The results of a large number of experiments designed to examine the effects of elevated CO2 concentrations on crops have generally confirmed high confidence in a net beneficial effect of CO2 fertilization, up to some level. Sustained plant response under field conditions to concentrations beyond 2xCO2 would likely be dependent on species as well as water and nutrient status and is highly uncertain.
A mean value yield response of C3 crops (most crops except maize, sugar cane, millet, and sorghum) to doubled CO2 is reported to be approximately +30% (range -10% to +80%). There is reason to expect, however, that this value represents an upper estimate unlikely to be achieved under field conditions. Factors known to affect the magnitude of CO2 response in crops include the availability of plant nutrients, the crop species, temperature, precipitation, and other environmental factors, such as air pollution, soil quality, weeds, insect pests, and diseases (IPCC 1996, WG II, Section 13.2.1). Increased WUE is a result of elevated CO2 as well, though in many regions of North America, higher temperatures associated with elevated CO2 can be expected to increase evaporative demand and transpiration, resulting in minimal benefit from the increase in WUE (Brklacich et al., 1997b).
Changes in soils (e.g., loss of soil organic matter, leaching of soil nutrients, salinization, and erosion) are likely consequences of climate change for some soils in some climatic zones. Cropping practices such as crop rotation, conservation tillage, and improved nutrient management are technically effective in combating or reversing such deleterious effects (IPCC 1996, WG II, Section 13.3; Matson et al., 1997).
Livestock production could be affected by changes in grain prices, changes in the prevalence and distribution of livestock pests, and changes in grazing and pasture productivity. Livestock are sensitive to stress from warmer, drier conditions, as well as reduced range forage quality and water availability. Warmer winter temperatures may enhance winter survival of range livestock. Taking action to improve forage quality or water supply could benefit livestock. Analyses indicate that intensively managed livestock systems such as those in North America have more potential for adaptation than crop systems because of their mobility in terms of access to food and water (IPCC 1996, WG II, Section 13.5).
The risk of losses due to weeds, insects, and diseases is sensitive to temperature and moisture (including rainfall, humidity, and dew); the risk is likely to increase in subregions where these factors become more favorable for specific disease organisms but may decrease under drier conditions. Increased climate variability may provide additional challenges for pest-management adaptation to climate change.
Elevated CO2 levels may enhance the growth of C3 weeds, based on the results of controlled exposure experiments. Evidence also exists, however, that other factors determining plant productivity may be more important in controlling plant response in the field (e.g., water- and nutrient-use efficiency) than the differential CO2 response (Bazzaz and McConnaughay, 1992). There currently is little experimental evidence to directly evaluate the effects of elevated CO2 on weed infestation, insect pests, or plant diseases under field conditions. Less severe winters may increase the range and severity of insect and disease infestations. Temperature and moisture are critical to the spread and development of many plant diseases (IPCC 1996, WG II, Section 13.4.3). Successful disease development requires convergence of a susceptible host, a virulent pathogen, and suitable environmental conditions. Increased variability of precipitation, for example, could affect the host-parasite interaction positively or negatively, leading to more or less disease development (Shriner, 1980). Increased climate variability also could render less effective disease-forecasting models currently used to manage some diseases and require increased reliance on pesticides. North American agriculture will need to address these concerns in the context of increasing pressure on agriculture to reduce chemical inputs.
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