Desertification

Drylands (defined as those areas where precipitation is low and where rainfall typically consists of small, erratic, short, high-intensity storms) cover about 40% of the total land area of the world and support over one-fifth of the world's population. Figure 7.10 shows how these arid areas are distributed over the continents.

Desertification in these drylands is the degradation of land brought about by climate variations or human activities that have led to decreased vegetation, reduction of available water, reduction of crop yields and erosion of soil. The United Nations Convention to Combat Desertification (UNCCD) set up in 1996 estimates that over 70% of these drylands, covering over 25% of the world's land area, are degraded34 and therefore affected by desertification. The degradation can be exacerbated by excessive land use or increased human needs (generally because of increased population), or political or economic pressures (for instance, the need to grow cash crops to raise foreign currency). It is often triggered or intensified by a naturally occurring drought.

The progress of desertification in some of the drylands will be increased by the more frequent or more intense droughts that are likely to result from climate change during the twenty-first century.

Recent research has demonstrated the complex nature of the effects of climate change on dryland ecosystems, on the interactions between the species they contain and with the local human communities who live in the dryland areas. Much more understanding is required to assess what is likely to occur and how adverse effects can be minimised.35

Figure 7.10 The world's drylands, by continent. The total area of drylands is about 60 million square kilometres (about 40% of the total land area), of which 10 million are hyper-arid deserts.

North America

Europe

Australia

Asia Africa

South America

Asia Africa

South America

North America

Europe

Australia

What will be the effect of climate change on agriculture and food supply? With the detailed knowledge of the conditions required by different species and the expertise in breeding techniques and genetic manipulation available today, there should be little difficulty in matching crops to new climatic conditions over large parts of the world. At least, that is the case for crops that mature over a year or two. Forests reach maturity over much longer periods, from decades up to a century or even more. The projected rate of climate change is such that, during this time, trees may find themselves in a climate to which they are far from suited. The temperature regime or the rainfall may be substantially changed, resulting in stunted growth or a greater susceptibility to disease, pests and fires. The impact of climate change on forests is considered in more detail in the next section.

An example of adaptation to changing climate is the way in which farmers in Peru adjust the crops they grow depending on the climate forecast for the year.38 Peru is a country whose climate is strongly influenced by the cycle of El Niño events described in Chapters 1 and 5. Two of the primary crops grown in Peru, rice and cotton, are very sensitive to the amount and the timing of rainfall. Rice requires large amounts of water; cotton has deeper roots and is capable of yielding greater production during years of low rainfall. In 1983, following the 1982-3 El Niño event, agricultural production dropped by 14%. By 1987 forecasts of the onset of El Niño events had become sufficiently good for Peruvian farmers to take them into account in their planning. In 1987, following the 1986-7 El Niño, production actually increased by 3%, thanks to a useful forecast.

Four factors are particularly important in considering the effect of climate change on agriculture and food production. The availability of water is the most important of the factors. The vulnerability of water supplies to climate change carries over into a vulnerability in the growing of crops and the production of food. Thus the arid or semi-arid areas, mostly in developing countries, are most at risk. A second factor, which tends to lead to increased production as a result of climate change, is the boost to growth that is given, particularly to some crops, by increased atmospheric carbon dioxide (see box below). A third factor is the effect of temperature changes; as temperatures rise, yields of some crops are substantially reduced.39 A fourth factor is the influence of climate extremes, heatwaves, floods and droughts that seriously interfere with food production.

Detailed studies have been carried out of the sensitivity to climate change during the twenty-first century of the major crops which make up a large proportion of the world's food supply (see box below). They have used the results of climate models to estimate changes in temperature and precipitation. Many of them include the effect of carbon dioxide fertilisation and some also model

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