Volcanic eruptions and climate - AGU Publications

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


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



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


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.


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.


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


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.


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



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


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 .-,

'

.,', :-o.2.,' 50N :,, IO,.2/

40Nlo:,

SON j

i

"'V' ' ' '•'

'"½.,' 4½, •

ß :

:

:

o-' i :

:

: .

": .....

"

'

:'ø

'

;""

/ ¾,/i' !..,•',.i",ii ø:':•/

; '

i } :'.o.:•.

:

:

•ON, •.

20S-!. -o., "' -

ß ", ,, ' D :o• : ""ø?!•-" "' : : : : :"'/ ',, . :)?:,'.:,-.•'• ::,,v,.',. ,'.";" ::•:;': ;: :::::

,,..., ...... .,

50S -

: :"; ...... ' : : "'::• : : 0 ....... •"''

....ß-'= . .. ,. ......

:' ;,• ,'.'•'

-5

-4

-',/'/::'œ:2-',:

........

-,.3

-• -\;'.-03,;:•)•",::; ' -0 '• •:--:'-', ;•'ß::;f::/'::. ...... :::,3,6•2 ,. ..... ......,.:---o-,,. .... '.. .. ,,.,., .............. ., -.... -œ0.2 '---

-2

ß

':-0 1•',•'., "•,

-1

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


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


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.



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,



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



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


z o

z o

z o

Robock:VOLCANIC ERUPTIONSAND CLIMATEß 211

o ta.I

(/• o

(/) o

212 ß Robock' VOLCANIC ERUPTIONS AND CLIMATE


.. .• 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.


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-



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.



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.

REFERENCES

1200 A.D.

Ahn, M. H., A new SO2 retrieval algorithm usingtotal ozone mappingspectrometerradianceand its applications,Ph.D. dissertation,192 pp., Dep. of Meteorol., Univ. of Md., Opticaldepth: A measureof the amountof extincCollege Park, 1997. tion of radiation along a path throughthe atmosphere, Andronova,N. G., E. V. Rozanov,F. Yang, M. E. Schlesinger, and G. L. Stenchikov,Radiativeforcingby volcanicaerosols proportionalto the amountof material in the path. from 1850 to 1994, J. Geophys.Res., 104, 16,807-16,826, Pyrheliometer: Radiation measuring instrument

Mt: Mcgaton= 106T = 109 kg = 10•2g = tcra-

gram.

1999.

that measures the direct solar flux.

Angell, J. K., Impact of E1 Nifio on the delineationof tropoPyroclasticflow: A hot, ash-ladeneruptioncloud sphericcoolingdue to volcaniceruptions,J. Geophys.Res., from a volcano that descendsdown the mountainside, in 93, 3697-3704, 1988. spite of its temperature,due to the weight of the sus- Angell, J. K., Estimated impact of Agung, E1 Chich6n, and Pinatubo volcanic eruptions on global and regional total pended aerosols. ozone after adjustmentfor the QBO, Geophys.Res. Lett., Quasi-biennialoscillation(QBO): Areversal of the 24, 647-650, 1997a.

wind direction in the tropical stratospherethat occurs Angell, J. K., Stratosphericwarming due to Agung, E1 Chiapproximatelyevery 27 months and influencestemperch6n, and Pinatubo taking into accountthe quasi-biennial ature and chemistry. oscillation, J. Geophys. Res.,102, 9479-9485,1997b.

StratosphericAerosoland Gas ExperimentII:

A

satellite instrumentin spacesince 1984 that measures the amount

of radiation

in four

visible

and ncar-IR

Asaturov, M. L., M. I. Budyko, K. Y. Vinnikov, P. Y. Groisman, A. S. Kabanov, I. L. Karol, M.P. Kolomeev, Z. I. Pivovarova, E. V. Rozanov, and S.S. Khmelevtsov, Volca-

nics,Stratospheric AerosolandEarth'sClimate(in Russian),

frequenciesthat is transmittedhorizontallythroughdif256 pp., Gidrometeoizdat,St. Petersburg,Russia, 1986. ferent altitudesby limb scanning.The amount and size Barkstrom, B. R., The Earth Radiation Budget Experiment distribution

of volcanic

aerosols

can be derived

from

(ERBE), Bull. Am. Meteorol.Soc.,65, 1170-1185, 1984. Barnes, J. E., and D. J. Hoffman, Lidar measurements of

these data.

stratosphericaerosol over Mauna Loa Observatory,Geophys.Res.Lett., 24, 1923-1926, 1997. SO4. Sulfat½aerosolsconsistof H20 and H2SO4. Bekki, S., J. A. Pyle, and D. M. Pyle,The role of microphysical and chemicalprocessesin prolongingthe climate forcingof Teleconnectionpattern: Patternthat is correlated the Toba eruption,Geophys. Res.Lett.,23, 2669-2672, 1996. but not contiguous. G. J. S., S. D. Doiron, S.C. Schnetzler,A. J. Krueger, Teragram:Equalto 10TMg = 109 kg = 106T = Bluth, and L. S. Walter, Global tracking of the SO2 cloudsfrom megaton. the June 1991 Mount Pinatubo eruptions, Geophys.Res.

