the 1982E1Chich6nand 1991Pinatuboeruptionsare clearlyseen.
Years on abscissaindicate Januaryof that year. Data courtesyof E. Dutton.
reduceddownwardnear-IR (producinglessabsorption throughtropopausefolds [Sassenet al., 1995],the global
by water vapor in the troposphere)is compensatedby effect has not been quantified.
the additional downward longwaveradiation from the
aerosol cloud. At the surface the large reduction in
direct
shortwave
downward
diffuse
radiation
shortwave
overwhelms
the
additional
flux and the small
4.
INDICES
OF PAST VOLCANISM
addi-
tional downward longwave radiation from the aerosol
cloud, except in the polar night, where there is no
sunlight,whichwasfirst shownby Harshvardhan[1979].
This net cooling at the surface is responsiblefor the
well-knownglobal coolingeffect of volcaniceruptions.
These calculationsagree with observationsof surface
flux changesmade at Mauna Loa by Dutton and Christy
[1992].They alsoagreewith observations
of Minnis et al.
[1993] made with Earth Radiation Budget Experiment
satellitedata [Barkstrom,1984], if the effectsof stratosphericwarming are considered.
To evaluate the causesof climate changeduring the
pastcenturyand a half of instrumentalrecordsor during
the past2000 years,includingthe MedievalWarmingand
the "Little Ice Age," a reliable record of the volcanic
aerosol loading of the atmosphereis necessary.Five
suchindices(Table 2) have been compiled,based on
different data sourcesand criteria, but none is perfect.
Robockand Free [1995, 1996] describetheseindicesin
detail and compare them, and here I summarizethem.
Another index describedby Robock and Free [1995],
beingusedby someclimatemodelinggroupsat the time,
Plate 5 also shows,for Pinatubo, that the lower strato- wasnever publishedand so is not includedhere. Pollack
sphericheatingis much larger in the tropicsthan at the et al. [1976] also compileda record of volcanicoptical
poles.It is this latitudinal gradientof heatingwhich sets depth,but it was limited to 1880-1925 and 1962-1972.
A perfect index would convey the radiative forcing
up a dynamicalresponsein the atmosphere,resultingin
the winter warming of NH continentalregions,due to associatedwith each explosiveeruption. The radiative
advectiveeffects,which dominate over the radiative ef- forcing is most directly related to the sulfur content of
fects in the winter.
emissionsthat reachinto the stratosphereand not to the
The possibleeffect of the aerosolson seedingcirrus explosivityof the eruption. However, all indirect indices
cloud formation [Mohnen,1990] is indicatedin Plate 1. are either incompletein geographicalor temporalcovWhile evidence exists for individual cases of cirrus cloud
erage or are a measure of some property of volcanic
formationby volcanicaerosolsenteringthe troposphere eruptionsother than their stratosphericaerosolloading.
200 ß Robock: VOLCANIC ERUPTIONS AND CLIMATE
TABLE 2.
38, 2 / REVIEWSOF GEOPHYSICS
Indices of Past Volcanic Eruptions
Name
Dust veil index (DVI)
How Calculated
Reference
Sapper[1917, 1927],sunsets,eruption,and radiation
Lamb [1970, 1977, 1983]
Units
Krakatau
= 1000
observations
Mitchell
aerosol mass
basedon H. H. Lamb (personalcommunication,1970) Mitchell [1970]
Volcanic explosivity
index (VEI)
Krakatau
explosivity,from geologicand historicalreports
Sato
-r (X = 0.55 txm)
Ice core volcanic
index (IVI)
Direct
radiation
= 6
-r (X = 0.55 txm)
Newhalland Self [1982]
Simkinet al. [1981]
Simkinand Siebert[1994]
Mitchell[1970],radiationand satelliteobservations
Satoet al. [1993]
averageof ice core acidityor sulfatemeasurements
Robock and Free
[1995, 1996]
measurements
would
be the best
technique, and combinationsof surface, aircraft, balloon, and satellitemeasurementshaveclearlyquantified
the distributionsand optical propertiesof the aerosols
from the 1982 E1 Chich6n [Robock,1983a] and 1991
Pinatubo[Stenchikov
et al., 1998]eruptions(seealsotwo
specialsectionsin Geophysical
Research
Letters:"Climatic Effectsof the Eruption of E1 Chich6n,"10(11), 9891060, 1983;and "The Stratosphericand ClimaticEffects
of the 1991 Mount Pinatubo Eruption: An Initial Assessment,"
19(2), 149-218, 1992).The brightness
of the
Moon during lunar eclipsescan be used as an index of
stratosphericturbidity,but suchobservations
can onlybe
made about onceper year, sometimesmissingthe maximum aerosol loading [Keen, 1983]. Even these most
recent large eruptions, however, have deficienciesin
their observations[Stenchikovet al., 1998]. In the past,
however, such measurementsare lacking, and indices
effects of the volcanic aerosols, and Lamb in addition
used reports of atmosphericeffects. Still, up to the
present,all the indicesmay misssomeSouthernHemisphere (SH) eruptions,as they may not be reported.
Even in the 1980s,the December 1981 aerosolsfrom the
eruption of Nyamuragirawere observedwith lidar but
were reported as the "mysterycloud" for severalyears
until the source was identified by reexamining the
TOMS satelliterecord [Kruegeret al., 1996].As late as
1990,volcanicaerosolswere observedwith Stratospheric
Aerosoland GasExperimentH (SAGE I1), but it hasnot
been possibleto identify the source[Yue et al., 1994].
Before 1978,with no satelliteor lidar records,there may
be importantmissingeruptionsevenin the NH averages.
This problem does not exist for individual ice core
records,becausethey are objectivemeasuresof volcanic
sulfuricacid, exceptthat the farther back in time one
goeswith ice cores,the fewer suchrecordsexist, and
have had to use the available surface radiation measureeach ice core record is extremely noisy and may have
ments combinedwith indirect measuressuchas reports otherproblems.Plate 6 showsthe five indices(Table 2)
of red sunsetsin diaries and paintings,and geological for the NH for the past 150 years.Here they are briefly
evidence.Geologicalmethods,basedon examinationof described.
the depositsremainingon the ground from eruptions,
canprovideusefulinformationon the total masserupted 4.1. I.amb's Dust Veil Index
and the date of the eruption,but estimatesof the atmoLamb [1970, 1977, 1983] createda volcanicdustveil
sphericsulfur loading are not very accurate.This "pet- index (DVI), specificallydesignedfor analyzingthe efrologic method" dependson the assumptionthat the fects of volcanoes on "surface weather, on lower and
difference in sulfur concentrationbetweenglassinclu- upper atmospherictemperatures,and on the large-scale
sionsin the depositsnear the volcanoand the concen- wind circulation"[Lamb, 1970,p. 470].Lamb [1970]and
trationsin the depositsthemselvesare representativeof Pollack et al. [1976] suggested
with data analysesthat
the total atmosphericsulfur injection,but this has been volcanismwas an important causeof climatechangefor
shownnot to work well for recenteruptionsfor whichwe the past500years,andRobock[1979]usedLamb'sindex
to force an energy-balancemodel simulation of the
have atmosphericdata [e.g.,Luhr et al., 1984].
For all the indicesthe problem of missingvolcanoes Little Ice Age, showingthat volcanicaerosolsplayed a
and their associateddustveilsbecomesincreasinglyim- major part in producingthe cooling during that time
portantthe farther backin time theygo.There mayeven period.The methodsusedto createthe DVI, described
have been significantvolcanicaerosolloadingsduring by Lamb [1970]and Kellyand Sear [1982],includehisthe pastcenturythat do not appearin any or mostof the torical reports of eruptions,optical phenomena,radiavolcanicindices.Volcanoesonly appear in most of the tion measurements
(for the period 1883 onward),temindices if there is a report of the eruption from the perature information, and estimatesof the volume of
ground.For recent eruptions,Lamb [1970] and Sato et ejecta.
