Climate Change & Desertification
One of the impacts which global warming may have on the surface of the Earth is to exacerbate the world-wide problem of desertification. A decrease in the total amount of precipitation in arid and semi-arid areas could increase the total area of drylands world-wide, and thus the total amount of land potentially at risk from desertification. In addition, desertification may enhance regional warming, through a variety of climate feedbacks.
Desertification has been defined by the United Nations Conference on Environment and Development (Rio de Janeiro, 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.
The Fragility of Drylands
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.
Drylands cover over 40% of the total land area of the world (6,150 million hectares). They are most prevalent in Africa and Asia. They are defined as those areas where precipitation is low and where rainfall typically consists of short, erratic, high-intensity storms. 70% of these drylands are affected by degradation, which support over 1 billion people in more than 110 countries. Traditional farming and grazing techniques, suitable for wetter regions, are becoming increasing less sustainable owing to inadequate precipitation in these areas. Although climatic extremes may exert considerable pressure upon those who farm the land, weather conditions are not usually cited as direct causes of desertification. Rather, it is factors such as overcultivation, overgrazing, deforestation and poor irrigation practices which arise due to a variety of socio-economic reasons that are the immediate cause.
Causes of Desertification
Desertification is considered separately from drought with respect to the way in which the two phenomena are initiated. Drought is caused by the direct effect of climate through a period of at least two years of below average rainfall upon an area of land. On a longer timescale a process of aridification lasting decades is known as desiccation. The cause of desertification, however, is more complex to unravel.
The management of drylands requires flexibility to be able to adapt to natural changes in climatic conditions. In the past a range of techniques has been employed to protect valuable water resources, vegetation, soil quality and crops. However, economic and political pressures as well as changing cultures, population sizes, trends towards more settled communities and a lack of respect for the land have resulted in increasing mismanagement of land and the failure to adapt to fluctuations in climate. This often leads to the adoption of land use practices unsuitable for the existing climatic regimes, or the over-intensification of existing practices until they become damaging to the land.
Overcultivation of crops and excessive tilling of the land leads to exhaustion of soil nutrients. Crops harvested in drylands are often grown in soils already depleted in nutrients. The pressure to exploit the land in this way can be brought about by increases in food demand due to an increasing population, and monetary pressures such as the development of a cash-crop economy. However, under-cultivation can also lead to desertification in areas where there are no longer enough people to adequately manage the land. Soils, especially those of a sandy nature exploited in this way can become prone to wind erosion, whilst overcultivation of clay soils may well cause water erosion leading to land degradation. Low levels of technical know-how can often be a major cause behind poor land management practices such as irrigation. Poverty and underdevelopment are also major factors, but developed countries, including Spain, Australia and the United States, also suffer from desertification.
The Role of Climate Change
Climate change brought about by increasing greenhouse gas concentrations 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. This enhanced variability will place greater strain on already stressed environments. The Intergovernmental Panel on Climate Change stated that through a change in climate "there is a distinct possibility that, as a result of high rates of evapo-transpiration, some regions in the tropics and subtropics could be characterised by a higher frequency of drought or similar frequency of more intense drought than at present". Computer models designed to simulate the effects of enhanced atmospheric greenhouse gas concentrations on climate cannot, as yet, identify with an acceptable degree of reliability those parts of the Earth where drought frequency is likely to increase. However, many scientists have concluded that the "possibility of large changes in the severity or frequency of drought should be taken seriously, since it is through extreme events that climate change will have its most important impact".
The Effects of Desertification
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 vital soil nutrients and bring about a loss of vegetation cover. This undermines local food production and can act as a contributing factor towards famine as well as reduced biodiversity.
Desertification can also initiate regional shifts in climate which may enhance climate changes due to greenhouse gas emissions. Reduced topsoil can lead to a fall in water availability. The resulting decrease in local atmospheric humidity and increase in surface albedo (reflectivity of sunlight) has the potential to further reduce the regional precipitation. This alteration of the energy balance and change in land surface can play a part in generating a self-sustaining drought. Furthermore, desertification reduces the availability of removal sinks for carbon dioxide, the main greenhouse gas.
In June 1994, the United Nations Convention to Combat Desertification was adopted. By May 1995 this had been signed by 105 countries. The Convention 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. The Convention also emphasises the need to co-ordinate research efforts and action programmes involving a transfer of scientific and technological expertise.
Desertification is a problem that is expected to worsen under the stresses of greenhouse gas enhanced climate change. The effects of desertification are not necessarily restricted to local communities, but may influence populations world-wide as the climatic effects of desertification enhance man-made causes of global warming. However, an awareness of actions that could exacerbate these problems, or actions that could further restrict the ability of affected communities to adapt to climatic changes may well prove invaluable in preventing possibly irreversible physical, social and climatic change in the future.