Roughly 40% of the world population lives in the tropics, and agriculture is a very important sector for the economies of most countries in the tropics. For example, in tropical Asia, more than half of the labour force is employed in agriculture, accounting for 10-63% of the GDP in most countries of the region (IPCC, 1998). Given the current scenarios of enhanced temperatures and increased frequency of extreme events, climate change is likely to have significant impact in the tropics.

Sivakumar et al. (2005) pointed out that the arid and semiarid regions account for approximately 30% of the world total area and are inhabited by 1.1 billion people or approximately 20% of the total world population. The arid and semiarid 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). According to Zhao et al. (2005), humid and subhumid tropical conditions are found over nearly 50% of the tropical land mass and 20% of the earth's total land surface. Tropical Central and South America contain about 45% of the world's humid and subhumid tropics; Africa, about 30% and Asia, about 25%.

Sivakumar et al. (2005) described the agricultural climate of the arid and semiarid tropical regions in Asia, Africa and Latin America, which is characterized by low and variable rainfall and consistently high temperatures during the growing season. Climate variability—both inter- and intra-annual—is a fact of life in these regions with a traditionally low agricultural productivity. The projected climate change and the attendant impacts on water resources and agriculture in the arid and semiarid tropical regions add additional layers of risk and uncertainty to agricultural systems that are already affected by land degradation due to growing population pressures.

It is interesting to note the observations of Sivakumar et al. (2005) that in certain agroecological zones such as the southern Sahelian zone of West Africa, where the predominant soils are sandy in nature, increased mean temperature could affect the maximum temperatures at the soil surface substantially. They pointed out that surface soil temperatures could exceed even 60°C and that under such temperatures, enzyme degradation will limit photosynthesis and growth. Increased temperatures will also result in increased rates of potential evapotranspiration. In the long term, the very establishment and survival of species in both the managed and unmanaged ecosystems in this region may be threatened, resulting in a change in the community structure.

The prospect of global climate change has serious implications for water resources and regional development (Riebsame et al., 1995). Projected temperature increases are likely to lead to increased open water and soil/plant evaporation. As a rough estimate, potential evapotranspiration over Africa is projected to increase by 5-10% by 2050. Since Africa is the continent with the lowest conversion factor of precipitation to runoff (averaging 15%), and precipitation in some areas may decrease, the dominant impact of global warming is predicted to be a reduction in soil moisture in subhumid zones and a reduction in runoff.

The general conclusion is that climate change will affect some parts of Africa negatively, although it will enhance prospects for crop production in other areas (see Downing, 1992, for case studies of agriculture in Kenya, Zimbabwe, and Senegal). Expansion of agriculture is important in the east African highlands. For example, agroecological suitability in the highlands of Kenya would increase by perhaps 20% with warming of 2.5 °C based on an index of potential food production (Downing, 1992). In contrast, semiarid areas are likely to be worse off. In eastern Kenya, 2.5 °C of warming results in a 20% decrease in calorie production.

The water and agriculture sectors are likely to be most sensitive and hence vulnerable to climate change-induced impacts in arid and semi-arid tropical Asia. Croplands in many of the countries in the region are irrigated because rainfall is low and highly variable (IPCC, 2001b). The agriculture sector here is potentially highly vulnerable to climate change because of degradation of the limited arable land. The predicted increase in frequency and/or severity of extreme events coupled with any increase in intensity of tropical cyclones could further exacerbate adverse impacts of climate change on the agricultural sector.

Sivakumar et al. (2005) referred to several studies aimed at understanding the nature and magnitude of gains/losses in yield of particular crops at selected sites in Asiaunder elevated CO2 conditions (e.g., Luo and Lin, 1999). These studies suggest that, in general, areas in mid- and high- latitudes will experience increases in crop yield, whereas yields in areas in the lower latitudes will decrease. Generally climatic variability and change will seriously endanger sustained agricultural production in tropical Asia in coming decades. The scheduling of the cropping season as well as the duration of the growing period of the crops would also be affected. Studies conducted in India, Indonesia and the Philippines confirmed that spikelet sterility and reduced yields negate any increase in dry-matter production as a result of CO2 fertilization. Amien et al. (1996) found that rice yields in east Java could decline by 1% annually as a result of increases in temperature. Sivakumar et al. (2005) referred to studies in Asia that showed adverse effects on sorghum in rainfed areas of India, for corn yields in the Philippines and on the tea industry of Sri Lanka.

Agricultural production in lower-latitude and lower-income countries is more likely to be negatively affected by climate change (IPCC, 1998; 2001b). It is seen that climate variability and climate change, particularly in terms of frequency/intensity of droughts, have larger impacts on the subhumid than on the humid regions. If climate variability-induced disasters become more common, widespread, and persistent, many countries in the humid and subhumid tropical regions will have difficulty in sustaining viable agricultural and forest practices. A good number of researchers have all concluded that climate change would affect agriculture as a result of increased temperatures, changes in rainfall patterns and increased frequency of extreme events, which could cause changes in pest ecology, ecological disruption in agricultural areas and socioeconomic shifts in land-use practices.