Lamb's DVI has been often criticized[e.g.,Bradley,
al. [1993] used actual measurementsof the radiative
38, 2 / REVIEWSOF GEOPHYSICS
1988] as havingused climaticinformation in its derivation, therebyresultingin circularreasoningif the DVI is
usedas an index to comparewith temperaturechanges.
In fact, for only a few eruptionsbetween1763 and 1882
was the NH averagedDVI calculatedbased solely on
temperatureinformation,but for severalin that period
the DVI wascalculatedpartially on the basisof temperature information.Robock [1981b] created a modified
version of Lamb's DVI which excluded temperature
Robock: VOLCANIC ERUPTIONS AND CLIMATE ß 201
1982], it had a negligiblestratosphericimpact [Robock,
1981a].
4.4.
Sato Index
Sato et al. [1993] produced monthly NH and SH
averageindices.Their index, expressedas optical depth
at wavelength0.55 p•m,is basedon volcanologicalinformation about the volume of ejectafrom Mitchell [1970]
from 1850 to 1882, on optical extinctiondata after 1882,
information. When used to force a climate model, the
and on satellitedata startingin 1979. The seasonaland
results did not differ significantlyfrom those using
latitudinal distributionsfor the beginningof the record
Lamb's original DVI, demonstratingthat this is not a
are uniform and offer no advantagesover the DVI and
seriousproblem.
in fact showlessdetail than the latitudinallydependent
indexof Robock[1981b],who distributedthe aerosolsin
4.2.
Mitchell Index
latitude with a simplediffusivemodel. The more recent
Mitchell[1970]alsoproduceda time seriesof volcanic
part of the record would presumablybe more accurate
eruptionsfor the period 1850-1968 using data from
than the DVI or VEI, as it includes actual observations
Lamb. As discussed
by Robock[1978, 1981b]andSatoet
of the latitudinal and temporal extent of the aerosol
al. [1993],the Mitchell volcaniccompilationfor the NH clouds.
is more detailed than Lamb's, because Lamb excluded
all volcanoeswith DVI < 100 in producing his NH
annual averageDVI. Mitchell provided a table of the 4.5. Ice Core Volcanic Index
order of magnitude of total mass ejected from each
Robockand Free [1995] examinedeight NH and six
volcano, which is a classificationsimilar to the volcanic SH ice core recordsof acidityor sulfatefor the period
explosivityindex.
1850to the presentin an attemptto identifythe volcanic
signalcommonto all records.They explainin detail the
possibleproblemswith using these recordsas volcanic
4.3. VolcanicExplosivityIndex
A comprehensivesurveyof past volcanic eruptions indices,includingother sourcesof acidsand bases,other
[Simkinet al., 1981;Simkinand Siebert,1994]produced sourcesof sulfate, dating, local volcanoes,variable ata tabulationof the volcanicexplosivity
index(VEI) [Ne- mosphericcirculation,the stochasticnature of snowfall
whalland Self,1982]for all knowneruptions,whichgives and dry deposition,mixing due to blowing snow, and
a geologicallybasedmeasureof the power of the volca- uncertaintiesin the electricalconductivitymeasuresof
nic explosion.This index has been used without any the ice. For the NH, although the individual ice core
modificationin many studies[seeRobock, 1991] as an records are, in general, not well correlated with each
index of the climatologicalimpact of volcanoes.A care- other or with any of the indices,a compositederived
ful readingof Newhalland Self[1982],however,will find from averagingthe cores, the ice core volcano index
the following quotes:"We have restrictedourselvesto (IVI), showedpromiseasa newindexof volcanicaerosol
considerationof volcanologicaldata (no atmospheric loading.This new index correlatedwell with the existing
data)..." (p. 1234)and "Sincethe abundanceof sulfate non-ice-corevolcanicindices and with high-frequency
aerosolis importantin climateproblems,VEI's mustbe temperaturerecords.Still, it is clear that high-latitude
combinedwith a compositional
factorbeforeusein such volcanoesare given too much weight, and it is only
studies"(pp. 1234-1235).In their Table 1, Newhalland possibleto adjustfor them if their signalscan unambigSelf list criteria for estimatingthe VEI in "decreasing uouslybe identified. For the SH the individualice cores
order of reliability,"and the very last criterion out of 11 and indices were better correlated. The SH IVI was
is "stratosphericinjection."For VEI of 3, stratospheric againhighlycorrelatedwith all indicesand individualice
injectionis listedas "possible,"for 4 it is "definite,"and coresbut not with high-frequency
temperaturerecords.
for 5 and larger it is "significant."If one attempts to
Robockand Free [1996] attemptedto extendthe IVI
work backwardand use a geologicallydeterminedVEI farther into the past. They comparedall the ice cores
to give a measure of stratosphericinjection, serious availablefor the past 2000 yearswith the DVI and the
errors can result. Not only is stratosphericinjectionthe VEI for this period. An IVI constructedfor the period
least reliable criterion for assigninga VEI, but it was 453 A.D. to the presentshowedlittle agreementwith the
never intended as a descriptionof the eruption which DVI or VEI. They determinedthat exceptfor a very few
had a VEI assignedfrom more reliable evidence.Nev- eruptions,the ice core record currently availableis inertheless,Robockand Free [1995] found the VEI posi- sufficientto delineate the climatic forcing by explosive
tively correlatedwith other indices,but imperfectly.For volcanic eruptionsbefore about 1200 for the NH and
example,the Mount St. Helens eruption of 1980 has a before about 1850 for the SH. They also point out,
large VEI of 5, and while it had a large local tempera- however, that the record of past volcanism remains
ture impact [Robockand Mass, 1982;Massand Robock, buried in the ice of Greenland and Antarctica, and more
202 ß Robock' VOLCANIC ERUPTIONS AND CLIMATE
TABLE 3.
38, 2 / REVIEWSOF GEOPHYSICS
Effects of Large Explosive Volcanic Eruptions on Weather and Climate
Effect
Reduction of diurnal cycle
Mechanism
blockageof shortwaveand emissionof longwave
Begins
Duration
immediately 1-4 days
radiation
Reducedtropical precipitation
Summer coolingof NH tropicsand
subtropics
Stratosphericwarming
blockageof shortwaveradiation, reducedevaporation 1-3 months
blockageof shortwaveradiation
1-3 months
3-6 months
stratosphericabsorptionof shortwaveand longwave
1-2 years
1-3 months
1-2 years
radiation
Winter warming of NH continents
stratospheric
absorptionof shortwaveand longwave
radiation, dynamics
Global cooling
blockageof shortwaveradiation
Global coolingfrom multiple eruptions blockageof shortwaveradiation
dilution, heterogeneouschemistryon aerosols
Ozone depletion, enhancedUV
1
• year
oneor two
winters
immediately 1-3 years
immediately 10-100 years
1 day
1-2 years
deep coresthat analyzethe sulfur or acid contenthave other eruptions[Robocket al., 1995;Robockand Free,
the promiseof producinga reliable record of the past. 1995;Selfet al., 1997].Volcaniceruptionscan still have
a large local effect on surfacetemperaturesin regions
near the eruptionfor severaldays,as Robockand Mass
5. WEATHER AND CLIMATE RESPONSE
[1982]andMassandRobock[1982]showedfor the 1980
Mount St. Helens eruption. In this section I briefly
Volcanic eruptionscan affect the climate systemon summarizetheseclimaticeffects,startingfrom the shortmanytimescales(Table 3). The greatestknowneruption est timescale.
