Global models that have incorporated both biophysical and socio-economic parameters have predicted that negative impacts on food production from climate change will largely be felt in the developing world, but positive impacts will be felt in the developed world . These studies conclude that the magnitude of this disparity will be determined by which future IPCC’s emissions scenario is adopted and the degree to which crops will respond to CO2 fertilization. Low latitude regions of the world may not benefit from CO2 fertilization, because the benefits are overshadowed by the predicted detrimental effects of increased temperature and/or precipitation changes . As a result, regions such as Africa or parts of Asia are predicted under the GHG intensive A1fi scenario to experience yield reductions up to 30% of 1990 levels by 2080 . The population at risk of hunger in Mali, for example, is predicted to increase from the current 34% to 44%, due to land degradation and then up to 72% due to the additional impacts of climate change by 2030 . These regions are at particular risk because their lack of infrastructure and technology impedes their producers’ ability to adapt to adverse and/or altered climate conditions. In contrast, the stress caused by A1fi climatic conditions is expected to be offset for some crops such as cereals , by the effects of CO2 fertilization, resulting in small increases in yield in Australia, North America, and South East South America.Assessments of agricultural production in the United States have used an Integrated Assessment approach, which includes complex interactions of temperature and precipitation changes with increased climate variation, changes in pesticide use, environmental effects caused by agriculture , changing global markets, societal responses, and technological adaptation, to model agricultural response to climate change . Consistent among these studies are the conclusions that there will be a dramatic difference in regional impacts,blueberry packaging box but agricultural production in the United States overall will increase, commodity prices will fall and irrigation use will go down due to increased precipitation and potentially higher water use efficiency that results from CO2 fertilization .
Climate change is therefore expected to be economically positive for U.S. consumers and negative for producers, but will entail increased pesticide use and result in increased environmental degradation . Regional-level forecasts could be quite different in California than nationally, due to its limited water resources and its focus on specialty crops.Water supply is central to the success of Californian agriculture. In addition to changes in precipitation, water availability will likely be influenced by rising temperatures, and consequential increases in water demand from other sectors . Increased temperatures will affect the amount of water collected and stored in the Sierra snow pack. By the end of the century, the Sierra snow pack is predicted to be 30% to 70% lower than the current winter total, due to an increase in rainfall vs. snowfall, and earlier melting of the snow pack . This will be most prominent in the southern Sierra Nevada, and at elevations below 3,000 m where 80% of California’s snow pack storage currently occurs. The changing availability of water both within California and to California agriculture, may lead to heavy reliance on groundwater resources, which are currently over drafted in many agricultural areas . Approximately 42% of current ground and surface water is used for agricultural purposes . Demand for water resources will be further exacerbated by an increase in the population of California in the coming century, which is projected to be > 46 million people by 2030, and may reach 90 million by 2100 . As will be discussed below, gradual shifts in climate over the next hundred years will necessitate adaptations that may not necessarily require direct government intervention, and could be driven, largely by market forces, changing management practices, and technological advances . California agricultural producers have had a history of adapting to new locations, development of water resources, and changes in markets. New adaptations will be made easier and more efficient by the availability of predictive information to producers, and an appropriate policy environment. Some sectors also lend themselves to more rapid change than others. For example, perennial tree crops and vines, of which many are unique to California in the U.S. context, may be particularly vulnerable to problems.
The adaptation to rapid change or extreme climatic events, such as floods, droughts, and heat waves are much more difficult to predict. Such extreme events may exceed the adaptive capacity of markets and be much more difficult for producers to cope with . Thus, development of risk and response strategies to various extreme climate change scenarios may gain more attention in the coming years. Beyond responding to changes in climate, California producers will most likely find opportunities to mitigate the release of GHG. Agriculture will play a significant role in a portfolio of national mitigation strategies, for example, as a first step to sequestering carbon . United States agriculture and forestry could remove more than 425 million metric tons of carbon equivalents of combined greenhouse gases , based on modeling of extreme increases in carbon prices. Carbon trading could have substantial impacts for agriculture, such as increased crop value and reduction of environmental externalities. Greenhouse gases include carbon dioxide , nitrous oxide , methane , and high global-warming-potential gases such as sulfur hexafluoride , hydrofluorocarbons , and chlorofluorocarbon . Since these gases absorb the terrestrial radiation leaving the earth’s surface, changes in the atmospheric concentrations of these gases can alter the balance of energy transfer between atmosphere, land, and oceans. All atmospheric GHG concentrations are increasing each year due to anthropogenic activity, which, in turn, leads to climate changes at the local, regional and global scale . The present section focuses in particular on CO2, N2O, and CH4 because they are the three major bio-genic GHGs produced by the agricultural sector in California and across the globe. It summarizes current sources and sinks of GHGs, i.e., total amounts of emissions by each type of gas, the contribution of the agricultural sector to California GHGs emissions, and consider agriculture and forests as potential sinks of GHGs. Potential impacts of climate change on CO2, N2O, and CH4 emissions and possible mitigation strategies for GHGs produced by the agricultural and other sectors are presented. California produced 493 million metric tons of CO2-equivalent GHGs emissions and in 2002 was ranked as the second largest U.S. state emitter after Texas . Most emissions were CO2 produced from the combustion of fossil fuels from industrial and transportation sources. Overall, the contribution of the agricultural sector to GHG emissions as a whole in California is relatively small. Taken together, agriculture and forestry contributed approximately 8% of the state’s total GHG emissions, including GHGs from all agriculturally related activities such as fossil fuel combustion associated with crop production, livestock production,blueberry packaging containers and soil liming . Emissions arising from transportation of agricultural commodities are not included in this estimate. CO2 emissions from non-fossil fuels, including agricultural activities, were 2.3% of the total GHG emissions of California. Of the 2.3% of CO2 from non-fossil fuels, agricultural activities contributed about 38%. Thus, the total contribution of CO2 from agricultural activities to the total GHG emissions was 0.9% in 2002 .