Extremes in climate variability already severely affect agriculture in Latin America (IPCC, 2001b). The largest area with marked vulnerability to climate variability in Latin America is northeast Brazil. Sivakumar et al. (2005) pointed out that periodic occurrences of severe El Nino-associated droughts in northeastern Brazil have resulted in occasional famines. Under doubled-CO2 scenarios, yields are projected to fall by 17 to 53%, depending on whether direct effects of CO2 are considered. Lemos et al. (2002) show the difficulties of absorption of seasonal climate forecasts in this region.

In Africa, most mid-elevation ranges, plateaus, and high-mountain slopes are under considerable pressure from commercial and subsistence farming activities. Mountain environments are potentially vulnerable to the impacts of global warming. This vulnerability has important ramifications for a wide variety of human uses— such as nature conservation, mountain streams, water management, agriculture, and tourism (IPCC 1998).

Zhao et al. (2005) referred to studies on the survival rate of pathogens in winter or summer which could vary with an increase in surface temperature. Higher temperatures in winter will not only result in higher pathogen survival rates but also lead to extension of cropping area, which could provide more host plants for pathogens. Thus, the overall impact of climate change is likely to be an enlargement of the source, population, and size of pathogenic bacteria. Damage from diseases may be more serious because heat-stress conditions will weaken the disease-resistance of host plants and provide pathogenic bacteria with more favourable growth conditions.

Sivakumar et al. (2005) and Zhao et al. (2005) pointed to the environmental and social stress caused by climate change in many of Asia's rangelands and drylands. Precipitation is scarce and has a high annual variance in dryland areas of the tropics. Very high daily temperature variance is recorded with frequent sand storms and intense sunshine. The combination of climatic variability and human land use make rangeland ecosystems more susceptible to rapid degeneration of ecosystem properties. For example, because of an alteration in the amount and pattern of rainfall, the occurrence of extreme events (e.g., hurricanes, drought), and the ENSO which could become more frequent and bring more severe weather under the 2 x CO2-climate, the northern South America Savannas could fail to function as they do now (Aceituno, 1988).

Climate affects livestock in four ways: through (i) the impact of changes on availability and price of feedgrain, (ii) impacts on livestock pastures and forage crops, (iii) the direct effects of weather and extreme events on animal health, growth, and reproduction, and (iv) changes in the distribution of livestock diseases.

It was pointed out that almost two-thirds of domestic livestock are supported on rangelands, although in some countries a significant share of animal fodder also comes from crop residue. The combination of elevated temperature and decreased precipitation in arid and semiarid rangelands could cause a manifold increase in potential evapotranspiration, leading to severe water stress conditions. Many desert organisms are near their limits of temperature tolerance. Because of the current marginality of soil water and nutrient reserves, some ecosystems in semiarid regions may be among the first to show the effects of climate change. Climate change has the potential to exacerbate the loss of biodiversity in this region.

Zhao et al. (2005) mentioned that for developing countries, the impact of climate variability on livestock is generally negative in the humid and subhumid tropics, particularly in the latter. For animals, heat stress has a variety of detrimental effects with significant effects on milk production and reproduction in dairy cows, and swine fertility. Moreover, warming in the tropics during warm months would likely affect livestock reproduction and production negatively (e.g., reduced animal weight gain, dairy production, and feed conversion efficiency). Impacts however may be minor for relatively intense livestock production systems.

Livestock in humid areas in Africa are prone to disease such as those carried by the tsetse fly. With warming, its distribution could extend westward in Angola and northeast in Tanzania but with reductions in the prevalence of tsetse in some current areas of distribution.

Because of the increasing trend for meat consumption, there is a higher demand for livestock feed. Production of feed grain in Asia, especially maize, is adversely affected by climate variability and climate change.

Tropical forests represent about 40% of the world's forested area and contain about 60% of global forest biomass. As many as 16 countries of tropical Asia are situated within the humid tropical forest region. Climate change is expected to affect the boundaries of forest types and areas, primary productivity, species population and migration, the outbreak/incidence of pests and diseases and forest degeneration in these countries.

Zhao et al. (2005) referred to the results of research from Thailand which suggest that climate change would have a profound effect on the future distribution, productivity, and health of Thailand's forests. It was estimated that the area of subtropical forest could decline from the current 50% to either 20% or 12% of Thailand's total forest cover (depending on the model used), whereas the area of tropical forests could increase from 45% to 80% of total forest cover. Estimates from Sri Lanka showed a decrease in tropical rainforest of 2-11% and an increase in tropical dry forest of 7-8%. A northward shift of tropical wet forests into areas currently occupied by tropical dry forests also is projected. In semiarid regions of Tropical Asia, tropical forests generally are sensitive to changes in temperature and rainfall, as well as changes in their seasonality.

