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The atmospheric concentrations of CO2, CH4, N2O, chlorofluorocarbons, and other gases capable of trapping terrestrial infrared radiation have risen markedly since preindustrial times. Theoretical and numerical analyses agree that these changes will cause a warming of the global average temperature by perhaps 1.5 to 4.5 °C during the 21st century, with actual changes dependent on rates of emissions. Evaporation and precipitation rates will also increase, but there is no consensus as to what specific changes will occur in different regions. Although many uncertainties remain, changes in temperature and soil water availability seem likely to induce changes in agricultural and natural ecological systems, with the rate, pattern, and magnitude of the changes being the most important—but still uncertain—determinants.
Most general circulation models of the atmosphere predict that a 300 to 600 μmol CO2/mol air doubling of the atmospheric CO2 concentration will increase Earth's mean surface air temperature by about 4°C, and that concomitant increases in certain other trace gases will produce another 4°C warming. Because of a number of demonstrable model deficiencies, however, this conclusion should not be accepted as definitive. For one thing, many well-known processes of importance to world climate are not properly represented in the models. A number of other pertinent phenomena are not even included; and the models fail to temper their calculations in accordance with the requirements of certain large-scale empirical constraints. As a result, there is reason to believe that current climate model predictions of the strength of the CO2/trace gas greenhouse effect may be an order of magnitude too large.
The effects of increasing trace gas concentrations and concomitant climate change on agriculture are likely to be substantial. With cropland and pasture now covering > 30% of the Earth's land surface, agricultural activity is also a significant factor in producing the observed increases in the greenhouse gases CO2, CH4, and N2O. Land clearing for agriculture and other purposes is responsible for 10 to 30% of total net CO2 emissions; the rest is due to fossil fuel combustion. In addition, intentional burning of agricultural wastes, grasslands, and forests makes a significant contribution to global emissions of CO, CH4, NOX, and N2O. Methane emissions from anaerobic respiration in rice (Oryza sativa L.) paddies and domestic animal rumens account for 30 to 50% of the global total, making agriculture the dominant anthropogenic source of this gas. The amount of N2O emitted as a result of N fertilizer applications is highly uncertain, but may be on the order of 10% of total N2O emissions. Future agricultural greenhouse gas emissions will be affected by population growth, economic development, and agricultural practices. Greenhouse gas emissions are likely to increase substantially in the future unless steps are taken to control them. Investigating potential approaches to reducing these emissions while expanding production presents a major challenge to the agricultural research community.
The increasing concentrations of CO2 and other “greenhouse gases” are expected to change temperature, rainfall, and cloudiness. Since the late 17th century we have been collecting empirical information on crop response to all these climatic factors except CO2 concentration ([CO2]). The primary effects of high [CO2] are an increase in photosynthetic rate and a decrease in transpiration rate. The former response is more pronounced in C3 plants than in C4 plants. The extra C fixed increases the dry weight of all organs, with disproportionately more going to roots and stems in many plants. The sizes of most vegetative organs and the numbers of many organs increase unless limited by other environmental factors. The environmental factors that can limit plant response to high [CO2] include nutrients, water, temperature, and light, although the effects of some of these are ameliorated by high [CO2. These factors and [CO2] interact in a complex way, so it is necessary to understand the mechanism of plant response to [CO2] to predict growth and yield. The net result is that in a future high-[CO2] world, we can expect larger plants and higher yields whatever organs are harvested, with higher water use efficiency but higher fertilizer requirements. The C3 crops might displace maize (Zea mays L.) where water is adequate, because their yields will probably be stimulated more. However, the increased [CO2] might make maize more drought-tolerant.
The climate variables predicted to experience major modification as a result of future increases in atmospheric CO2 and other radiatively active trace gases are temperature and precipitation. Predicted changes in these two parameters should intensify the hydrologic cycle over the globe, but could produce opposite trends in certain regions. A “worst-case” scenario of consequent local reductions in summer soil moisture is evaluated in terms of the beneficial effects of atmospheric CO2 enrichment on plant water use efficiency and the interactive effect of air temperature increase on the growth-enhancing effects of atmospheric CO2 enrichment. It is demonstrated that the direct biological impacts of concomitant increases in CO2 and air temperature are probably sufficient to offset the adverse effects of summer soil moisture reductions predicted by state-of-the-art climate/water balance models. It is also noted that the worst-case climate scenario is unrealistic. Consequently, plant growth the world over should be significantly stimulated by atmospheric CO2 enrichment, a phenomenon that many people feel is already evident in a number of ecological indicators.
Warming of the atmosphere will increase mean annual soil temperatures. Large areas in the Northern Hemisphere could become suitable for cultivation, assuming little change in rainfall patterns. A 3 °C temperature rise is predicted to cause an overall 11% decrease in soil organic matter content to a 30-cm depth in the temperature zone, but this increase is well within the standard deviations of the data. If so, a 3 °C temperature increase in the temperate zone is predicted to increase atmospheric CO2 content by approximately 8% over the present (1990) levels during a 50-yr period, provided no increase in biomass production occurs. Carbon/N ratios of soil organic matter are expected to narrow slightly, and total N in the soil is predicted to increase by 10% in the temperate region. However, an average increase of 568 kg ha−1 above and belowground organic residue return to the soil is sufficient to offset changes in soil organic C content. No major changes in fertilization practices are predicted, because modern fertilizer rates greatly exceed weathering rates in providing nutrients to plants. Increased solubility of soil minerals is expected to be slight. Redder soil colors may result from hematite formation.
Future increases in the CO2 concentration of the Earth's atmosphere will directly affect the physiological processes and growth of plants. Indirect climatic effects, including global warming and changes in precipitation patterns and the frequency of weather extremes, may have greater impact than direct effects of CO2 on physiological processes. Carbon dioxide enrichment up to twice ambient levels or more generally increases plant growth, although the magnitude of growth stimulation varies greatly with species, photosynthetic pathway, growth stage, and water and nutrient status. In both natural and managed ecosystems, differential growth responses to both CO2 concentration and climatic change will affect future competitive ability and fitness of plants. The relative importance of various weed species in agroecosystems may change, but selection of adapted crop varieties and management methods may minimize negative impacts. In natural ecosystems, species extinctions probably will increase, because migration and adaptation through natural selection may be too slow to accommodate the rapid climatic changes involved. Weedy species with broad ecological amplitudes are likely to prosper at the expense of endemic species or those already in marginal habitats.
Green plants need CO2 to grow. A higher concentration of atmospheric CO2 will stimulate the photosynthetic process, promoting plant growth and agricultural productivity without increasing the water demand for crop transpiration. On the otherhand, expected climatic warming may have adverse effects on agriculture, partly offsetting the positive direct CO2 effects. The availability of water resources depends on precipitation and potential evaporation, but also on many other factors. Interannual variability cannot be used to extract information of the impact of gradual climatic change. About 5 to 10% of the actual rate of increase of agricultural productivity worldwide can be ascribed to the fertilizing effect of rising atmospheric CO2. Thepositive direct effect of CO2 on plant growth is often smaller when crops are poorly fertilized, but it is fully retained when water shortage limits productivity. Beneficial and detrimental effects of climatic change will not be evenly distributed over the world. Cool and temperate climatic zones will benefit, but in the tropics a further increase in temperature will be undesirable. These changes will exert their influence at such a slow rate that they will be hardly noticeable compared with changes in technology and in economy. Yet they will gradually affect the range of options available.