1. INTRODUCTION

Volcanic eruptions have been an important cause ofweather and climate change on timescales ranging fromhours [Robock and Mass, 1982] to centuries [Humphreys,1913]. Their atmospheric aerosol and gas inputs, in additionto being potentially very dangerous to people nearby and to

Introduction: Mount Pinatubo as a Test of Climate Feedback Mechanisms

Alan Robock

Department of Environmental Sciences, Rutgers University, New Brunswick, New Jersey

The June 15, 1991 Mount Pinatubo eruption was a large but relatively short-lived shock to the Earth’s atmosphere. It thus provided an excellent opportunityto study the workings of the climate system, to test climate models, and to exam-ine the impacts of climate change on life. The largest eruption of the 20th Centuryinspired a large amount of research on the connection between volcanic eruptionsand the Earth’s atmosphere in the 12 years since that eruption, as exemplified bythe chapters in this book. Here several additional examples of our new under-standing of these connections are presented. While the global cooling afterPinatubo was not surprising, the observed winter warming over NorthernHemisphere continents in the two winters following the eruption is now under-stood as a dynamic response to volcanically produced temperature gradients inthe lower stratosphere from aerosol heating and ozone depletion, and to reducedtropospheric storminess. Interactions of the Quasi-Biennial Oscillation of tropicalstratospheric winds with the climate system are also now better understood byexamining their role in the Pinatubo response. We have more confidence in thesensitivity of climate models used for attribution and projection of anthropogeniceffects on climate because the strength of the water vapor feedback has been val-idated with Pinatubo simulations. The response of the biosphere to the Pinatuboeruption also illustrates its sensitivity to climate change and clarifies portions ofthe carbon cycle. Death of coral in the Red Sea in the winter of 1991–1992 andan unusually large number of polar bear cubs born in the summer of 1992 weretwo responses to the characteristic winter and summer temperature responses ofthe climate system. This strengthens our concern about negative impacts of glob-al warming on polar bears and other wildlife. Enhanced vegetation growth frommore diffuse and less direct solar radiation took more carbon dioxide out of theatmosphere than normal, temporarily reducing the observed long-term increase incarbon dioxide. Continued research on the Pinatubo eruption and its aftermathwill undoubtedly enhance our understanding of the climate system.

Volcanism and the Earth’s AtmosphereGeophysical Monograph 139Copyright 2003 by the American Geophysical Union10.1029/139GM01

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aircraft during explosive eruptions, produce local air pollu-tion and haze that not only can have a strong atmosphericimpact while being erupted, but also when concentrated inpeat that is much later dried and burned [Langmann andGraf, 2003]. Major eruptions, such as the Santorini blast inthe 17th Century BCE, have had major impacts on civiliza-tion and changed history.

For 51 years before the 1963 Gunung Agung eruption inBali, Indonesia, there had not been a major volcanic erup-tion on Earth. Starting then, Agung and the 1980 Mt. St.Helens, 1982 El Chichón and 1991 Mount Pinatubo erup-tions have stimulated research on the impacts of volcaniceruptions on the atmosphere. The 12 years since the June15, 1991 Mount Pinatubo eruption in the Philippines havebeen particularly productive, and the chapters in this book,based on presentations at the Chapman Conference onVolcanism and the Earth’s Atmosphere in Thera, Greece,June 17–21, 2002, give many examples of this excellentwork. Here I present examples of additional recent researchto paint a broader picture of the new understanding we havegained.

A comprehensive survey of the effects of volcanic erup-tions on climate was presented by Robock [2000], updatedby Robock [2002a, 2003]. While we have gained much newunderstanding, the work presented in this book and at theChapman Conference raises more sophisticated questionsthat will inspire continued work on the subject [Robock,2002b].

2. VOLCANIC ERUPTIONS AND CLIMATE

Most causes of climate change are gradual shifts inatmospheric composition or land surface characteristics.Volcanic eruptions, however, can produce a very large, butshort-lived, perturbation to the Earth’s radiative balance.While we cannot use these perturbations to test long-termprocesses, such as changes in the thermohaline circulation,we can take advantage of them to examine some short time-scale feedback processes and impacts.

