© Dee Breger

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MONGOLIAN FROST RINGS: Lamont dendrochronologist Gordon Jacoby collected this sample of tree rings from a Siberian pine in Mongolia, which records the years 534-539 C.E. (left to right). The narrow, distorted rings for 536 and 537 indicate a drastic cooling in the northern hemisphere that froze sap in the cells during the growing season. Evidence for this abrupt climate change points to a massive eruption of the volcanic precursor of Krakatoa. Another theory invokes a cosmic impact that possibly triggered the eruption. Magnification ~ 27.6 X