Quaternary Science Reviews 19 (2000) 417}438

Use of paleo-records in determining variability within the volcanism}climate system Gregory A. Zielinski* Climate Change Research Center, Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, NH 03824, USA

Abstract

Volcanic eruptions that inject large quantities of sulfur-rich gases into the stratosphere have the capability of cooling global climate by 0.2}0.33C for several years after the eruption. Equatorial eruptions will impact global climate whereas mid-latitude eruptions can cool climate in the hemisphere of origin. Magnitude of cooling varies by latitude and it is possible for warming to occur in certain regions, primarily during the winter. Although instrumental records have been used to quantify the volcanic impact on climate, they are limited in their temporal and spatial coverage, and the style and magnitude of eruptions occurring over the two centuries of instrumental records is limited. A thorough evaluation of the range of variability in the volcanism}climate system requires a multidisciplinary approach that includes the analysis of ice core records, geological data, atmospheric measurements and visual phenomena, tree-ring records and other proxy data. Evaluation of these longer time series indicates that multiple volcanic eruptions have the potential to force climate over decadal to multi-decadal time frames, especially when these eruptions enhance or extend pre-existing cool conditions. On the other hand, a lack of climatically signi"cant eruptions may result in warmer average temperatures over decadal time frames because the volcanic-cooling component within the climate system is absent. Mega-eruptions, like the Toba eruption of &71,000}73,000 yr ago, may impact climate on centennial time frames through positive feedback mechanisms. Evidence exists which indicates that environmental changes associated with rapid climatic #uctuations, such as crustal loading/unloading with glaciation/deglaciation and variability in glacial meltwater loading on ocean basins, may cause an increase in volcanic activ- ity.
1999 Elsevier Science Ltd. All rights reserved.

1. Introduction be in operation when an eruption occurs. It is for these reasons that we need to look into the past to determine Much of our present understanding of how volcanic the full range of variability in the volcanism}climate eruptions force climate comes from the evaluation of the system and use these data to make reliable predictions of up to 200# yr of instrumental records in existence (e.g., the climatic impact of future eruptions. Angell and Korshover, 1985) and evaluation of the This paper summarizes our present understanding of 20# yr of information available from satellite data (e.g., the volcanism}climate system and the techniques used to Bluth et al., 1993) and other technical advances, such as reach these conclusions as well as to expand on our lidar (e.g., McCormick et al., 1993). Unfortunately, our present knowledge. It begins with a brief introduction understanding of the entire volcanism}climate system is and overview on how eruptions perturb the atmosphere, not complete (e.g., Self and Rampino, 1988; Bradley and in general, and more speci"cally how and why particular Jones, 1992) because of the limited number and styles of types of eruptions impact either global or hemispheric eruptions occurring during these time frames, the moder- climate. This is followed by a look at the various tech- ate magnitude of these eruptions compared to others niques that can be used to characterize the impact of past over the last 100,000# yr, and the fact that climatic eruptions; the key being a multidisciplinary approach variability over the last few centuries is quite limited since no single technique provides a complete unequivo- when compared to the possible climatic modes that may cal record of the impact of a past eruption. I then present several speci"c examples of how this approach works followed by additional evidence that indicates the poten- * Corresponding author. Tel.: 001-603-862-1012; fax: 001-603-862- tial climatic forcing of volcanism over time periods 2124. beyond a few years (i.e., decadal- to centennial-scale for- E-mail address: [email protected] (G.A. Zielinski) cing). The paper ends with a discussion on the opposite

0277-3791/99/$- see front matter
1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 (9 9 ) 0 0 0 7 3 - 6 418 G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438 relationship to volcanism forcing climate, that is The climatic perturbation is dominantly in the form of the probability that environmental changes associated cooling at the Earth's surface as incoming solar radiation with changing climatic conditions force volcanism. This is either re#ected back or absorbed in the stratosphere may be especially true during the extreme environmental (e.g., Sigurdsson, 1990, Fig. 1, p. 278). This will produce changes that occur between glacial and interglacial a warming of the stratosphere and cooling of the tropo- climates. sphere. Additional cooling of the Earth's surface may result from the re#ection of incoming solar radiation in the troposphere from clouds that may form when 2. Climatic impact of volcanism tropospheric HSO acts as condensation nucleii. How- ever, the amount of cooling at the Earth's surface from A single eruption injects various quantities of insoluble this process is probably a small portion of the total silicate matter (tephra) and gases into various levels of cooling. the atmosphere (e.g., Hofmann, 1987; McCormick et al., The type of eruption that most readily injects debris 1995). Because the silicate matter is much larger and into the stratosphere is the very explosive plinian erup- heavier than the aerosols produced by the oxidation of tion. However, there are two key points that must be the gases released, the ash component quickly settles out emphasized when evaluating the type of eruption that of the atmosphere. As a result, this material has very will have an in#uence on climate. Firstly, many of the limited climatic in#uence except for areas in the immedi- largest plinian eruptions will develop extensive pyroclas- ate vicinity of the eruption. The work by Robock and tic #ows with the collapse of the plinian column because Mass (1982) and Mass and Robock (1982) on the 1980 of the increased density of the column produced by a very Mt. St. Helens eruption showed that the strong interac- high eruption rate. Nevertheless, the buoyant co-ignim- tion between the larger ash particles and infrared and brite clouds produced from these pyroclastic #ows reach visible radiation in the troposphere (e.g., Pollack et al., heights that may be similar to the plinian phase of an 1976) led to surface warming in areas close to the vol- eruption (see Simarski, 1992, Fig. 3, p.7). Secondly, debris cano. Similarly, greenhouse gases emitted during an also may reach the stratosphere from e!usive Icelandic eruption (i.e., CO) can contribute to surface warming eruptions because of the very buoyant clouds generated (e.g., Sigurdsson, 1990; Fig. 1, p. 278), but the total vol- above large "re fountains (e.g., Stothers et al., 1986; canic contribution of these gases pales in comparison to Thordarson and Self, 1993), and, because of the lower anthropogenically produced CO (e.g., Cadle, 1980). tropopause at higher latitudes. Furthermore, the basaltic Furthermore, sedimentation of the silicate matter has the composition of these Icelandic-type eruptions is much potential to scavenge large quantities of the more soluble more sulfur-rich than magma that often produces very acids produced by the eruption as observed following the explosive plinian eruptions. As a result, it is an explosive, 1974 Fuego eruption (Rose, 1977). This especially ap- sulfur-rich eruption that has the greatest impact on cli- pears to be the case for highly soluble aerosols such as mate. HCl, as modeled by Tabadazeh and Turco (1993). How- The spatial extent of the climatic perturbation is ever, there is evidence from ice cores that some Cl\ may a function of the location of the eruption with an equato- remain aloft for a year or two following the eruption (e.g., rial eruption having the capability to a!ect global climate Lyons et al., 1990), albeit in lower concentrations than since the aerosols produced can spread into both hemi- that initially injected into the atmosphere. spheres. However, the dispersal of these aerosols into On the other hand, the acids formed from the sulfur each hemisphere can be asymmetrical as a function of gases produced (SO and HS) are less soluble than HCl time of the year, location of the intertropical convergence and they will remain aloft for longer periods of time. zone (ICTZ), and the quasi-biennial oscillation (QBO). Oxidation of the sulfur gases to HSO is quick, occur- A mid-to-high-latitude eruption will primarily impact the ring completely within about a month following the hemisphere of origin since there will not be much inter- eruption (see Bluth et al., 1992 and references therein). hemispheric transport of the aerosols. This is not to say Once in the stratosphere, they will remain there for that mid-to-high-latitude eruptions are not important in several years thus providing the source for the climatic the overall volcanism}climate system as they can have perturbation as well as providing a nucleus for various a severe impact on the hemisphere of origin. From a chemical reactions that can lead to ozone loss (e.g., human perspective, any large, explosive northern Hofmann et al., 1992). Material remaining in the tro- hemisphere eruption in the future, as occurred in the past posphere, where the Earth's weather occurs, will be in Kamchatka and Alaska, would have a very severe washed out quickly and have a very limited impact on impact on the large population centers of the northern climate. Consequently, I will focus on the stratospheric hemisphere. Furthermore, the lower height of the component of the volcanism}climate system (e.g., Ram- tropopause at high latitudes allows less explosive erup- pino and Self, 1982,1984; Rampino et al., 1985; Pyle et al., tions in those regions to inject material into the strato- 1996). sphere. G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438 419