Sulfate: Containingoxidizedsulfur,in the form of

•otal Ozone MappingSpectrometerfrOMS): An

instrument

that has flown

on satellites

since 1979 and

Lett., 19, 151-154, 1992.

Bradley,R. S., The explosivevolcaniceruptionsignalin North-



eruption of Italian and Icelandicvolcanoesin 1783, Quaern Hemisphere continental temperature records, Clim. ternaire, 9, 25-35, 1998. Change,12, 221-243, 1988. Bryson,R. A., and G. J. Dittberner,A non-equilibriummodel Groisman, P. Y., Regional climate consequencesof volcanic eruptions(in Russian),Meteorol.Hydrol., No. 4, 39-45, of the hemisphere,J. Atmos. Sci., 33, 2094-2106, 1976. 1985. Coffey, M. T., Observationsof the impact of volcanicactivity of the on stratosphericchemistry,J. Geophys.Res., 101, 6767- Groisman,P. Y., Possibleregionalclimateconsequences 6780, 1996. Pinatuboeruption:An empiricalapproach,Geophys.Res. Lett., 19, 1603-1606, 1992. Crowley, T. J., and K.-Y. Kim, Modeling the temperature responseto forced climate changeover the last six centu- Haigh, J., The impact of solarvariability on climate, Science, 272, 981-984, 1996. ries, Geophys.Res.Lett., 26, 1901-1904, 1999. Crutzen,P. J., The possibleimportanceof CSO for the sulfate Halpert, M. S., and C. F. Ropelewski,Surface temperature layerof the stratosphere,Geophys. Res.Lett., 3, 73-76, 1976. patternsassociated with the SouthernOscillation,J. Clim., 5, 577-593, 1992. D'Arrigo, R. D., G. C. Jacoby,M. Free, and A. Robock, Northern Hemispheretemperaturevariabilityfor the past Handler, P., Possible associationbetween the climatic effects three centuries: Tree-ring and model estimates,Clim. of stratosphericaerosolsand sea surfacetemperaturesin Change,42, 663-675, 1999. the easterntropicalPacificOcean,J. Climatol.,6, 31-41, 1986. Dutton, E.G., A coherencebetweenthe QBO and the ampli- Hansen, J. E., W.-C. Wang, and A. A. Lacis, Mount Agung tude of the Mauna Loa atmospherictransmissionannual providesa test of a global climatic perturbation,Science, 199, 1065-1068, 1978. cycle,Int. J. Climatol.,12, 383-396, 1992. Dutton, E.G., and J. R. Christy, Solar radiative forcing at Hansen, J., I. Fung, A. Lacis,D. Rind, S. Lebedeff, R. Ruedy, G. Russell,and P. Stone,Global climatechangesasforecast selected locations and evidence for global lower troposphericcoolingfollowingthe eruption of E1 Chich6n and by the Goddard Institute for Space Studiesthree-dimenPinatubo, Geophys.Res.Lett., 19, 2313-2316, 1992. sionalmodel, J. Geophys.Res.,93, 9341-9364, 1988. Dutton, E.G., J. J. DeLuisi, and A. P. Austring,Interpretation Hansen,J., A. Lacis,R. Ruedy, and M. Sato, Potential climate of Mauna Loa atmospherictransmissionrelative to aeroimpactof Mount Pinatuboeruption,Geophys. Res.Lett., 19, sols,usingphotometricprecipitablewater amounts,J. At215-218, 1992. mos. Chem., 3, 53-68, 1985. Hansen, J., et al., A Pinatubo climate modeling investigation, Ellsaesser,H. W., Isolating the climatologiceffects of volcain The Mount PinatuboEruptionEffectson theAtmosphere noes,Rep. UCRL-89161, 29 pp., LawrenceLivermore Natl. and Climate, NATO ASI Set., Set. I, 42, 233-272, 1996. Lab., Livermore, Calif., 1983. Harington, C. R. (Ed.), The Year Withouta Summer?World Forsyth,P. Y., In the wakeof Etna, 44 B.C., ClassicalAntiq.,7, Climate in 1816, 576 pp., Can. Mus. of Nature, Ottawa, 49-57, 1988.

