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Paleoclimatic Perspective
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The southwestern United States has experienced unusually dry conditions for much of the last four years. By some measures, the dryness has been as bad as the drought of the 1950s (considered to be the worst drought episode of the 20th century), but by other measures (the Palmer Drought Index) it can be considered worse. Extensive reliable instrumental records go back only about 100 years, but tree rings are a form of paleoclimatic data that can reliably extend the climate record back several more centuries.
Precipitation data, Palmer Drought Indices, and tree ring data were analyzed for stations in southwestern New Mexico and southeastern Arizona to create a subregion called "Southwest New Mexico". The Palmer Drought Index computed over the instrumental record (graph below left) shows that the 1950s was the worst drought of the last 100 years for this region. However, worse dry spells have occurred over the last 300 years when the tree-ring reconstruction is examined (graph below right). |
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Total winter (November-March) precipitation for Southwest New Mexico was reconstructed from a set of moisture-sensitive trees in this region (Woodhouse 1996). The reconstruction explains 66% of the variance in the instrumental record, and extends from 1702-1983 (blue line in the graph above right). The values shown are departures from average, so negative values are drier than average and positive values are wetter than average. The instrumental record for this region, 1896-2004, is shown in red. In the period of overlap, the match between the tree-ring-based reconstruction and the instrumental record can be assessed. In general, the two records vary in the same ways (e.g., low values in the instrumental record are also low values in the reconstruction), but the variance not explained by the tree rings is typically in the magnitude of extreme values (e.g., 1941). The Pearson correlation coefficient between the reconstruction and the instrumental record is 0.76, which is a very good match.
The reconstruction is useful for placing 20th century drought events into a long term context. Since 2000, there has been a sequence of severe drought years, but the drought in the 1950s was as severe and even more persistent. Looking back to years prior to the instrumental record, single drought years including 1861, 1818, 1752, and 1748 were more intense than any year in the 20th century. In particular, winter rainfall in 1748 was more than two and a half standard deviations below average, and the driest year in the 282-year reconstruction. The reconstruction also shows that some of these pre-instrumental period droughts were more persistent than any in the 20th century. The longest sequence of consecutive years below average was the eight-year period between 1750 and 1757. In contrast, the longest drought period in the 20th century is the five-year period from 1953-1957. Reference: Woodhouse, C.A., 1996. "Climate Variability on the Southwestern United States as Reconstructed from Tree-Ring Chronologies." Ph.D. Dissertation. University of Arizona, 227 pp. Additional Sources: Additional paleoclimatic information can be found at the NOAA Paleoclimatology Program web site: and the University of Arizona Climate Assessment for the Southwest web site: |
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