Finally, the relative importance of volcanic aerosols in house gases over this time period was about #2.2 W/m the climate system is signi"cant compared to other cli- with solar irradiance accounting for another mate-forcing factors. Hansen et al. (1985) modeled the #0.3 W/m. Thus, the climatic-forcing potential of the magnitude of various climatic-forcing components and Pinatubo aerosols was greater than or equal to these found that stratospheric HSO can cool temperatures other forcing mechanisms (i.e., warming mechanisms) by about !0.63C. This cooling is equivalent to other during the "rst two years following the event. Interesting- forcing factors such as anthropogenically produced ly, the Pinatubo eruption is only moderate-sized from ! tropospheric HSO aerosols ( 0.73C) and cooling that a geological perspective. can be obtained by increased surface albedo (!0.63C). Although cooling is obvious following certain erup- Factors leading to the greatest amount of warming were tions there is regional variability in the timing and modeled to be CO (up to 1.23C) and variability in solar amount of cooling particularly given an equatorial versus irradiance (0.73C). Thus, stratospheric volcanic aerosols a high-latitude eruption. Bradley (1988) looked at the have the potential to be one of the greatest climatic magnitude and timing of volcanically induced temper- forcing components in the climate system at any one ature depression over the last roughly 200 years at low, time, albeit the forcing from a single eruption is generally mid, and high latitudes as well as the northern hemi- short-lived. sphere as a whole. He found that peaks in the cooling from an equatorial eruption were cyclical, coinciding 2.1. Direct evidence: instrumental records with the hemispheric distribution of the aerosols sea- sonally. Peak cooling occurs at higher latitudes during Direct evidence that volcanic eruptions cool climate is the summer of the "rst and second years following an shown by the lower temperatures in northern hemisphere eruption as stratospheric aerosols are dispersed from the instrumental records over the last 200# yr following tropics into higher latitudes each spring. By comparison, almost all major eruptions. Angell and Korshover (1985) the northern hemisphere would feel a quicker and more compared the time series of temperatures from New pronounced cooling immediately following a high north- Haven, CT (USA), northern and central Europe and ern latitude eruption compared to that from an equato- central England with all major eruptions since 1740 (Fig. rial eruption. 1). In nearly every case, there were cool years below the The complexity of the climatic response to an eruption long-term mean following the eruption or a cooling be- is shown by the results of Robock and Mao (1992) for 12 low that of the preceding few years. This becomes even eruptions 1883}1992 and of Groisman (1992) for the more apparent when the northern hemisphere is viewed Pinatubo and El ChichoH n eruptions. Both studies as a whole (Fig. 1). However, there are other forcing showed that Eurasian winters following a major eruption factors at work during this time period as, in some cases, are often warmer than average. The perturbation to the there are cooler pre-existing conditions for some of these latitudinal energy budget from the presence of stratos- eruptions. This suggests that certain eruptions over the pheric aerosols in the tropics leads to frequent advection last few centuries may have enhanced and possibly ex- of warm air into Eurasia, although this same area, as well tended the cool climate existing at the time of the erup- as the northeastern US, exhibits much colder than aver- tion. age summers following a major eruption. These condi- Quanti"cation of the climatic impact of many of these tions seem to occur regardless of the location of the same eruptions across various latitudinal bands was eruption. undertaken by Self et al. (1981). They used composites An additional factor that can modify the climatic im- from a series of eruptions in the last two centuries to pact of an eruption is its coincidence with an El Nino show that the global cooling from a major explosive event. This would diminish the climatic cooling as ob- eruption may be 0.2}0.33C in the year of the eruption or served following the El ChichoH n eruption in 1982 (Mass in the "rst year after the eruption. The complete impact and Portman, 1989; Portman and Gutzler, 1996). Angell may last for up to 4 yr (Fig. 1) with the magnitude of the (1988) showed that both northern and southern hemi- annual cooling becoming less in each subsequent year sphere surface temperatures were surprisingly warm after following the peak period of cooling. El ChichoH n in comparison to the cooling following the The most recent evidence of the amount and longevity 1963 Agung event. The amount of sulfur produced by the of cooling following an eruption comes from an analysis El ChichoH n eruption is estimated to be slightly less than of the forcing potential of the aerosols produced by the or possibly greater than that from Agung (e.g., Rampino 1991 Pinatubo eruption as compared to that for other et al., 1988; McCormick et al., 1995). Identifying the cause forcing factors (McCormick et al., 1995). Pinatubo aero- of the lack of cooling following the El ChichoH n eruption sols produced a forcing of !2.4 W/m in August of was undertaken by evaluating the impact of warmer sea 1991, '!3W/m in August of 1992 and then surface temperatures on global temperature. By remov- &!1W/m in August of 1993, two years after the ing the El Nino impact, Angell calculated the magni- eruption. The combined forcing component of all green- tude of cooling that was associated with El ChichoH n 420 G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438

Fig. 1. (Top) Northern hemisphere surface temperature trends (3C) as related to major volcanic eruptions over the last &250 yr (reprinted from Angell and Korshover, Journal of Applied Climate and Meteorology, 1985, vol. 24, Fig. 1, p. 939, courtesy of the American Meteorological Society). Temperature data are smoothed, primarily through a 1-2-1 weighting. Recent work by Zielinski et al. (1994b) suggests that the 1783 Asama eruption had no impact on climate in the early 1780s meaning that the 1783 Laki eruption was responsible for the climatic perturbation at that time. (Bottom) Composite of changes in northern hemisphere temperature for 4 yr before and after major eruptions of the last two centuries. Reprinted from Journal of Volcanology and Geothermal Research, vol. 11, Self et al., The possible e!ects of large 19th and 20th century volcanic eruptions on zonal and hemispheric surface temperatures, p. 41}60, 1981, with permission from Elsevier Science.

