Tree-Ring Evidence for Climatically Effective Volcanic Eruptions

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Abstract
Ringwidth variations from temperature-sensitive upper timberline sites in the Sierra Nevada show a marked correspondence to the decadal pattern of volcanic sulfate aerosols recorded in a Greenland ice-core acidity profile and a significant negative growth response to individual explosive volcanic events. The appearance of single events in the mid-latitude tree-ring record, in connection with ice-core evidence from the arctic and historical records from the Mediterranean, indicates that the majority of these events represent climatically effective volcanic eruptions, producing temperature decreases on the order of 1°C for up to 2 yr after the initial eruption. Clusters of climatically effective volcanic events may serve as a trigger to glaciation and are consistently associated with lowered ringwidths and late-Holocene glacier advance in the Sierra Nevada. The tree-ring record strongly suggests forcing of solar radiation receipt and temperatures by increased volcanic aerosols, especially during the Recess Peak advances and Matthes (Little Ice Age) advances from 1400 to 1850 A.D. Intervals with an absence of significant volcanic aerosol production or historically documented eruptive activity correspond to intervals of significantly increased indexed ringwidth values, minimal numbers of severe annual negative ringwidth anomalies, and an absence of glacial deposits in the southern Sierra Nevada.