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Box 5-4. Elevated CO2 Impacts on Crop The short-term responses to elevated CO2 of plants grown in artificial conditions are notoriously difficult to extrapolate to crops in the field (Körner, 1995a). Moreover, with field-grown plants, enclosures tend to modify the plant's environment (Kimball et al., 1997). However, even the most realistic free-air CO2 enrichment (FACE) experiments undertaken to date create a modified area (Kimball et al., 1993), analogous to a single irrigated field in a dry environment, and impose an abrupt change in CO2 concentration. A cotton crop exposed to FACE increased biomass and harvestable yield by 37 and 48%, respectively, in elevated (550 ppm) CO2. This effect was attributed to increased early leaf area, more profuse flowering, and a longer period of fruit retention (Mauney et al., 1994). At 550 ppm CO2, spring wheat increased grain yields by 8-10% under well-watered conditions (Pinter et al., 1996). More recent studies with optimal nitrogen and irrigation increased final grain yield by 15 and 16% for two growing seasons at elevated CO2 concentration (550 ppm), compared with control treatments (Pinter et al., 1996). If these latter results are linearly extrapolated to the possible effect of a doubling (700 ppm) of the current atmospheric CO2 concentration, yields under ideal conditions would be 28% greater—in agreement with previous statements by Reilly et al. (1996). In grass-clover mixtures, the proportion of legume increased significantly under elevated CO2 (Hebeisen et al.,1997)—a conclusion also reached by several experimental studies with temperate and fertile managed grasslands (Newton et al., 1996; Soussana and Hartwig, 1996; Stewart and Potvin, 1996). |
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.
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.
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.
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).
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