(i.e., about 0.333C). This same exercise showed that the 2.2. Direct evidence: satellite data and other technological magnitude of cooling following other eruptions in the advances last two centuries was lessened to some degree by a sub- sequent El Nino. However, there is no convincing evid- Recent advances in technology have given the scienti"c ence that a volcanic eruption may induce an El Nino (Self community a much better opportunity to characterize et al., 1997), as previously hypothesized by Handler the climatic and atmospheric impact of volcanism. Satel- (1989). lite data, such as that provided by the total-ozone G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438 421 mapping spectrometer or TOMS unit, are now available In addition to satellite data, direct stratospheric \ to make real-time estimates of the quantity of SO injec- measurements of SO aerosols have been undertaken in ted into the stratosphere by an eruption. Bluth et al. the very recent past, although such measurements are (1993) developed a time series of volcanic SO emissions discontinuous both spatially and temporally. Sedlacek et over the last 25 years to show that for an eruption to be al. (1983) did seasonal sampling of the stratosphere in climatically e!ective (such as Pinatubo and El ChichoH n), both hemispheres between 1971 and 1981 to conclude \ it needs to inject more than 1}5MtSO into the strato- that volcanism contributed over 50% of the SO mea- sphere. The mass of gaseous HSO eventually produced sured in the stratosphere during that decade. They also in the stratosphere is about double the amount of SO found that several eruptions injected material to higher emitted, whereas the mass of the total HSO aerosol elevations than originally thought (e.g., 1980 eruption of eventually produced in the stratosphere is approximately Gareloi, Alaska; Sedlacek et al., 1981), and that there was ; another 1.25 the mass of HSO produced. For evidence of stratospheric injections that could not be example, an eruption that releases 5 Mt of SO into the reliably linked to a known source. It is likely that stratosphere will produce about 10 Mt of HSO and Nyamuragira was the source of one of those injections. about 12.5 Mt of HSO aerosol. The calculation to Nevertheless, the fact that a remote eruption can occur determine the total HSO aerosol produced is based on without detection in the modern world makes knowing an aerosol composition of about 75% HSO and 25% the complete volcanic record and its relationship to past HO (Self et al., 1996). climatic change very challenging. Other types of satellite data used in evaluating the atmospheric perturbation from a volcanic eruption are SAGE I and II and SAM II measurements (e.g., Kent and 3. Evaluating past volcanism: a multidisciplinary McCormick, 1984; McCormick et al., 1993). One speci"c approach advantage of these measurements is that they provide real-time estimates of variability in optical depth (). Instrumental meteorological records in combination Optical depth is the value given to the di!erence in with satellite data have provided much insight into the solar radiation at the top of the stratosphere from climatic impact of volcanism. Unfortunately, these con- that leaving the stratosphere with a greater thickness clusions are based on a limited sampling of eruption coinciding with a greater stratospheric loading of aero- types during the 200# yr of instrumental records and sols. Lidar data also provide an estimate of the optical especially during the 20}25 yr of satellite availability. depth associated with recent eruptions via the amount Furthermore, we are not able to look at how a speci"c of backscatter of the laser beam (e.g., McCormick et al., type of eruption would impact climate if it occurred when 1993 and references therein). Optical depth is one of the Earth's climate was in one particular mode versus the the more important parameters needed to determine contrasting mode (such as glacial vs. interglacial condi- the climatic impact of an eruption especially when tions to use the extremes). Consequently, it is necessary putting the impact of volcanic aerosols into climatic to look into the past and in doing this, it is necessary to models. take a multidisciplinary approach in evaluating the cli- The development and improvement of general circula- matic impact of past volcanism. tion models (GCMs) has given much additional informa- The reason for looking at many di!erent proxy data is tion on how a volcanic eruption may impact climate and that no one record provides an all-encompassing, de- particularly on how the presence of stratospheric aero- tailed record of past eruptions and the climatic impact of sols may modify circulation systems and ocean}atmo- those eruptions. Multiple data sets for any single erup- sphere exchange. Details on the nature of particular tion also provide much more information on its charac- models and their use in predicting temperature changes teristics, as well as its climatic impact. Two examples associated with volcanic eruptions can be found in sev- highlight potential problems that can occur with a single eral sources (e.g., Hansen et al., 1978; Hansen et al., 1992; index. Lamb (1970) developed the Dust Veil Index (DVI) Robock, 1979; Overpeck and Rind, 1992; Rind et al., as the "rst single number to evaluate and compare the 1992; Graf et al., 1993). atmospheric and climatic impact of past eruptions. He A noteworthy item that has been brought to the scient- used various parameters to produce an arbitrary scale i"c community's attention by these advances in high with the standard being the 1883 Krakatau eruption technology (speci"cally satellite data) is the ability for an (DVI"1000). However, Lamb's use of temperature data eruption to go undetected even in this day and age. as one criterion in the process may make some of the Satellite data in the early 1980s showed the presence of DVI values questionable, because temperature may be a stratospheric aerosol cloud, but the source eruption highly in#uenced by other forcing factors, and thus was not known. Eventually this `mystery clouda was the potential for circular reasoning (Robock, 1981a). On linked to the 1980 eruption of Nyamuragira in Africa the other hand, Lamb speci"cally noted when temper- (Schnetzler et al., 1991). ature was used in determining the DVI for a particular 422 G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438 eruption, thereby making this potential bias known to Table 1 the user of the DVI data set. The use of the DVI term also Parameters Used in a Multidisciplinary Approach to Evaluating Cli- is misleading since aerosols are the major forcing com- matic Forcing by Volcanism, and the Information Available from Each Parameter ponent. Similarly, the volcanic explosivity index (VEI) developed by Newhall and Self (1982) never was intended A. Direct products of the eruption: to be used as a tool for evaluating the climatic impact of 1. Ice cores: an eruption. Any agreement between climatic perturba- a. Volcanic aerosols measured: HSO and other acids tions and VEI numbers simply re#ects the greater poten- (HCL, HF), b. Tephra. tial for a larger eruption to have an impact on climate. The classic example of the problem with using VEI to 2. Geologic deposits: identify the potential for climate forcing is that the more a. Volume erupted, composition of magma, height of voluminous 1980 Mt. St. Helens eruption (VEI"5) had plume, direction of dispersal. much less of a climatic impact (e.g., Robock, 1981b) than b. Chronology of volcanism through radiometric ages, c. Estimates of degassing through the petrological technique. the 1963 Agung eruption (VEI"4) (e.g., Volz, 1970). Following is a brief description of the various methods 3. Atmospheric measurements and historical observations: used in a multidisciplinary approach with speci"c exam- a. Direct impact of the stratospheric loading through optical ples of the records produced. I begin with those para- depth values derived from meters that record direct products of an eruption phyrheliometirc measurements and lunar eclipses, b. Indications of the presence of stratospheric aerosols as including ice cores, geological phenomena and direct available from atmospheric phenomena atmospheric measurements/observations. Next is a dis- such as red sunsets, Bishop's rings, stellar visibilities, green cussion of those parameters that record the climatic suns, dry fogs. impact of the eruption, and thus are a secondary signal. The two proxy data I discuss are tree rings, a well- B. Secondary impacts of the eruption: climatic signal: 1. Tree-ring characteristics including frost rings, narrow rings, established data source, and corals, a potential volcanic maximum latewood density, light rings. proxy that is just now being investigated. Table 1 sum- marizes the parameters used in this approach and the 2. Corals information available with each one.

3.1. Ice core records excellent temporal resolution available from the larger Ice cores arguably provide the best chronology of ice sheets, and there may be summer melting that hinders climatically e!ective volcanism because they record the development of the core chronology. Nevertheless, these deposition of the stratospheric aerosols directly respon- records provide additional sites useful in evaluating the sible for the climate forcing, and their records are con- large-scale, spatial variability of signals from particular tinuous, highly resolved (sub-seasonal to annual to eruptions. The abundance of local alkaline dust from decadal) as well as lengthy (up to 100,000#yr). After areas close to many low-latitude sites is detrimental to migration of the stratospheric cloud from the latitude of developing the volcanic record in those ice cores, because the eruption to the polar regions, sedimentation of these the dust may readily neutralize volcanic acids. Yet, the aerosols into the troposphere results in the eventual presence of acid and volcanic glass from the AD 1600 scavenging by snowfall and deposition on the glacier Huaynaputina eruption in the Peruvian Quelccaya ice surface (see Delmas, 1992, Fig. 1, p. 2). Freshly fallen core (Thompson et al., 1986) has contributed to our snow is then compacted by subsequent snowfalls, event- understanding of the extent and climatic impact of that ually forming glacial ice. The temporal resolution avail- eruption, as discussed in more detail later. able in a single core and the total length of discernible The ice core parameters which document the presence \ record is a function of accumulation rates, with relatively of volcanic aerosols are: SO (a direct measurement of high accumulation rates providing great detail over the HSO component of the record, e.g., Herron, 1982; a shorter period of time (e.g., summit of Greenland) Legrand and Delmas, 1987; Lyons et al., 1990; Delmas et compared to relatively low accumulation rates that pro- al., 1985; Delmas et al., 1992; Mayewski et al., 1993a,b; vide less resolved records over a longer time period (e.g., Zielinski et al., 1994a,1996a) and electrical properties of East Antarctica plateau). the ice, such as conductivity (i.e., ECM, which measures Volcanic records are not only available from the major total acids, e.g., Hammer, 1980; Hammer et al., ice sheets, but they exist in ice cores collected from 1978,1980; Taylor et al., 1992; Hammer et al., 1997) and smaller ice caps in the Canadian Arctic (e.g., Fisher and the dipolar method (i.e., DEP, which measures total salts, Koerner, 1994; Fisher et al., 1995) and from high-altitude, e.g., Moore et al., 1991). The detection of volcanic acid low latitude sites (e.g., Thompson et al., 1986). However, deposition in ice cores was pioneered by Hammer and many of the records from smaller ice caps do not have the colleagues through their work on several Greenland ice G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438 423 cores in the late 1970s (Hammer, 1980; Hammer et al., 1978). More recently, volcanic records in ice cores have reached a new level of detail through the measurement of \ both SO (e.g., Zielinski et al., 1996a) and electrical properties (e.g., Clausen et al., 1997) on the Summit, Greenland, cores (GISP2 and GRIP). The ability to obtain volcanic records from both polar regions (e.g., Langway et al., 1995) enhances the capabili- ties of detecting past eruptions that may have had a glo- bal impact (i.e., when the signal is found in both Greenland and Antarctica ice cores). Such is the case for the equatorial Tambora and Krakatau eruptions (Fig. 2). On the other hand, when the signal is found in only one of the polar regions, a source eruption from the hemi- sphere of the ice core is implied. That is the case for the Icelandic Laki and Alaskan Katmai eruptions, signals found in almost all Greenland ice cores, and the New Zealand Tarawera eruption found in many Antarctica cores (Fig. 2). Although ice cores provide excellent records there are particular aspects of these records that must be taken into consideration. Foremost, the limited number of ice cores that can be collected (compared to tree-ring re- cords) restricts the ability to correlate among multiple cores (and thus increases the error in a single core's chronology) and restricts the ability to extract the signal from the noise. A small number of cores also complicates Fig. 2. Comparison of volcanic records in ice cores from each polar determining the spatial variability of the signal for a par- region emphasizing the capabilities of reconstructing both global and ticular eruption. The variable transport of volcanic aero- hemispheric chronologies through bi-polar comparisons (Delmas, Re- sols from an equatorial eruption into each hemisphere, views of Geophysics, vol. 30, p. 10, 1992, Fig. 11, copyright by the due to the stratospheric quasi-biannual oscillation American Geophysical Union). Eruptions noted are: Agung (AG), Santa Maria (SM), Tarawera (TA), Krakatau (KR), Coseguina (C), (QBO), can produce di!erent records among cores and Tambora (T), 1809 unidenti"ed event (UN), Hekla (HE), Katmai (KA), the tropospheric transport of aerosols to a glacier from Laki (LA). The Dome C core was matched to the dated ($1 yr) Cre( te local or hemispheric eruptions can lead to an enhanced core via the Tambora signal. signal in some areas. Lack of scavenging when the aero- sol cloud is over the eventual coring site may lead to an incomplete record, and wind scouring of the material once deposited may modify the signal. Despite these volcanism are far from complete, as many ice core signals concerns, ice cores provide perhaps the most useful re- cannot be easily matched with a probable source erup- cords for evaluating past volcanism, although at the same tion, particularly beyond the last 2000 yr. For example, time, these concerns emphasize the need for a multi- the AD 79 Vesuvius eruption is the oldest large eruption parameter approach that crosses disciplinary lines. that is previously dated from historical records. The longest, existing continuous record of volcanism However, even in the more recent past, there are ice from an ice core is the 110,000-yr record from the Green- core signals that cannot be matched to a documented land GISP2 core which re#ects high sulfur-producing eruption. Dai et al. (1991) identi"ed a very large volcanic equatorial and northern hemisphere eruptions (Fig. 3; signal in the layer dated as 1809 in cores from both polar Zielinski et al., 1994a,1996a). It was developed by regions, but whether this was a large equatorial eruption \ smoothing the original SO time series and using the or coincident eruptions in each hemisphere has yet to be residuals over that smoothing to identify the volcanic determined (e.g., Moore et al., 1991). Interestingly, this \ input to the SO record. Because of the sampling re- eruption could have been the initial trigger for a series of gime used (biyearly over the last &12,000 yr and up to years with very cold temperatures in the northern hemi- about 50 yr/sample at 100,000 yr ago), the GISP2 core sphere beginning around 1809 (Fig. 1) and culminating provides only a minimum record of past volcanism. with the `year without a summera following the Tam- Nevertheless, one of the key contributions of this record bora eruption (e.g., Stommel and Stommel, 1983; Harrin- is the high resolution for its length. The record clearly gton, 1992). Similarly, the source eruption is not known shows that existing geological and historical records of for an eruption dated at AD 1259, often the largest 424 G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438

