Transient extremes linked to volcanic explosivity

Modified from Biffra et al. (1998) Nature 393: 450-455.

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Caption: Over the last 600 years, all the known (and several, as yet unidentified) major volcanic eruptions are marked by changes in mean tree ring density (linked to lower spring/summer temperatures) for a set of records covering much of the boreal forest zone in the Northern hemisphere. The Volcanic Explosivity Index VEI is a rough measure of magnitude. Note that most events have an annual impact, but the impact of strong events, as in 1641 and 1816, lasted for 3 years. The general down-turn in the curve over the last 30 years or so is not explicable in climatic terms, since it contrasts with every other line of evidence for the same period. The likely implication is that there is a change in physiological response linked perhaps to CO2 enrichment and/or hydrology/soil moisture status.

K. Briffa et. al. (1998) Influence of volcanic eruptions on Northern Hemisphere summer temperature over the past 600 years, Nature,  393, 450-455