vegetation and the ground. Once established, an arid climate regime, such as the one that prevailed during the Dust Bowl, appears to be capable of perpetuating itself until a well-timed series of rainstorms enables the vegetation to regain a foothold (Dirmeyer and Shukla, 1996; Wang and Eltahir, 2000a,b; Clark et al., 2001).

The onset and termination of the 1930s Dust Bowl are examples of abrupt regime shifts from a climate conducive to agriculture to a climate more characteristic of a desert region and back again. During the time covered by instrumental records, such shifts have occurred rather infrequently in the United States but more regularly in semiarid agricultural regions, such as the Sahel, northeast Brazil, and the Middle East (Nicholson et al., 1998; Street-Perrott et al., 2000). If such dry regimes are sufficiently frequent or long, the cumulative loss of topsoil due to wind erosion makes it increasingly difficult for vegetation to thrive, and difficult-to-reverse “desertification” occurs (United Nations, 1980). Thus far, the United States has experienced relatively little true desertification, but other regions of the globe have not been as fortunate. For example, it is well documented that the Sahara expanded northward and engulfed formerly productive agricultural regions of North Africa during the last few centuries of the Roman Empire (Reale and Dirmeyer, 2000); this transition might well have involved a series of prolonged drought episodes analogous to the Dust Bowl.

Agricultural practices influence the retention of topsoil. Poor cultivation practices and overgrazing have been blamed for the desertification that has plagued North Africa, the Sahel, and other semi-arid regions (Otterman 1981; Wendler and Eaton, 1983; Balling, 1988; Bryant et al., 1990; BenGai et al., 1998; Nicholson et al., 1998; Pickup, 1998), and the planting of hedgerows designed to impede the flow of wind-blown dust has been credited with sparing much of the US Great Plains from suffering a similar fate. Whether adherence to environmentally sound agricultural practices will be sufficient to prevent further desertification is less clear.

Global warming could render such regions as the western and central United States more vulnerable to extended drought episodes by increasing temperatures during the growing season, and thereby increasing the rate of evapotranspiration. There is no conclusive evidence of such behavior in response to the rapid warming of the last two decades, but simulations with climate models indicate that more pronounced warming like that predicted to occur by the end of the twenty-first century could serve to increase the frequency of drought episodes and the risk of irreversible desertification (Rind et al., 1989; Henderson-Sellers et al., 1995; Bounoua et al., 1999).



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