Plates 1 and 2 show the lower tropospheric temperatureanomalies for the Northern Hemisphere winters of 1991/92and 1992/93. Before the Pinatubo eruption, cooling was theonly well-understood response of climate to volcanic erup-tions. The large continental warming seen in this plate wasnot predicted, but we now understand it as a dynamicalresponse to the radiative perturbation from the volcanicaerosols and ozone depletion from the eruption. Stenchikovet al. [2002] recently presented atmospheric general circu-lation model simulations which illustrate that this winterwarming phenomenon, a positive mode of the Arctic

Oscillation [Thompson and Wallace, 1998], can be pro-duced by the enhanced temperature gradient in the lowerstratosphere from aerosol heating in the tropics, or fromaerosol-induced ozone depletion in the higher latitudes, orby reduced winter synoptic wave activity (storminess) in thetroposphere caused by reduced latitudinal temperature gra-dients (Figure 1). The Arctic Oscillation is the dominantmode of interannual variability of Northern Hemisphereatmospheric circulation, and a positive mode corresponds toa stronger than normal polar vortex, with strong westerlywinds in the midlatitudes in the lower stratosphere andupper troposphere. The references in that paper and inRobock [2000] provide a detailed description of other obser-vational and modeling studies that support this conclusion.We might expect similar Arctic Oscillation responses tolong-term ozone depletion and increases in greenhousegases, a mechanism now better understood by examiningthe response of the climate system to the Pinatubo eruption.Stenchikov et al. [2003] have shown that the Quasi-BiennialOscillation of tropical lower stratospheric winds [Angelland Korshover, 1964] significantly modifies the response ofstratospheric winds to the volcanic aerosols, making thepolar vortex stronger in the second winter following theeruption, amplifying the response to a diminished aerosolloading and producing winter warming for the second win-ter, too. Thus, our understanding of stratospheric dynamicshas also been enhanced by studying the reaction to an explo-sive volcanic eruption.

Plate 3 shows the lower troposphere temperature anom-alies for the Northern Hemisphere summer of 1992, oneyear after the Pinatubo eruption. Virtually the entire planetwas cooler than normal, as expected. But the amount of

2 MOUNT PINATUBO AS A TEST OF CLIMATE FEEDBACK MECHANISMS

Figure 1. Ways volcanic eruptions cause a positive mode of theArctic Oscillation. (Figure 13 from Stenchikov et al. [2002].)

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4 MOUNT PINATUBO AS A TEST OF CLIMATE FEEDBACK MECHANISMS

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cooling depended on the sensitivity of the climate system toradiative perturbations, and the strength of the most impor-tant positive feedback in the climate system, the watervapor-greenhouse feedback [Schneider and Dickinson,1974]. As the planet cools, the amount of water vapor in theatmosphere goes down, reducing the greenhouse effect, andamplifying the cooling. (Of course, this positive feedbackalso works for warming.) The timing and amplitude offuture global warming depend on this sensitivity of the cli-mate system. Soden et al. [2002] used the global coolingand drying of the atmosphere that was observed after theeruption of Mount Pinatubo to test model predictions of theclimate feedback from water vapor. By comparing modelsimulations with and without water vapor feedback, theydemonstrated the importance of atmospheric drying inamplifying the temperature change and showed that, with-out the strong positive feedback from water vapor, theiratmospheric general circulation model was unable to repro-duce the observed cooling (Figure 2). These results providequantitative evidence of the reliability of water vapor feed-back in current climate models, which is crucial to their usefor global warming projections.

3. IMPACTS OF CLIMATE CHANGE

The impacts of global warming depend on the response ofvarious natural and human systems to climate changes.Pinatubo was a short, but large, perturbation to the climatesystem, but we can still take advantage of this natural cli-mate experiment to learn about the impacts of anthro-pogenic forcings.

The winter pattern for 1991/92 (Plate 1) actually is awave response to an enhanced polar vortex, and has both

warm and cold anomalies. While the lower stratospheric cir-culation is quite symmetric, in the troposphere the enhancedcirculation is affected by mountain ranges, which producessoutheasterly (warm) and northeasterly (cold) winds. One ofthese cold regions was in the eastern Mediterranean. It wasso cold that winter that Jerusalem experienced rare snowfalland coral at the bottom of the Red Sea died, because thewater at the surface cooled and convectively mixed theentire depth of the water [Genin et al., 1995]. The resultingenhanced supply of nutrients produced huge algal and phy-toplankton blooms, which smothered the coral. This coraldeath had only happened before in winters following largevolcanic eruptions [Genin et al., 1995]. As some globalwarming scenarios predict more frequent positive modes ofthe Arctic Oscillation, this volcanic response informs usabout possible biological responses to future climate.

