Drought in the continental United States (U.S.) is a recurring phenomenon that has shown large natural variability on both the instrumental and paleoclimatic time scales. Recent research examining global changes in drought suggest that drought conditions have increased globally in the last 50 years (Dai et al. 2004). In the U.S. as a whole there is no indication that drought has become more frequent. However, in some regions, such as the Southwestern U.S. there is evidence that drought has become more prevalent, while in other areas drought has become less frequent.

Over the 20th century the U.S. has experienced statistically significant increases in both the annually averaged mean temperature and the total annual precipitation. The most commonly used index is the Palmer Drought Severity Index (PDSI), which incorporates both precipitation amounts, and an estimate for evapotranspiration, and requires only monthly temperature and precipitation as input. Here we examine the possibility that with the observed increase in temperature and without the observed increase in precipitation, drought conditions since 1950, as defined by the PDSI, over the United States would have been more prevalent. The starting point of 1950 is chosen to include the major droughts of the early 1950s and includes the period in the U.S. temperature record that is roughly the start of a slight temperature decline until the 1970s when U.S. temperatures, like global temperatures, began a strong increase. Results indicate that without the increase in precipitation, the percentage of U.S area in drought would increase substantially, in some years by as much as a 25% increase in the area in severe and extreme drought.