To encroach upon something means to advance past its usual or natural limits, so when we pair the words “encroachment” and “desert,” we refer to the spread of a desert into surrounding lands.Though “desert” usually conjures up an image of sand, camels, or Lawrence of Arabia; a desert is really any place, regardless of temperature, where average annual precipitation is less than 250mm (about ten inches). Deserts are surrounded by semi-arid regions, which have up to twice the annual rainfall.
The boundaries of deserts have always changed through time. These changes occur naturally as climate shifts and alters rainfall patterns. Changing vegetation patterns can also cause subtle change in the arbitrary line defining a desert. Within the past few thousand years, however, desertification has become the most common cause of desert encroachment.
Though there are known examples of natural desertification, human activity is the most common cause. Farming and grazing often upset the delicate ecosystem of semi-arid regions; creating conditions that allow nearby deserts to spread, or encroach, beyond their boundaries. Let’s consider a typical desertification and desert encroachment scenario.
Semi-arid regions are home to grasses evolved for a harsh environment. These plants survive on limited water because of their widespread root systems and tough stems that baffle, or slow, runoff from infrequent rainstorms. In undisturbed grasslands, the water sinks into the soil where it can be taken up by the roots of plants. Trapped moisture returns to the atmosphere through evaporation of water from the upper soil layers and transpiration from plant leaves. The two processes, together called evapotranspiration, raise local humidity. Higher humidity increases the frequency and quantity of rain, and moderates the local climate because humid air’s temperature is harder to change than that of dry air – as coastal dwellers know, water vapor acts as a temperature buffer. The vegetation and local climate in semi-arid regions have a feedback effect: good grass cover contributes to more rain, which leads to more grass.
When human activity breaks the feedback loop, local climate changes. Poor tilling practices, for instance, remove grass and allow increased runoff, so local humidity decreases. Once water leaves the area in streams and rivers, it cannot contribute to the local weather patterns –- rainfall decreases, eventually to the point where the area becomes a desert. Overgrazing by livestock also removes grass cover and increases runoff, with similar results.
Large regions surrounding the world’s great deserts have undergone desertification since the advent of farming and herding, and the phenomenon continues today in many third-world countries. Non-governmental organizations such as the Peace Corps are actively teaching farmers and herders in arid regions practices designed to prevent desertification.
Human-caused desertification is not the only cause of desert encroachment. Grasslands in semi-arid regions can be destroyed by prolonged drought or natural wildfires. Overgrazing by herds of wild herbivores can also contribute. However, man’s hand is behind the great majority of desertification observed through recorded history.
Desert encroachment caused by natural or human-caused desertification is local, although certainly devastating in the long term. Parts of the Middle East, such as Syria, are hotter and more arid than two millennia ago, almost entirely as a result of desertification caused by humans. This differs from climatological predictions of global climate change, which warn of large-scale and rapid desert encroachment caused by shifting climate patterns. Based on observation of local desert encroachment caused by desertification, there is no denying that, if large-scale desert encroachment were to occur, the face of the world would be radically altered.
