VOLCAN IC ERUPTIONS AN D CLIMATE Alan Robock Departmentof EnvironmentalSciences RutgersUniversity New Brunswick,New Jersey Abstract. Volcaniceruptions are an importantnatural an enhancedpole-to-equator temperature gradient, es- cause of climate change on many timescales.A new pecially in winter. In the Northern Hemisphere winter capability to predict the climatic responseto a large this enhancedgradient produces a strongerpolar vortex, tropical eruption for the succeeding2 years will prove and this strongerjet stream producesa characteristic valuable to society.In addition, to detect and attribute stationarywave pattern of troposphericcirculation, re- anthropogenicinfluences on climate,including effects of sultingin winter warming of Northern Hemispherecon- greenhousegases, aerosols, and ozone-depletingchem- tinents.This indirect advectiveeffect on temperatureis icals, it is crucial to quantify the natural fluctuationsso strongerthan the radiative coolingeffect that dominates as to separatethem from anthropogenicfluctuations in at lower latitudes and in the summer. The volcanic the climate record. Studyingthe responsesof climate to aerosolsalso serveas surfacesfor heterogeneouschem- volcanic eruptions also helps us to better understand ical reactions that destroy stratosphericozone, which important radiative and dynamical processesthat re- lowers ultraviolet absorptionand reducesthe radiative spondin the climate systemto both natural and anthro- heating in the lower stratosphere,but the net effect is pogenicforcings. Furthermore, modeling the effects of still heating.Because this chemicaleffect dependson the volcanic eruptions helps us to improve climate models presenceof anthropogenicchlorine, it has only become that are needed to study anthropogeniceffects. Large important in recent decades.For a few days after an volcanic eruptions inject sulfur gasesinto the strato- eruptionthe amplitudeof the diurnal cycleof surfaceair sphere,which convertto sulfate aerosolswith an e-fold- temperature is reduced under the cloud. On a much ing residencetime of about 1 year. Large ash particles longer timescale,volcanic effects played a large role in fall out muchquicker. The radiativeand chemicaleffects interdecadalclimate changeof the Little Ice Age. There of this aerosol cloud produce responsesin the climate is no perfect index of past volcanism,but more ice cores system.By scatteringsome solar radiation back to space, fromGreenland and Antarctica will improve the record. the aerosolscool the surface,but by absorbingboth solar There is no evidencethat volcaniceruptions produce E1 and terrestrial radiation, the aerosol layer heats the Nifio events, but the climatic effects of E1 Nifio and stratosphere.For a tropical eruption this heating is volcaniceruptions must be separatedto understandthe larger in the tropicsthan in the high latitudes,producing climatic responseto each. 1. INTRODUCTION the main text.) Mitchell [1961] was the first to conducta superposedepoch analysis,averaging the effectsof sev- Volcanism has long been implicated as a possible eral eruptionsto isolate the volcanic effect from other causeof weather and climatevariations. Even 2000 years presumably random fluctuations. He only looked at ago,Plutarch and others[Forsyth, 1988] pointed out that 5-year averageperiods, however, and did not have a very the eruption of Mount Etna in 44 B.C. dimmed the Sun long temperaturerecord. Severalprevious reviews of the and suggestedthat the resultingcooling caused crops to effects of volcanoeson climate include Lamb [1970], shrivel and produced famine in Rome and Egypt. No Toon and Pollack [1980], Toon [1982],Ellsaesser [1983], other publicationson this subjectappeared until Ben- Asaturovet al. [1986],Kondratyev [1988], Robock [1989, jamin Franklin suggestedthat the Lakagigareruption in 1991],and Kondratyevand Galindo [1997].Past theoret- Iceland in 1783 might have been responsiblefor the ical studies of the radiative effects include Pollack et al. abnormallycold summerof 1783 in Europe and the cold [1976], Harshvardhan[1979], Hansenet al. [1992], and winter of 1783-1784 [Franklin,1784]. Humphreys [1913, Stenchikovet al. [1998].The work of H. H. Lamb, in fact, 1940]associated cooling events after largevolcanic erup- was extremely influential in the modern study of the tionswith the radiative effectsof the stratosphericaero- impact of volcanic eruptions on climate [Kelly et al., solsbut did not have a sufficientlylong or horizontally 1998]. Sincethese reviews, a deeper and more complex extensivetemperature databaseto