Increased Temperature And Co₂ Concentrations

Global warming will entail complex changes in the physical environment upon which agriculture depends. The implications of these changes for crops and livestock will also be complex. In the following sections, the effects of changes in temperature, CO₂ , and precipitation are first treated separately, then combined in sections that reflect a more systems-level approach.

Increased Temperature

Negative Effects on Crop Physiology . For biological systems the most important effect of increased concentrations of greenhouse gases in the atmosphere will be the rise in global temperatures. Most of the physiological processes involved in the development of organisms are driven by temperature: development occurs within a range between low and high lethal temperatures, and the rate of development within this favorable range is directly, if not linearly, related to temperature. Beyond this favorable range, incremental increases in temperature cause rapid decreases in growth rates until, finally, mortality occurs. An increase of two to four degrees Celsius could therefore have important implications for many California crops.

Along with average temperatures, short periods of intense heat occurring

during critical times in the growth cycle of a susceptible plant can have negative results. Short periods of high temperatures can retard morphological development, induce male sterility in flowers, reduce grain fill or fruit development, and cause fruit drop. Because the variability of weather patterns is expected to increase along with average temperatures under climate change, damage due to extreme temperatures may be an increasingly severe problem.

The number of crops that could be affected by increased average and peak temperatures is extensive. Virtually every category of crop grown in California includes species that are susceptible to temperature changes in the realm of those predicted under a global warming scenario. Pears, kiwi, apples, oranges, cherries, most leafy vegetables, cotton, and melons are just a few of the many crops at risk.

California's desert valleys already experience some of the highest summer temperatures of any major crop production zones in the world. In the Imperial Valley, cotton, okra, sudan grass, alfalfa, and sorghum consistently produce economic yields, but many cultivars produce suboptimally owing to high temperatures. Of the cropping systems found in the southern San Joaquin Valley, many are not successful in the Imperial Valley, owing in part to a three-degree (Celsius) difference in average temperature. If average temperatures shift upward, agricultural production in the Imperial Valley could be greatly restricted, and parts of the San Joaquin could approximate current conditions in the Imperial Valley.

In addition to average and peak temperatures, other heat-related variables can limit crop growth; low temperatures and diurnal variation (the difference between day and night temperatures) are important to some species. Most deciduous fruit and nut crops in California have a chilling requirement that must be met to ensure an orderly, compact bloom period in spring. Many of these same crops require a certain spread between night and day temperatures. When this is not met, yield and fruit quality may suffer.

Warmer temperatures may affect the incidence and type of pest populations occurring in a given region. The pink bollworm, for example, has ravaged cotton in the Imperial Valley but has never been established in the San Joaquin Valley, although bollworm moths are consistently found there. Researchers believe that the San Joaquin Valley may have escaped serious infestation owing in part to inability of the bollworm to survive the colder, longer winters there. In the San Joaquin Valley, autumn light and temperature conditions do not provide the right signal to the bollworm to begin storing energy needed to enter diapause, the insect equivalent of hibernation. Because of this (together with a sterile moth release program and other management practices), relatively few of the moths survive the winter to emerge in the spring, and populations

stay naturally low. Scientists fear that an increase in average temperatures could allow the bollworm to better survive the winter in the San Joaquin Valley.

Increased temperature can also cause a change in the peaking of pest cycles with respect to the life cycles of host crops. Lygus bug, another insect known to devastate cotton crops, overwinters in the hilly range-lands of California and in the mature crowns of alfalfa plants. Those bugs that overwinter in grasslands migrate to irrigated crops such as cotton when the grasses dry in late spring. If this migration coincides with the reproductive phase in the development of the cotton crop, serious yield losses can result. High temperatures, together with late spring rainfalls, could result in earlier maturation of the cotton crop and a later migration of lygus bugs, increasing the likelihood of infestation at the reproductive stage.

Implications . Potential responses to the types of problems described above will likely be variants on three basic strategies: 1) genetic manipulation through plant breeding and biotechnology, providing better tolerance to environmental stimuli; 2) management strategies changed to optimize production under new constraints; or 3) cropping patterns shifted to follow the movement of climatic regions. The potential for breeding crops tolerant to heat is great, although few resources have been devoted to this effort to date. Likewise, few resources have been devoted toward understanding and improving field management at the cropping systems level, and the potential for finding adaptive improvements is high. The final option, shifting cropping patterns geographically, will require higher farm investments during the transition period and will probably impact the total acreage sown to various crops. A few species could actually be eliminated from California, while others might be successfully grown for the first time. Although it is tempting to think of regions of adaptation for crops as simply moving north along the new isotherms, diverse geology and geography will likely interpose microclimate features that reduce the success of such migrations. There are no Salinas or Napa Valley equivalents, for example, north of Santa Rosa, extending as far north as Washington State. Soil types, which can vary widely within a small area, will provide additional complexity to the crop-adaptation puzzle.

Photoperiod requirement (number of hours plants are exposed to darkness) is another variable that might constrain man's ability to shift cropping regions latitudinally. Although genetic improvement of many crops has brought virtual day-neutrality, the adaptation of other varieties to regions where the cropping season is too long or too short has included use of photoperiod response as a mechanism to control days-to-maturity. For these varieties, as well as new species that might be introduced

to California, day-length requirements could affect the short-term potential for geographic shifts in cropping patterns.