\ > Fig. 3. (Top) 110,000-yr volcanic SO record from the GISP2 core compared to the Ca record (modi"ed from Zielinski et al., Journal of Geophysical Research, vol. 102, p. 26,628, 1997a, Fig. 1, copyright by the American Geophysical Union). Ca> is a proxy for climatic conditions with high Ca> corresponding to cold conditions and low values corresponding to warm conditions. Signals for the eruptions responsible for the Z2-ash equivalent and for the Toba (T) eruption are noted and discussed in the text. The larger size of the Z2 signal compared to Toba is a function of the Icelandic source for Z2 and the "ner temporal resolution of that sample compared to the Toba sample. (Bottom) Intermediate estimates of the optical depth (
) produced by volcanic eruptions over the last 2100 years as recorded in the GISP2 ice core (modi"ed from Zielinski, Journal of Geophysical Research, vol. 100, p. 20,945, 1995, Fig. 1, copyright by the American Geophysical Union). See text and Zielinski (1995) for an explanation of how optical depth values were calculated and an explanation of the range of potential  values. Multiple eruptions closely spaced in time that are responsible for closely-spaced signals in the GISP2 core (given scale of the "gure) are noted together. volcanic signal over the last 5000 years in both Green- Zielinski, 1995). Identifying where these eruptions occur- land and Antarctica ice cores, (e.g., Langway et al., 1988), red should be a major goal of the volcanological and and for one of the largest signals over the last 5000 yr in paleoclimatological communities as these events are con- Greenland cores (&53 BC, Hammer et al., 1980; sistently found in almost all ice cores. G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438 425

In addition to volcanic records available in single snow studies. The Pinatubo signal has been found in cores, Robock and Free (1995a, b) used all ice cores to southern hemisphere snow studies (Dibb and Whitlow, develop a composite volcanic record for each hemisphere 1996; Cole-Dai et al., 1997a) and in fact, Cole-Dai et al., (the Ice-core Volcanic Index (IVI)). The study was ini- 1997a felt that they could separate the Pinatubo (erupted tially done for the last 150 yr and then later updated to in 1991) signal from that of the 1991 Cerro Hudson span the last 2000 yr. Their primary conclusion is that (Argentina) eruption. A problem exists in that there is not the largest sulfur-producing eruptions, like the equatorial yet a clear method to `back-outa the mass of stratos- Tambora eruption (recorded in both polar regions) or the pheric HSO produced from an eruption using south- northern hemisphere Katmai eruption (recorded in all ern hemisphere ice cores. This de"ciency needs to be northern hemisphere cores), are the events that are most addressed in the near future to be able to do more reliable consistently observed in the ice core record, and thus the bipolar comparisons of stratospheric loading and optical easiest eruptions to evaluate. Unfortunately, these are depth estimates. not the only eruptions that may have had a climatic Optical depth estimates from the GISP2 core for the impact on a region, and certainly are not the only events last 2100 years provide the longest continuous time series that have an impact on local to regional biogeochemical of this parameter presently available (Fig. 3). The most cycles. It is for this reason that a complete highly resolved obvious trend in this time series is the increased volcanic volcanic record needs to be developed as far back in time activity over the last 600 yr compared to the 1500 yr as possible. prior (noting that temporal sampling resolution was con- Another important contribution ice cores make to the stant over this part of the GISP2 core). This increase in study of the volcanism}climate system is the ability to volcanism played a role in the overall cooler northern estimate optical depth values for past eruptions. Portions hemisphere climate over that time, as suggested by others of the technique were initially developed by Hammer et and discussed later. In addition, there are several very al. (1980) and Clausen and Hammer (1988). They used the large signals over this entire record that undoubtedly had record of bomb fallout in Greenland to obtain a mass of an impact on northern hemisphere and/or global climate. HSO produced in the stratosphere from an individual For instance, some eruptions, like the Philippine eruption. They then accounted for the latitude of the Babuyan eruption of 1831, have not been well studied, eruption by employing an appropriate multiplier within but yet appear to be part of a high sulfur producing the calculations. Zielinski (1995) expanded on the tech- system. Future eruptions from this volcano, and others nique by calculating the total HSO aerosol loading like it, need to be monitored closely. This is also the case  (M") and then ultimately, the optical depth ( ") using the for the large signal at AD 1641, formerly thought to be relationship de"ned by Stothers (1984a), where from the Awu eruption. It is now known that Mt. Parker,  " ;  " M"/1.5 10 g. However, when the resulting ice- Philippines, was the major eruption in that year (Del"n core-derived -values for the GISP2 core were calibrated et al., 1997; Bri!a et al., 1998), an eruption very similar in with other independent optical depth measurements it magnitude and style to that of Pinatubo. More detailed was found that equivalent optical depth measurements discussion on the 1259 signal and especially the dual were obtained in some cases, but the ice core estimates signals 53}44 BC (a period characterized by an abund- were 2}5; greater in others (Zielinski, 1995). This was ance of atmospheric observations suggestive of a very especially true for mid-latitude northern hemisphere large volcanic perturbation) may be found in Zielinski eruptions where there may have been some tropospheric (1995). However, it is worth noting that there are limited transport of HSO to polar ice sheets, and thus an proxy data indicating a period of major cooling around enhanced signal. However, Zielinski et al. (1997b) de- AD 1259 (e.g. Scuderi, 1990). One can speculate that this  veloped a composite record of volcanic HSO #ux to eruption coincided with a large El Nino event that would Greenland from the El ChichoH n eruption through the have been able to lessen the volcanic cooling (e.g. Angell, analysis of a series of snow pits across the Greenland Ice 1988). I will talk about the very large Kuwae and Sheet. They found that by using a composite average Huaynaputina eruptions later, as examples of the multi- \ annual-#ux value of volcanic SO compared to back- disciplinary approach to evaluating their impact on cli- ground annual-#ux levels, stratospheric loading and op- mate. tical depth estimates using the modi"ed `bomb-fallouta The "nal type of volcanic data available in ice cores is & technique (i.e., 20 Mt HSO with a peak optical the presence of tephra (i.e., silicate matter including vol- depth of 0.13) were similar to those obtained from satel- canic glass). By locating volcanic glass in a section of ice lite data. and determining its chemical composition via rigorous Unfortunately, additional calibration of ice core re- microscopic techniques, the source eruption can be deter- cords with satellite data has not occurred because, to my mined by matching composition of the ice-core derived knowledge, fallout from the only other major eruption in tephra with that of glass from the suspected eruption the satellite era (i.e., Mt. Pinatubo, Philippines) has not (DeAngelis et al., 1985; Palais et al., 1990; Palais et al., been unequivocally found in any northern hemisphere 1991; Palais et al., 1992; Fiacco et al., 1993,1994; 426 G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438

