There has been great interest among climatologists in recent years in interpreting the historic temperature record. Much of this interest has been stimulated by concerns over the possibility of an enhancement of the greenhouse effect (e.g., Ramanathan 1988; Schneider 1989). Global temperature records show an irregular warming of around .5[degrees] C over the last 100 years. During the same period, the combined level of radiatively active trace gases in the atmosphere has increased by up to 50%. Although it is tempting to attribute the historic warming to this change in atmospheric composition, it is clear that temperature is influenced by many factors other than atmospheric composition. To gain some understanding of the historic relationship between atmospheric composition and temperature, it is necessary to identify and quantify the effects of these factors on temperature.
Among the processes that may influence climate is volcanism (e.g., Lamb 1970; Toon and Pollack 1980). Most of the empirical work on the climatic effects of volcanism has been based on the careful study of regional temperatures following a few of the largest eruptions (e.g., Angell and Korshover 1985; Mass and Portman 1989). The results of this work show that, although explosive volcanism appears to have a cooling effect, the magnitude and duration of the effect depend on a number of factors including the magnitude of the eruption, the composition of the ejecta, the season during which the eruption occurred, and other factors.
Having established, at least provisionally, that individual explosive eruptions have an effect on temperatures, the next step in understanding the volcanic effects on the historic temperature record is to assess the overall historic pattern of explosive volcanism. The purpose of this article is to take a step in that direction. Specifically, this article presents an exploratory analysis of the frequency of explosive volcanism in the Northern Hemisphere for the period 1851-1985.
The remainder of the article is organized as follows: Section 2 contains a brief description of the data, methods of analysis are described in Section 3, the results of the analysis are presented in Section 4, and Section 5 contains some discussion.
The primary short-term mechanism by which volcanic eruptions influence climate is the injection of dust and aerosols into the stratosphere (e.g., Harshvardhan and Cess 1976; Pollack et al. 1976). For this reason, in constructing a record of eruptions capable of influencing climate, it is most important to include large-scale explosive eruptions. This is fortuitous, because it is less likely that such eruptions would be missed in the construction of the record. It is important to note, however, that factors other than magnitude may influence the climatic effect of a volcanic eruption.
Simkin et al. (1981) constructed a chronology of known volcanic events over the past 8,000 years. Using a classification criterion--the Volcanic Explosivity Index--developed by Newhall and Self (1982), Bradley (1988) extracted a record of major explosive eruptions in the Northern Hemisphere for the period 1851-1981. The record analyzed in this article has been updated through 1985.
The restriction to the Northern Hemisphere is necessitated by the poor quality of Southern Hemisphere data on volcanic activity. The long time-scale of transequatorial stratospheric transport ensures that Northern Hemisphere temperatures generally will be free from the influence of Southern Hemisphere volcanism.
Although this record is among the best available, it is certainly incomplete. The accuracy of the record probably improves with time, although as recently as 1982 elevated sulfate aerosol levels from an undetected eruption were discovered by high altitude sampling (Mroz, Mason, and Sedlacek 1983). Although this data set is unlikely to …