Positive Effects on Crops . Not all of the changes due to global warming will be negative. Although significant dislocations will occur in the short run if climate change is rapid, long-term results could be favorable when (and if) the climate stabilizes at a new equilibrium. Temperature increases in southern parts of the state could allow for introduction of tropical species, and winter vegetables and avocados, which are currently limited to southern California, could become viable in the San Joaquin Valley. More double cropping could be accommodated owing to warmer winters, and some pest species might be inhibited, rather than helped, by higher temperatures. Interactions of microclimates with crop species, while limiting distribution of some types, would also assure that many specialty crops will continue to be grown in the state.

Impact on Animals . Increased temperature can have both direct and indirect effects on livestock. Indirect effects result from changes in crop production zones and nutritional value. For example, alfalfa loses quality in hot weather through increases in cell wall content, meaning that additional feed is required to meet the nutritional and caloric needs of ruminants that consume the alfalfa. Because most animals raised for dairy and meat production are homoiothermic (warm-blooded), they are highly adaptable to environmental changes within the ranges assumed for global warming. Nonetheless, high temperatures can cause molt and mortality in poultry, reduced milk production in dairy cattle, and reproductive inefficiencies in swine. Feedlot cattle do not gain weight as rapidly during prolonged periods of elevated temperatures as during cooler periods.

Fish, which are poikilothermic (cold-blooded), may be the most influenced by temperature change of any food animals. Salmon, steelhead, and other species that need cold water to survive are the most at risk. Decreases in streamflows together with increased ambient temperatures will result in warmer water. If these fish are to survive, greater releases will be required from reservoirs in order to keep stream temperatures at acceptably low temperatures. Warmer water will also contribute to development of algal blooms and increased biological oxygen demand in fishponds (reducing the oxygen available for fish).

Just as plant pests will be affected by global warming, the distribution of some livestock pests may change with warmer temperatures: culicoides, mosquitoes, and other pests may move north, increasing existing problems and creating potential new problems. Some of these pests carry important infectious diseases of sheep, cattle, horses, or humans, such as the bluetongue and encephalitis viruses. New strains of these viruses as well as viruses from subtropical climates may appear in California.

Warmer climates will increase the length of the seasons that some of these vector-borne diseases are active, resulting in a need for more extensive control programs and consequent costs to producers and the environment.

Possible responses to protect livestock from the effects of excessive temperature are likely to be costly. Livestock may be bred for greater heat tolerance: those animals with greater surface area per weight and greater capacity to perspire might be favored over larger species. However, such breeding efforts will require time. Many of the smaller breeds of heat-tolerant animals, for instance, are inefficient milk producers. In addition to improvement of current California livestock, new breeds from tropical climates such as Brahman cattle could be adapted and improved for California production. Where breeding efforts are ineffective or time-consuming, livestock that are not adapted to increased temperatures may be raised in altered environments. This may be accomplished by changing the location where the animals are raised, or by building shelters to maintain them.

Increased CO₂ Concentrations

Physiological Effects on Crops . According to some scientists, present-day crop species evolved under significantly higher concentrations of atmospheric CO₂ than exist today. If true, this helps explain why availability of CO₂ is limiting under most conditions for current crop species. The effects of increased CO₂ concentrations on crop development and production have been studied, giving the following rule of thumb. Assuming other factors nonlimiting, a twofold increase in atmospheric CO₂ will raise the rate of photosynthesis so that a 50 percent increase in plant biomass might result. The partitioning of this increased mass through the plant is fairly uniform, making the estimate valid for harvested portions of the plant. However, variation in response to CO₂ exists between different crop species. Legumes, normally in an energy-deficit state due to the requirements of nitrogen fixation, may reduce their deficiency through the effects of increased CO₂ on photosynthetic rates. Positive effects for tree crops may be small, owing to the many nonphotosynthesizing parts of the tree, such as shaded leaves, trunk, and branches. C₄ plants (those like sorghum and corn which use four carbon acids to fix CO₂ ) will lose some of the advantage they presently have over C₃ plants (plants that use three carbon acids to fix CO₂ ). However, this relative loss in efficiency by C₄ plants may be compensated for by increased water-use efficiency; trials on sorghum under increased atmospheric CO₂ have shown such an increased efficiency of plant-water use. Acting to counterbalance this effect is the increase in plant-leaf temperatures that could result from partial stomatal closure—the primary factor contributing to increased water-use efficiency.

Although plant biomass may increase in response to higher concentrations of atmospheric CO₂ , that mass may have a modified chemical composition. Carbohydrate content is likely to increase disproportionately with respect to nitrogenous compounds. Pest species could respond to this change by eating more plant. Soybean looper (Pseudoplusia includens ) larvae fed soybean foliage grown at 650 ppmv CO₂ consumed 80 percent more than did larvae fed on leaves grown at 350 ppmv CO₂ . In other experiments, plants grown under higher CO₂ levels had heavier infestations of aphids than did controls. The effects seem to be insect-and plant-dependent, however, making generalizations difficult. Additionally, the short duration of the studies has not permitted buildup of insect predator and parasite species, so it is difficult to predict the net long-term effects of increased and modified biomass on pest infestations.

Impact on Livestock . Just as insects may need to increase their intake of plant matter to compensate for reduced nutritional value, livestock may need to eat more, or at least eat a more diverse diet. No direct metabolic effects of increased CO₂ are expected for animals, since the partial pressure of oxygen is not expected to drop below necessary levels.

Combined Effects of Increased Temperature and CO₂

Unfortunately, little information is available on the combined effects of increased temperature and CO₂ . It may be assumed, however, that although response by individual species will differ, increased rates of maintenance respiration could offset to some extent the benefits to plants of elevated CO₂ levels.