4.1. Crop Climate Relationship and Yield Impacts
Global crop production needs to double by 2050 to meet the projected demand for food from rising population, diet shifts, and increasing biofuel consumption [
32]. In the agriculture sector, climate change will lead to a major spatial shift and extension of croplands, which precludes a favorable environment for crop growth across different regions [
33]. Most of the permanent crops in California require several years to reach maturity and profitable production. Their market value may also depend upon several quality-related factors such as size, color, chemical composition, firmness, and aesthetic features. Most of these attributes are sensitive to even relatively small temperature changes during critical development stages and/or close to harvest. An evaluation of climate change impacts on 8 out of the 20 major permanent crops grown in California showed that temperature variations of 2 °C were most closely related to yield reductions in almonds, wine grapes, strawberries, hay, walnuts, table grapes, freestone peaches, and cherries [
34].
Individual crops have specific optimum temperature ranges (temperature thresholds) at which vegetative and reproductive growth thrive and exposure to extremely high temperatures during these growth stages can affect growth and yield. Acute exposure to extreme temperature may be most detrimental during the crop reproductive stages.
Table 2 outlines important temperature thresholds for some vegetable crops grown in California. This table also implies that crop species differ in their cardinal temperatures, which are unique in different stages of development [
35]. Research by [
36] portrays the relationship of yield versus two weather variables deemed most important for each crop. Tomatoes show favorable yields during the warmer April and June months. Pistachios require temperatures of between 0 and 7 °C for about 700 h each winter, but for the past four years there have been less than 500 chill hours [
37]. It is noted that influence of climate change on vine phenology and grape composition affects metabolite accumulations under extremely hot temperatures and may affect wine aroma and color [
38]. A temperature sensitivity study [
39] shows that the yields for wine grapes, strawberries, and walnuts are expected to be reduced due to warm winters, while warm summers improve yields. Warmer January and February weather reduces almond yield, while warm summer temperatures are detrimental to peach yields. Cherries and table grapes do not benefit at any time of the year from a warmer climate; cherries are especially harmed by warm November to February weather due to chilling hour requirements [
39].
The impacts of climate change on annual crop yields are typically analyzed with a process-based model, in which analyses of many perennial crops typically involve statistical models [
34]. The relationship between crop yield and climatic variables such as minimum temperature, maximum temperature, and precipitation for twelve major California crops (wine grapes, lettuce, almonds, strawberries, table grapes, hay, oranges, cotton, tomatoes, walnuts, avocados, and pistachios) was derived from historical records from 1980–2003 using regression models [
36]. These climatic trends have mixed effects on crop yields for orange, walnut, and avocado, as compared to other crops.
Figure 10 shows how climate change is expected to impact yields of almonds, walnuts, avocados, oranges, and grapes by 2050. Median projections for wine grape yields exhibited very small changes over the next century due to climate change, while the other five crops exhibited moderate to substantial yield declines. The impact of climate uncertainty on projections was substantial but not overwhelming. While uncertainties were slightly negative, the differences in climate uncertainty between crops reflect the fact that each crop responds in different ways to climate uncertainties.
The impacts of climate change on crop yields for different field crops such as alfalfa, cotton, maize, wither wheat, tomato, rice, and sunflower in Yolo County and throughout the Central Valley as seen in
Figure 11 [
40,
41] were modeled using a process-based crop model named Daycent. The model provided best estimates of yields for the period from 2000 through 2050 under high- and low-emission scenarios. While alfalfa yields were predicted to increase under climate change, yields from tomato and rice remain unaffected. The effect on wine grape yield is not expected to be high; temperature increases might adversely influence fruit quality. Heat waves in May predicted yield losses of 1–10% for maize, rice, sunflower, and tomato, whereas heat waves in June affected maize and sunflower yields [
41]. Overall, a 4 °C increase in temperature may reduce yields from most fruits by more than 5%, and this figure may reach up to 40% in some important regions [
42].
4.2. Impacts on Chill Hours
Many fruit and nut crops require cold temperatures in winter to break dormancy. This requirement defines a location’s suitability for the production of many tree crops [
43,
44]. These fruit and nut species adapt to temperate or cool subtropical climates where chilling each winter is needed to achieve homogeneous and simultaneous flowering and steady crop yields. Quantifying chilling requirements is crucial for the successful cultivation of such crops, and temperature records are converted into a metric of coldness. The lack of adequate chilling hours can delay pollination and foliation, reducing fruit yield and quality [
45]. The effects of insufficient winter chill can vary among species. Walnuts and pistachios depend on synchronization between male and female flowering that is regulated by the number of chilling hours. For various stone fruits, a lack of winter chill results in delayed foliation, reduced fruit set, and poor fruit quality. In many cases, insufficient winter chilling hours result in reduced tree crop performance.
Figure 12 portrays historic and projected future changes in winter chill in California according to two different chilling models: chilling hours and dynamic models [
44]. This research aimed at determining time-line management measures, such as the spraying of dormancy-breaking chemicals, as a predictor of crop yield potential for the season. The study reported that climatic conditions by the end of the 21st century would no longer support some of the main tree crops currently grown in California. As seen in
Figure 12, winter chill hours in 1950 and those projected to occur between 2080 and 2099 will vary spatially between the Sacramento and San Joaquin valleys.
