Overview of climate change impacts on agricultural crops of the Southwestern United States
Cropland composes 8% of the non-federal rural land in the Southwestern Climate Hub region (Arizona, California, Hawaii, Nevada, New Mexico, and Utah) (USDA 2009). Nearly 70% of the regional crop area is located in California. The total value of crops produced in the region in 2012 was $34.5 billion (B), with $30.4 B supplied by California. The most lucrative commodity groups in the region include fruit, tree nuts, and berries ($17.9 B); and vegetables, melons, and potatoes ($7.2 B). Forage-land occupies the largest land area in the region (3.6 million (M) acres) followed by vegetables (1.3 M acres), grapes (0.94 M acres), and almonds (0.94 M acres).
Southwestern agriculture is defined by water scarcity. More than 92% of the region’s cropland is irrigated and agricultural uses account for 79% of all water withdrawals in the region (Kenny et al. 2009; USDA 2010). A warmer, drier climate accelerates large transfers of irrigation water to urban areas, which directly affects local agriculture and associated communities (Garfin et al. 2014). Water availability may drive transformational shifts such as abandoning irrigated agriculture on certain regions of the southwest and the Pacific Islands.
By the end of this century, the amount of time between rainfall events is projected to increase. In the southwest, this shift further limits scarce water resources (Hatfield et al. 2014).
Soil and water are essential resources for agricultural production and subject to new conditions as climate changes. Along with precipitation and temperature, the amount of available water also depends on soil type, soil water holding capacity, and the rate at which water filters through the soil. Soil characteristics are sensitive to changing climate conditions; changes in soil carbon content and soil loss will be directly affected by climate change (Hatfield et al. 2014).
High nighttime temperatures during the grain-filling period (the period during which the kernel matures) increase the rate of grain-filling and decrease the length of the grain-filling period, resulting in reduced grain yields (Hatfield et al. 2014). High nighttime temperatures also effectively increase the minimum temperatures. The increase in minimum temperatures causes snowmelt earlier in the year which may be a time that irrigators cannot effectively make use of the runoff.
Hotter climate conditions may reach temperature thresholds for warm-season vegetable crops, thereby limiting growth and viability (Dominati et al. 2010; O’Neal et al. 2005). Temperature increases beyond optimum thresholds, like those projected for the decades beyond 2050, can cause large decreases in crop yields (Garfin et al. 2014).
Perennial specialty crops have a winter chilling requirement (a minimum period of cold weather after which a fruit-bearing tree will blossom) ranging from 200 to 2,000 cumulative hours. Yields decline if the chilling requirement is not completely satisfied, because flower emergence and viability is low. Winter chill periods are projected to fall below the duration necessary for many California trees to bear nuts and fruits, which will result in lower yields. Projections show that chilling requirements for fruit and nut trees in California will not be met by the middle to the end of this century (Luedeling et al. 2009; Luedeling et al. 2012). In 2012, fruit, tree nuts, and berries accounted for $17.9 B in sales, the largest revenue from a commodity group in the southwest.
The southwest produces more than half the nation’s high-value specialty crops. Drought and extreme weather affect the market value of fruits and vegetables because sales depend on good visual appearance. Extreme weather events in Hawaii and the Pacific Trust Territories, for example storm surges, can lead to saltwater intrusion in giant taro patches. Since taro has a two to three year growing period from planting to harvest and it may take up to two years of normal rainfall to flush brackish water, there may be a five-year gap before the next taro harvest. Drought threatens traditional food sources such as taro and breadfruit. It is very likely that agriculture on the Pacific Islands will be adversely affected by climate change (Wairiu et al. 2012).
Climate change projections suggest an increase in extreme heat, severe drought, and heavy precipitation. The timing of extreme events will be critical because they may occur at sensitive stages in the life cycles of agricultural crops. Extreme events at vulnerable times could result in major impacts on growth or productivity.
In Hawaii and the associated islands, coastal agricultural activity on low islands will be affected as sea level rise decreases the land area available for farming.
The cumulative effects of a longer frost-free season, less frequent cold spells, and more frequent heat waves accelerates crop ripening and maturity, reduces yields of corn, tree fruit, and wine grapes and increases agricultural water consumption. This combination of climate changes is projected to continue and intensify, possibly requiring a northward shift in crop production, displacing existing growers and affecting farming communities (Garfin et al. 2014).
Rising temperatures and shifting precipitation patterns, especially in the southern portion of the region, will alter crop-water requirements, crop-water availability, crop productivity, and costs of water access. Higher temperatures will increase both evaporative and transpiration losses. The resulting shift in crop health will, in turn, drive changes in cropland allocations and production systems.
On most Pacific Islands, increased temperatures coupled with decreased rainfall and increased drought will reduce the amount of available freshwater available. Freshwater supplies are already constrained and will become more limited on many islands.
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- Garfin, G., G. Franco, H. Blanco, A. Comrie, P. Gonzalez, T. Piechota, R. Smyth, and R. Waskom, 2014: Ch. 20: Southwest. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 462-486. doi:10.7930/J08G8HMN
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