Most tropical forests are likely to be more affected by changes in soil water availability (e.g., seasonal droughts). Some evergreen species of the humid forest clearly will be at a disadvantage in those areas that experience more severe and prolonged droughts. Significantly, drought affects the survival of individual species; those without morphological or physiological adaptations to drought often die. Species in moist tropical forests, including economically important hardwoods, are the least drought-adapted in the tropics, and their survival in some areas must be considered at risk from climate change. Droughts would favor forest fire; therefore, with a likely increase of droughts, the incidence of forest fires may also increase.

More than 50% of the world's terrestrial plant and animal species are in the frontier forests in Asia. There already are trends of increasing risks to this rich array of living species being seen in China, India, Malaysia, Myanmar and Thailand, partly due to the degeneration of their habitat (IPCC, 2001b). Since distribution of species are limited to a narrow range of environmental conditions, there are possibilities that climate change could alter these conditions which could make them unsuitable. This could cause the loss of a large number of unique species that currently inhabit the world's tropical forests.

There is high confidence that if the extent of deforestation in Amazonia expands to substantially larger areas, reduced evapotranspiration would lead to less rainfall during its dry periods. If this dry period becomes larger and more severe, it could have deleterious impacts on the forest. Many trees could die due to increased water stress. Greater severity of droughts coupled with deforestation could lead to erosion in what remains of the forests in this region. Moreover, occasional severe droughts likely to occur during the EI Ninos would kill many trees of susceptible species and would result to a replacement of tropical moist forests with drought-tolerant species (Shukla et al. 1990).

There are other features of agricultural vulnerability to climate change which are also likely to vary across people, regions, continents and countries. One of these is vulnerability to food security because there would then be rapid changes in supply and demand structures most especially in the developing countries, especially in the tropics. For example, food-importing countries like those in Africa are at risk of adverse impacts of climate change, especially because these impacts are intricately linked with changes in world markets as with changes in local and regional biophysical systems. The already deficient food production in many areas of Africa could also this way result in worsening problems of food security.


Maracchi et al. (2005) and Motha and Baier (2005) presented overviews of impacts for Europe and North America, respectively. Increased climate variability has resulted in greater fluctuations in crop yields during recent decades. Extreme weather events such as drought, flooding, and heat waves have had severe impacts on agriculture and forestry, as have changes in drought tendencies, soil moisture availability and frost-free growing seasons. Agriculture has also played a role in greenhouse gas emissions. The clearing of forests, the draining of wetlands, and the ploughing of rangelands have led to a significant increase in atmospheric CO2, as organic carbon was decomposed. Nitrous oxide (N2O) originates as a by-product of nitrogen fertilizer application and in water-logged soils. Thus, in higher latitudes, a spring burst of N2O emissions occurs with rapid snowmelt. Heavy rains in low-lying areas also cause a N2O burst of emissions. Methane (CH4) emitted from agriculture is produced by the microbial breakdown of plant material and in the digestive system of cattle.

There are measures to mitigate agriculture's role in greenhouse gas emissions. Atmospheric CO2 can be returned to the land by afforestation, conservation tillage by a cover crop, and no-till conservation practices. Soil microorganisms can remove both N2O and CH4 with improved pasture conditions and cover crops on cultivated land to lower the amount of inorganic nitrogen in the soil. Higher quality cattle feeds can reduce CH4 emissions from domestic livestock.

The combined effect of climate change and enhanced CO2 on crop production varies. Yields of C3 crops (vegetables, wheat, grapes) generally increase. Yields of C4 crops (corn, sugarcane, tropical grasses) generally decrease. However, annual variability of crop yields increase.

Distinct regional patterns by latitude were discernible in future climate scenarios for Europe and North America (Maracchi et al., 2005; Motha and Baier, 2005). Temperatures are expected to increase in nearly all areas but the largest temperature increases are projected over southern portions of both the United States and Europe. Consequently, the extreme cold of winter is expected to diminish but a greater likelihood of heat waves is projected in summer. An increase in the frequency and intensity of heavy precipitation is expected, even in southern Europe and the southern United States, despite projections of total precipitation to decrease.

Northern crop areas of both Europe and the United States will have a longer growing season and an expansion of suitable area for crop production. With higher crop production, however, the increased risk of nutrient leaching and an accelerated breakdown of soil organic matter may affect the quality of northern agricultural lands. Lower crop yields are expected in southern crop areas due to the warmer and drier summers.

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