Ont., Canada, 1992.

Franklin, B., Meteorological imaginations and conjectures, Harshvardhan,Perturbationsof the zonal radiation balanceby Manchr.Lit. Philos.Soc.Mem.Proc.,2, 122, 1784.(Reprinta stratosphericaerosollayer,J. Atmos.Sci.,36, 1274-1285, ed in Weatherwise, 35, 262, 1982.) 1979. Free, M., and A. Robock, Global warmingin the contextof the Hirono, M., On the triggerof E1Nifio-SouthernOscillationby Little Ice Age, J. Geophys.Res.,104, 19,057-19,070,1999. the forcing of early E1 Chich6nvolcanicaerosols,J. GeoGates, W. L., AMIP: The AtmosphericModel Intercompariphys.Res., 93, 5365-5384, 1988. sonProject,Bull. Am. Meteorol.Soc., 73, 1962-1970,1992. Holton, J. R., P. H. Haynes,M. E. Mcintyre, A. R. Douglass, Genin, A., B. Lazar, and S. Brenner, Vertical mixingand coral R. B. Rood, and L. Pfister, Stratosphere-troposphere exdeath in the Red Sea following the eruption of Mount change,Rev. Geophys.,33, 403-439, 1995. Pinatubo, Nature, 377, 507-510, 1995. Houghton,J. T., L. G. Meira Filho, B. A. Callander,N. Harris, Gilliland, R. L., Solar, volcanic, and CO2 forcing of recent A. Kattenberg, and K. Maskell (Eds.), Climate Change climaticchanges,Clim. Change,4, 111-131, 1982. 1995raThe Scienceof ClimateChange,584 pp., Cambridge Gilliland, R. L., and S. H. Schneider,Volcanic, CO2, and solar forcingof Northern and SouthernHemispheresurfacetemperatures,Nature, 310, 38-41, 1984.

Univ. Press, New York, 1996.

Humphreys,W. J., Volcanic dust and other factors in the productionof climaticchanges,and their possiblerelation Graf, H.-F., I. Kirchner, A. Robock, and I. Schult, Pinatubo to ice gases,J. Franklin Inst., Aug., 131-172, 1913. eruptionwinter climate effects:Model versusobservations, Humphreys, W. J., Physicsof theAir, 676 pp., Dover, Mineola, Clim. Dyn., 9, 81-93, 1993. Graf, H.-F., J. Perlwitz, I. Kirchner, and I. Schult, On the inter-

N.Y.,

1940.

of a relationshipbetweenrecentclimatetrends,ozonechangeand Hunt, B. G., A simulationof the possibleconsequences volcaniceruption on the general circulationof the atmoincreasedgreenhousegasforcing,in AtmosphericOzoneAs sphere,Mon. WeatherRev., 105, 247-260, 1977. a ClimateGas, edited by W. C. Wang and I. S. A. Isaksen, Hurrell, J. W., Decadal trends in the North Atlantic Oscillapp. 163-179, Springer-Verlag,New York, 1995. tion' Regionaltemperaturesandprecipitation,Science,269, Graf, H.-F., J. Feichter, and B. Langmann, Volcanic sulfur 676-679, 1995. emission:Estimatesof sourcestrengthand its contribution to the global sulfate distribution,J. Geophys.Res., 102, Hurrell, J. W., and H. van Loon, Decadal variations in climate 10,727-10,738, 1997.

associated with the North Atlantic Oscillation, Clim.

Change,36, 301-326, 1997. Grant, W. B., Tropical stratosphericozone changesfollowing the eruption of Mount Pinatubo,in The Mount Pinatubo Kalnay, E., et al., The NCEP/NCAR 40-yearreanalysisproject, Bull. Am. Meteorol. Soc., 77, 437-471, 1996. EruptionEffectson theAtmosphereand Climate,NATO ASI Set., Set. I, 42, 161-175, 1996. Keen, R. A., Volcanicaerosolsand lunar eclipses,Science,222, Grant, W. B., et al., Observations of reduced ozone concen-

trations in the tropical atmosphereafter the eruption of Mount Pinatubo,Geophys.Res.Lett., 19, 1109-1112, 1992. Grattan, J., M. Brayshay,and J. Sadler, Modelling the distal impactsof pastvolcanicgasemissions: Evidenceof Europewide environmentalimpactsfrom gasesemitted during the

1011-1013, 1983.

