Atmospheric particulate matter within the San Joaquin Valley is correlated with annual precipitation patterns , possibly suggesting that this relationship led to increased CCN, less atmospheric scrubbing , or a feedback between the two. Agriculture is an important source of dust in California. However, the amounts of dust can be reduced by employing conservation or minimum tillage practices. In the San Joaquin Valley, conservation tillage decreased amounts of both total and respirable dust to approximately one third of that compared to adjacent standard tillage treatments . Inclusion of a cover crop reduced this difference to approximately two thirds and three quarters of total and respirable dust, respectively . Precipitation is the main agent removing aerosols from the atmosphere, particularly smaller aerosols, which are more effective at scattering light. Thus, as more aerosols enter the atmosphere, potentially decreasing precipitation events by the ”second indirect effect,“ aerosol concentrations could increase as the rain’s natural scrubbing ability is lessened. This may be exacerbated in summer, especially if precipitation decreases in California due climate change . However,hydroponic grow systems as predicted surface temperatures increase, water vapor entering the atmosphere through evaporation will also increase , which could potentially cause increases in precipitation as global warming takes effect.
The net effect upon precipitation is difficult to predict accurately, as it is a function of many factors, which are understood and predicted with differing degrees of confidence. Although there are large uncertainties associated with precipitation , changes in the absolute volume of precipitation, in the number and/or duration of storm events, or a lengthening of the rainy season, could affect air quality.The complex interactions between agriculture, climate change, and air quality are a product of the mechanisms involved with source, sink , and the feed backs between air quality constituents ; California Air Resources Board ; Krupa ; Mennon. For instance, black carbon and organic carbon aerosols are formed in similar combustive processes, yet BC aerosols are generally associated with global heating, whereas OC aerosols are associated with global cooling, due to their respective absorptive and reflective properties . Although vegetation generally reduces climate change by sequestering C , isoprene emissions from vegetation, including crop and grassland , are nearly three times as reactive in smog formation than a weighted average of VOCs emitted from vehicle exhaust . Isoprene production from oak stands growing along the western slope of the Sierra Nevada, mixed with a combination of anthropogenic VOCs and NOx emissions, blown up-wind from the Central Valley, produce high levels of O3 concentrations in the Sierra foothills . Furthermore, both biogenic isoprene emissions and O3 formation can be accelerated from increased sunlight and heat, and the contribution of both to future exacerbations in air quality, regardless of current anthropogenic source reductions, are important variables in California climate change scenarios.
Based upon data from the South Coast Air Quality Management District, a slight increase in tropospheric O3 concentrations over the Los Angeles Basin, as a result of increased surface temperatures, is projected . Conversely, increased temperatures shift the solid form of ammonium nitrate, a dominant wintertime PM constituent, to its gaseous precursors, thereby reducing atmospheric PM concentrations and altering the adsorptive quality of reactive atmospheric N in ecosystems. The processes outlined above not only contribute to climate change, but their rates and extents, and hence, their impacts upon California agriculture, may be altered . The amount of sunlight available for photosynthesis directly affects crop growth cited in Chameides et al., and can be increased or decreased through cloud-aerosol interactions . For example, the direct effect of aerosols in China may be reducing current yields of rice and winter wheat by between 5% and 30%, by reducing the amount of light available to drive photosynthesis . However, because aerosols both scatter and absorb light, depending on their relative optical and absorptive properties , they are also predicted to have positive effects on net primary productivity , with increased light scattering resulting in increased productivity on under story leaves, and on cloudy days . While the air over the Central Valley is likely cleaner than that over China, crops with sufficient sun exposure can be expected to show similar negative growth trends, as estimated by Chamiedes et al , as the rate of aerosol production increases in California. Aerosols also play a role in determining the nature and magnitude of changes in the precipitation cycle, and their indirect effects , and as such represent a significant source of the uncertainty in current precipitation models.
Changes in the hydrological cycle may result in unexpected challenges for the ability of California to maintain its current agricultural productivity, and could be heavily influenced by aerosol levels. Agriculture complicates the interaction of aerosols and cloud formation by impacting the latter mechanism directly. In southeastern Australia, Lyons found there to be many more clouds over native vegetation than agricultural fields, probably because native vegetation is typically darker than cropland, leading to convective cloud formation. Another factor is that native vegetation is more ”rough” than cultivated fields, slowing down wind more effectively. In California, however, summer drought may change these relationships, as grasslands are lighter colored and more even in stature than irrigated cropland—especially perennial fruit and nut crops. As with the effects of CCN and increased temperatures on precipitation patterns, feed backs between aerosols emitted over agricultural fields, particularly through tillage , may counteract any decreases in precipitation by promoting cloud formation. However, the net effects remain highly uncertain.California agriculture has the potential to improve air quality, and help mitigate climate based plant and animal stresses, by implementing best management practices such as reduced till farming ,hydroponic growing water conservation , and crop rotations. High-input intensive agriculture has the potential to adversely affect air quality and climate stability by increasing smog formation, aerosol production, and reactive N introduced to the biosphere. Efforts to reduce tillage, lower soil N concentrations, maximize N use efficiency and optimize manure management, are simple approaches that could be implemented Decisions need to be made, and policies developed and implemented, if we are to effectively mitigate climate change impacts upon air quality and California agriculture. Statewide, one of the most promising agricultural practices that could enhance air quality is to reduce tillage, a best management practice that minimizes the number times a field is cultivated. Reduced tillage has the potential to mitigate poor air quality by limiting soil disturbance and formation of atmospheric aerosols , reducing fuel combustion, and decreasing NOx formation. Baker et al. found conservation tillage in the San Joaquin Valley to decrease amounts of both total and respireable dust, compared to adjacent standard tillage treatments. Currently, only 16% of California’s total farm acreage employs conservation tillage practices , creating a large potential for air quality improvement through reduced tillage incentives. Regulations are also a critical means to achieve air quality mitigation, specifically regarding tropospheric O3 reductions. Ozone concentrations in the San Bernardino Mountains peaked in 1978, and have been decreasing ever since, despite large increases in both vehicle miles traveled per capita and population in Southern California. Improvements in vehicle efficiency , as mandated by the California legislature, account for this trend . Complete elimination of O3 precursors from motor vehicles are estimated to remove $2.9 billion in crop damage in California .