Figure 13 shows that around the year 1950, growers in the Central Valley could rely on having between 700 and 1200 chilling hours, depending on the location of their orchard in the valley. Information about chilling requirements is presented in
Table 3 for different tree species [
35].
Figure 13 suggests that winter chill conditions for cultivars requiring 200 chilling hours (almond, fig, olive, persimmon, and pomegranate) are unlikely to become critical by the end of the 21st century. For chilling requirements of 500 h (chestnut, pecan, and quince) only about 78% of the Central Valley will be suitable for production by the end of the 21st century. For cultivars that require more than 700 h (apricot, kiwifruit, peach, nectarine, plum, and walnut), only 23–46% of the valley remains suitable and only 10% will remain viable by 2080–2095. Only 4% of the area of the Central Valley was suitable in the year 2000 for species such as apples, cherries, and pears, which have chilling hour requirements of more than 1000 h. However, virtually no areas will remain suitable by 2041–2060 under any emissions scenario [
44]. Among the most climate-sensitive trees and vines, walnuts require the highest number of chill hours, implying a future decline in walnut acreage within the valley [
46].
4.3. Impacts on Plants, Pesst, and Diseases
Climate change may have impact on the incidence and severity of plant disease and influence the further co-evolution of plants and their pathogens [
47]. Climate change may affect pathogen development and survival rates, modify host susceptibility, and result in a spread of diseases such as sudden oak death to the forests of Coastal California and Southwestern Oregon [
48,
49]. Moreover, plant diseases could be used as indicators of climate change in which the environment can move from being disease-suppressive to disease-conducive, or vice versa [
50,
51].
Plant diseases, insects, and invasive weeds are mostly caused by temperature-related climate factors, with the invasion of previously uninhabitable areas, for example, Yolo County [
52]. For instance, while milder winters help many frost-sensitive insects to survive, and increased temperature may help promote more rapid reproduction in other insects [
53]. Crop diseases including animal, fungal, bacterial, or viral pathogens are often spread through an insect vector, wind, or anthropogenic activities. In Yolo County, it was noted that soybean rust spread throughout the world due to increases in severe weather events such as hurricanes [
52]. Recently, stem nematode has been reported in alfalfa in Yolo County, which can spread in various ways including through waterways and irrigation runoff, contaminated farm equipment, and other anthropogenic means. Statewide integrated pest management (IPM) involves listing common diseases and insects, and allows us to elucidate potential plant disease and manage all kinds of pests elsewhere within the state [
54].
4.4. California Agricultural Vulnerability to Climate Risks
Volatility in agricultural markets and cost of energy, fertilizers, and other inputs can have a multitude of unpredictable biophysical and social consequences [
41,
55]. In order to reflect the aspects of exposure, sensitivity, and adaptive capacity to climate risks, agricultural vulnerability indices are often assessed by examining biophysical and social indicators over time and space. Among various indices, the Social Vulnerability Index (SVI) is used to explore vulnerability to environmental hazards [
56]. To date, there is no single data point to measure crop vulnerability across the landscape. Merging coupled crop and economic models may provide a better picture of specialty crop vulnerability [
55].
The Agricultural Vulnerability Index (AVI) for California integrates a broad set of biophysical and social indicators relevant to state and local efforts to adapt to changes in climate, land use, and economic forces. As such, the California AVI is meant to be a starting point for “place-based” adaptation planning throughout California. The AVI assigns each variable to one of four sub-indices: climate vulnerability, crop vulnerability, land use vulnerability, and socioeconomic vulnerability based on an a priori judgment and spatial resolution [
56].
Figure 14 establishes some of the most vulnerable agricultural regions to climate change from multiple perspectives. The Salinas Valley and the San Joaquin Valley are identified as two of the most vulnerable agricultural regions [
56]. While agriculture in the Imperial Valley and the corridor between Fresno and Merced are found to be very vulnerable to climate change, northern regions of the state may provide hospitable environments for fruits (wine grapes) and vegetables.
It is also useful to note how Southern California shrub lands could move to higher elevations as a response to cooler climates and greater precipitation because of rising temperatures and reduced precipitation in their current environments. As noted, non-native grasslands could be converted into shrub lands and consequently exhibit reduced range and proficiency [
57]. Various model scenarios suggested that forage production for cattle grazing might decline because of decreases in annual precipitation [
58]. Forest diseases such as pitch canker are vulnerable to warmer winter temperatures and affect the root and stem-base of a wide range of broad-leaved and coniferous species [
59]. The grassland habitat in the Sacramento Valley may decline by 1–20% by 2070 due to warmer winter temperatures and variable precipitation [
60]. The eastern edge of the Central Valley might become climatically unsuitable for grassland habitats including valley oak under drier conditions and the northern Central Valley to a large degree may become unsuitable for such habitats under wetter conditions [
61]. It is estimated that 24–59% of current California foothill, valley forests, and woodlands will not be climatically suitable for oak woodlands by the end of the century.