Several tree ring studies have documented the spatial patterns of climatic effects following major volcanic episodes. Frost rings, micro- or narrow rings and light density latewood rings have all been used to identify severe cold periods of several years or more which are likely associated with volcanism. In northern North America, spatial variations in tree growth were found to reflect cooling influenced by atmospheric circulation patterns induced by volcanic events around AD 1640, 1783, 1815 and other years. Further back in time, historical accounts, augmented by tree ring and ice core evidence from western Europe and North America, suggest that three of the largest eruptions in the last two millennia occurred around AD 536, 934 and 1258. These events are believed to have had profound climatic and demographic repercussions over much of the globe. New tree ring chronologies from Mongolia and northern Siberia demonstrate that the climatic impact of these eruptions also extended into these remote regions.