Droughts More Severe Than Dust Bowl Likely, NOAA Reports

Connie Woodhouse, a University of Colorado research scientist working at NOAA's National Geophysical Data Center in Boulder, Colo., and Jonathan Overpeck, head of NOAA's Paleoclimatology Program, report the results of their research in the December issue of the Bulletin of the American Meteorological Society.

The authors reviewed existing paleoclimatic literature, including a variety of data sources, to determine what droughts were like before instruments were invented, and to compare droughts of the past 2000 years with more recent droughts. The data sources consist of historical documents, tree rings and archaeological remains, as well as lake, river and wind-blown sediments.

The authors found a greater range of drought variability in the past than found in the instrumental record. Droughts of the 20th century have been only moderately severe and relatively short, compared with droughts of much longer ago. Woodhouse said that paleoclimatic records of the past 400 years strongly indicate that the severe droughts of the 20th century, the 1930s Dust Bowl and the l950s drought, were not unusual events and suggest that we can expect to have droughts of this magnitude once or twice a century.

"However, when we look even farther back in time, we see indications of droughts with much greater duration," said Woodhouse. During the 13th to 16th centuries, there is evidence for two major droughts that probably significantly exceeded the severity, length, and spatial extent of 20th century droughts, the authors report. The most recent of these "megadroughts" occurred throughout the western United States in the second part of the 16th century. This drought appears to have been the most severe and persistent drought in the Southwest in the past 1000 to 2000 years. Another megadrought occurred in the last quarter of the 13th century.

"Conditions that lead to severe droughts such as that of the late 16th century could recur in the future, leading to a natural disaster of a dimension unprecedented in the 20th century," Overpeck said. "Besides the fact that natural variability could have more severe droughts in store for us in the future, two human factors could make the Great Plains even more susceptible to a severe drought in the future. These are land use practices and global warming."

"Even in the absence of significant greenhouse warming, however, future droughts may be much more severe and last much longer than what we have experienced this century," Woodhouse said. Overpeck said that paleoclimatic data in combination with instrumental data, satellite observations, and climate models are essential to understanding the full range of natural drought variability, and also to reduce uncertainty with respect to what human-induced and natural climatic change will occur in the future.