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Influencing Factors
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Carbon Dioxide, Temperature, and Crop YieldsYields Increasing atmospheric CO2 levels are expected to influence crop production in many different ways. The response to an initial increase in temperature by itself in isolation should generally be positive for crop yields. (The magnitude of the response varies from crop to crop and can change from positive to negative if the temperature change is too great.)
In terms of plant growth and development, higher rates of photosynthesis are found in entire canopies placed in a CO2-enriched atmosphere-due to the CO2 "fertilization" effect. In general "C3" crops (such as wheat, rice, and soybeans) respond more to CO2 enrichment than "C4" crops (such as maize, sorghum, sugarcane, and millet). Other things being equal (and typically they are not), this will be a net positive for plant growth and agricultural production. Important uncertainties include: (1) What happens to the rate of photosynthesis over the longer term? The initial increase is often found to diminish or disappear under long-term exposure. (2) How significant is the potential for "C4" crops to become more vulnerable to increased competition from "C3" weeds?
Another important positive factor is an overall increase in plant water-use efficiency. This increase generally occurs as increased CO2 leads to the partial closure of stomates, the small openings in leaf surfaces through which CO2 is absorbed and water vapor is released. Thus, transpiration (water evaporation from foliage) may be reduced even while the rate of photosynthesis rises.
Long-term studies are needed to better define the roles of short-term acclimation and long-term adaptation of crop processes to higher CO2 levels. Research topics include the following areas:
- Response to different genotypes of the same crop
- Below ground processes (root systems and the soil)
- Litter characteristics and composition
- Carbon and nutrient allocation within the plant
- Plant response to high-temperature stress
- Influence of lengthened growing season on plant development
- Differential effects due to changes is max/min temperatures
- Response to changing hydrological regimes & climate variability
Effects on Weeds, Insects, and DiseaseRising atmospheric CO2 and climate change will also affect the associated agricultural pests. Distribution and proliferation of weeds, fungi, and insects are determined to a large extent by climate. Much more research has been done on the potential changes in weed growth than on changes in the spread of insects and diseases.
Weeds will be directly affected by changes in climate and in CO2 levels. Insects and diseases are not likely to be directly affected by CO2 changes, but may be affected indirectly because of altered host plant metabolism, development and morphology. New, previously unobserved combinations of climate, atmospheric constituents, and soil conditions may result and lead to new infestations of various pests. The overall importance of such developments is unclear at this point, but crop losses due to weeds, insects, and disease are likely to increase.
Effects on Soil ResourcesSoil is a complex and dynamic system, consisting of a solid phase (both mineral and organic, particulate and amorphous), a liquid phase (water and solutes), and a gaseous phase (air with associated water vapor, often enriched with carbon dioxide and sometimes with methane as well). Soil responds to both short-term events such as the episodic infiltration of rainfall and long-term processes, such as physical and chemical weathering.
Only rough, qualitative estimations of the predicted climate change effects on soil are practical now, due to the uncertainties in the forecasts but also to the complex, interactive influences of hydrological regime, vegetation, and land use. Factors that need to be considered include:
- Temperature Effects on Soil Nutrients
- Hydrological effects on soil nutrients
- Soil carbon accretion/depletion
Effects on Water ResourcesPotential changes in soil moisture may be as important for agriculture as the projected temperature changes. Since climate change is likely to alter the hydrological regimes of entire regions, it should be factored into water-resource planning and polices for the future-at least on a contingency basis. Parameters that will affect evaporative demand of crops include:
As global climate change will be manifested in these parameters, changes in the water regimes of crops will ensue. In the global hydrological cycle, water evaporated must be precipitated; hence, more evaporation implies more rainfall overall. However, increases in potential evapotranspiration and in rainfall may not be commensurate or concurrent in many locations.
Climate models suggest that potential evapotranspiration tends to rise most where the temperature is already high (i.e., low to mid latitudes), while precipitation tends to increase most where the air is cooler and more readily saturated by the additional moisture (i.e., in higher latitudes and near seacoasts).
Thus, drier conditions may occur in many of the world's most important agriculture regions, a consequence that could have great practical importance. Both demand for and the supply of water for irrigation will be affected by changing hydrological regimes. Water quality tends to deteriorate under conditions of low flows and higher water temperatures, predicted for arid areas. In such areas, the effect of climate change on water quality may be especially significant.
For U.S. agriculture, such hydrological changes are likely to lead to an increase in the overall acreage under irrigation. If so, this is likely to exacerbate current overdraft and groundwater quality problems in many regions of the West. By way of contrast, other agriculture regions may need to adapt to an increased risk of severe flooding.
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