5.3.2.1. Degradation of Natural Resources

Degradation of natural resources—taken here as soils, forests, marine fisheries, air, and water—diminishes agricultural production capacity (Pinstrup-Andersen and Pandya-Lorch, 1998). Soil degradation emerges as one of the major challenges for global agriculture. It is induced via erosion, chemical depletion, water saturation, and solute accumulation. In the post-World War II period, approximately 23% of the world's agricultural land, permanent pastures, forests, and woodland were degraded as defined by the United Nations Environment Programme (UNEP) (Oldeman et al., 1991). Various estimates put the annual loss of land at 5-10 Mha yr^-1 (Scherr and Yadav, 1997). Although irrigated land accounts for only 16% of the world's cropland, it produces 40% of the world's food. There are signs of a slowing in the rate of expansion of irrigation: 10-15% of irrigated land is degraded to some extent by waterlogging and salinization (Alexandratos, 1995). Degradation of natural resources is likely to hinder increases in agricultural productivity and could dim optimistic assessments of the prospects of satisfying growing world food demand at acceptable environmental cost.

5.3.2.2. Other Global Change Factors

Regional scenarios of seasonal temperature and precipitation change for 32 world regions analyzed in Chapter 3 show the current variability of climate and the range of changes predicted by GCMs for 30-year time periods centered on 2025, 2055, and 2085. This background information is essential to interpret the potential impacts of climate change on crops and livestock production. Equally important background information is provided by agroclimatic indices. Agroclimatic indices are useful in conveying climate variability and change in terms that are meaningful to agriculture. They give a first approximation of the potential effects of climate change on agricultural production and should continue to be used (Sirotenko et al., 1995; Sirotenko and Abashina, 1998; Menzhulin, 1998).

Several other climate-related global environmental changes are likely to affect the agriculture sector in coming years. Reilly et al. (1996) reviewed the exposure of crops to tropospheric ozone (O₃). Progress in sorting out interactions between O₃, CO₂, and climate variability is reviewed below.

Climate change is likely to interact with other global changes, including population growth and migration, economic growth, urbanization, and changes in land use and resource degradation. Döös and Shaw (1999) use an accounting system to estimate the sensitivity of agricultural production to various aspects of global change, including loss of cropland from soil degradation and urbanization. Imhoff et al. (1997) use remote-sensing techniques and soils data to show that urbanization in the United States has occurred primarily on high-quality agricultural lands.

5.3.3.1. Interaction between Rising CO₂ Concentrations and Climate Change

Advances in knowledge of CO₂ effects on crop and forage plants establish convincingly, although incompletely, that it is no longer useful to examine the impacts of climate change absent their interactions with rising atmospheric CO₂ (see Boxes 5-3 and 5-4). Crop and forage plants are likely to be forced to deal with the combined effects of climate change and rising atmospheric CO₂ concentrations. In this section, emphasis is placed on understanding basic interactions between plant productivity, climate change, and rising CO₂ concentrations. The direct effects of climate change on livestock also are considered.

5.3.3.1.1. Interactive effects of temperature increase and atmospheric CO₂ concentration

Because temperature increase enhances photorespiration in C₃ species (Long, 1991), the positive effects of CO₂ enrichment on photosynthetic productivity usually are greater when temperature rises (Bowes et al., 1996; Casella et al., 1996). A rise in mean global nighttime temperatures (Horton, 1995) could enhance carbon losses from crops by stimulating shoot dark respiration (Amthor, 1997). Despite possible short-term effects of elevated CO₂ on dark respiration (Amthor, 1997; Drake et al., 1997), the long-term ratio of shoot dark respiration to photosynthesis is approximately constant with respect to air temperature and CO₂ concentration (Gifford, 1995; Casella and Soussana, 1997). With moderate temperatures, long-term doubling of current ambient CO₂ under field-like conditions leads to a 30% enhancement in the seed yield of rice, despite a 5-10% decline in the number of days to heading (Horie et al., 2000). The grain yield of CO₂-enriched rice shows about a 10% decline for each 1°C rise above 26°C. This decline is caused by a shortening of growth duration and increased spikelet sterility. Similar scenarios have been reported for soybean and wheat (Mitchell et al., 1993; Bowes et al., 1996). With rice, the effects of elevated CO₂ on yield may even become negative at extremely high temperatures (above 36.5 °C) during flowering (Horie et al., 2000). However, in some cropping systems with growth in the cooler months, increased rates of phenological development with warm temperatures and/or earlier planting dates may tend to move the grain fill period earlier into the year during the cooler months, offsetting at least part of the deleterious effects of higher temperatures (Howden et al., 1999a).

IPCC Homepage