11.3 Global change issues
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Loss of biodiversity
The projections indicate two particular pressures that warrant attention, namely deforestation and the loss of wetlands. Forests, biodiversity and climate change (discussed below) are closely interlinked because forests have a dual role as habitats and as major carbon sinks. The many levels of forest canopy, especially those in the tropics, with their varying light intensities and moisture levels, allow a multitude of habitats to co-exist in a small area rich in biodiversity. The closed tropical forests, for example, cover only 7 percent of the earth's surface, yet contain at least 50 percent and possibly 90 percent of the world's species. Many of these species have not been described or assessed for their utility as foods, medicines or other purposes. It is impossible to say which of them may be redundant for long-term sustainable development, though it is also difficult to argue that life as we know it could not go on without some of these unassessed resources.
Although the area of tropical forest that may be taken up by agriculture during the period to 2010 would be only a relatively small percentage of the remaining global stock, there are grounds for attempting to minimize such losses. A small proportion of the developing countries' wetlands will also probably be drained and used for crop production, since they represent one of their few remaining resources suitable for permanent agriculture. The wetlands are both a source of biodiversity and an ecosystem providing environmental services such as the purification of water, flood mitigation, fisheries and wildlife habitat and amenity. Their role needs to be protected, but as noted the areas likely to be converted to agriculture will probably be a small share of the total, and will provide a vital contribution to staple food security.
Climate change: potential global and regional impacts
Agricultural activities are a major contributor to anthropogenic sources of greenhouse gases, which in turn contribute to radiative forcing and hence climate change. In turn, climate change will have an impact on agriculture. Apart from the release of carbon dioxide caused by biomass burning, mainly through deforestation and savanna fires, which together account for about 30 percent of the total amount of the CO2 emitted, agriculture's main contribution to radiative forcing is through the emission of methane (CH4, about 70 percent of the total emitted) and nitrous oxide (N2O, about 90 percent of the total).
Rice cultivation appears to be the largest anthropogenic source of global methane emissions, influenced by a complex set of factors that primarily affect methane-generating and methane-absorbing bacteria. These interactions are very sensitive to the physical, chemical and biological conditions of the rice paddy environment and hence can be manipulated by management practices. For example, methane emissions to the atmosphere are higher in deep water than in shallow water rice cultivation.
The harvested area for rice in the developing countries, excluding China, is expected to increase by about 11 million ha by 2010, an increase of about 10 percent. Hence the projected increase of methane emissions from this source is relatively modest also because some of the increase in rice area will. arise from the conversion of wetlands which produce some methane in their natural state. The modest incremental contribution from this source will be further depressed by the expected move away from deep water rice cultivation, mainly because of increasing competition from aquaculture.
A greater increase in methane emissions to the atmosphere than from rice cultivation may arise from ruminant livestock, the numbers of which are projected to increase by more than 35 percent by 2010 in the developing countries. Methane is released as a result of incomplete decomposition of plant material by aerobic fermentation, a process favoured by the fibrous and forage feeds of varying quality widely used for ruminants in developing countries. Thus, more methane is released from poorly fed than from better fed ruminants. The overall effect of the projections on radiative forcing is difficult to assess, being the combined effect of the numbers of ruminant livestock and the nutritional composition of their feeds, which is expected to improve moderately under the pressure of the rising demand for livestock products.
The annual rise in atmospheric nitrous oxide is relatively small and precise measurements of sources of this gas are scarce. Emissions are estimated to be mainly from biotic sources, related to nitrification and denitrification processes. These take place in the sorts of natural ecosystems such as tropical rainforests, tropical/subtropical savannas and intensive fertilizer agroecosystems. Applications of nitrogenous fertilizers are estimated to account for about 20 percent of the current annual increase in atmospheric N2O. Such applications are expected to rise little in industrialized countries, but significantly in the developing countries, particularly on the better quality arable land. However, typically these lands do not have the seasonally hydromorphic soil conditions that specially favour N2O emissions.
There is still a great deal of uncertainty regarding the nature, regional distribution and distribution over time of the potential impacts of climate change on agriculture. In spite of the extensive research stimulated by the Intergovernmental Panel on Climate Change (IPCC) and by national concerns, it will probably take many more years before these uncertainties are resolved. Nonetheless, there is a scientific consensus that global warming is a real phenomenon that could have a range of both negative and positive impacts on agriculture (FAO, 1990d). Some of the most severe negative impacts could be in those regions already vulnerable to present-day climate variation, notably sub-Saharan Africa, and with the least ability to research or buy their way out of trouble. In broad terms, the developing countries may be more affected by the possible changes than the developed countries, even though the temperature rise from global warming is projected to be greatest at the higher latitudes where most of the developed countries are located. The potentially greater impact on developing countries is, on the physical side, because most of them are in low rainfall areas or have significant areas of arid land, which are already subject to major agricultural production problems because of rainfall variability and associated constraints, and on the economic side because of their greater dependence on agriculture (see Chapter 3).
All of these potential climate impacts probably lie outside the time frame of this study, since they are not currently envisaged to play a notable role before about 2030. There is, however, one positive impact which may already be infiuencing agricultural production, namely the boost to crop growth arising from the enhanced carbon dioxide levels in the atmosphere that are one of the forcing factors for global warming and climate change. The higher CO2 levels lead to faster growth of plant biomass and better water utilization in many crops, contributing to stronger root growth and denser ground cover. Some scientists believe that this effect accounts for a proportion of crop yield gains in recent decades, possibly as much as 10 to 25 percent of the cumulative increase. Moreover, even if the developed countries succeed in stabilizing CO2 emissions in the early part of the twenty-first century, it is unrealistic to expect the developing countries to do so before the end of the century. Hence this CO2 fertilization will have an important impact throughout the current projection period and beyond.
Finally, although the main negative impacts of climate change are likely to be beyond 2010, there are agricultural measures which can be justified against current socioeconomic needs and yet would help to minimize the potential impacts of climate change. These are considered in Chapters 12 and 13.
Two aspects of the discussion in this chapter are worthy of emphasis. First, as discussed in Chapter 4, the future rate of growth of land expansion for agriculture will likely be lower than in the past. The pressures on water, however, will grow considerably as will those on the environment arising from the intensification of land use. Secondly, the analysis has focused primarily on pressures on natural resources in the aggregate, which does not identify clearly the distribution of such pressures among countries arising through international trade. Trade contributes to transfer pressures on resources from importing countries to exporting ones, as for example happened with the production of cassava in Thailand for export to Europe. Such effects may be significant and are partly addressed in Chapter 8 in the context of the discussion of trade issues.
1. FAO's suggested definition is "The sum of geological, climatic, biological and human factors which lead to the degradation of the physical, chemical and biological potential of lands in arid and semi-arid areas, and endanger biodiversity and the survival of human communities." (FAO, 1993g).
2. The role of forests and forestry in relation to the carbon cycle is discussed in Chapter 5.
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