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Desertification is generally viewed as an advanced stage of land degradation. The UN has defined desertification as a “diminution or destruction of the biological potential of the land which can lead ultimately to desert-like conditions.” Desertification can occur when extended periods of drought in arid, semi-arid, or dry sub-humid areas — known as drylands — sap the land’s productivity until it becomes “dead” soil. In addition, human activities often contribute to the process. While people have managed fragile drylands successfully for millennia in many parts of the world, pressure upon the land is much greater today as roughly 2 billion people inhabit drylands around the world.

Human activities that contribute to desertification include the expansion and intensive use of agricultural lands, poor irrigation practices, deforestation, and overgrazing. These unsustainable land uses place enormous pressure on the land by altering its soil chemistry and hydrology. Eventually, overexploited drylands suffer from erosion, soil salinization, loss of productivity, and decreased resilience to climatic variations. Land management plays a particularly large role in highly populated regions of less developed countries, where population growth is placing increased pressures on marginal lands. Global warming due to the atmospheric buildup of carbon dioxide and other gases emitted by fossil fuel combustion threatens to complicate this picture in the future. A rise in global temperatures is likely to accelerate the process of desertification as evaporation rates increase.

While these various causal factors have been identified, the actual process of desertification remains poorly understood. For instance, it is difficult to pinpoint when drought, which is driven by temporary changes in atmospheric circulation patterns, may become a long-term, permanent condition. Some meteorologists and soil scientists measure the impacts and length of a drought to determine if it is an example of desertification. Droughts can persist for months or years, but eventually subside; lands undergoing desertification never recover past productivity. In the United States, for example, 65 percent of the country was affected by drought in the 1930s, but the Great Basin eventually recovered, and modern occurrences of drought usually affect no more than 10 percent of the land.

As social and political forces intensify the pressures on the land that lead to desertification, land degradation itself can lead to further disruption of societal and political stability. The loss of fertile soil, water, and other resources, both for subsistence and for commercial use, leaves many people in dryland areas without the means to support themselves and their children. These displaced populations often migrate to urban areas or to other countries, adding to population pressures and sometimes increasing the likelihood of social and political conflict. According to the Natural Heritage Institute, many of the illegal immigrants who enter the United States from Mexico each year are fleeing the country’s severely degraded lands, which make up 60 percent of the country’s territory. Worldwide, the International Committee of the Red Cross estimates that 25 million of the world’s refugees — 58 percent — are fleeing degraded lands.

April Reese is a freelance environmental journalist based in Washington, DC.