Figure 7. Gridded U.S. drought reconstructions based on a new 388-site network of treering records from across the coterminous U.S. Time series showing the number of 155 2°x3° grid points characterized by (a) exessive moisture (Palmer Drought Severity Index <+1) or (b) drought (PDSI >-1) for each of the 278 years reconstructed. (c) Mapped areas characterized by drought for at least 80 years of the total AD 1700 to 1978 record. Together, these reconstructions suggest that the most widespread drought of the last three centuries occurred during the "dust-bowl" years of the 20th century, and that the southwestern U.S. and High Plains are most susceptible to drought. Other data (see text) suggest that droughts both more extreme and persistent occurred earlier in the Holocene (figure after Cook et al., in press).