A gridded network of tree-ring reconstructions of Palmer Drought Severity Index (PDSI) for the last 300 years has been used to create a set of maps of the spatial pattern of PDSI for each year, back to AD 1700. This set of maps enables an assessment of the droughts of the 20th century compared to droughts for the past 300 years. An inspection of the maps shows that droughts similar to the 1950s, in terms of duration and spatial extent, occurred once or twice a century for the past three centuries (for example, during the 1860s, 1820s, 1730s). However, there has not been another drought as extensive and prolonged as the 1930s drought in the past 300 years.

Longer records show strong evidence for a drought that appears to have been more severe in some areas of central North America than anything we have experienced in the 20th century, including the 1930s drought. Tree-ring records from around North America document episodes of severe drought during the last half of the 16th century. Drought is reconstructed as far east as Jamestown, Virginia, where tree rings reflect several extended periods of drought that coincided with the disappearance of the Roanoke Colonists, and difficult times for the Jamestown colony. These droughts were extremely severe and lasted for three to six years, a long time for such severe drought conditions to persist in this region of North America.

Coincident droughts, or the same droughts, are apparent in tree-ring records from Mexico to British Columbia, and from California to the East Coast (See examples in the graph to the right). Winter and spring drought conditions appear to have been particularly severe in the Southwestern U.S. and northwestern Mexico, where this drought appears to have lasted several decades. In other areas, drought conditions were milder, suggesting drought impacts may have been tempered by seasonal variations.

Assessment of the droughts of the 20th century compared to droughts for the past 300 years