Drought during Medieval Times In North America: Results from An Expanded High-Resolution Grid of PDSI Reconstructions from Tree Rings

The protracted 21st century drought in the western United States, now in its 9th year, has focused attention on the importance of drought monitoring and assessment. This includes monitoring droughts as they develop in near real-time and assessing how they compare to past droughts. To this end, a new one-half degree resolution blended-living North American Drought Atlas (NADA) is being developed from long tree-ring reconstructions of the Palmer Drought Severity Index (PDSI) blended with continuously updated instrumental PDSI. This new drought atlas is based on 1,845 tree-ring records, with over 1,000 not used in a previous lower-resolution version of the NADA, including 60 new ones that begin before A.D. 1000. Because this blended-living drought grid extends back 1,000 years or more over large portions of North America, it is possible to examine in much finer detail some of the remarkable Medieval “megadroughts” that occurred in the past. These megadroughts dwarf all historic droughts known to have occurred based on instrumental records that only extend back into the latter part of the 19th century. They differ from those in historic times not in annual intensity, but in duration, with some of the Medieval megadroughts lasting several decades. These “no analog” droughts present a challenge to the climate science community to explain why they lasted so long. Modeling evidence supports the importance of La Niña-like cool tropical Pacific sea surface temperatures (SSTs) in the development and maintenance of megadroughts in the American West (especially in the Southwest), with SSTs elsewhere probably contributing as well to some extent depending on the location of the drought. Increased solar forcing over the tropical Pacific during Medieval times coupled with low explosive volcanism may also have been a contributor. The possible return of North American megadroughts due to global warming will be addressed in this context.