quantify the effects. understandingof the impactsof volcaniceruptions on (Termsin italicare definedin the glossary,which follows weather and climate has resulted, driven by the many Copyright2000 by the AmericanGeophysical Union. Reviewsof Geophysics,38, 2 / May'2000 pages 191-219 8755-1209/00/1998 RG000054 $15.00 Papernumber 1998RG000054 ß 191 ß 192 ß Robock: VOLCANIC ERUPTIONS AND CLIMATE 38, 2 / REVIEWS OF GEOPHYSICS TABLE 1. Major Volcanic Eruptions of the Past 250 Years Year of VolcaFlo Eruption VEI D VI/Emax IVI Grimsvotn[Lakagigar], Iceland 1783 4 2300 0.19 Tambora, Sumbawa, Indonesia 1815 7 3000 0.50 Cosiguina,Nicaragua 1835 5 4000 0.11 Askja, Iceland 1875 5 1000 0.01' Krakatau, Indonesia 1883 6 1000 0.12 Okataina [Tarawera], North Island, New Zealand 1886 5 800 0.04 Santa Maria, Guatemala 1902 6 600 0.05 Ksudach, Kamchatka, Russia 1907 5 500 0.02 Novarupta [Katmai], Alaska, United States 1912 6 500 0.15 Agung, Bali, Indonesia 1963 4 800 0.06 Mount St. Helens, Washington,United States 1980 5 500 0.00 E1 Chich6n, Chiapas,Mexico 1982 5 800 0.06 Mount Pinatubo,Luzon, Philippines 1991 6 1000 ... The officialnames of the volcanoesand the volcanicexplosivity index (VEI) [Newhalland Self, 1982]are from Simkinand Siebert[1994]. The dustveil index(DVI/Emax) comes from Lamb [1970,1977, 1983], updated by Robockand Free [1995].The ice corevolcanic index (IVI) is the averageof Northern and SouthernHemisphere values and is representedas opticaldepth at X - 0.55 pom[from Robockand Free, 1995, 1996]. *SouthernHemisphere signal only; probablynot Askja. studiesof the impactof the 1991 Pinatuboeruption and London [Symons,1888; Simkin and Fiske, 1983]. This continuinganalyses of the 1982 E1 Chich6n eruption in wasprobably the loudestexplosion of historictimes, and Mexico. the book includescolor figuresof the resultingpressure This paper reviewsthese new results,including the wave'sfour circuitsof the globe as measuredby micro- indirect effect on atmosphericcirculation that produces barographs.The 1963 Agung eruption produced the winter warmingof the Northern Hemisphere(NH) con- largeststratospheric dust veil in more than 50 yearsand tinentsand the new impactson ozonedue to the strato- inspired many modern scientific studies. While the sphericpresence of anthropogenicchlorine. A better Mount St. Helens eruptionof 1980was very explosive,it understandingof the impactsof volcaniceruptions has did not inject much sulfurinto the stratosphere.There- importantapplications in a numberof areas.Attribution fore it had very small global effects[Robock, 1981a]. Its of the warming of the past century to anthropogenic troposphericeffects lasted only a few days[Robock and greenhousegases requires assessment of other causesof Mass, 1982;Mass and Robock, 1982], but it occurredin climate changeduring the past severalhundred years, the United States and so received much attention. includingvolcanic eruptions and solar variations.After Quantificationof the size of theseeruptions is difficult, the next major eruption,new knowledgeof the indirect as different measures reveal different information. For effectson atmosphericcirculation will allow better sea- example,one could examinethe total massejected, the sonalforecasts, especially for the NH in the winter. The explosiveness,or the sulfur input to the stratosphere. impactsof volcaniceruptions serve as analogs,although The limitationsof data for each of thesepotential mea- imperfectones, for the effectsof other massiveaerosol sures,and a descriptionof indicesthat have been pro- loadings of the atmosphere, including meteorite or duced, are discussedlater. comet impactsor nuclearwinter. Volcanic eruptionscan inject into the stratosphere The largesteruptions of the past250 years(Table 1) tens of teragramsof chemicallyand microphysicallyac- 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-speakingworld. (Simkin et al. [1981] and stratosphericchemical equilibrium. The volcaniccloud Simkinand Siebert[1994] provide a comprehensivelist of forms in several weeks by SO2 conversionto sulfate all known volcanoesand their eruptions.) The 1783 aerosol and its subsequentmicrophysical transforma- eruptionin Iceland producedlarge effectsall that sum- tions [Pintoet al., 1989;Zhao et al., 1995].The resulting mer in Europe [Franklin,1784; Grattan et al., 1998].The cloudof sulfateaerosol particles, with an e-foldingdecay 1815 Tambora eruption producedthe "year without a time of approximately1 year [e.g.,Barnes and 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 affects surfacetemperatures extensivestudy