Kelly, P.M., and C. B. Sear, The formulationof Lamb's dust veil index, in AtmosphericEffects and Potential Climatic Impactof the 1980Eruptionsof Mount St.Helens,editedby A. Deepak, NASA Conf.Publ. 2240, 293-298, 1982. Kelly, P.M., P. D. Jones,A. Robock, and K. R. Briffa, The


Robock:VOLCANIC ERUPTIONS AND CLIMATE ß 21 7

contributionof Hubert H. Lamb to the study of volcanic Mass, C., and A. Robock, The short-term influence of the effects on climate, Weather, 53, 209-222, 1998. Mount St. Helensvolcaniceruptionon surfacetemperature in the northwest United States, Mort. Weather Rev., 110, Kinne, S., O. B. Toon, and M. J. Prather, Buffering of strato614-622, 1982. sphericcirculationby changingamountsof tropicalozone:A Pinatubocasestudy,Geophys. Res.Lett.,19, 1927-1930,1992. Mcintyre, M. E., How well do we understandthe dynamicsof Kirchner, I., and H.-F. Graf, Volcanos and E1 Nifio: Signal stratosphericwarmings?,J. Meteorol.Soc.Jpn., 60, 37-65, 1982. separationin Northern Hemispherewinter, Clim. Dyn., 11, 341-358, 1995. Kirchner, I., G. L. Stenchikov,H.-F. Graf, A. Robock, and J. C.

Miles, M. K., and P. B. Gildersleeves, Volcanic dust and

Antufia, Climate model simulationof winter warming and summercoolingfollowingthe 1991 Mount Pinatubovolcanic eruption,J. Geophys.Res.,104, 19,039-19,055, 1999.

735-736, 1978. Minnis, P., E. F. Harrison, L. L. Stowe, G. G. Gison, F. M.

Kitoh, A., H. Koide, K. Kodera, S. Yukimoto, and A. Noda,

changesin Northern Hemispheretemperature,Nature,271,

Denn, D. R. Doelling, and W. L. Smith Jr., Radiative climate forcing by the Mount Pinatubo eruption, Science,

259, 1411-1415, 1993. Interannualvariabilityin the stratospheric-tropospheric circulation in a coupled ocean-atmosphere GCM, Geophys. Mitchell, J. M., Jr., Recent secular changesof the global Res. Lett., 23, 543-546, 1996. temperature,Ann. N.Y. ,4cad. Sci., 95, 235-250, 1961. Kodera, K., Influenceof volcaniceruptionson the troposphere Mitchell, J. M., Jr., A preliminary evaluationof atmospheric throughstratosphericdynamicalprocesses in the Northern pollutionasa causeof the globaltemperaturefluctuationof the past century,in Global Effectsof EnvironmentalPolluHemispherewinter, J. Geophys. Res.,99, 1273-1282, 1994. Kodera, K., M. Chiba, and K. Shibata, A general circulation tion, edited by S. F. Singer,pp. 139-155, D. Reidel, Norwell, Mass., 1970. model study of the solar and QBO modulation of the stratosphericcirculationduring the Northern Hemisphere Mohnen, V. A., Stratosphericion and aerosol chemistryand winter, Geophys.Res.Lett., 18, 1209-1212, 1991. possiblelinks with cirrus cloud microphysics--A critical

Kodera, K., M. Chiba, H. Koide, A. Kitoh, and Y. Nikaidou,

Interannualvariability of the winter stratosphereand tropospherein the Northern Hemisphere,J. Meteorol.Soc. Jpn., 74, 365-382, 1996. Kondratyev,K. Y., Volcanoes and climate, WCP-54, WMO/ TD-166, 103 pp., World Meteorol. Org., Geneva, 1988. Kondratyev,K. Y., and I. Galindo, VolcanicActivityand Climate, 382 pp., A. Deepak, Hampton, Va., 1997. Krueger, A. J., C. C. Schnetzler,and L. S. Walter, The December1981eruptionof NyamuragiraVolcano(Zaire) and the origin of the "mysterycloud" of early 1982,J. Geophys. Res., 101, 15,191-15,196, 1996. Labitzke, K., and H. van Loon, Association between the l 1-

assessment,J. ,4tmos. Sci., 47, 1933-1948, 1990.

Newhall, C. G., and S. Self, The volcanic explosivityindex (VEI): An estimateof explosivemagnitudefor historical volcanism,J. Geophys.Res., 87, 1231-1238, 1982. Nicholls,N., Low latitudevolcaniceruptionsand the E1 NifioSouthern Oscillation, J. Climatol., 8, 91-95, 1988.

Oliver, R. C., On the responseof hemisphericmean temperature to stratosphericdust:An empiricalapproach,J. Appl. Meteorol., 15, 933-950, 1976.

