One of the impacts which global warming may have on the surface of the Earth is to exacerbate the worldwide problem of desertification. A decrease in the total amount of rainfall in arid and semi-arid areas could increase the total area of drylands worldwide, and thus the total amount of land potentially at risk from desertification.
Desertification was defined at the Rio Earth Summit in 1992 as "land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors including climatic variations and human activities". Desertification involves the depletion of vegetation and soils. Land degradation occurs all over the world, but it is only referred to as desertification when it takes place in drylands. This is because these areas are especially prone to more permanent damage as different areas of degraded land spread and merge together to form desert-like conditions.
Global warming brought about by increasing greenhouse gas levels in the atmosphere is expected to increase the variability of weather conditions and extreme events. Many dryland areas face increasingly low and erratic rainfalls, coupled with soil erosion by wind and the drying up of water resources through increased regional temperatures. Deforestation can also reduce rainfall in certain areas, increasing the threat of desertification. It is not yet possible, using computer models, to identify with an acceptable degree of reliability those parts of the Earth where desertification will occur. Existing drylands, which cover over 40% of the total land area of the world, most significantly in Africa and Asia, will probably be most at risk to climate change. These areas already experience low rainfall, and any that falls is usually in the form of short, erratic, high-intensity storms. In addition such areas also suffer from land degradation due to over-cultivation, overgrazing, deforestation and poor irrigation practices.
The direct physical consequences of desertification may include an increased frequency of sand and dust storms and increased flooding due to inadequate drainage or poor irrigation practices. This can contribute to the removal of topsoil and vital soil nutrients needed for food production, and bring about a loss of vegetation cover which would otherwise have assisted with the removal of carbon dioxide from the atmosphere for plant photosynthesis. Desertification can also initiate regional shifts in climate which may enhance climate changes due to greenhouse gas emissions.
The United Nations Convention to Combat Desertification intends to tackle the problem of desertification, by adopting a partnership approach between governments and local populations. The Convention aims to encourage local communities to regain a sense of respect for, and understanding of, their land and the climatic factors which affect it.