The upper three-panel graph (1400-1990 A.D.) shows values of the tree-ring reconstructed June-August Palmer Drought Severity Index (PDSI) from each of three grid points along longitude 102.5°W at latitudes 35°N, 40°N and 45°N. Values to 1978 are tree-ring reconstructed data while those from 1979-1990 are instrumental PDSI data for the same grid points.

In the lower diagram, the time series in green shows the average of tree-ring reconstructed PDSI data (1250-1978) from the same three grid points. The time series in blue shows instrumental Modified Palmer Drought Severity Index (PMDI) for the same area as the grid points (western Great Plains) computed from June-August weather observations over the period 1900-2002. The variance of the tree-ring reconstruction has been scaled to that of the instrumental record over the calibration interval (1928-1978).

(The methodology is an updated version of that discussed in an article by Cook et al., 1999; details of the original method are available at the NOAA Paleoclimatology web site. The new data set is still in development and will be available in 2003. The tree ring data successfully reproduce the wet and dry periods during the overlap between the instrumental and reconstructed records.)

The 1930s drought is the worst drought in the 20th century instrumental record. However, the reconstructed index shows that droughts as severe as the worst during the last 100 years have occurred many times during the prior centuries. More importantly, prolonged severe droughts lasting many years (as long, or longer, than the 1930s) have occurred, notably during the 1840s and 1860s, and the 13th, 15th and 16th centuries.

Cook, E.R., Meko, D.M., Stahle, D.W. and Cleaveland, M.K. 1999. "Drought reconstructions for the continental United States." Journal of Climate, 12:1145-1162.