Zielinski, 1995; Cole-Dai et al., 1997b). When this glass is derived from other techniques such as ice cores and direct found in the same layer as a large acid signal, it can verify optical measurements (Fig. 4). Similarly, re"nements in the source eruption for that particular chemical signal. the technique now appear to be able to provide estimates Visible layers are only occasionally found in Greenland of sulfur degassing that closely match other independent ice cores, but they occur much more frequently in Antarc- estimates (e.g., Self and King, 1996; Mandeville et al., tic ice cores (e.g., Kyle et al., 1981). For example, one of 1996). Nevertheless, potential problems with the petrol- the visible ash layers in the GISP2 core contained glass ogic technique emphasizes the need for the multidiscip- whose composition matched that of glass found in the linary approach presented here. GRIP core from Greenland, in the Z2 layer in North Atlantic deep sea cores, and in terrestrial deposits on 3.3. Atmospheric measurements and observations Iceland (i.e., GroK nvold et al., 1995; Zielinski et al., 1997a). This is a superb example of how tephra found in ice cores One of the most bene"cial data sets compiled from can provide absolute time lines for correlating climatic direct atmospheric measurements is the pyrheliometric proxy records from many sources. data set of Stothers (1996) and Sato et al. (1993). Direct- beam measurements of the Sun began in 1883 in Mon- 3.2. Volcanological data tpellier, France, and thus this is the oldest data set that can be used to derive a direct time series of the volcanic Volcanological data, particularly those obtained from perturbation to incoming solar radiation at a monthly "eld studies, provide much critical information necessary resolution. Fig. 5 shows the pyrheliometric optical depth  to evaluate the climatic impact of an eruption. Several ( ) measurements from the Montpellier station that critical pieces of information include estimates of the resulted from the atmospheric perturbation by the 1883 volume erupted (e.g., Pyle, 1989a), magma type (i.e., is it Krakatau eruption. In addition to the chronology of a composition that often contains high sulfur amounts), volcanic perturbations to the northern hemisphere atmo- primary direction of cloud dispersal for the various sphere, the many pyrheliometric time series now avail- phases of an eruption (e.g. Bogaard and Schmincke, able also show the latitudinal variability in optical depth 1985), column height via the dispersal of tephra (e.g., for a single eruption. Variation in optical depth as a func- Carey and Sparks, 1986), and radiometric (numerical) tion of latitude is particularly well shown for the three ages of past events. Several of these parameters are sum- years around the 1912 Katmai eruption (Fig. 5 and also marized in the compilation of global volcanism for the Volz, 1975). Higher latitude sites ('453N) display Holocene at the Smithsonian Institution (Simkin and a greater perturbation from that eruption compared to Siebert, 1994). That data base includes VEI estimates for mid-latitude sites between 30 and 453N, as expected many of these eruptions. given the 583N latitude of the volcano.  Although all of the aforementioned volcanological The  values for the Katmai and Krakatau erup- parameters provide necessary information for evaluating tions, once converted to visual optical depth (Stothers, the climatic impact of an eruption, it is the estimate of the 1996), closely match the intermediate optical depth esti- amount of sulfur degassed via the petrological technique mates from the GISP2 ice core (Fig. 3; Zielinski, 1995) that provides the most relevant data (Devine et al., 1984; (Fig. 6). These results give con"dence in extending the Sigurdsson et al., 1985). This technique compares the optical depth time series of Stothers (1996) and Sato et al. sulfur content of inclusions in a phenocryst, thought to (1993) back to 2100 yr ago by using the ice core record. represent the initial sulfur composition of the magma, Such a reconstruction is not without complications as with that found in matrix glass. The di!erence is thought there is a greater number of unknown eruptions and to be the amount degassed into the atmosphere and when an increase in dating uncertainty of past eruptions combined with estimates of the volume erupted can lead with time. Optical depth estimates from volcanic signals to estimates of the mass of SO and HSO produced by in ice cores thus become more uncertain because higher the eruption. The technique also estimates the amount of latitude eruptions can produce enhanced signals from Cl\ (HCl) and F\ (HF) produced. However, there are tropospheric transport of the aerosols. For example, some problems with the technique as estimates of the if a very large ice core signal in a Greenland ice core amount of sulfur produced by the El ChichoH n eruption cannot be linked to a known event, the magnitude of that by the petrologic technique were much lower than those signal may be a function of the latitude of the eruption estimated from satellite data (Luhr et al., 1984). This (if the source eruption was in the mid-latitudes of the "nding suggests that there is a sulfur source not picked northern hemisphere) and not a true representation of up by this method (e.g., Gerlach et al., 1994). However, it the climatic impact (i.e., stratospheric loading) of that does not mean that the technique is always unreliable as eruption. Palais and Sigurdsson (1989) showed that there can be A second atmospherically derived measurement that a predictable relationship between mass loading esti- has provided very promising results for estimating the mates derived from the petrological technique and that optical depth of past eruptions comes from the nature of G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438 427

Fig. 4. (Top) Comparison between total acid estimates from the petrol- ogic method and those derived from direct optical depth measurements (Palais and Sigurdsson, Understanding Climate Change, p. 43, 1989, Fig. 1, copyright by the American Geophysical Union). Eruptions noted are: Tambora (T), Santa Maria (SM), Katmai (K), Krakatau (KK), Rabaul (R), Agung (A), Fuego (F), Mount St. Helens (MSH), Soufriere (S). Regression coe$cient is 0.96. (Bottom) Comparison be- tween total acid estimates from the petrologic method and global acid fallout derived from ice cores (Palais and Sigurdsson, 1989, p. 44, Fig. 2). Eruptions noted as in top "gure with addition of EldgjaH (E) and Coseguina (C). Regression coe$cient is 0.92. 1 : 1 line shown in both Fig. 5. (Top) Pyrheliometric optical depth series from Montpellier for "gures for comparison with best-"t lines. the 1883 Krakatau eruption (Stothers, Journal of Geophysical Re- search, Vol. 101, p. 3908, 1996, Fig. 3, published by the American Geophysical Union). Circles are annual averages, dashed lines repres- ent largest and smallest monthly means in each year and solid line represents adjusted annual averages following the Krakatau eruption. lunar eclipses when volcanic aerosols are present in the (Middle and Bottom). Mean pyrheliometric optical depth series from stratosphere (Keen, 1983). In fact, this method was used various stations between 473 and 603N (middle) and between 303 and by Sato et al. (1993) to develop the latest part of their 433N (bottom) for the 1912 Katmai eruption (modi"ed from Stothers, optical depth time series. The technique relies on the 1996, Fig. 6, p. 3911). Arrow indicates timing of the Katmai eruption. change in the umbra of lunar eclipses (as a result of the stratospheric aerosols) as a function of the observing latitude for the eclipse. Fig. 7 is the most recent record between data points from individual eclipses, thus pro- produced by Keen (1997) showing the probable optical viding an estimate of maximum optical depth from an depth of Pinatubo aerosols compared to others. Keen eruption if there was not a lunar eclipse that coincided (1997) developed a decay model to be able to interpolate with that time period. There is great potential in using 428 G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438

1816, and a  of &0.4 in September 1817 from dim star observations. All of the atmospheric methods discussed so far can quantify the atmospheric perturbation of a volcanic eruption. However, there are other atmospheric phe- nomena, recorded in historical records (e.g., Pang et al., 1986; Pang, 1991), that provide evidence of the presence of stratospheric volcanic aerosols. Consequently, they are very useful in developing an overall chronology of past volcanism that may have perturbed the stratosphere. These phenomena include such items as red sunsets, green celestial bodies, Bishop's rings around the sun, and dry fogs. These phenomena are a result of the refraction of the sun's rays passing through or tangent to the stratospheric aerosols. In fact, a very distinct sequence of colors is observed during the afterglow following sunset when volcanic aerosols are present in the stratosphere (Simkin and Fiske, 1983). This sequence of colors can be used to estimate height of the aerosol cloud (Simkin and Fiske, 1983). All of these phenomena have been noted in many historical records and diaries and continue to be a great asset in identifying years when an explosive vol- canic eruption may have occurred. Stothers and Ram- Fig. 6. Perturbations in the mean annual optical depth (visual, "0.55 pino (1983a, b) used accounts of dry fogs in historical  m) for both hemispheres resulting from volcanic eruptions since 1880 records to reconstruct the volcanic record in the Mediter- (Stothers, 1996, Fig. 7, p. 3915, with the portion from 1961 to 1994 from Sato et al., 1993). Maxima are labeled with the source volcano. ranean region over the last few millennium. Even more interesting were the accounts of dry fogs by Benjamin Franklin following the Laki eruption, the "rst meaning- ful link made between the volcanic and climatic systems (e.g., Sigurdsson, 1982).