Parker, D. E., and J. K. L. Brownscombe,Stratosphericwarming followingthe E1 Chich6nvolcaniceruption,Nature,301, 406-408, 1983.

Pawson,S., et al., The GCM-Reality IntercomparisonProject for SPARC (GRIPS): Scientificissuesand initial results,

year solarcycle,the QBO, and the atmosphere,part I, The troposphereand the stratospherein the Northern HemiBull. Am. Meteorol. Soc., 81,781-796, 2000. spherein winter, J. Atmos. Ten'.Phys.,50, 197-207, 1988. Perlwitz, J., and H.-F. Graf, The statistical connection between Lamb, H. H., Volcanic dust in the atmosphere,with a chrotroposphericand stratosphericcirculationof the Northern nology and assessmentof its meteorologicalsignificance, Hemispherein winter, J. Clim., 8, 2281-2295, 1995. Philos. Trans. R. Soc. London, Set. A, 266, 425-533, 1970. Pinto, J.P., R. P. Turco, and O. B. Toon, Self-limitingphysical Lamb, H. H., Supplementaryvolcanicdust veil index assessand chemicaleffectsin volcaniceruptionclouds,J. Geophys. ments, Clim. Monit., 6, 57-67, 1977. Lamb, H. H., Update of the chronologyof assessments of the volcanic dust veil index, Clim. Monit., 12, 79-90, 1983.

Res., 94, 11,165-11,174, 1989.

Pollack,J. B., O. B. Toon, C. Sagan,A. Summers,B. Baldwin, and W. Van Camp, Volcanic explosionsand climatic change: A theoretical assessment,J. Geophys.Res., 81,

Li, X. S., and A. Berger, Simulatingclimatic impact of the Toba eruption with the LLN 2-D climate model, Sci. Rep. 1071-1083, 1976. 1997/14, Inst. d'Astron. et de Geophys.G. Lemaitre, Univ. Pollack, J. B., D. Rind, A. Lacis, J. E. Hansen, M. Sato, and R. Cath. de Louvain, Louvain-la-Neuve,Belgium, 1997. Ruedy, GCM simulationof volcanicaerosolforcing,part I, Lough, J. M., and H. C. Fritts, An assessment of the possible Climate changes induced bv steady-stateperturbations, effects of volcanic eruptionson North American climate J. Clim., 6, 1719-1742, 1993. usingtree-ring data, 1602 to 1900 A.D., Clim. Change,10, Quiroz, R. S., The isolation of stratospherictemperature 219-239, 1987. changedue to the E1 Chich6nvolcaniceruption from nonLuhr, J. F., I. S. E. Carmichael,and J. C. Varekamp, The 1982 volcanicsignals,J. Geophys.Res.,88, 6773-6780, 1983. eruptionsof E1 Chich6nvolcano,Chiapas,Mexico: Mineralogy and petrology of the anhydrite-bearingpumices,J. Ramaswamy,V., M.D. Schwarzkopf,and W. J. Randel, Fingerprint of ozone depletion in the spatial and temporal Volcanol. Geotherm. Res., 23, 69-108, 1984. pattern of recent lower-stratosphericcooling,Nature, 382, MacCracken,M. C., and F. M. Luther, Preliminaryestimateof 616-618, 1996. the radiative and climatic effectsof the E1 Chich6n eruption, Geofis.Int., 23, 385-401, 1984. Rampino, M. R., and S. Self, Sulphur-richvolcaniceruptions Mao, J., and A. Robock, Surfaceair temperature simulations and stratosphericaerosols,Nature, 310, 677-679, 1984. by AMIP general circulationmodels:Volcanic and ENSO Rampino, M. R., and S. Self, Volcanicwinter and accelerated signalsand systematicerrors,J. Clim., 11, 1538-1552, 1998. glaciationfollowing the Toba super-eruption,Nature, 359, 50-52, 1992. Marcus,S. L., Y. Chao, J. O. Dickey, and P. Gegout,Detection and modeling of nontidal oceaniceffects on Earth's rota- Rampino, M. R., S. Self, and R. W. Fairbridge, Can climatic tion rate, Science, 281, 1656-1659, 1998. change causevolcanic eruptions?,Science,206, 826-828, 1979. Mass,C. F., and D. A. Portman,Major volcaniceruptionsand climate: A critical evaluation, J. Clim., 2, 566-593, 1989. Rind, D., N. K. Balachandran,and R. Suozzo,Climate change



Tahiti-Darwin Southern Oscillation Index, Mon. Weather and the middle atmosphere,part II, The impact of volcanic aerosols,J. Clim., 5, 189-208, 1992. Rev., 115, 2161-2165, 1987. Robock,A., Internally and externallycausedclimatechange,J. Rosenfield,J., D. Considine,P. Meade, J. Bacmeister,C. JackAtmos. Sci., 35, 1111-1122, 1978. man, and M. Schoeberl,Stratospheric effectsof Mount PinaRobock, A., The "Little Ice Age": Northern Hemisphereavtuboaerosolstudiedwith a coupledtwo-dimensional model,J. Geophys. Res.,102, 3649-3670, 1997. erage observationsand model calculations,Science,206, 1402-1404, 1979.