of the past 100,000yearswasthe Toba eruptionof about
71,000yearsB.P. [Zielinskiet al., 1996],whichoccurred 5.1. Reductionof Diurnal Cycle
intriguingly
closeto the beginning
of a majorglaciation, The Mount St. Helens eruption in May 1980, in
and while Rampino and Self [1992] suggesteda cause WashingtonState in northwesternUnited States,was a
and effect relationship,it has yet to be established[Li very powerful lateral blast which produceda huge local
and Berger, 1997]. Many papers, as discussedin the troposphericloadingof volcanicash.In Yakima, Washintroduction, have suggestedthat volcanic aerosolscan ington, 135 km to the east,it was so dark that automatic
be importantcausesof temperaturechangesfor several streetlightswent on in the middle of the day. This thick
years followinglarge eruptionsand that even on a 100- aerosollayer effectivelyradiativelyisolatedthe Earth's
year timescale,they can be important when their cumu- surfacefrom the top of the atmosphere.The surfaceair
lative effects are taken into account. The effects of
temperature in Yakima was 15øCfor 15 straighthours,
volcanic eruptions on climate are very significant in independentof the normal diurnal cycle (Figure 3).
analyzingthe globalwarmingproblem,asthe impactsof Robockand Mass [1982] and Mass and Robock [1982]
anthropogenic
greenhouse
gasesand aerosolson climate examined the errors of the model output statistics
must be evaluated against a backgroundof continued (MOS) forecastsproduced by the National Weather
natural forcingof the climatesystemfrom volcanicerup- Service. As the MOS forecasts did not include volcanic
[ions,solarvariations,and internalrandomvariations aerosolsas predictors,they were able to interpret these
from land-atmosphereand ocean-atmosphere
interac- errors as the volcaniceffect. They found that the aerotions.
solscooledthe surfaceby asmuchas 8øCin the daytime
Individuallarge eruptionscertainlyproduceglobalor but warmed the surfaceby as much as 8øC at night.
hemisphericcoolingfor 2 or 3 years [Robockand Mao,
The reductionof the diurnal cycleonly lasted a cou1995], and this signalis now clearer.The winter follow- ple of days,until the aerosolclouddispersed.The effect
ing a large tropical eruption is warmer over the NH wasalsoobservedafter the Krakataueruptionin Batavia
continents, and this counterintuitive effect is due to (now know asJakarta),Indonesia[seeSimkinand Fiske,
nonlinearresponsethroughatmosphericdynamics[Rob- 1983, Figure 58]. While the Mount St. Helens eruption
ock and Mao, 1992; Graf et al., 1993;Mao and Robock, had a large local effecton temperature,no other impact
1998;Kirchneret al., 1999].Volcanic aerosolsprovidea was identified on precipitationor atmosphericcirculasurface for heterogeneouschemical reactions that de- tion. Its stratosphericinput of sulfurwasvery small,and
stroy ozone, and observationsfollowing Pinatubo have hencethisvery explosiveeruptionhad a minimal impact
documentedmidlatitude ozone depletion causedby a on globalclimate [Robock,1981a].
volcaniceruption[Solomonet al., 1996;Solomon,1999].
While the large 1982-1983 E1 Nifio amplifiedjust after 5.2. SummerCooling
the 1982 E1 Chich6n eruption in Mexico, there is no
It has long been known that the global averagetemevidenceof a causeand effectrelationshipfor thisor any perature falls after a large explosivevolcaniceruption
38, 2 / REVIEWS OF GEOPHYSICS
Robock: VOLCANIC ERUPTIONS AND CLIMATE ß 203
c
_.1._. I
i
II
iii
I
o
-T-
,,i
•
o
o
o
..,....
ix)
..-
204 ß Robock: VOLCANIC ERUPTIONS AND CLIMATE
M•Y
17
Y
A
38, 2 / REVIEWS OF GEOPHYSICS
1980
18
19
and there have been so few large eruptionsin the past
century.For severalrecent eruptions,Angell [1988],Nicholls [1988], and Mass and Portman [1989] demonstratedthat the ENSO signalin the pastclimaticrecord
partially obscuresthe detectionof the volcanicsignalon
a hemisphericannual averagebasisfor surfaceair tem-
20
•
K
I
M
I0
A
S gO
P
0
perature.
• •o
More recently,Robockand Mao [1995] removedthe
ENSO signalfrom the surfacetemperaturerecord to
extractmore clearlythe seasonaland spatialpatternsof
the volcanicsignalin surfacetemperaturerecords.This
observedsignal matches the general cooling patterns
found from GCM simulations[Hansenet al., 1988;Robock and Liu, 1994]. Robock and Mao found, by superposing the signalsof Krakatau, Santa Maria, Katmai,
Agung, E1 Chich6n, and Pinatubo, that the maximum
cooling is found approximately1 year following the
eruptions.This cooling,of 0.1ø-0.2øC,followsthe solar
declinationbut is displacedtowardthe NH (Figure 4);
the maximumcoolingin NH winter is at about 10øNand
in summeris at about 40øN.This pattern is becauseof
the distribution of continents:Land surfacesrespond
more quicklyto radiationperturbationsandthusthe NH
N
GF
RA
œL
A k
TS
•
•0
0
I
S •
E
•0
12
0
12
0
12
0
12
LST
Figure 3. Time seriesof surfaceair temperaturefor Yakima
and Spokane,Washington;Great Falls, Montana; and Boise,
Idaho, for May 17-20, 1980, under the plume of the 1980
Mount St. Helens eruption,from Figure 3 of Robockand Mass
[1982].Time of arrivalof the plumeis indicatedwith an arrow.
LST is local standard time. The plume never passedover
Boise,which is includedas a control.Note the dampingof the
diurnalcycleafter the arrivalof the troposphericaerosolcloud. is more sensitive to the radiation
reduction
from volca-
nic aerosols.
[e.g.,Humphreys,1913, 1940;Mitchell, 1961].The direct
radiativeforcingof the surface,with a reductionof total
downward radiation, cools the surface. For example,
Hansenet al. [1978],usinga radiative-convective
climate
model, successfullymodeled the surface cooling and
stratospheric
warmingafter the 1963Agungeruption.In
the tropicsand in the midlatitude summertheseradiative effectsare larger than most other climaticforcings,
as there is more sunlightto block. In somelocationsin
the tropics,however,eventhe effectsof a large eruption
like E1 Chich6n can be overwhelmedby a large E1 Nifio,
Robock and Liu [1994] analyzedthe Hansen et al.
[1988] GCM simulationsand found reduced tropical
precipitation for 1-2 years following large eruptions.
The global hydrologicalcycleis fueled by evaporation,
and the coolingafter the eruptionsproducedthis effect.
They evenfound a reductionin Sahelprecipitationthat
matched
the
observed
enhancement
of
the
Sahel
drought following the E1 Chich6n eruption, but this
result needs further
confirmation
before
it can be con-
sidered robust.
5.3. StratosphericHeating
It haslongbeenknownthat the stratosphereis heated
studyby Vupputuriand Blanchet[1984] also simulated after injection of volcanic aerosols[e.g., Quiroz, 1983;
coolingat the surfaceand warmingin the stratosphere. Parker and Brownscombe,1983;Angell, 1997b]. As exEnergy-balancemodels[Schneiderand Mass, 1975; Ol- plained above,this heating is causedby absorptionof
iver, 1976;Brysonand Dittberner,1976;Miles and GiM- both near-IR solar radiation at the top of the layer and
as was the case in 1983. The radiative-convective
model
ersleeves,1978;Robock, 1978, 1979; Gilliland, 1982; Gilli-
land and Schneider,1984]have all showncoolingeffects
for up to severalyears after major eruptions.An early
zonally averageddynamicclimate model [MacCracken
and Luther, 1984] and GCM studiesby Hunt [1977],
Hansenet al. [1988, 1992, 1996],Rind et al. [1992], and
Pollacket al. [1993]alsoshowedvolcaniccoolingeffects.