3.4. Climate proxy data

Two main types of climatic proxy data, tree rings and corals, are able to record the climatic impact of past eruptions. Although these methods are extremely bene"- cial in constructing a volcanic chronology and evaluating the climatic impact of an eruption, they are secondary signals in that they do not record a direct product of the eruption. Nevertheless, each of these types of data pro- vides an annual resolution of various aspects of past climatic conditions. In the case of tree rings, it has been Fig. 7. Optical thickness for volcanic eruptions since 1960 using 35 shown that certain characteristics of tree rings correlate total or near-total lunar eclipses between 1960 and 1997 (Keen, 1997, well to volcanic activity; the use of corals is just in the Fig. 17, p. 15). initial stages of development. The "rst use of tree rings to produce a record of explosive volcanic eruptions was derived from the pres- this type of historical data over the last few centuries as is ence of frost rings in bristlecone pines of the western US discussed below for the AD 1600 Huaynaputina eruption (LaMarche and Hirschboeck, 1984). Frost rings re#ect (de Silva and Zielinski, 1998). a below freezing event during the early part of the growth In addition to lunar eclipses, such items as the histori- season and good match of the years when these occur cal record of the dimness of stars may be used to estimate versus the volcanic record known at that time (Fig. 8), led the optical depth of a past eruption. Stothers (1984b) did LaMarche and Hirschboeck to believe that frost rings are this for the 1815 Tambora eruption to help de"ne the excellent recorders of the climatic cooling associated with decay curve of the stratospheric aerosols. He estimated an eruption. This relationship implies that latitudinal a  of &1.0 from naked-eye star observations for June insolation gradients induced by the presence of stratos- G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438 429

Fig. 8. (Top) Timing of frost rings in bristlecone pine of the western U.S. compared to the DVI record of Lamb (1970) to show the good agreement between the two records (modi"ed from LaMarche and Hirschboeck, 1984, Fig. 3, p. 123). A `NOa indicates that there was not a known eruption for the respective signal or that there was no frost ring that corresponded to any known major eruption, based on the DVI record, at that time. Permission for reproduction granted from Nature and from K.K. Hirschboeck for V. LaMarche, Jr. (deceased). (Bottom) Northern hemisphere density chronology (NHD1) and regression-based estimates of northern hemisphere mean temperature anomalies compared to major volcanic eruptions over the last 600 yr (Bri!a et al., 1998, Fig. 1, p. 451). Curve is bidecadally smoothed. DVI values from Simkin and Siebert (1994). Permission for reproduction granted from Nature and from K. R. Bri!a.

pheric aerosols will allow strong anticyclones to become teristics in the tree ring record. Moreover, the strongest established in the northern US/southern Canada with signals found in the composite records of Bri!a et al. the resulting strong northerly #ow into the western US (1998) for the northern boreal forest match well with the Caution is needed, however, in that such cold outbreaks GISP2 ice core record (Fig. 3), particularly the very large can occur unrelated to explosive eruptions. Kuwae (&AD 1453), Huaynaputina (AD 1600), Parker Two other characteristics of the tree ring record being (AD 1641) and Tambora (AD 1815) eruptions. The good utilized more recently are narrow ring widths and parti- agreement between these two, recently developed, highly cularly, low maximum latewood-density (e.g., Scuderi, resolved volcanic records is encouraging, especially when 1990; Bri!a et al., 1992a,1998,1992b; Jones et al., 1995). the chronology of volcanic signals developed in ice core The presence of either of these characteristics suggests and tree-ring records from just 10# yr ago did not cold summers leading to a year of poor growth. An match up this well (see discussions by Pyle, excellent correlation between narrow rings and/or low 1989b,1990,1992). Other climatically e!ective eruptions densities with that of summer temperatures veri"es the are found in both data sets including the Katmai, forcing factor responsible for these phenomena. Transfer Krakatau and possibly the Coseguina eruption, although functions have been developed in several studies (e.g., Self et al. (1989) suggests that despite the explosiveness of Jones et al., 1995) to quantify the cooling in summers the Coseguina eruption the small amount of juvenile following volcanic eruptions. These authors (e.g., Bri!aet material erupted probably led to a low sulfur output. al., 1998) acknowledge that there are other forcing factors Interestingly the large signals around 1831 in the GISP2 that can lead to cool summers, but the coincidence of the core are not evident in the 600-yr, northern hemisphere, lowest densities and known volcanic eruptions (Fig. 8) composite tree-ring record of Bri!a et al. (1998). There is strongly indicates that volcanic eruptions are probably a very strong El Nino event in 1828 and a moderate-plus the cause of the cool summers that produce these charac- event in 1832 (Quinn and Neal, 1992). It is possible that 430 G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438