Robock,A., The Mount St. Helensvolcaniceruptionof 18 May 1980:Minimal climaticeffect,Science,212, 1383-1384,1981a. Robock,A., A latitudinallydependentvolcanicdustveil index, and its effect on climate simulations,J. Volcanol. Geotherm. Res., 11, 67-80, 1981b.

Robock,A., The dustcloudof the century,Nature,301,373374, 1983a. Robock, A., Ice and snow feedbacks and the latitudinal and

seasonaldistributionof climatesensitivity, J.Atmos.Sci.,40, 986-997, 1983b.

Robock,A., Review of VolcanoWeather:The Storyof 1816, the Year Withouta Summer,Natl. WeatherDig., 9, 5-6, 1984a.

Russell, P. B., et al., Global to microscale evolution of the Pinatubo

volcanic

aerosol

derived

from

diverse

measure-

mentsandanalyses, J. Geophys. Res.,101, 18,745-18,763,1996. Sapper,K., Beitrfigezur Geographieder tfitingenVulkane, Z. Vulkanol., 3, 65-197, 1917.

Sapper,K., Vulkankunde,Englehorn Verlag, Stuttgart,Germany, 1927. Sassen,K., D. O. Starr, G. G. Mace, M. R. Poellot, S. H. Melfi,

W. L. Eberhard, J. D. Spinhirne, E. W. Eloranta, D. E. Hagen, and J. Hallett, The 5-6 December 1991 FIRE IFO II jet streamcirruscasestudy:Possibleinfluencesof volcanic aerosols,J. Atmos. Sci., 52, 97-123, 1995. Sato, M., J. E. Hansen, M.P. McCormick, and J. B. Pollack,

Robock, A., Climate model simulationsof the effects of the E1

Stratosphericaerosol optical depths, 1850-1990, J. Geophys.Res.,98, 22,987-22,994, 1993. Schatten,K. H., H. G. Mayr, I. Harris, and H. A. Taylor, A winter, Nature, 310, 667-670, 1984c. zonallysymmetricmodel for volcanicinfluenceupon atmosphericcirculation,Geophys.Res.Lett., 11,303-306, 1984. Robock, A., Volcanoes and climate, in Climate and GeoSciences:A Challengefor Scienceand Societyin the 21st Schneider,S. H., and C. Mass, Volcanic dust, sunspotsand temperaturetrends,Science,190, 741-746, 1975. Century,NATO ASI Ser., Ser. C, 285, 309-314, 1989. Robock,A., The volcaniccontributionto climatechangeof the Self, S., M. R. Rampino, J. Zhao, and M. G. Katz, Volcanic aerosolperturbationsand strongE1 Nifio events:No genpast 100 years, in Greenhouse-Gas-Induced Climatic eral correlation, Geophys.Res.Lett., 24, 1247-1250, 1997. Change:A CriticalAppraisal of Simulationsand Observations, edited by M. E. Schlesinger,pp. 429-444, Elsevier Shelley, M., Frankenstein,Bantam Books, New York, 1818. (Classicedition,209 pp., BantamBooks,New York, 1981.) Sci., New York, 1991. Robock, A., Review of Year Withouta Summer?World Climate Shindell, D., D. Rind, N. Balachandran, J. Lean, and P. Lonergan, Solar cyclevariability, ozone, and climate, Science, in 1816, Clim. Change,26, 105-108, 1994. Chich6n eruption, Geofis.Int., 23, 403-414, 1984b. Robock,A., Snowand ice feedbacksprolongeffectsof nuclear

Robock, A., Stratosphericcontrol of climate, Science,272, 972-973, 1996a.

Robock, A., Nuclear winter, in Encyclopediaof Weatherand Climate, vol. 2, edited by S. H. Schneider,pp. 534-536, Oxford Univ. Press,New York, 1996b. Robock, A., and M.P. Free, Ice cores as an index of global

volcanismfrom 1850 to the present,J. Geophys.Res.,100, 11,549-11,567, 1995. Robock, A., and M.P. Free, The volcanic record in ice cores

284, 305-308, 1999.

