Preface

Climate was long defined as the mean weather conditions in a given location. A corollary to this definition is that climate is an inherent and invariant aspect of environment. More and more, climate is being recognized instead as an environmental variable, and an overwhelming body of evidence points to accelerated rates of climate change during the past century. At the same time, unmistakable environmental changes have occurred in dryland regions in response to human behavior. Some of the human factors include the sedentarization of nomads, the continual increase in urbanization, population increase, land-use changes, and technological means of exploiting the environment. The result is worldwide changes in the land surface, soils, and vegetation. One of the future challenges is to understand the interplay between these various aspects of the environment and managing the environmental resources in ways that provide for human well-being while conserving and protecting the resources.

There are pressing reasons for focusing on the drylands. First, because the essential resource – water – is discontinuously available in time and space, environmental processes are quite fragile. The semi-arid regions, in particular, are expected to be among those most sensitive to future climate change and increasing intensity of land use (IPCC 1996). Thus, the ability to predict changes in dryland landscapes is one of the top priorities for global change research (Breshears and Barnes 1999). Further, perhaps more than anywhere else, the environment of drylands is a consequence of closely tuned feedbacks among biological, geomorphological, hydrological, and human systems. Changes in any of these systems can readily upset the feedback loops, creating serious disturbances in the environment (Graetz 1991). Finally, an increasing body of evidence suggests that this sensitivity is such that, when critical thresholds of certain variables are surpassed, abrupt and irreversible changes in the ecosystem can occur (e.g., Rietkerk et al. 2004; Scheffer et al. 2001). Perhaps more importantly, misinformation concerning this sensitivity abounds, mainly as a result of an incomplete understanding of the role played by climate in the dryland environment. The issue of desertification, discussed in detail in this book, is a case in point.