The problem of detectionand attributionof a volcanic signal in the past is the same as the problem of
identifyinga greenhousesignalin the past:identifyinga
uniquefingerprintof the forcingand separatingout the
effects of other potential forcings. It is necessaryto
separatethe volcanicsignalfrom that of other simultaneousclimaticvariationsbecausethe climatic signalof
volcaniceruptionsis of approximatelythe same amplitude as that of E1 Nifio-SouthernOscillation(ENSO)
terrestrial radiation at the bottom of the layer. Figure 5
and Plate 7 showsobservationsof lower stratospheric
temperaturefor the past20 years.Two strongsignalscan
be seen.After the 1982E1 Chich6neruptionthe globally
averaged stratospherictemperature rose by about IøC
for about 2 years. After the 1991 Pinatubo eruption a
warmingof equallengthbut abouttwicethe amplitudeis
clearlyvisible.These large warmingsare superimposed
on a downward trend in stratospherictemperature
caused by ozone depletion and increased CO2 [Ramaswamyet al., 1996;Vinnikovet al., 1996].This cooling,
at 10 timesthe rate of troposphericwarmingfor the past
century,is a clearsignalof anthropogenicimpactson the
climate system.
Plate 7 showsthe stratospherictemperaturesseparated by latitude bands.After the 1982 E1 Chich6nand
38
2 / REVIEWS OF GEOPHYSICS
Robock: VOLCANIC
ERUPTIONS
AND CLIMATE
ß 205
60N
ß-0
'" '4 \ ,',",.... 0 2 .-,
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.,', :-o.2.,'
50N
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40Nlo:,
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::•:;': ;: :::::
,,...,
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-',/'/::'œ:2-',:
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0
;","
,'. ,.-, .... ;,- - . .
1
2
3
--. '-'e,-,..
4
-,
5
Log (years)
Figure 4. Zonal averagesurfaceair temperatureanomalies,averagedfor the six largestvolcaniceruptions
of the pastcentury.Anomalies(øC) are with respectto the mean of the 5-yearperiodbefore eacheruption,
with the seasonalcycleremoved.Values significantlydifferent (95% level) from 0øC are shaded.Contour
interval is 0.1øC,with the 0øCinterval left out and negativecontoursdashed.From Figure 6 of Robockand
Mao [1995].
1991Pinatuboeruptionsthe tropicalbands(30øS-30øN),
shownby the greenand black curves,warmedmore than
the 30øN-90øNband (blue curve), producingan enhanced pole-to-equatortemperature gradient. The resultingstrongerpolar vortex producesthe tropospheric
winter warming describednext.
'
5.4.1. Observations. It was first suggestedby
Groisman [1985], with reference to previous Russian
studies, that warm winters over central Russia were a
consequenceof large volcaniceruptions.Using surface ß
air temperaturedata for stationsin Europe (including
the Europeanpart of Russia)and northeasternNorth
America for averagesof two or three winters after the
volcanic years of 1815, 1822, 1831, 1835, 1872, 1883,
5.4. Winter Warming
Robock [1981b, 1984b] used an energy-balancecli- 1902, 1912, and 1963, he showeda significantwarming
mate model to examine the seasonal and latitudinal
over the central European part of Russiaand insignifiresponseof the climate systemto the Mount St. Helens cant changesin the other regions,includingcoolingover
and E1 Chich6n eruptionsand found that the maximum northeasternNorth America. In an update, Groisman
surfacecoolingwas in the winter in the polar regionsof [1992] examinedthe winter patterns after the 1982 E1
both hemispheres.This was due to the positivefeedback Chich6nand 1991 Pinatuboeruptionsand found warmof sea ice, which lowered the thermal inertia and en- ing over Russia again but found warming over northhancedthe seasonalcycleof coolingand warmingat the eastern North America.
By averagingover severalwintersand only examining
poles[Robock,1983b].Energy-balance
models,however,
are zonallyaveragedand parameterizeatmosphericdy- part of the hemisphere,Groisman [1992] did not disnamicsin terms of temperaturegradients.Thus they do coverthe completewinter warmingpattern but did innot allow nonlinear dynamical responseswith zonal spireother studies.His explanationof the warmingover
structure. While the sea ice/thermal inertia feedback is Russia as due to enhancedzonal winds bringingwarm
indeed part of the behavior of the climate system,we maritime air from the north Atlantic over the continent
now know that an atmosphericdynamicalresponseto was correct,but he explainedtheseenhancedwinds as
large volcaniceruptionsdominatesthe NH winter cli- due to a larger pole-to-equatortemperature gradient
mate response,producingtroposphericwarming rather causedby polar cooling resulting from the eruptions.
than an enhancedcoolingover NH continents.
However, if the larger pole-to-equatortemperaturegra-
206 ß Robock: VOLCANIC
ERUPTIONS AND CLIMATE
38 2 / REVIEWS OF GEOPHYSICS
GlobalAvg. Lower StratosphericTemp. Anomalies
with respect to 1984-1990
mean
1.4
1.2
1
•
0.8
-•
o.6
o
0.4
•
0.2
(D
0
I--
-0.2
-0.4-
-0.6-
-0.8
1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000
Figure 5. Globalaveragemonthlystratospheric
temperatures
from microwave
soundingunit satelliteobservations,channel4 [Spencer
et al., 1990](updatedin 1999).Anomalies(øC) are with respectto the 1984-1990
nonvolcanic
period.Timesof 1982E1Chich6nand 1991Pinatuboeruptionsare denotedwith arrows.Yearson
abscissa
indicateJanuaryof that year.
Lower Stratospheric Temp. Anomalies - Latitude Bands
with respect to 1984-1990 meon
1.5
.•-
0.5
o
•
0--
-2
1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000
38, 2 / REVIEWS OF GEOPHYSICS
dientwascausedby polar cooling,whywere the temperatures in the polar region over Eurasia warm and not
cool? Why was northeasternNorth America cool after
someeruptionsand warm after others?
Lough and Fritts [1987], usingtree-ring data and selecting 24 volcanicyearsbetween 1602 and 1900, found
winter warming over western North America for the
averageof years 0-2 after the eruptionyears.The pattern they found is quite similar to that during ENSO
years, and they made no attempt to correct for this
factor. They also found springand summercoolingin
the centralUnited Statesand summerwarmingover the
United Stateswestcoast.They did not useanyvolcanoes
south of 10øS,believing them not important for NH
temperaturevariations.
Robockand Mao [1992] were the first to systematically examinethe global surfaceair temperaturerecord
for the NH winter warming phenomenon.They examined the winter surfaceair temperature patterns after
the 12 largesteruptionsof the pastcentury,removedthe
ENSO signal,and found a consistentpattern of warming
over the continentsand coolingover the oceansand the
Middle East,when combiningthe firstwinter after tropical eruptionsand the secondwinter after high-latitude
eruptions.Robock and Mao [1995] showed that this
pattern is mainly due to the tropicaleruptions.
The winter warming pattern is illustratedin Plate 8,
which showsthe global lower tropospherictemperature
anomaly pattern for the NH winter of 1991-1992, followingthe 1991 Mount Pinatuboeruption.This pattern
is closelycorrelated with the surface air temperature
pattern where the data overlap, but the satellite data
allow global coverage. The temperature over North
America, Europe, and Siberia was much warmer than
Robock: VOLCANIC ERUPTIONS AND CLIMATE ß 207
sponsein the climate systemto the radiative forcing
from the eruptions.The direct radiativeresponsein the
winter wouldbe a smallcooling,asit is winter becauseof
the low insolation.At the pole the surface radiative
forcingis actuallywarming,asthere is a smallincreasein
downwardlongwaveradiation.
5.4.2. lheory.
The winter warmingpatternsde-
scribed above are closelyrelated to troposphericand
stratosphericcirculation.Both Perlwitzand Graf [1995]
and Kodera et al. [1996] examinedthe observationsof
NH winter stratosphericand troposphericcirculationfor
the past 40 years and found that the dominant mode of
circulationof the stratosphereis a strongpolar vortex
(polar night jet), which occurssimultaneously
with a
500-mbarpattern with a low anomaly over Greenland
and high anomaliesover North America, Europe, and
east Asia. The associatedsurfaceair temperature pattern is exactlythat shownin Plate 8. Perlwitz and Graf
call this pattern the "baroclinicmode." The same pattern had previouslybeen identifiedasthe North Atlantic
Oscillation(NAO) [Hurrell,1995,and referencestherein] and is now also called the Arctic Oscillation(AO)
[Thompsonand Wallace,1998, 2000a,b].