Nitrous oxide and CH4 emissions contributed 6.8% and 6.4%, respectively, to the total GHG emissions in 2002, with approximately 59% and 38%, respectively, originating from agricultural activities. An estimated 18.6 and 0.9 metric tons of CO2-equivalent GHG came from agricultural practices and manure management, respectively, in 2002 . Thus, the agricultural contribution to the state’s 2002 emissions of N2O and CH4 was 4% and 3%, respectively. Methane emissions from California flooded rice fields constituted a total of 0.5 metric tons of CO2-equivalent GHG and constituted less than 2% of total CH4 emission in California in 2002 . Methane emissions from animal production included 7.3 and 6.6 metric tons of CO2-equivalent from enteric fermentation and manure, respectively .Greenhouse gases are produced primarily by soil microorganisms carrying out oxidation-reduction reactions, including nitrification, denitrification, methanogenesis and organic matter decomposition . Because changes in temperature and precipitation alter the activity of soil microorganisms, GHG emissions from agriculture would likely be affected by climate change. This section considers, in general terms, the impacts of climate change on respiration and soil organic matter dynamics, as well as N2O and CH4 emissions. Carbon dioxide is the end product of respiration by soil biota .A potential impact of increased temperature is loss of carbon from the large reservoir of C contained in SOM in agricultural and forest soils. Although forests are currently a sink for CO2 , they might become a source of CO2 with temperature increases from global warming . This issue is critical, because SOM contains roughly two-thirds of the terrestrial C and two to three times as much C as atmospheric CO2 . Many researchers have investigated the effects of temperature on decomposition rates of SOM in mineral soils. Trumbore et al. reported that temperature is a major controller of turnover for a large component of SOM, as long as soil moisture is not a limiting factor. It is hypothesized that the decomposition of soil labile C is sensitive to temperature variation whereas resistant components are less sensitive. However, Fang et al. suggested that the temperature sensitivity for resistant SOM pools does not differ significantly from that of labile pools, and that both pools of SOM will therefore respond similarly to global warming. If this conclusion is correct, more C will be released than predicted by the HadCM3 model that assumes an insensitivity of resistant C pools to temperature. In contrast to observations that decomposition is enhanced by increases in temperature, Giardina and Ryan reported, based on analyses of data from locations across the world, that rates of SOM decomposition in mineral soils were not controlled by temperature limitation to microbial activity and that estimates made from short term studies may overestimate temperature sensitivity. Because moisture content in soils strongly affects the activities of soil microorganisms in direct, and perhaps more importantly in indirect ways , changes in precipitation patterns due to global warming may be one of the main impacts of climate change on SOM decomposition in mineral soils. Predictions of changes in precipitation are problematic, differing among climate models, thus making influences on SOM difficult to predict. Other factors influence SOM decomposition , some of which may be affected by climate change, must also be considered in projections of how SOM will behave. Temperature should not be viewed in isolation from other factors . Unfortunately, the magnitude and relative importance of these factors in governing SOM dynamics have received little attention in the literature.Nitrous oxide is produced primarily during denitrification, an anaerobic microbial process in soils or sediments, in which nitrate is used as an electron acceptor in the absence of oxygen . Though nitrification, the oxidation of ammonium to nitrate, also produces some N2O, this process is thought to be less important than denitrification . Agricultural activities—soil emissions from fertilizer use, residue burning and animal production—are responsible for an estimated 80% of anthropogenic emissions of N2O . Few studies have investigated N2O emissions in agroecosystems in California. In a comparison of organic and conventional managed tomato soils in the Central Valley, N2O emissions were found to be of short duration; followed addition of organic or mineral fertilizer in the organic and conventional systems, respectively; and occurred immediately after irrigation events . There are, however, no published extensive, systematic studies collecting field measurements of N2O over the growing season in different soil types of California to permit identification of relationships between fluxes, management practices, and environmental variables. Other studies outside of California have indicated that emissions of N2O are primarily controlled by soil moisture content, in particular the water-filled pore space , temperature , organic carbon availability , and concentration of mineral nitrogen . The latter factor is often optimal in agricultural soils for N2O fluxes because addition of synthetic N fertilizers and organic manures lead to elevated mineral N concentrations at least temporarily. In addition, California agricultural systems are frequently irrigated, leading to ideal moisture conditions for denitrification and potentially N2O fluxes .