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Paleoclimatic Perspective
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West North Central-Northwest Illinois |
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Since March 2005, drought conditions have gripped portions of the upper Midwest, particularly northern Illinois. For northwest Illinois (Division 1), persistently dry conditions for the last six months resulted in a rapid decrease in the Palmer Drought Severity Index (PDSI) (see graph to left). By the end of August, the PDSI for Illinois Division 1 had reached -3.64, which is classified as severe drought. |
| How unusual is the current drought? The instrumental record can provide a century-long context to answer this question. On this time scale, the current summer (June-August) ranked as the fifth driest summer in the 1895-2005 record (see graph below left), while 2005 had the second driest March-August on record (graph below right). When long-term moisture conditions are factored in, the PDSI is not as extreme due to wetter conditions during the previous couple years, with the droughts of the 1930s, 1960s, 1970s, and 1980s reaching more severe magnitudes (see graph above). A longer window into past droughts requires proxy data such as tree rings. |
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| The graph to the right shows a 329-year tree-ring reconstruction (1650-1978) of summer (June-August average) PDSI (annual values in light red; 5-year weighted mean in dark red) for an area centered on the northern border of Illinois, including most of Division 1. The reconstruction is one developed by Cook et al. (1999, 2004) in a gridded network covering much of North America. The reconstruction is based on tree-ring data from 28 sites in the Midwest. The correlation between the reconstructed and instrumental PDSI record over their common period (1900-1978) is very high (0.832). This particular instrumental record (annual values shown in light blue; 5-year weighted mean in dark blue) was calculated through only 2003, so the very similar Division 1 summer PDSI record is used to represent 2005 (blue dot). |
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Both the instrumental and reconstructed mean summer PDSI records for the 20th century show six years in the severe drought range (between -3.00 and -3.99) and a single year, 1934, that qualifies as extreme drought (below -4.00). For the period 1650-1900, the reconstruction indicates another four years of severe drought and seven years of extreme drought, with only one of those years (1736) comparable to 1934. Overall, drought conditions similar to or worse than those of 2005 occurred, on average, five years per century. The 20th century was notable for having a higher proportion of severe/extreme drought years than in the preceding 250 years, including the Dust Bowl years of 1930-1934, during which the average instrumental summer PDSI was -2.48. The tree-ring record shows only one period longer than 5 years, 1816-1821, with a lower average PDSI.
The current drought was very unusual in the speed of its onset, with PDSI dropping from slightly wet to severe drought in just five months. In the Illinois Division 1 instrumental record, this has occurred only once before, in 1936. But the drought's current severity has been matched or exceeded on numerous occasions in the past, according to both the instrumental and tree-ring records. |
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NCDC /
Clim. Monitoring /
Climate-2005 /
Aug /
U.S. Regional Drought /
Search /
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