188.8.131.52. Direct and Indirect Effects
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.