Simarski,L. T., Volcanismand climatechange,specialreport, 27 pp., AGU, Washington,D.C., 1992. Simkin, T., and R. S. Fiske, Krakatau 1883: The Volcanic

Eruptionand Its Effects,464 pp., Smithson.Inst., Washington, D.C.,

1983.

Simkin,T., and L. Siebert,Volcanoesof the World,2nd ed., 349 pp., GeosciencePress,Tucson,Ariz., 1994. Simkin, T., L. Siebert, L. McClelland, D. Bridge, C. G. Newhall, and J. H. Latter, Volcanoes of the World,232 pp., Van

for the past 2000 years,in ClimaticVariationsand Forcing Nostrand Reinhold, New York, 1981. Mechanismsof the Last 2000 Years,edited by P. D. Jones, Solomon,S., Stratosphericozone depletion:A review of conR. S. Bradley,and J. Jouzel,pp. 533-546, Springer-Verlag, ceptsand history,Rev. Geophys.,37, 275-316, 1999. New York, 1996.

Solomon, S., R. W. Portmann, R. R. Garcia, L. W. Thomason,

Robock, A., and Y. Liu, The volcanic signal in Goddard L. R. Poole, and M.P. McCormick, The role of aerosol Institute for SpaceStudiesthree-dimensional model simuvariations in anthropogenicozone depletion at northern lations,J. Clim., 7, 44-55, 1994. midlatitudes,J. Geophys.Res., 101, 6713-6727, 1996. Robock,A., and J. Mao, Winter warmingfrom large volcanic Spencer,R. W., J. R. Christy,and N. C. Grody, Global atmoeruptions,Geophys.Res.Lett., 19, 2405-2408, 1992. spheric temperature monitoring with satellite microwave Robock, A., and J. Mao, The volcanicsignalin surfacetemmeasurements:Method and results 1979-1984, J. Clim., 3, peratureobservations, J. Clim., 8, 1086-1103, 1995. 1111-1128, 1990. Robock, A., and C. Mass, The Mount St. Helens volcanic Stenchikov, G. L., I. Kirchner, A. Robock, H.-F. Graf, J. C. eruptionof 18 May 1980:Large short-termsurfacetemperAntufia, R. G. Grainger, A. Lambert, and L. Thomason, ature effects, Science,216, 628-630, 1982. Radiative forcingfrom the 1991 Mount Pinatubovolcanic Robock,A., and M. Matson, Circumglobaltransportof the E1 eruption,J. Geophys.Res.,103, 13,837-13,857,1998. Chich6nvolcanicdust cloud, Science,221, 195-197, 1983. Stommel,H., and E. Stommel,VolcanoWeather.'The Storyof Robock,A., K. E. Taylor, G. L. Stenchikov,and Y. Liu, GCM 1816, the Year Withouta Summer,177 pp., SevenSeasPress, evaluationof a mechanismfor E1 Nifio triggeringby the E1 Newport, R. I., 1983. Chichdnashcloud,Geophys. Res.Lett.,22, 2369-2372, 1995. Stothers,R. B., The great Tambora eruption in 1815 and its aftermath, Science,224, 1191-1198, 1984. Roeckner,E., K. Arpe, L. Bengtsson,M. Christoph,M. Claussen, L. Duemenil, M. Ecsh, M. Giogetta, U. Schlese,and Stowe,L. L., R. M. Carey,and P. P. Pellegrino,Monitoringthe Mount Pinatubo aerosol layer with NOAA 11 AVHRR U. Schulzweida,The atmosphericgeneralcirculationmodel data, Geophys. Res.Lett., 19, 159-162, 1992. ECHAM4: Model descriptionand simulationof presentday climate,Rep. 218, 90 pp., Max-Planck-Inst.ffir Meteo- Strong, A. E., Monitoring E1 Chich6n aerosol distribution rol., Hamburg, Germany, 1996. usingNOAA-7 satellite AVHRR sea surfacetemperature observations,Geofis.Int., 23, 129-141, 1984. Ropelewski, C. F., and P. D. Jones, An extensionof the


Strong,A. E., The effect of E1 Chich6n on the 82/83 E1 Nifio

Robock: VOLCANIC ERUPTIONS AND CLIMATE ß 219 van Loon, H., and K. Labitzke, Association between the l 1-

