What was done
Using tree-ring wood density data from over 380 boreal forest locations in the Northern Hemisphere as a surrogate for summertime temperatures, the authors of this paper looked for effects of large volcanic eruptions on climate going back to 1400 AD.

What was learned
Major documented volcanic eruptions resulted in significant Northern Hemispheric cooling trends in the year following their occurrence.  The largest temperature departure in the record was 0.81°C in 1601, the year following the eruption of Huaynaputina in Peru.  Of the six centuries examined, the seventeenth century experienced the greatest number of climatically significant eruptions, exhibiting a total of six such events.  Strong temperature anomalies also suggest that there were three other major eruptions during the late seventeenth century that have not been reported in historical accounts.  It is also noteworthy that every Northern Hemispheric temperature departure of 0.3°C or more since 1400 (19 total events) followed a confirmed major volcanic eruption.

What it means
This study demonstrates the strong linkage between volcanic activity and large-scale temperature variability and may help to explain the period of cool temperatures referred to as the Little Ice Age, as it illustrates how closely-spaced multiple eruptions can reduce hemispheric temperatures on decadal and multi-decadal time scales and how a lack of such eruptions can result in periods of warmer global temperatures.  It thus raises a warning signal about the difficulties inherent in determining the causes of temperature changes over periods of a century or less.