Introduction

Drylands are thought to be the regions that global change will impact most severely. As described later in this chapter, our predictive capabilities are not up to the task of offering a view of these impacts. By evaluating the general nature of climatic variability, its causes, and its long-term trends in the drylands, this chapter provides a view of the broadest range of conditions that would likely prevail, as well as more realistic ones. The extremes are represented by conditions of the last 20,000 years, the late Pleistocene and Holocene. The historical period, roughly the last 2000 years, represents the more realistic scenarios. The majority of the examples in this chapter come from Africa because of the very visible and controversial changes over that continent, the large amount of available information, and the author’s extensive experience in the region. However, these examples are used to illustrate broadly applicable concepts of climate variability. A discussion of El Niño and its impact is also included because of its almost ubiquitous influence on climatic variability in the drylands.

This chapter presents little information on recent climatic variability. The reason is that the available data and literature with analyses thereof are so voluminous that a brief synthesis is not feasible. Also, most of the literature on recent change is readily available in the meteorological literature and is therefore easy for the reader to access. A few notable papers looking at long-term variability during the instrumental period examine northern Eurasia (Wang and Cho 1997), the Tien Shan (Aizen and Aizen 1997), the US Pacific Coast (Chen et al. 1996), India (Krishnamurthy and Shukla 2000), the Mojave Desert (Hereford et al. 2006), the central United States (Hu et al.1998), China (Ye et al. 2003), Iran (Modarres and Sarhadi 2009), South America (Aguillar et al. 2005), and Africa (Nicholson 2000a, 2001a).