Both
studies also found
a second mode
with weak
stratosphericanomaliesbut a strongmidtropospheric
pattern of wave anomalies generated in the tropical
Pacific and propagating acrossNorth America. This
secondbarotropicmode [Perlwitzand Graf, 1995] has
previouslybeen identified as the PacificNorth America
teleconnection
pattern associatedwith ENSO [Wallace
and Gutzler,1981]. It is confinedto the Western Hemisphere and only influences extratropical regions over
North America. Kitoh et al. [1996] found the samepatterns in a GCM experiment,with the baroclinicmode
normal, and that over Alaska, Greenland, the Middle
dominating and no relationship between the ENSO
East, and China was cold. In fact, it was so cold that mode and stratosphericcirculation.
winter that it snowedin Jerusalem,a very unusual ocThe picture that emergesis one of a characteristic
currence. Coral at the bottom of the Red Sea died that
baroclinicmode, or NAO or AO circulationpattern, in
winter [Genin et al., 1995], becausethe water at the the troposphere(Figure 6) that producesanomalously
surfacecooled and convectivelymixed the entire depth warm surfaceair temperaturesover the continentsin the
of the water.The enhancedsupplyof nutrientsproduced NH winter. It is a natural mode of oscillation of the
anomalouslylarge algal and phytoplankton blooms, winter atmosphericcirculation,so externalstratospheric
which smotheredthe coral. This coral death had only forcing apparentlycan push the systeminto this mode
happened before in winters following large volcanic without too much trouble. As Perlwitzand Graf [1995]
eruptions[Geninet al., 1995].
explainin detail, the theoreticalexplanationis that the
While the tropicalregionscoolin all seasons[Robock strongpolar vortextrapsthe verticallypropagatingplanand Mao, 1995] (Figure 4), in the winter following a etary waves,which constructivelyinterfere to produce
large eruption,the zonal averagetemperaturechangeis this stationary wave pattern. When there is a strong
small,but the wave pattern of anomaliesproduceslarge polar night jet, it preventssuddenstratosphericwarmwarm and cool anomalies,indicating a dynamical re- ings[Mcintyre,1982]later in the winter and perpetuates
Plate 7. (opposite)Stratospheric
temperatureanomaliesfor four equal-areazonalbands,shownas5-month
runningmeans,from microwavesoundingunit satelliteobservations,
channel4 [Spencer
et al., 1990](updated
in 1999).Anomalies(øC)arewith respectto the 1984-1990nonvolcanic
period.Timesof 1982E1Chich6nand
1991Pinatuboeruptionsare denotedwith arrows.Note that after theseeruptionsthe tropicalbandswarmed
more than the 30ø-90øNband, producingan enhancedpole-to-equatortemperaturegradient.Years on
abscissaindicateJanuaryof that year.
208 ß Robock: VOLCANIC ERUPTIONS AND CLIMATE
38, 2 / REVIEWS OF GEOPHYSICS
itself throughout the winter, keeping the polar lower
stratospherecold in the isolatedvortexcenter.When the
pattern occursnaturallywithout externalforcing,it more
frequently breaks down due to these sudden stratosphericwarmings.
The winter warmingcirculationresponseoccursafter
all large tropical eruptions that have occurred since
radiosondeobservationsbegan,Agung in 1963, E1 Chich6n in 1982, and Pinatuboin 1991 [Kodera,1994]. It
shouldbe noted, however,that other forcingscan also
work in the same direction. One is the l 1-year solar
cycle,as the l 1-year cyclein ultraviolet flux and ozone
amount combine to produce anomalousstratospheric
heating and circulation [Kodera et al., 1991; Robock,
1996a;Haigh, 1996;Shindellet al., 1999].Thesecirculation changeshelp to explain the solar cycle-climate
relationsdiscoveredby Labitzke and van Loon [1988]
and vanLoon andLabitzke[1990],but their patternsare
still not completelyunderstood.Another forcing is simply globalwarming,which increasesthe thicknessof the
tropical tropospheremore than at higher latitudes,also
enhancing the pole-to-equator gradient [Graf et al.,
1995]. Ozone depletion in the lower stratospherehas
producedmore coolingat the poles [Ramaswamyet al.,
1996], also enhancingthe pole-to-equatorgradientand
producinga trend in the strengthof the polar vortex
[Graf et al., 1995]. These global warming and ozone
depletiontrendshelp explainthe observedtrend in the
NAO [Hurrell,1995;Hurrelland vanLoon, 1997]and the
AO [Thompsonand Wallace,1998, 2000b] of the past
b
30
several decades and in the related cold ocean-warm
land
patternof Wallaceet al. [1995,1996].Nevertheless,after
a large tropicalvolcaniceruption, the forcingis so large
.%
that it overwhelms
these more subtle trends and domi-
nates the NH winter circulationof the succeedingyear.
The connectionbetweenstratosphericcirculationand
surface air temperature anomaliesin the NH winter
after a large tropical volcanic eruption is illustrated
schematicallyin Plate 9. The heating of the tropical
lower stratosphereby absorptionof terrestrialand solar
Figure 6. (a) Observed 500-mbar geopotential height
(meters)anomalypatternfor the 1991-1992NH winter (DJF)
followingthe 1991 Mount Pinatuboeruption.Data are from
the National Centersfor EnvironmentalPredictionreanalysis
[Kalnayet al., 1996], and anomaliesare with respectto the
period1986-1990.(b) Generalcirculationmodel(GCM) simulationsfrom Kirchneret al. [1999]. Shownis the difference
between
the two GCM
ensembles
with
and without
strato-
sphericaerosols.In both panels,regionssignificantlydifferent
from 0 at the 80% levelare shaded,with positivevaluesshaded
lighter and negativevalues shadeddarker. The simulations
reproducedthe observedpattern of circulationanomalies,but
not perfectly.
near-IR radiation(Plate 5) expandsthat layer and producesand enhancedpole-to-equatortemperaturedifference. The strengthened polar vortex traps the wave
energyof the troposphericcirculation,and the stationary wave pattern known as the NAO or AO dominates
the winter circulation,producingthe winter warming.
5.4.3. Modeling. To testthe abovetheory,Grafet
al. [1993] presentedresultsfrom a perpetual January
GCM calculationof the effectsof stratosphericaerosols
on climatethat showedwinter warmingover both northern Eurasia and Canada and cooling over the Middle
East, northern Brazil, and the United States.They used
forcing with a stratosphericaerosolloading in the pattern of the E1 Chich6n
volcano
and the low-resolution
(T21) ECHAM2 GCM, and the circulationresponsein
Januaryfollowingvolcaniceruptionswaswell simulated
[Kirchnerand Graf, 1995].Kirchneret al. [1999]repeated
this calculationin more detail with an improvedmodel,
38, 2 / REVIEWSOF GEOPHYSICS
Robock: VOLCANIC ERUPTIONS AND CLIMATE ß 209
in an experimentwith better definedaerosolparameters
[Stenchikovet al., 1998], interactivecalculationof the
aerosol radiative effects, and an improved GCM
(ECHAM4) [Roeckneret al., 1996], includingthe full
seasonalcycle.They were successful
in reproducingthe
observedcirculation(Figure6) and surfacetemperature
patterns (Figure 7) after the 1991 Pinatubo eruption
when usingclimatologicalseasurfacetemperatures.Althoughthe simulatedpatternsdid not exactlymatchthe
observations,the positiveheight anomaliesover North
AmericaandEurope andnegativeanomaliesoverBaffin
Island and the Middle East (Figure 6) are in the right
location,which correspondto warm and cold anomalies
overthe samelocations(Figure7). The simulationswere
not as successfulwhen using the actual observedsea
surface temperatures,which included a moderate E1
Nifio. Thus GCMs have not yet demonstratedan ability
to successfully
simulate the completeresponseof the
climatesystemto surfaceand stratosphericforcing.This
is an area of active current research,includingthe new
Pinatubo Simulation Task of the GCM-Reality IntercomparisonProjectfor the Stratosphere(GRIPS) [Pawson et al., 2000].
