Weather patterns deduced from tree rings
By Peter Cleary | March 27, 2003With the help of James Madison, a group of researchers from the University of Virginia and the University of Arkansas have developed a history of central Virginia precipitation for the past two centuries. Their study, published recently in the Bulletin of the American Meteorological Society, shows a shift in the timing of spring precipitation from May to June. At the urging of Thomas Jefferson, Madison recorded over 16000 weather observations at his Montpelier plantation between 1784 and 1802. The modern-day researchers used these 18th century observations, along with 20th century instrumental records, to relate the width of growth rings in white oak trees at Montpelier to seasonal precipitation. Precipitation levels are a major factor dictating the width of the ring a tree adds to its trunk each growing season. "More precipitation generally leads to a wider growth ring," Environmental Sciences student, Dan Druckenbrod, the study's main author, said. Druckenbrod feels fortunate to have Madison's records because they allow him to calibrate what he saw in the tree rings with actual precipitation. "These were guys who lived the ideals of the enlightenment," he said, indicating that the records of Madison and Jefferson met the highest scientific standards of their time. Work at Madison's plantation has provided a unique and valuable data set, which includes both a long period of continuous tree growth and centuries-old weather records. "Madison was both interested enough in weather and wise enough not to clear-cut," Environmental Science Professor and co-author of the study, Michael Mann said. Mann added that studies of this nature usually only have 20th century data available to relate ring width to precipitation. Due to a lack of data, other studies are forced to assume that the relationship between precipitation and ring width has not changed significantly over the past few centuries. Based on Madison's journals and information gleaned from ring width, the recent study shows that there was a significant shift in precipitation over the past 200 years. Today, spring rains fall mostly in June.According to Mann, however, the growth rings from the late 18th century showed "an extremely high correlation with May precipitation." This observation agrees with what Madison recorded in his diaries. Mann indicated there were many reasons such a shift could have occurred, but that these speculations were outside the scope of this study. The joint University of Virginia/University of Arkansas research is an important contribution to our knowledge of Virginia's climate in that it provides a long record of drought. While the drought of the past few years was too recent to be included in this study, Druckenbrod noticed other years of drought in the tree rings."1911 and 1918 were both years of extreme drought," as indicated by small growth rings, he said. The results also show the dynamic nature of Virginia's climate."This suggests the climate is variable, or has been variable for the past 200 years," Druckenbrod added. The largest drought noted by the researchers occurred in the early 1790s. "That's also known to be [the period of] one of the most severe El Niños in the past three centuries," Mann said.