The largest cooling in the summer of 1992 was in the cen-ter of North America (Plate 3). As a result, the ice on HudsonBay melted almost a month later than normal that year. Polarbears, who feed and have babies on the ice, were much heav-ier and had more healthy cubs that summer. Biologists callthose now 11-year olds “Pinatubo Bears” [Stirling, 1997].The cool conditions in the summer after the Pinatubo erup-tion was very beneficial for the Hudson Bay polar bears, andthere were many more bears born that year than the yearbefore or after. The long-term concern for these bears, andmany other plants and animals in the Arctic, however, is theopposite impact from global warming. This temporary posi-tive impact from Pinatubo strengthens the argument of thenegative impacts of the predicted warming. Pinatubo pro-duced global cooling, but impacts work in both directions, sothe benefits of Pinatubo from global cooling teach us aboutthe negative impacts of anthropogenic global warming.

6 MOUNT PINATUBO AS A TEST OF CLIMATE FEEDBACK MECHANISMS

Figure 2. Observations (MSU – Microwave Sounding Unit [Spencer et al., 1990] and MSU (ENSO removed) – withthe effects of sea surface temperatures removed) and climate model simulations (GCM – general circulation model). Thesimulation was only successful with the positive water vapor feedback. (Figure 4 from Soden et al. [2002].)

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4. CARBON CYCLE

The carbon dioxide concentration has been measured at theMauna Loa observatory since 1958 [Keeling and Whorf,2002], and is representative of global trends since CO2 hassuch a long atmospheric lifetime (~100 years). Figure 3 showsthat the Mauna Loa CO2 concentration has a large upwardtrend superimposed on the terrestrial biologically inducedseasonal cycle. This trend is not uniform however (Figure 4),and changes in emissions and El Niño and La Niña episodesexplain part, but not all, of these interannual variations.

The tropical ocean warming associated with El Niño resultsin enhanced emissions from the ocean. The terrestrial compo-nent of the carbon cycle, however, is less well understood, andthe Pinatubo eruption has provided another opportunity tolearn about a part of the climate system that increases ourunderstanding of the global warming problem. Enhanced for-ward scattering of incoming solar radiation caused by thePinatubo aerosols increased the diffuse radiation reaching thesurface and decreased the direct flux [Robock, 2000]. Thisallowed plants to photosynthesize more of the time, increasingthe CO2 sink [Gu et al., 2002, 2003; Farquhar and Roderick,2003]. Jones and Cox [2001] model experiments showed thatthe cool temperatures over land following volcanic eruptionsproduced reduced soil and plant respiration globally andenhanced gross primary productivity in the tropics, both ofwhich would also reduce atmospheric CO2 concentrations.These effects can be seen in the reduced rate of increase ofCO2 following each large volcanic eruption (Figure 4). Thisreponse somewhat mimics the effect of tropospheric clouds,and illustrates a biospheric climate feedback.

5. SUMMARY

This brief introduction presents just a few examples ofhow studies of the responses to a large, recent volcaniceruption have deepened our understanding of the climatesystem. As a result, our confidence in our projections of thecarbon cycle, the climatic response to anthropogenic green-house gases and land surface changes, and the biologicalresponse to the predicted warming has increased. Futurestudies, addressing outstanding issues in the impacts of vol-canic eruptions on the atmosphere [Preface; Robock,2002b], will surely add to this understanding.

Acknowledgments. Supported by NASA grant NAG 5-9792 andNSF grant ATM-9988419. I thank Juan Carlos Antuña for helpingto draw Plates 1, 2, and 3.

REFERENCES

Angell, J. K., and J. Korshover, Quasi-biennial variations in tem-perature, total ozone, and tropopause height, J. Atmos. Sci., 21,479-492, 1964.

Farquhar, G. D., and M. L. Roderick, Pinatubo, diffuse light, andthe carbon cycle, Science, 209, 1997-1998, 2003.

Genin, A., B. Lazar, and S. Brenner, Vertical mixing and coraldeath in the Red Sea following the eruption of Mount Pinatubo,Nature, 377, 507-510, 1995.

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Figure 3. Record of CO2 concentration at the Mauna LoaObservatory. Data are from Keeling and Whorf [2002].

