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.