Such shifts have occurred rather infrequently over the United States, but more regularly in semiarid agricultural regions such as the Sahel, Northeast Brazil, and the Middle East. If such dry regimes are sufficiently frequent or prolonged, the cumulative loss of topsoil due to wind erosion makes it increasingly difficult for vegetation to thrive, and irreversible 'desertification' occurs. Thus far the United states has experienced relatively little true desertification, but other regions of the global have not been so fortunate. 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. This transition may well have involved a series of dry 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 and the Sahel, and the planting of hedgerows designed to impede the flow of wind blown dust has been credited with sparing much of the U.S. Great Plains from suffering a similar fate. But whether adherence to environmentally sound agricultural practices will be sufficient to present further desertification is less clear. It is conceivable that climate change could act increase the frequency of dry regimes, thereby increasing the long term rate of wind erosion. By increasing summertime temperatures, greenhouse warming would increase the mean rate of evapotranspiration, rendering the climate of areas such as the western and central United States more vulnerable to extended drought episodes. While there is no conclusive evidence of such behavior in response to the warming of the past two decades, simulations with climate models indicate that more pronounced warming such as that predicted to occur by the end of the 21st century could result in widespread desertification over these regions, increasing the frequency of drought episodes and the risk of irreversible desertification.