Mao and Robock [1998] conductedanother model
test of the winter warmingpattern. Patternssimilar to
that in Plate 8 appeared over North America in the
winters following the 1982 E1 Chich6n and 1991 Pinatubo eruptions,which were both duringE1 Nifios. This
led someto suggestthat the temperaturepatternswere
producedby the PacificNorth American teleconnection
pattern connectedwith the E1 Nifio. As both forcings
were actingat the sametime, observations
alone cannot
distinguishthe causeof the pattern. Mao and Robock
took advantageof the designof the AtmosphericModel
IntercomparisonProject (AMIP) [Gates,1992], which
was conductedfor the period 1979-1988. Thirty different GCMs simulatedthe weatherof this 10-yearperiod,
forced only with observed sea surface temperatures
(SSTs), which includedtwo E1 Nifios, 1982-1983 and
1986-1987. No volcanicaerosolforcingwas used in the
stratosphericheating during the winter of 1783-1984
and a reduced pole-to-equatortemperature gradient.
This theory suggeststhat the opposite phase of the
pattern discussedabove was produced, with a large
negativeNAO anomaly.This suggestionis now being
testedwith GCM experiments.
5.5. tittle Ice Age
Sincevolcanicaerosolsnormallyremain in the stratosphere no more than 2 or 3 years, with the possible
exceptionof extremelylarge eruptionssuch as that of
Toba, approximately71,000 years ago [Bekki et al.,
1996], the radiative forcing from volcanoesis interannual rather than interdecadal
in scale. A series of volca-
nic eruptions could, however, raise the mean optical
depthsignificantlyover a longerperiod and therebygive
rise to a decadal-scalecooling. If a period of active
volcanismendsfor a significantinterval,the adjustment
of the climate systemto no volcanicforcingcould produce warming. This was the casefor the 50 years from
1912 to 1963, when global climate warmed. Furthermore, it is possiblethat feedbacksinvolving ice and
ocean,which act on longer timescales,could transform
the short-termvolcanicforcinginto a longer-termeffect.
As a result, the possiblerole of volcanoesin decadalscaleclimate changeremainsunclear.In particular,the
current centuryis the warmestof the past five, with the
previousfour centuriesearningthe moniker of the Little
Ice Age due to its coldness.This period has never been
satisfactorilyexplained.What was the role of volcanism
in this climate change?
Until recently,Schneider
andMass[1975]andRobock
[1979]were the onlymodelsto usevolcanicchronologies
to investigateperiodsbefore the midnineteenthcentury.
At the same time, they examined the hypothesisthat
solarconstantvariationslinked to the sunspotcyclewere
alsoan importantcauseof climatechangeon this timescale. Schneider
and Mass used a zero-dimensional
en-
ergy-balancemodel and found agreement of several
large-scalefeatures of their simulationswith climate
simulations, so the results of the simulation should re- records. They concluded that while volcanoes had a
flect ENSO patternsonly. Mao and Robock found that weak relationshipto climate,the solar-climateeffectwas
the North American patterns simulatedby the models not proven.Robockuseda latitudinallyresolvedenergywere very similar for both E1 Nifio winters. They were balance model with volcanic [Mitchell, 1970] and solar
very close to the observedpattern for 1986-1987, a forcing (proportional to the envelope of the sunspot
winter with only E1 Nifio forcing and no volcanicaero- number) and found the volcanic forcing explained a
sols,but did not resemblethe observedpattern of 1982- much larger share of the temperaturevariability since
1983 (Plate 8) at all. Thereforethey concludedthat the 1620 than did the solar series. Both the Schneider and
major warming over the North American and Eurasian Mass and Robock models used a simple mixed-layer
continents
in the 1982-1983
NH winter
is not an ENSO-
dominantmode, but rather a pattern associatedwith the
enhancedstratosphericpolar vortexby the larger equator-to-pole temperaturegradientproducedby volcanic
sulfateaerosolsin the stratosphere.
Franklin [1784] noted that the winter in Europe following the 1783 Lakagigareruptionwas extremelycold
rather than warm. BecauseLakagigarwas a high-latitude eruption, it could have produced high-latitude
ocean.
Three new studies[Crowleyand Kim, 1999;Free and
Robock,1999;DMrrigo et al., 1999] have now made use
of new volcanochronologies,new solar constantreconstructions,and new reconstructionsof climate change
for the Little Ice Age to addressthis problem again.
They used upwelling-diffusionenergy-balanceclimate
models to simulate the past 600 years with volcanic,
solar, and anthropogenicforcings and compared the
210 ß Robock: VOLCANIC ERUPTIONS AND CLIMATE
38, 2 / REVIEWS OF GEOPHYSICS
75N
60N
45N
30N
15N
EQ
180
Figure 7. Temperaturepatternsfor NH winter (DJF) 1991-1992followingthe Mount Pinatuboeruption.
(a) Observedanomalies(with respectto the averagefor 1986-1990)from channel2LT of the microwave
soundingunit satelliteobservations(same as in Plate 8), representativeof the lower troposphere.(b)
Anomaliessimulatedby a GCM forcedwith observedPinatuboaerosols[Kirchneret al., 1999] expressedas
GCM simulatedchannel2LT temperatures.Shownare differencesbetweenthe two GCM ensembles
with and
without stratosphericaerosols.Contour interval is IøC, and shadingis as in Figure 6. The locationsof
anomalouswarm and cold regionsare quite closeto thoseobserved.
resultswith paleoclimaticreconstructions,
mainly based
on tree rings.They conclude,subjectto the limitationsof
the forcingand validationdata sets,that volcaniceruptions and solarvariationswere both important causesof
climate changein the Little Ice Age. Furthermore, the
warming of the past century cannot be explained by
natural causesand can be explainedby warming from
anthropogenic greenhouse gases. Further work is
neededin this area, however,linking ocean-atmosphere
interactionswith better volcanicand solar chronologies.
Ropelewski,1992]. Owing to the coincidenceof the beginningof the ENSO eventand the E1Chich6neruption,
severalsuggestions
were made as to a causeand effect
relationship,evengoingsofar as to suggestthat mostE!
Nifios were causedby volcaniceruptions.
How could a volcaniceruption produce an E1 Nifio?
Would the mechanisminvolve the stratosphericaerosol
cloud, troposphericaerosols,or the dynamicalresponse
to surfacetemperature changes?What is the evidence
based on the past record of volcanic eruptions and.
ENSO
6.
DO
VOLCANIC
ERUPTIONS
PRODUCE
EL NINOS?
The April 1982 E1 Chich6nvolcaniceruptionwas the
largestof the centuryup to that time. Beginningin April
1982, as indicatedby the SouthernOscillationIndex, an
unpredictedand unprecedentedE1 Nifio began, resulting in the largestwarm ENSO eventof the centuryup to
that time, with recordwarm temperaturesin the eastern
equatorial Pacific Ocean and remote temperature and
precipitationanomaliesin distantlocations[Halpertand
events? Was the simultaneous
occurrence
of the
E1 Chich6n eruption and large E1 Nifio just a coincidence,or was it an exampleof a mechanismthat works
after many other large eruptions?
Hirono [1988] proposed a plausible mechanism
whereby troposphericaerosolsfrom the E1 Chich6n
eruption would induce an atmosphericdynamical responseproducinga trade wind collapse,and the trade
wind reduction would produce an oceanographicresponsewhich affected the timing and strength of the
resultingE1 Nifio. He suggestedthat large tropospheric
aerosolsfalling out of the E1 Chich6n eruption cloud
into the regionwest of Mexico in the eastPacificin the
38, 2 / REVIEWSOF GEOPHYSICS
z
o
z
o
z
o
Robock:VOLCANIC ERUPTIONSAND CLIMATEß 211
o
ta.I
(/•
o
(/)
o
212 ß Robock' VOLCANIC ERUPTIONS AND CLIMATE
38, 2 / REVIEWS OF GEOPHYSICS
.. .• Aerosol
===============================
.•..
