During the last two million years - the Pleistocene period - the earth underwent a series of alternate cycles of cooling and warming, induced by variations in the planet's orbit and in the inclination of its axis. During the colder periods - known as the "Ice Ages" - most of the high-latitude regions of the world became covered by massive glaciers, and temperate ecosystems such as cold grasslands and conifer forests moved towards the equator. Because of the higher proportion of land-mass in the northern hemisphere, this phenomenon was more conspicuous in North America and Eurasia (Figure 1.9).

From the late Pleistocene glacial period to present times significant changes occurred in the climate and ecosystems of the planet. During the Last Glacial Maximum (LGM) period, ( 5-17 000 years before present [yBP]), the presence of large ice sheets and lower concentration of atmospheric CO had given rise to a colder climate and a reduced summer monsoon resulting in low global forest cover. The tropical belt narrowed and the deserts moved towards the equator, shrinking in the mid-latitudes, where they were replaced by grasslands, semiarid scrubs, open woodlands, or cold steppes. The ancestors of the modern-day desert biota found refuge in what are now dry subtropical habitats, especially in places where arid conditions persisted under the rain shadow of large mountain ranges, or in areas that are now covered by dry tropical savannas which, lacking intense monsoons, were then more arid than at present. The last glaciation ended around 15 000 years ago, when the glaciers retreated, giving place to the warm interglacial period that followed: the Holocene, our current global climate.

As a general rule, during the LGM, the high-latitude borders of the world's deserts became colder and wetter than they are at present, while the tropical fringes became drier than they are today. Thus, during the LGM large dunes developed in the southern Sahara-Sahel zone and in the Thar Desert of India, while the Mediterranean coast of Algeria and Morocco became wetter and colder. Similarly, during the late Pleistocene the Chihuahuan Desert of North America witnessed woodlands of piñon pines, junipers, and oaks, while the tropical dry woodlands of central Mexico became drier, evolving the rich cactus flora that characterizes the region today.

During the warm early to mid-Holocene (8 000- 5 000 yBP), the global climate that resulted from glacial retreat brought an increase in the intensity of the monsoon throughout the sub-tropical arid lands. Lake Chad became a freshwater inland lake bigger than today's Caspian Sea, in an area that has again become a complete desert. Tropical forests and dry woodlands around the equator expanded north and south, while deserts moved into the mid-latitudes. During that period, the southern Sahara and the Sahel were much wetter than today, with extensive vegetation cover, thriving animal communities, and numerous human settlements.

Sometime between 6 000 and 5 000 yBP, there was again a transition to more arid conditions. Mesic vegetation communities disappeared rapidly, lake levels declined dramatically, and highly mobile pastoralist cultures started to dominate and replace sedentary lacustrine and riparian traditions. The Liwa region of the United Arab Emirates, for example, experienced phases of sand deposition that lead to the formation of a large (up to 160 m high) mega-dune. A similar transition towards more arid conditions occurred in North America, where the Holocene brought the arrival of Mojave, Chihuahuan and Sonoran desert scrub elements from the south, such as the agaves, cacti, ocotillos (Fouquieria), and creosote bushes that characterize the area today.

An explanation for these climatic variations is that changes in incoming solar radiation, associated with slow shifts in the Earth's orbit, enhanced the strength of the summer monsoon rains at the beginning of the Holocene. These rains, in turn, increased the extent of vegetation cover and wetlands, and this had two major effects - a reduction in surface albedo (reflectance) and an increased ability to recycle water back to the atmosphere through evapotranspiration. Both effects helped fuel the monsoons with additional energy and moisture, increasing the summer rains. In Africa, the climate-vegetation system maintained a "green Sahara" climatic regime through the middle Holocene, when a sudden transition occurred to a "desert Sahara," the regime that we know at present. The aridization trend of the mid- Holocene fed back into the deserts themselves by decreasing vegetation cover, reducing local inputs of moisture into the atmosphere, and further increasing the dry conditions.