Figure 4. Annual rate of increase of CO2 in the atmosphere. Dataare from Keeling and Whorf [2002]. Thin line is monthly valuesand thick line is 9-month running mean. Minima following thelargest volcanic eruptions of the period are indicated. Otherinterannual variations are partially due to El Niños, La Niñas, andemission variations.

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Gu, L., D. Baldocchi, S. B. Verma, T. A. Black, T. Vesala, E. M.Falge, and P. R. Dowty, Advantages of diffuse radiation for ter-restrial ecosystem productivity, J. Geophys. Res., 107(D6),10.1029/2001JD001242, 2002.

Gu, L., D. D. Baldocchi, S. C. Wofsy, J. W. Munger, J. J.Michalsky, S. P. Urbanski, and T. A. Boden, Response of adeciduous forest to the Mount Pinatubo eruption: Enhancedphotosynthesis, Science, 299, 2035-2038, 2003.

Humphreys, W. J., Volcanic dust and other factors in the produc-tion of climatic changes, and their possible relation to ice ages,J. Franklin Institute, August, 131-172, 1913.

Jones, C. D., and P. M. Cox, Modeling the volcanic signal in theatmospheric CO2 record, Glob. Biogeochem. Cycles, 15, 453-465, 2001.

Keeling, C. D., and T. P. Whorf, Atmospheric CO2 records fromsites in the SIO air sampling network. In Trends: A Compendiumof Data on Global Change (Carbon Dioxide InformationAnalysis Center, Oak Ridge National Laboratory, U.S.Department of Energy, Oak Ridge, Tenn., U.S.A.), 2002.

Langmann, B., and H. F. Graf, Indonesian smoke aerosols frompeat fires and the contribution from volcanic sulfur emissions,Geophys. Res. Lett., 30(11), 1547, doi: 10.1029/2002GL016646, 2003.

Robock, A., Volcanic eruptions and climate, Rev. Geophys., 38,191-219, 2000.

Robock, A., Pinatubo eruption: The climatic aftermath, Science,295, 1242-1244, 2002a.

Robock, A., Blowin’ in the wind: Research priorities for climateeffects of volcanic eruptions. EOS, 83, 472, 2002b.

Robock, A., Volcanoes: Role in climate, in Encyclopedia ofAtmospheric Sciences, J. Holton, J. A. Curry, and J. Pyle, Eds.,(Academic Press, London), 10.1006/rwas.2002.0169, 2494-2500, 2003.

Robock, A., and C. Mass, The Mount St. Helens volcanic eruptionof 18 May 1980: Large short-term surface temperature effects,Science, 216, 628-630, 1982.

Schneider, S. H., and R. E. Dickinson, Climate modeling, Rev.Geophys. Space Phys., 12, 447-493, 1974.

Soden, B. J., R. T. Wetherald, G. L. Stenchikov, and A. Robock,Global cooling following the eruption of Mt. Pinatubo: A test ofclimate feedback by water vapor, Science, 296, 727-730, 2002.

Spencer, R. W., J. R. Christy, and N. C. Grody, Global atmospher-ic temperature monitoring with satellite microwave measure-ments: Method and results 1979-1984, J. Climate, 3, 1111-1128,1990.

Stenchikov, G., A. Robock, V. Ramaswamy, M. D. Schwarzkopf,K. Hamilton, and S. Ramachandran, Arctic Oscillation responseto the 1991 Mount Pinatubo eruption: Effects of volcanicaerosols and ozone depletion, J. Geophys. Res., 107 (D24),4803, doi:10.1029/2002JD002090, 2002.

Stenchikov, G., K. Hamilton, A. Robock, V. Ramaswamy, and M.D. Schwarzkopf, Arctic Oscillation response to the 1991Pinatubo eruption in the SKYHI GCM with a realistic Quasi-Biennial Oscillation, Submitted to J. Geophys. Res., 2003.

Stirling, I., The importance of polynyas, ice edges, and leads tomarine mammals and birds, J. Marine Systems, 10, 9-21, 1997.

Thompson, D. W. J., and J. M. Wallace, The Arctic Oscillation sig-nature in the wintertime geopotential height and temperaturefields, Geophys. Res. Lett., 25, 1297-1300, 1998.

Alan Robock, Department of Environmental Sciences, RutgersUniversity, 14 College Farm Road, New Brunswick, NJ 08901E-mail: robock@envsci.rutgers.edu

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