What was done
The authors reviewed what we know about the frequency and severity of drought in the central United States over the last two thousand years based upon empirical evidence of drought from various proxy data indicators.

What was learned
Data from a number of different proxy sources indicated the presence of numerous "multidecadal- to century-scale droughts," leading the authors to conclude that "twentieth-century droughts are not representative of the full range of drought variability that has occurred over the last 2000 years."  In addition, the authors note that the most recent century has been characterized by droughts of "moderate severity and comparatively short duration, relative to the full range of past drought variability."

With respect to causes of drought, the authors suggest a number of different possibilities that either directly or indirectly induce changes in atmospheric circulation and moisture transport.  However, they caution that "the causes of droughts with durations of years (i.e., the 1930s) to decades or centuries (i.e., paleodroughts) are not well understood."

What it means
According to the authors, "the full range of past natural drought variability, deduced from a comprehensive review of the paleoclimatic literature, suggests that droughts more severe than those of the 1930s and 1950s are likely to occur in the future."  Because severe and long-lasting drought for this region has been shown to be more normal than conditions of the past century, any intensification of droughts that might occur there in the future ought not be suggested to be the product of human-induced global warming, especially in light of our ignorance of the causes of past droughts in this region.