The World Atlas of Desertification (UNEP, 1992) defines arid regions as the areas where the ratio of mean annual rainfall (R) to mean annual potential evapotranspiration (PET) varies between 0.05 and 0.20 and the semi-arid regions as those where the ratio ranges between 0.2 and 0.5. In an assessment of population levels in the world's drylands, the Office to Combat Desertification and Drought (UNSO) of the United Nations Development Programme (UNDP) showed that the arid and semi-arid regions account for approximately 30% of the world total area and are inhabited by 1.10 billion people or approximately 20% of the total world population. The arid and semi-arid regions are home to about 24% of the total population in Africa, 17% in the Americas and the Caribbean, 23% in Asia, 6% in Australia and Oceania, and 11% in Europe (UNSO, 1997).

Climate variability has been, and continues to be, the principal source of fluctuations in global food production in the arid and semi-arid tropical countries of the developing world. Throughout history, extremes of heat and cold, droughts and floods, and various forms of violent weather have wreaked havoc on the agricultural systems in these regions. In conjunction with other physical, social and political-economic factors, climate variability and change contribute to vulnerability to economic loss, hunger, famine and dislocation. Hence, it is imperative that these aspects are well understood in order to formulate more sustainable policies and strategies to promote food production in the arid and semi-arid tropics.

Increasing greenhouse gas accumulation in the global atmosphere and increasing regional concentrations of aerosol particulates are now understood to have detectable effects on the global climate system (Santer et al., 1996) and global average temperatures and sea level are projected to rise under all the scenarios from the Special Report on Emission Scenarios (IPCC, 2000). According to IPCC (2001a), the global average surface temperature increased over the 20th century by about 0.6 °C and temperatures have risen in the past four decades in the lowest eight kilometers of the atmosphere. New analyses of data for the northern hemisphere indicate that the increase in temperature in the 20th century is likely to have been the largest during any century since 1000 AD.

In this paper, we examine the issues of present and future climate variability and change on agriculture and forestry in the arid and semi-arid tropics of the world. For the sake of convenience, the discussion under each of these issues had been presented separately for Asia, Africa and Latin America. In view of the relatively smaller proportion of area, no attempt has been made to cover the arid and semi-arid tropics in the developed world.

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