El Nino circulation patterns around this time could have information to delineate the climatic impact of a past modi"ed the climatic impact of eruptions during eruption. The two eruptions that I discuss are the AD 1829}1831. 1600 Huaynaputina eruption and the &AD 1453 Several di!erent ways to evaluate variability in the Kuwae eruption. As previously noted, these are two of climatic impact of volcanism using the tree-ring record the largest climate-forcing eruptions over the last 600 yr. can be seen from the following examples. Lough and In fact, the magnitude and potential climatic impact of Fritts (1987) used tree-ring time series for the western US these two events are just now becoming realized, unlike from 1601 to 1900 to evaluate the spatial variability of that for the more `famousa Tambora (Harrington, 1992) v0olcanic forcing in North America, in contrast to devel- and Santorini eruptions (Hardy and Renfrew, 1990; oping a hemispheric composite record. They concluded Hardy et al., 1990). that low-latitude eruptions provide the greatest forcing, resulting in cool summers in the central and eastern US 4.1. Huaynaputina, Peru: AD 1600 and warm winters in the western US Other proxy data from North America supported their conclusions. Sim- An overwhelming number of climate proxy records ilarly, Luckman (1996) and D'Arrigo and Jacoby (1999) provide evidence of exceptionally cold years in the early found that the spatial variability in the volcanic signal 1600s with the summer of 1601 being one of the coldest found in trees along the northern tree line could easily be over the last millennium, even colder than 1816 the year accounted for by the distribution of Arctic outbreaks without a summer (e.g., see the summary in de Silva and controlled by large-scale upper-air wave con"gurations. Zielinski, 1998; Bri!a et al., 1998; Pyle, 1998). The stron- These "ndings further emphasize the seasonal variability gest evidence of these extreme conditions comes from in the climatic response to an eruption, variability in the composite tree-ring records across the northern hemi- direction (either warming or cooling) of the climatic sphere (Jones et al., 1995; Bri!a et al., 1998), many of change, and spatial (e.g., longitudinal) variability in the which indicate that the summer of 1601 was, in fact, the climatic impact. coldest of the last 400}600 yr. Links to the possibility Several other characteristics of the tree-ring record that volcanism may have been at least partially respon- have been used in paleovolcanic research. Filion et al. sible for this cooling come from the large acidity signals (1986) and Yamaguchi et al. (1993) used the correlation in ice cores from both polar regions, as well as the between light rings in conifers (i.e., very few latewood identi"cation of volcanic glass from Huaynaputina with- cells in the growing season) and cool conditions in in a large microparticle peak at the AD 1600 level in the Quebec to indicate the impact of volcanism on climate. Peruvian Quelccaya ice core (Thompson et al., 1986). These studies were able to quantify the cooling. For Further evidence of the presence of stratospheric aerosols example, they suggested a 4.23C drop in summer temper- comes from historical records of dim suns in Scandinavia, atures at the northern Quebec treeline during the sum- reddish suns and moons in other areas of the northern mer of 1601. Additional tree-ring work by Yamaguchi hemisphere, and a darkened lunar eclipse in 1601 (e.g., de (1983) provided one of the only direct records of volcan- Silva and Zielinski, 1998). These data all indicate a very ism available in tree-ring chronologies. Yamaguchi sam- large perturbation to the atmosphere. pled Douglas "r trees in the path of ash fallout from some Using the known geologic record, it was apparent that of the past eruptions of Mt. St. Helens to improve on the the most likely source of the perturbation may have been dating of these particular eruptions via the presence of the 1600 Huaynaputina eruption in Peru. Although this disturbed rings. was known to be a large eruption, the true magnitude of An archive of the volcanic impact on climate that may the eruption was not apparent before recent "eld studies. become available after additional investigations is the De Silva and Zielinski (1998) suggest that the volume of annual record of coral growth. Crowley et al. (1997) erupted material was on the order of eruptions like San- found a reasonable agreement between years following torini and Mt. Mazama (e.g., Zdanowicz et al., 1999), thus an eruption and cooler sea surface temperatures recorded it is one of the "ve largest historical eruptions. However, in the O time series from corals in New Caledonia. linking the eruption to the global perturbation was not However, they stated that the impact of El Nino events veri"ed until tephra from the eruption was found in on the record needs further investigation, thus there still Antarctica ice core layers containing a large acid spike needs to be additional work before coral records can be (Palais et al., 1990). The magnitude of the signal contain- reliably used in paleovolcanic research. ing that glass suggests a stratospheric loading of 100 Mt of HSO, thus a major sulfur-producing eruption sim- ilar in magnitude to Tambora. However, tephra found in 4. Case studies one of the two acid spikes around 1601}1603 in the Greenland GISP2 ice core did not match that from I now present two speci"c examples to show how Huaynaputina. By assuming that the acid signal not a multidisciplinary approach can provide the necessary containing tephra was from Huaynaputina, conservative G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438 431 estimates on the stratospheric loading from that eruption historical eruptions and that it highly perturbed the at- were made. The GISP2 and other Greenland ice core mosphere in the mid-1450s. records of the Huaynaputina eruption suggest about a 40 Mt loading on average for the northern hemisphere, an estimate twice that of Pinatubo. Average global loading 5. Decadal-to-centennial-scale climate forcing for Huaynaputina would then be about 70 Mt HSO. Thus, it took the combined results from geological map- The time series of recent volcanic eruptions compared ping, ice cores, tree rings and records of atmospheric to instrumental records indicate that the length of the phenomena to determine that the Huaynaputina erup- volcanic forcing is generally on the order of several years. tion not only had a greater climatic impact than any However, what are the implications when there are sev- eruption in the 20th century, but it was one of the largest eral eruptions closely spaced in time? Does the length of in historical time. the cooling episode increase to the point that volcanism can force climate over longer time frames, such as dec- 4.2. Kuwae, Vanuatu, South Pacixc: AD 1450s adal-scale forcing? The early work by Self et al. (1981) suggested that there was not increased cooling during the The initial evidence that there was a major volcanic late 1800s when there were a series of eruptions closely eruption in the AD 1450s came from high acidity levels in spaced in time. I present more recent "ndings that look at ice cores from Antarctica (Delmas et al., 1992). At the longer time frames and the frequency of eruptions over time of that work there was not a known eruption that those periods which suggest that volcanism plays a role could be responsible for these signals. Several years later, in decadal-scale cooling. This is followed by a discussion "eld work in the Vanuatu area of the South Paci"c of the possible forcing capacity of a single mega-eruption, revealed the presence of a large caldera, now submerged, and thus the possibility that such an eruption could force with 100# m-thick deposits on the existing islands large-scale climatic change. I will use the Toba eruption (Monzier et al., 1994). Studies of the deposits indicated as an example of how a mega-eruption may force climate. a major caldera-forming eruption, that is, Kuwae. The estimated volume is on the order of 30 km dry-rock 5.1. Multiple eruptions equivalent, that is Santorini and Mazama class erup- tions. Radiocarbon dates indicated an eruption around Evidence for the ability of multiple eruptions to force AD 1425}1430, but there is quite a range of variability in climate on decadal time scales comes from several sour- the calendrical equivalent of these dates. Nevertheless, it ces: ice cores, both alone and when combined with time appears that the Kuwae eruption probably is the major series of other forcing factors, climate proxy data (i.e., tree event responsible for the Antarctica ice core signals rings) and modeling results. The main premise is that around the 1450s. cooler summer temperatures resulting from a series of More recent research now has shown that the Kuwae eruptions within several decades, as occurred over the eruption was one of the largest climate-forcing eruptions last few hundred years (Figs. 3 and 7), will produce an of historical time. Additional ice core work by Zielinski overall lower average temperature for many regions of (1995) and Cole-Dai et al. (1997b) indicate a magnitude of the northern hemisphere. When this consistently occurs HSO #ux to each of the polar regions that is almost over 50}70 yr time frames, as is found in the GISP2 identical to that calculated for Tambora in these same ice volcanic record between AD 1400 and the present, there cores. There is a caveat to this conclusion, however, as are lengthy periods of much reduced average temper- the signals in the AD 1450s found in many ice cores are atures. These summer temperatures are thus lowered assumed to be from the Kuwae eruption; tephra has not often enough that long-term averages are cooler than been found in any ice core as of yet to verify the source of they would be without the eruptions. This may be espe- these signals. cially e!ective when other climate forcing factors are in Finally, two additional pieces of evidence are indica- place, leading to an enhanced cooling or possibly an tive of the magnitude of the Kuwae eruption. The recent extended period of cooling on decadal to even multi- tree-ring work by Bri!a et al. (1998) suggests that the decadal time frames. summer of 1453 was the fourth coldest of the last 600 yr, These trends become particularly apparent when look- only surpassed by the summers following the ing at the compilation of summer temperature proxies Huaynaputina, Tambora and Parker eruptions. The from the Arctic through multiple data sets (Overpeck et compilations of historical observations by Pang (1993) al., 1997) and through the tree-ring composite from the further indicate a tremendous atmospheric perturbation. northern boreal forest of the northern hemisphere (Bri!a Pang notes that red sky may have played a role in the et al., 1998). Overpeck et al. used lake sediment data, siege on Constantinople, providing an interesting tie-in tree-ring records and some ice core data to develop to history (e.g., Simarski, 1996). All of these records a 400-yr proxy record of summer temperatures in the support the conclusion that Kuwae was one of the largest Arctic. These data were compared to various forcing 432 G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438 factors including volcanic activity, greenhouse gases and Furthermore, even the minimum estimate of stratos- solar activity. Although none of the forcing factors com- pheric loading (1;10) would have reduced sunlight to pletely explains the reconstructed summer temperatures, about 1/10 of a cloudless day at high noon (Rampino the frequent volcanism over the last 400 yr led to many et al., 1988), a scenario that has signi"cant impli- cool summers thereby lowering mean temperatures over cations. decadal time frames. In addition, the lack of any climat- The results of Rampino and Self's work (1992, 1993) ically-e!ective volcanism in the period 1920s } early suggested a 3}53C regional to possibly hemispheric an- 1950s undoubtedly contributed to the overall warm con- nual cooling following the Toba eruption. The possibility ditions during those decades, as frequent cool summers that summer cooling on the order of *103C occurred is that would lower the overall average temperature did not enough to lead to increased perennial snowcover and exist in the northern hemisphere. Hirschboeck (1980) also sea-ice extent. In general, these factors easily could have noted that periods of low volcanic activity over the last accelerated the global cooling underway at that time 100# yr correspond to decades with generally warmer from changing orbital parameters. Unfortunately, at the temperatures. The clustering of volcanic eruptions in time of those studies there were no records of a high several decades of the 1600s and early 1800s led to enough temporal resolution to absolutely place the tim- periods with very low maximum latewood-density in the ing of the Toba eruption into an accurate climatic setting. composite tree-ring record of the northern hemisphere Sampling of the highly resolved GISP2 core showed (Bri!a et al., 1998) further documenting the potential for that there was an exceptionally large volcanic signal at a series of eruptions to modify climate on decadal to 71,000$5000 yr ago (Zielinski et al., 1996b), between multi-decadal time frames. interstadials 19 and 20 (Fig. 9). Because of the timing and In addition to the ice core and tree-ring data sets, an magnitude of the signal, Zielinski et al. hypothesized that early modeling study by Robock (1979) indicated that it was most likely from the Toba eruption. Schultz et al. volcanic eruptions played a role in forcing `Little Ice (1998) subsequently veri"ed that Toba erupted between Agea climate. He found that the volcanic dust veil pro- these two interstadial events based on the presence of duced by Mitchell (1970) provided the best correlation to Toba glass in sediment cores from the Arabian Sea at the northern hemisphere surface temperatures over the last same O level as that shown for the GISP2 core in Fig. 400 yr. This is not to say that volcanism caused the 9. Thus, the Toba eruption occurs at the beginning of `Little Ice Agea, but it provides further evidence that it a 1000-yr stadial (cold) event and not at the beginning of probably enhanced cooling during this time period. As the 10,000-yr glacial event (i.e., isotopic stage 4), thereby stated by Overpeck et al. (1997), none of the known eliminating the possibility that Toba could have initiated forcing factors can explain all the trends in temperature the glacial event. However, there still is the question as to over the last 400 yr. Other pieces of proxy evidence have whether or not the eruption could have initiated the been presented over the last few decades suggesting that 1000-yr stadial event indicated by the ice-core para- volcanism can modify climate over decadal time frames. meters shown in Fig. 9. These include the agreement between known glacier ad- Annual sampling on the GISP2 core over the section vances and volcanic eruptions (Bray, 1974,1976; Porter, containing the Toba signal indicates that high sulfur ;  1981,1986), and the potential agreement between records loading from a maximum of 2}4 10 gHSO may of longer-term climatic change (i.e., over 10 yr) and have existed in the stratosphere for a period of up to 7 yr. volcanism (Bryson and Goodman, 1980). Modeling work by Bekki et al. (1996) also suggested that the residence time of the aerosols could have been up to 5.2. Mega-Eruptions 7 yr. These are the "rst results that imply that the aero- sols from an eruption may have stayed aloft for such The eruption of Toba, Sumatra, that occurred a lengthy period of time. These "ndings may mean that 71,000}73,000$5000 yr ago, produced 2500}3000 km various feedback mechanisms could begin, such as cool- of magma (dense rock equivalent), almost two orders of ing of sea surface temperatures, which would then lead to magnitude greater than that produced by the largest a lengthy period of global cooling. The detailed record of known recent historical eruption (Tambora) (e.g., Ches- the Toba eruption in the GISP2 core shows that al- ;  ;  ner et al., 1991). The 1 10 to 1 10 gHSO that though the cooling associated with the stadial event may have been produced by the Toba eruption (Rose and following Toba began before the eruption, the form the Chesner, 1990; Rampino and Self, 1992) would have been stadial then took, compared to other stadial events, is enough to a!ect climate drastically. The timing of the di!erent (Fig. 9). In general, there seem to be several eruption at the boundary of isotopic stages 4 and 5, centuries of enhanced cooling following Toba that do together with the high amount of sulfur that could have not appear in other stadial events during the last been injected into the stratosphere, led Rampino and Self glacial. Zielinski et al. (1996b) speculated that Toba may (1992,1993) to evaluate whether or not it could have been have enhanced cooling through various feedback mecha- of su$cient magnitude to initiate the last glaciation. nisms on century time scales. Such an eruption and the G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438 433