(abstract),Eos Trans.AGU, 67(44), 880, 1986. year solarcycle,the QBO and the atmosphere,part IV, The Symons,G. J. (Ed.), TheEruptionof Krakatoa,and Subsequent stratosphere, not grouped by the phase of the QBO, J. Clim., 3, 827-837, 1990. Phenomena,494 pp., Triibner, London, 1888. Tabazadeh,A., and R. P. Turco, Stratosphericchlorineinjec- Vinnikov, K. Y., A. Robock, R. J. Stouffer, and S. Manabe, tion by volcaniceruptions:HC1 scavengingand implication Vertical patterns of free and forced climate variations, for ozone, Science, 260, 1082-1086, 1993. Geophys.Res.Lett., 23, 1801-1804, 1996. Thompson,D. W. J., and J. M. Wallace, The Arctic Oscillation signaturein the wintertime geopotential height and temperature fields,Geophys.Res.Lett., 25, 1297-1300, 1998. Thompson,D. W. J., and J. M. Wallace, Annular modesin the extratropicalcirculation,part I, Month-to-month variability, J. Clim., 13, 1000-1016, 2000a. Thompson,D. W. J., J. M. Wallace, and G. C. Hegerl, Annular modes in the extratropical circulation, part II, Trends, J. Clim., 13, 1018-1036, 2000b. Tie, X., and G. Brasseur,The responseof the stratospheric ozone to volcanic eruptions: Sensitivityto atmospheric chlorineloading,Geophys.Res.Lett., 22, 3035-3038, 1995.

Vogelmann, A.M., T. P. Ackerman, and R. P. Turco, Enhancementsin biologically effective ultraviolet radiation followingvolcaniceruptions,Nature, 359, 47-49, 1992. Vupputuri, R. K. R., and J.P. Blanchet,The possibleeffectsof the E1Chich6neruptionon atmospheric thermalandchemical structureand surfaceclimate,Geofis.Int., 23, 433-447, 1984. Wallace, J. M., and D. S. Gutzler, Teleconnections in the

geopotentialheightfield duringthe Northern Hemisphere winter, Mon. Weather Rev., 109, 784-812, 1981.

Wallace, J. M., Y. Zhang, and J. A. Renwick,Dynamiccontribution to hemisphericmean temperature trends, Science, 270, 780-783, 1995.

Tie, X., G. P. Brasseur, C. Granier, A. De Rudder, and N.

Wallace, J. M., Y. Zhang, and L. Balik, Interpretation of Larsen,Model studyof polar stratosphericcloudsand their interdecadaltrends in Northern Hemisphere surface air effect on stratosphericozone, 2, Model results,J. Geophys. temperature,J. Clim., 9, 249-259, 1996. Res., 101, 12,575-12,584, 1996. Yue, G. K., R. E. Veiga, and P.-H. Wang, SAGE II observaToon, O. B., Volcanoesand climate,in AtmosphericEffectsand tions of a previously unreported stratosphericvolcanic PotentialClimaticImpact of the 1980Eruptionsof Mount St. aerosolcloud in the northern polar summerof 1980, Geophys.Res.Lett., 21,429-432, 1994. Helens, edited by A. Deepak, NASA Conf. Publ. 2240, 15-36, 1982. Zerefos, C. S., K. Tourpali, and A. F. Bais, Further studieson Toon, O. B., and J. B. Pollack, Atmospheric aerosolsand possiblevolcanicsignalto the ozone layer,J. Geophys.Res., climate, Am. Sci., 68, 268-278, 1980.

99, 25,741-25,746, 1994.

Trepte, C. R., and M. H. Hitchman, Tropical stratospheric Zhao, J., R. P. Turco, and O. B. Toon, A model simulation of circulation deduced from satellite aerosol data, Nature, 355, Pinatubovolcanicaerosolsin the stratosphere,J. Geophys. 626-628, 1992.

Trepte, C. R., R. E. Veiga, and M.P. McCormick, The poleward dispersalof Mount Pinatubovolcanicaerosol,J. Geophys.Res.,98, 18,563-18,573, 1993. Turco, R. P., O. B. Toon, T. P. Ackerman, J. B. Pollack, and

C. Sagan,Nuclear winter: Global consequences of multiple nuclear explosions,Science,222, 1283-1292, 1983. Turco, R. P., O. B. Toon, T. P. Ackerman, J. B. Pollack, and C.

Sagan,Nuclearwinter: Climate and smoke:An appraisalof nuclear winter, Science,247, 166-176, 1990.

Res., 100, 7315-7328, 1995.

Zielinski, G. A., P. A. Mayewski, L. D. Meeker, S. Whitlow, M. S. Twickler, and K. Taylor, Potential atmosphericimpact of the Toba mega-eruption---71,000years ago, Geophys.Res.Lett., 23, 837-840, 1996.

A. Robock,Department of Environmental Sciences, Rutgers University,14 CollegeFarmRoad,New Brunswick, NJ089018551. ([email protected])