Ozone mitigation is a classic example of a strategy that requires careful consideration of aspects other than just lowering the concentration of its precursors . VOCs differ largely in their reactivity . While some VOCs are essentially unreactive , others are very potent O3 precursors . Therefore, policy makers should not solely consider total VOC concentration alone as an effective mitigation strategy, but rather concentrate on those precursors that are highly reactive. The production of O3 depends on the relative concentrations of VOCs to NOx . For this reason, anthropogenic production of VOCs is controlled by state law, along with NOx production. Under current regulatory standards in California, dairy cattle are assumed to produce 12.8 lbs/cow/yr of VOCs. This dairy emission factor means that dairies have surpassed all other sources as smog producers . However, this dairy emission factor is based on a 1938 study in which methane from cows and other ruminant animals were measured. Recent VOC research conducted indicates that this number is greatly inflated, and that the emission factor for dairies should be closer to 2-3 lbs cow-1 year-1 . They also found that most VOCs produced from dairies are compounds like acetone, acetic acid, and several alcohols that are known to be low in their ability to form O3. Be that as it may, cattle do play an important role in climate change, because both animals and their manure produce a large portion of agriculture’s 38% contribution to statewide methane emissions . Mitigation efforts to reduce global warming effects from livestock production should focus on methane, rather than VOC emissions.While the numerous predictions of climate change scenarios are in general agreement in forecasting a rise in the mean temperature of California , the predictions for changes in the future precipitation patterns of California vary widely . In comparing the three most recent precipitation predictions from the PCM, HadCM2, and HadCM3 models under a range of IS92 emissions scenarios, they range from mildly drier than present to a slightly more wet climate scenario and an extremely dry climate scenario . The large disparity in estimates reflects the difficulty of extrapolating predictions of GHGs and temperature to climatic patterns that generate precipitation. However, one trend is present in all scenarios: as temperatures rise, precipitation type changes increasingly from snow to rain . Higher temperatures will produce reductions in snow pack accumulation in the Sierra Nevada Mountains, with subsequent effects on water storage, stream flow, and supply . Water stored in the snow pack is a major natural reservoir for California. It is the presence of this large snow pack that provides the majority of the irrigation water for the dry Central Valley during the growing season. Additionally, the shift in precipitation type may increase the risk of winter flooding, especially in the Delta region, where a series of levees keep the subsided delta islands dry. The frequency of flooding and other “extreme” weather events, such as El Niño and heavier winter storms, has been projected to increase with rising temperatures , but this issue has not been adequately addressed by climate models. In this section, the issue of climate change and its potential effects on California agricultural water resources will be treated as an issue of future scarcity. Even in scenarios with higher precipitation levels , earlier snow melt and flood control allocation in reservoirs decreased surface water storage and the subsequent ability for water deliveries during the growing season . This section identifies the potential impacts, then discusses potential mitigation and adaptation strategies and data gaps in current analyses.The earlier snow melt and runoff from increased temperatures and decreased snow pack will likely create some challenges for state reservoir managers. Managers would be forced to operate storage space conservatively, losing more water downstream and leaving less water for deliveries during the summer growing season . Projecting future water use for California, Tanaka and colleagues predict that agricultural water allocation in California will continue to decline relative to urban and environmental uses. Apart from environmental flows, 70% of California’s water is currently directed towards agricultural production. The rise in demand for water from the urban and industrial sectors will result in decreased water allocation to agriculture. Escalating water demand will require shifts in water sources as the reservoir system reaches capacity . Conjunctive water use will increase with less surface water from decreased snow pack and snow melt capture.Desalination in coastal regions for urban use and for aquifer treatment is expected as technology improvements reduce energy costs for treating water, and as water costs increase . Taken together, these eventualities represent a significant challenge to water resource managers and California agriculture. Sea level rise may have a major impact on California water transfers through the Sacramento-San Joaquin Delta. Increased salinity intrusion into the San Francisco estuary and potential failure of levees protecting low-lying land may degrade the quality and reliability of fresh water transfer supplies pumped at the southern edge of the Delta, or may require more fresh water releases to repel ocean salinity .