.::i:!:!:i:i:i:i:!:i:i:i:i:!:i:i:i:!:i:i:i:!:i:!:i:i:!:•:i:i:i:!:i:!-
'.>:.:.:.:.:.>:.:.:..'.:.:.:.:.:-:.:->:.:-:-:-:.:-:.:q•-:-:-:-:-:-'
50mbbefore
heating
ß
. .....>¾:¾•¾..-..:..:..-.>:.-.--'
. -. -. -' -' -'
Tropopause
Jet stream ams
i
i
AerosolheatingincreasesEquator-Pole
temperaturegradient,increasing
heightgradient,and makingjet stream
(polar vortex) stronger.
II
North Pole
Tropics
Strongervortexamplifies"North
Atlantic Oscillation"
circulation
pattern,producingwinter warming.
III
I
II
Winter1991-92(DJF)SurfaceAir Temperature
Anomalies
(øC)
•!
-
.
Plate 9. Schcmaticdiagramof howtropicallowerstratospheric
heatingfrom volcanicaerosolsproducesthe
winter warming temperaturepattern at the surface.The map at the bottom is surfaceair temperature
anomalies(with respect to 1961-1990) for the 1991-1992 NH winter (DJF) following the 1991 Mount
Pinatuboeruption.The green curvesindicatethe anomaloustroposphericwind patternsresponsiblefor
horizontaladvectionthat producedthesetemperatureanomalies.This temperaturepatternis very similarto
the one shownin Plate 8 but is for the surface.Data courtesyof P. Jones.
38, 2 / REVIEWSOF GEOPHYSICS
Robock:VOLCANIC ERUPTIONSAND CLIMATE ß 213
weeks after the eruption would absorbterrestrial and
solar radiation,heating the atmosphereand inducinga
low-pressureregion,changingthe troposphericcirculation. The wind spiralinginto the low would reducethe
northeasttrade winds,producingan oceanographicresponsethatwouldcausean E1Nifio.Robocket al. [1995]
usedthe LawrenceLivermore National Laboratoryversion of the Community Climate Model, Version 1
(CCM1) GCM to investigatethis mechanismwith radiative forcing from the observedaerosol distribution.
Indeed, there was a trade wind collapseinducedin the
atmosphere,but it wastoo late andin the wrongposition
to have producedthe observedE1 Nifio.
It is interestingto note that when this study was
beginning,E. Rasmusson,
one of the world'sexpertson
Robock and Free [1995] calculated the correlation
betweenthe SouthernOscillationIndex [Ropelewski
and
Jones,1987], the best availableindex of past E1 Nifios,
and everyvolcanicindexdescribedabove,aswell aseach
E1 Nifios
7.
and whose
office was next to mine
at the
Universityof Maryland, the week before the 1991 Pinatubo eruption excitedlywalked down the hall announcing that a new E1 Nifio wasbeginning.Thus althoughin
1991 there was again a large volcaniceruption at the
sametime as a large E1 Nifio, the E1Nifio occurredfirst.
In fact, Rampino et al. [1979] once suggestedthat climatic changecouldcausevolcaniceruptionsby changing
individual ice core record that was available, and in no
casesfound significantcorrelations.Self et al. [1997]
recentlyexaminedevery large eruption of the past 150
yearsand showedthat in no caseswas there an E1 Nifio
that resultedas a consequence.
Therefore, as concluded
by Robocket al. [1995],the timing and locationof the E1
Chich6n eruption and the large ENSO event that followed were coincidental, and there is no evidence that
large volcaniceruptionscan produce E1 Nifios.
EFFECTS ON
STRATOSPHERIC
OZONE
Volcanic aerosolshave the potential to change not
only the radiative flux in the stratosphere,but also its
chemistry.The most important chemicalchangesin the
stratosphereare related to O3, which has significant
effectson ultraviolet and longwaveradiative fluxes.The
reactionswhich produceand destroyO3 dependon the
the stress on the Earth's crust as the snow and ice
UV flux, the temperature,and the presenceof surfaces
loading shiftsin responseto the climate. More recent for heterogeneousreactions,all of which are changedby
evidenceof relativelylarge changesin the Earth's rota- volcanicaerosols[Crutzen,1976; Tabazadehand Turco,
tion rate after E1 Nifios, in responseto the enhanced 1993; Tie and Brasseur,1995; Tie et al., 1996; Solomon et
atmosphericcirculation[Marcuset al., 1998],couldpro- al., 1996]. The heterogeneouschemistryresponsiblefor
duce an additional lithosphericstress.These specula- the ozone hole over Antarctica in October each year
tions have not been proven,however,and further study occurson polar stratosphericcloudsof water or nitric
acid, which only occur in the extremely cold isolated
is limited by availablelong-termrecords.
Schattenet al. [1984] and Strong[1986]suggested
that springvortex in the SouthernHemisphere.These reacthe atmosphericdynamicresponseto the stratospheric tions make anthropogenicchlorine availablefor chemiheating from the volcanicaerosolswould perturb the cal destructionof O3. Sulfate aerosolsproducedby volHadley cell circulationin the tropicsin sucha way as to caniceruptionscan alsoprovidethesesurfacesat lower
reducethe trade windsand triggeran E1 Nifio. Schatten latitudes and at all times of the year.
Solomon[1999] describesthe effects of aerosolson
et al.'s model, however,was zonally symmetricand did
not explain why the responsewould be so rapid and ozonein great detail. Here only a brief mention of some
of the issues related to volcanic eruptions is made.
mainly in the Pacific.
Handler[1986]suggested
that the climaticresponseto Quantifying the effects of volcanic aerosolson ozone
coolingover continentsproducesa monsoon-likecircu- amount is difficult, as chemical and dynamical effects
and the effectsare not muchlarger
lation that somehowproducesE1 Nifios, but he did not occursimultaneously
clearly explainthe physicalmechanismor demonstrate than natural variability. Nevertheless, attempts were
that a model can producethis effect.His claim to find a made after the 1991 Pinatubo eruption to estimatethe
statisticallink betweenoccurrenceof volcaniceruptions effectson ozone.Column O3 reductionof about5% was
and E1 Nifios is flawed in severalways.As there have observedin midlatitudes [Zerefoset al., 1994; Coffey,
been many volcaniceruptionsin the past few centuries 1996],rangingfrom about2% in the tropicsto about7%
and E1Nifiosoccurevery4-7 years,it is possibleto find in the midlatitudes [Angell, 1997a]. Therefore ozone
an eruptionclosein time to each E1 Nifio. However, to depletion in the aerosol cloud was much larger and
establisha cause and effect relationship,the eruption reachedabout20% [Grantet al., 1992;Grant, 1996].The
must be of the proper type and at the proper time to chemicalozone destructionis lesseffectivein the tropproducethe claimedresponse.Handler'sstudiesdid not ics,but lifting of low ozoneconcentrationlayerswith the
use a proper index of stratosphericsulfateloading.He aerosolcloud [Kinneet al., 1992] causesa fast decrease
selectedvery small eruptionsthat could not have had a in ozone mixing ratio in the low latitudes. Similarly,
climaticeffect, and he was not carefulwith the timing of subsidenceat high latitudesincreasesozone concentrathe eruptionshe chose,claimingeruptionsthat occurred tion there and masks chemical destruction.
Decrease of the ozone concentration
causes less UV
after particularENSO eventsor long before them were
their causes.
absorptionin the stratosphere,which modifiesthe aero-
214 ß Robock:VOLCANIC ERUPTIONSAND CLIMATE
38, 2 / REVIEWSOF GEOPHYSICS
sol heating effect [Kinne et al., 1992; RosenfieMet al.,
1997]. The net effect of volcanicaerosolsis to increase
surfaceUV [Vogelmann
et al., 1992].The subsequent
03
depletionallowsthroughmore UV than is backscattered
by the aerosols.