change. This is not only true for the Quaternary, but in other periods of earth history (e.g., Axelrod, 1981). The implication of this relationship is that various aspects of climatic change, and particularly rapid #uctuations in climate, can force volcanism. Perhaps the greatest cha- nges may occur during #uctuations from glacial to inter- glacial conditions and vice versa. It is possible that the combination of loading/unloading of the crust with ice sheet growth/decay and especially the increased loading on the ocean basins from greater meltwater input with melting ice sheets may lead to enough crustal stresses that magma chambers become much more active than during the middle of a climatic mode. Support for this hypothesis is available in several dif- ferent media including the geologic record, ice cores, and modeling results. For instance, Sigvaldason et al. (1992) found that lava production was much higher in Iceland in the millennium following deglaciation than under more recent conditions. This high rate continued for several thousand years after the initial impulse. McGuire et al. (1997) found evidence of increased frequency in volcanic deposition in the basins of the Mediterranean Sea during periods of rapid sea-level change associated with changes in the size of continental ice sheets. Even stronger evidence comes from ice cores in both polar regions. Gow and Williamson (1971) found an abundance of visible ash layers in sections of the Byrd ice core that correspond to the last glacial maximum. Sub- sequent analysis of these tephra layers by Kyle et al. \ >  Fig. 9. Time series of SO ,Ca , electrical conductivity and O (1981) indicated that Mt. Takahe, West Antarctica, was between 60,000 and 80,000 years ago showing the location and magni- tude of the Toba signal in the GISP2 core (Zielinski et al., Geophysical the most likely source. This conclusion led to the sugges- Research Letters, vol. 23, p. 838, 1996b, Fig. 1, copyright by the tion that the increased loading associated with thicken- American Geophysical Union). Arrows show timing of Toba aerosol ing of this part of the West Antarctic Ice Sheet during the deposition related to the non-volcanic Ca> and O records. 19 and late glacial may have trigged the eruptions that produced 20 represent interstadial events used as climatic markers in the Summit, the tephra eventually observed in the Byrd ice core. Greenland, ice cores. Note the isolated peak in Ca> values at the beginning of the stadial event containing the Toba signal (i.e., enhanced Zielinski et al. (1996a) evaluated the 110,000-yr volcanic cold conditions, as in caption for Fig. 3), a feature not observed in other record from the Greenland GISP2 core and also noted stadial events (compare to the Ca> record for the stadial event from that periods with the greatest number and magnitude of 73,600 to 72,400 yr ago). See text for discussion of signi"cance of the volcanic signals corresponded exceptionally well to peri- enhanced cooling at the beginning of the stadial coinciding with and ods of changing climatic conditions. In fact, the greatest following the Toba eruption. period of volcanic activity and many of the largest signals occur between about 7000 and 13,000 yr ago or during the main part of deglaciation, a time period when crustal resulting feedbacks and climatic change would have ex- stresses would be the greatest due to ice unloading and tremely serious social and economic repercussions meltwater loading. To account for changes in the tem- should it happen today. poral resolution in the earlier parts of the ice core record and the resulting decrease in the number of eruptions recorded, Zielinski et al. (1996a) sampled a portion of the 6. Climatic impact forcing of volcanism core at the same resolution as was done for the Holocene. They were then able to produce a multiplier to account The evidence for the climatic impact of past volcanism for potential missing signals. A time series of the possible is overwhelming; however, there is an opposite relation- number of eruptions per millennium that may have oc- ship in the overall system that has been proposed. Grove curred over the last 110,000 yr, was compared to an (1976) and then Rampino et al. (1979) were among the updated sea-level curve (Zielinski et al., 1997a). The peri- "rst to point out the fact that periods of increased vol- ods with the greatest number of eruptions correspond to canic activity often occur during periods of climatic the last deglaciation and to periods when sea level was 434 G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438 changing quickly (i.e., when continental ice sheets were technology, we still need to look into past records to "rst growing and during the build-up to the last glacial understand the complete range of variability in the vol- maximum). canism}climate system. No better example is available Finally, models developed by Nakada and Yokose than the fact that we have not felt the impact of a mega- (1992) suggest that the additional stresses generated both eruption, like Toba, in modern or even historical time. It by ice loading/unloading and meltwater changes in the is necessary to evaluate the system using information ocean basins were su$cient enough to trigger and/or from many disciplines, such as ice core research, vol- accelerate volcanic activity in island-arc areas that have canology, meteorology, dendrochronology, and poten- a thin lithospheric thickness. Consequently, the active tially other sources yet untapped (Table 1). No single volcanic zones of the northernmost Paci"c Rim (Japan, method is su$ciently comprehensive to provide the in- Kurile Islands, Kamchatka, Alaska) may have been espe- formation necessary to obtain a complete picture of the cially active during deglaciation. Evidence of increased climatic impact of an eruption, and to show how environ- volcanic activity in Iceland and the possibility of in- mental changes resulting from #uctuations in climate may creased volcanism in the northernmost Paci"c may be increase volcanic activity. Moreover, it may be di$cult to the reason why the largest signals are found in the latest produce a single volcanism}climate index given the many Pleistocene/early Holocene part of the GISP2 core (i.e., complexities in the system. Future work needs to proceed sources proximal or directly upwind of Greenland). It using a multidisciplinary approach to understand reliably may be that the increased volcanism occurred in volcanic the nature of the volcanism}climate system. regions of the northern hemisphere at di!erent times, thereby accounting for the extensive periods of increased volcanic activity in the GISP2 core. These data support Acknowledgements the suggestion that environmental changes associated with climatic change have the potential to increase vol- I thank the many individuals with whom I have collab- canism. orated with over the past few years, particularly S. Self, A. Robock, K. Hirschboeck, K. Pang, R. Keen. Special appreciation is given to members of the Climate Change 7. Conclusions Research Center at UNH for providing glaciochemical data from many of the ice cores we have analyzed and Volcanic eruptions are clearly able to force rapid cli- especially those from the GISP2 core. Their help in matic change (the type of change that has signi"cant evaluating these data sets is greatly appreciated as well. impact on humans) and to change climate on longer time These individuals are P. Mayewski, M. Twickler, S. scales. High sulfur-producing eruptions that inject large Whitlow, D. Meeker and J. Dibb. J. Palais gave me the quantities of material into the stratosphere may cool opportunity to become involved with the volcanic com- global or hemispheric climate by 0.2}0.33C for several ponent of ice core work. M. Germani collaborated on ice years after the eruption. On the other hand, multiple core tephra studies as has N. Dunbar, B. MacIntosh, R. eruptions closely spaced in time and especially mega- Esser and P. Kyle. I thank the many members of the eruptions, have the potential to impact social and eco- GISP2 ice coring community for their assistance. Fruitful nomic systems on decadal to possibly centennial scales, discussions with M. Rampino, D. Pyle, S. Carey, M. respectively. Adding to the complexity of the volcan- Baillie, H. Sigurdsson and K. Bri!a among others have ism}climate system is the fact that the resulting impact of helped in evaluating and compiling information event- an eruption is in#uenced by the climatic mode in exist- ually used in this summary. E. Mosley-Thompson and an ence at the time of the eruption. An eruption during anonymous reviewer provided many bene"cial com- a much warmer mode may have a more limited impact, ments that helped clarify the points discussed in this as would happen with the simultaneous occurrence of an paper. Support for this work has come from the O$ce of  eruption and an El Nino event. An eruption that occurs Polar Programs, U.S. National Science Foundation. Ad- during a cooler climatic mode may enhance or extend ditional support used to compile the multidisciplinary those cooler conditions. The latitude of the eruption is records of several past eruptions was provided by the also critical in the timing of the maximum cooling of the Division of Atmospheric Sciences, U.S. National Science eruption. The nature of the climatic change will also vary Foundation. spatially. Certain areas will feel maximum cooling during the summer or summers following the eruption, while other areas may experience an increase in temperature, References particularly during the winter. Although much information about the volcanic forc- Angell, J.K., 1988. Impact of El Nino on the delineation of tropospheric ing of climate is available through instrumental climatic cooling due to volcanic eruptions. Journal of Geophysical Research data and other sources of data from recent advances in 93, 3697}3704. G.A. Zielinski / Quaternary Science Reviews 19 (2000) 417}438 435

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