Volcanic Eruptions over the Last 5,000 Years from High Elevation Tree-Ring Widths and Frost Rings

Some tree-ring records, due to their great age, the annual resolution of their dates, and their sensitivity to the climatic effects of large volcanic eruptions, are useful in understanding the magnitude and frequency of large globally-effective volcanic eruptions. Two primary factors are thought to have forced much of late Holocene variation in climate prior to industrialization: solar output and volcanic eruptions (Free and Robock 1999; Crowley 2000; Shindell et al. 2001). While there is some debate regarding which of these forcings has played the dominant role (Shindell et al. 2003), there is little doubt that volcanism affects climate. Large explosive eruptions inject great quantities of sulfur compounds into the stratosphere, which combine with water to produce sulfuric acid aerosol (Rampino and Self 1982). This injection changes the radiative balance by increasing absorption and reflection of incoming short wave radiation by stratospheric aerosols, and generally has a cooling effect on climate (Lacis et al. 1992; Minnis et al. 1993; McMormick et al. 1995). Volcanism has also been reported to cause winter warming in the extratropical Northern Hemisphere due to cold season shifts in the Arctic Oscillation (Robock and Mao 1992; Kelly et al. 1996). However, radiative forcing dominates the net surface temperature changes from very large eruptions and leads to significant cooling (Shindell et al. 2003). There is some evidence that volcanic eruptions have played a major role in forcing past global temperatures. Pulses of volcanic activity, for example, contributed substantially to the decadal-scale climate variability of the Little Ice Age (LIA) interval (AD 1400–1850) (Porter 1986; Mann et al. 1998; Crowley 2000). Yet the climatic impact of past eruptions varies spatially and appears to be partly dependent on eruption frequency, size, location, seasonal timing, sulfur content, and the state of the climate system at the time of the eruption.