The tropics is a geographic region, defined by 23.5 degrees north and 23.5 degrees south latitude (Agnew, 1998). The tropics are home to some of the world's poorest populations. Although considerable variability exists within the tropics, their climates are generally distinguished by the persistence of high year-round temperatures and the convective and highly seasonal nature of rainfall. And, although precipitation is one of the more difficult variables to simulate accurately in climate models, rainfall is the principal biophysical determinant of tropical vegetation and crop production (Agnew, 1998).
The variability of precipitation in the tropics exposes countries of this region to the full range of extreme events — floods, droughts, cyclones, hurricanes, hail storms, and, in the areas of higher elevation, frost (Gommes, 1993). Among other driving forces, the influence of the El Nino-Southern Oscillation (ENSO) tends to be stronger in the tropics (Cane, 2001) and in recent years it has contributed to the high frequency of disasters that have exacted such a high toll on developing economies (Dilley and Heyman, 1995).
These characteristics pose particular challenges to agricultural production. As described by Rosenzweig and Liver-man (1992), agricultural productivity in the humid tropics is limited by fragile soils with low organic matter content and serious problems with soil leaching and erosion. In the subhumid and semi-arid tropical regions, yields are often affected by problems of soil water retention, limited irrigation potential due to poor geographic distribution of water resources and infrastructure costs and soil salinity. The year-round warm conditions that generate the rich biodiversity for which the tropics are well known also facilitate the proliferation of crop diseases, pests, and weeds that continually threaten agricultural yields (Rosenzweig and Liverman, 1992).
Although, as was already mentioned, the tropics exhibit a diversity of climates and regional characteristics, research has shown that these limitations are more likely to be exacerbated rather than mitigated under climate change. Small changes in mean climatic conditions are likely to mean disproportionately larger changes in climatic variability, and consequently an increase in the frequency of extreme events (Burton, 1997; IPCC, 2001; Parry and Carter, 1998). For example, should the ENSO change in temporal duration and frequency — causing several subsequent years of poor production conditions — the impact on food security and economic stability of highly sensitive regions, such as northeastern Brazil and southern Africa, would be severe.
Even changes in mean conditions are unlikely to be helpful. Production in the tropics already occurs near the higher temperature limits of many crops. Although in some areas precipitation is expected to increase, simultaneous increases in temperatures also mean increased evapotranspiration and thus perhaps a decrease in moisture available for plant growth. The prevalence of C4 crops in the tropics, coupled with nitrogen limitations, indicates that yields are unlikely to improve as much for C3 crops in temperate zones from increases in atmospheric carbon levels (IPCC, 2001).
In semi-arid regions, capturing any increase in rainfall for human use is also difficult, particularly if it arrives in concentrated storms as many models anticipate. In these cases, increases in temperature will do little to improve yields and, depending on the timing and frequency of heat spells during the crop growth cycle, may be quite damaging to annual tropical crops (IPCC, 2001; Rosenzweig and Liverman, 1992).
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