Kirchneret al. [1999] calculateda January1992 global
average70-mbarheatingof about2.7 K from the aerosol
heating following the 1991 Pinatubo eruption as comparedwith the observedheatingof only1.0 K. The GCM
calculationsdid not include considerationof the quasibiennialoscillation(QBO) or 03 effects.They then conducted
another
calculation
to see the
effects
of the
observed Pinatubo-induced 0 3 depletion on stratospheric heating and found that it would have cut the
calculatedheating by 1 K. If the GCM had considered
the observedQBO coolingand this 03 effect, the calculated heatingwould agree quite well with observations.
The volcaniceffecton 03 chemistryis a newphenomenon, dependent on anthropogenicchlorine in the
stratosphere.While we have no observations,the 1963
Agung eruption probably did not deplete 03, as there
was little anthropogenicchlorine in the stratosphere.
Becauseof the Montreal protocoland subsequentinternational agreements,chlorineconcentrationhaspeaked
in the stratosphereand is now decreasing.Therefore, for
the next few decades,large volcaniceruptionswill have
effects similar to Pinatubo, but after that, these 0 3
effectswill go away and volcaniceruptionswill have a
stronger effect on atmosphericcirculationwithout the
negativefeedbackproducedby 03 depletion.
8.
SUMMARY
AND
DISCUSSION
Large volcaniceruptionsinject sulfur gasesinto the
stratosphere,which convert to sulfate aerosolswith an
e-folding residencetimescaleof about 1 year. The climate responseto large eruptionslastsfor severalyears.
The aerosol cloud producescooling at the surfacebut
heatingin the stratosphere.For a tropical eruption this
heatingis larger in the tropicsthan in the high latitudes,
producing an enhanced pole-to-equator temperature
gradient and, in the Northern Hemisphere winter, a
strongerpolar vortex and winter warming of Northern
Hemispherecontinents.This indirectadvectiveeffect on
temperatureis strongerthan the radiativecoolingeffect
that
dominates
at lower
latitudes
and in the summer.
matic responserepresent a large perturbation to the
climate systemover a relativelyshort period, observations and the simulatedmodel responsescan serve as
important analogs for understandingthe climatic responseto other perturbations.While the climatic responseto explosivevolcaniceruptionsis a usefulanalog
for some other climatic forcings,there are also limitations.
The theory of "nuclearwinter," the climaticeffectsof
a massiveinjectionof sootaerosolsinto the atmosphere
from firesfollowinga globalnuclearholocaust[Turcoet
al., 1983, 1990;Robock,1984c,1996b],includesupward
injectionof the aerosolsto the stratosphere,
rapidglobal
dispersalof stratosphericaerosols,heatingof the stratosphere, and cooling at the surface under this cloud.
Becausethis theory cannot be tested in the real world,
volcaniceruptionsprovide analogsthat supportthese
aspectsof the theory.
Even though a climate model successfully
simulates
the responseto a volcaniceruption [e.g.,Hansenet al.,
1992], this does not guaranteethat it can accurately
simulatethe responseto greenhousegases.The abilityof
the samemodel to respondto decadal-and longer-scale
responsesto climate forcingsis not tested,as the interannualtime-dependentresponseof the climatesystem
dependsonly on the thermal inertia of the oceanicmixed
layer combinedwith the climate model sensitivity.The
long-termresponseto globalwarmingdependson accurate simulation of the deep ocean circulation, and a
volcanoexperimentdoesnot allow an evaluationof that
portion of the model. If a volcano experiment [e.g.,
Kirchneret al., 1999] simulatesthe winter warming responseof a climate model to volcanicaerosols,then we
can infer the ability of a model to simulate the same
dynamicalmechanismsin responseto increasedgreenhousegases,ozone depletion,or solarvariations.
As climate models improve, through programslike
the GCM-Reality Intercomparison Project for the
Stratosphere(GRIPS) of the StratosphericProcesses
and their Relation to Climate (SPARC) program[Pawson et al., 2000], they will improvetheir ability to simulate the climatic responseto volcanic eruptions and
other causesof climate change.
GLOSSARY
The volcanic aerosolsserve as surfacesfor heterogeneous chemical reactions that destroy stratospheric
500-mbar pattern: Wind circulationin the middle
ozone,which lowersultravioletabsorptionand reduces of the troposphere,typicallyat a heightof about5.5 km,
the radiativeheatingin the lower stratosphere.Sincethis which is indicativeof the generaldirectionof transport
chemicaleffect dependson the presenceof anthropo- of energy and moisture. Average surface pressureis
genic chlorine, it has only becomeimportant in recent about1000mbar,equalto l0s Pa.
decades.There is no evidencethat volcaniceruptions
Advectiveeffect: Changesproducedby horizontal
produceE1 Nifio events,but the climaticconsequences transportof energyby windsas opposedto thosecaused
of E1 Nifio and volcaniceruptionsare similar and must by radiation.
be separatedto understandthe climaticresponseto each.
Aerosol: A liquid droplet or solid particle susBecausevolcaniceruptionsand their subsequentcli- pended in a gas.
38, 2 / REVIEWS OF GEOPHYSICS
Robock: VOLCANIC ERUPTIONS AND CLIMATE ß 215
Apparenttransmission: Amountof total solarradi- measures backscattered ultraviolet radiation in several
ation transmittedto the surfacecorrectedfor geometry wavelengths,from whichthe total columnamountof 03
and time of day of measurements.
and SO2 can be derived.
December-January-February
(DJF): Winter in the
Northern Hemisphere.
ACKNOWLEDGMENTS. I thank Georgiy Stenchikov,
Ellsworth
Dutton, Steven Self, and the reviewers for valuable
forananomaly
to decaytoe- • of itsinitialperturbation.
e-foldingdecaytime: The amountof time it takes
This is typicallytaken to be the definition of the timescaleof a logarithmicprocess.
Glass inclusions: Pocketsof liquid or gas inside
glassformed when molten rock cools after a volcanic
eruption.
Heterogeneouschemical reactions: Chemicalreactionsthat occurwith interactionsinvolvingmore than
one phase. Gas reactionsthat occur on the surface of
aerosolparticlesare an example.
commentson the manuscript.My work on volcaniceruptions
and climate has been supportedby NSF and NASA for more
than 20 years,most recentlyby NSF grant ATM 99-96063 and
NASA grant NAG 5-7913. I particularly thank Jay Fein, Ken
Bergman,and Lou Walter for their supportas programmanagers.I thank Clifford Mass, Michael Matson, JianpingMao,
Yuhe Liu, Georgiy Stenchikov,Karl Taylor, Hans Graf, Ingo
Kirchner, Melissa Free, and Juan Carlos Antufia for their
collaborationswith me that made these results possible.I
thank John Christy and Phil Jones for temperature data. I
Interdecadal: Varying from one decade (10-year thank Ellsworth Dutton for radiation observations used in
Figures 1 and 2. Figures 1, 2, and 4-7 and Plates 5-8 were
period) to another.
Lithic: Pertainingto stone. In the contexthere, it drawn using GRADS, created by Brian Doty. I thank Juan
meansthat the aerosolparticlesare solid and consistof CarlosAntufia for helpingto draw Figures5 and 7 and Plates
7 and 8.
crustal material.
Little Ice Age: The period 1500-1900A.D., which
was cooler than the period before or after.
Magmaticmaterial: From the magma,the hot liquid from the interior of the Earth that often emerges
during a volcaniceruption.
MedievalWarming: Relativelywarm period,900-
Michael Coffey was the Editor responsiblefor this paper.
He thanks Glynn Germany for the cross-disciplinary
review
and O. B. Toon
and R. Stolarski
for the technical
review.
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A. Robock,Department
of Environmental
Sciences,
Rutgers
University,14 CollegeFarmRoad,New Brunswick,
NJ089018551. ([email protected])