Theme one: it is essential to evaluate climate impacts using a variety of methodologies and perspectives
Identifying climate risks helps frame adaptation to climatic changes in the agricultural sector by focusing on management for sustainable production and resilient working landscapes (Howden et al. 2007). Ensemble mean climate projections of temperature and precipitation are often used in climate analyses and adaptation planning (Knutti et al. 2010). Means are valuable because they allow for adaptation measures to average conditions; however, addressing different emissions scenarios can inform the possible range of future temperature/precipitation impacts on the sector of interest. For example, Thorne et al. (this issue) found that the higher emissions scenario (RCP8.5) led to an impacted forest area three times larger than the area predicted by the lower emissions scenario (RCP4.5). It is essential to craft adaptive measures for expected average conditions but also to consider the likely extreme events and resulting social vulnerabilities, systemic resilience, and cascading impacts. Extreme events are important for risk analysis; however, adaptation related to such events is often not successful (Schneider et al. 2000).
Kerr et al. (this issue) found that the choice of metric (relative vs absolute; winter vs summer, and area-based vs value-based) influences the spatial pattern of specialty crop vulnerability. This highlights the value of taking multiple viewpoints to evaluate and describe varying impacts. In this collection, specialty and field crop county-level impacts are reported using two impact metrics (absolute and relative impact). No county ranked in the highest quantile using both metrics for specialty crops (n = 58 counties), and only one county (Imperial County, California) ranked in the highest quantile using both metrics for field crops (n = 152 counties). Additionally, specialty crops analyses used both economic- and area-based perspectives and found that there is no single locus of agricultural vulnerability in California, but differing areas and crops appearing vulnerable depending upon assessment methods and assumptions (Kerr et al., 2017).
Using the county as a fundamental mapping unit encourages the use of climate change projections within existing institutional structures like cooperative extension (CE). Sometimes, a rapid analysis of climate impacts on a particular crop/location can provide enough information to support management or indicate if more in-depth analysis is necessary. For example, most prior assessments of specialty crops in California were conducted on perennial specialty crops, but analysis in this SI showed that annual strawberry production in California may impart significant vulnerability to Coastal California counties and warrants additional analysis.
Theme two: there will be widely varying impacts across the Southwest, often related to water availability
As highlighted in “Section 2,” water availability is paramount in the SW. The articles of this collection further illustrate this point based on the varied impacts of precipitation changed on differing production systems. Thorne et al. (this issue) show that in forested ecosystems of the region, the wetter modeling scenario predicts considerably less exposure (see Table 1) to climate change (27% vs. 78% exposed), thereby decreasing forest vulnerability (Thorne et al. 2016). As described by Havstad et al. (this issue), rangeland is similarly at the mercy of annual precipitation that supports forage production, thus supplemental feed is vital to support regional livestock production. In contrast, the large majority of specialty crops in the region are irrigated, and approximately half of this irrigation is supplied by groundwater. Thus, unlike rainfed agriculture, un-supplemented rangeland systems, and forest systems, specialty crops would not be expected to respond dramatically to precipitation changes in the short-medium term (Kerr et al. this issue). Vulnerability to precipitation changes is inherently varied and defined by production system.
As described by Elias et al. (this issue), changes in exposure will vary across the region, as temperature will not increase uniformly across all counties. Coastal counties are projected to have a 2C lower mean temperature increase than inland counties of the SW. Similarly, minimum temperatures are anticipated to increase more rapidly than maximum temperatures in the region (Gershunov et al. 2013) disproportionately affecting annual and perennial crops. Growing conditions may not be climatically suitable for many key perennial specialty crops in the region, whereas annual specialty crop temperature–yield relationships are more complex, with both positive and negative impacts of climate change, indicating a need to evaluate impacts on a crop-by-crop basis to account. Even among annual crops, impacts will be different for different crops because species exhibit varied thermal tolerance. For example, by midcentury, area suitable for maize cultivation is projected to decrease, while area suitable for cotton cultivation expands northward and nearly doubles in extent. Heat stress already reduced historic (1950–2005) yields in the region and is predicted to reduce cotton and maize yields by 37% and 27%, respectively, compared to potential yield by midcentury.
Growers can alleviate crop heat stress by increasing irrigation (Haim et al. 2008), which enhances evaporative cooling. This is a common adaptation strategy for many crops cultivated in the SW (Nicholas and Durham 2012; Radin et al. 1994). However, delivering adequate water to reduce heat stress and maintain yields may be a challenging or even impossible adaptation strategy with predicted reductions in water supply and increased water scarcity (Schoups et al. 2005). Water availability may be the most critical factor impacted by a changing Southwestern climate. Though beyond the scope of this collection, surface water quantity and quality, diminishing and over-tapped groundwater resources, and continued competing demands are critical to many sectors of the region and described more completely in articles of this collection and elsewhere.
From a livestock perspective, animal agriculture is highly adaptive, but producers need to understand the ecological characteristics of their specific landscapes to cope with emerging climatic changes. For example, Havstad et al. (this issue) report that the most lucrative beef cow revenues surpassed $30 M per county in the 14 counties of the Great Salt Lake Area (MLRA 28a). Surface water is used to grow animal feed crops as 27% of MLRA 28a is private cropland used in support of livestock production. Even in the more arid regions of the SW, regionally available-harvested forages from irrigated croplands are vital to support beef calf–cow operations and support the essential need for water for agriculture.
The hotter and drier conditions predicted in a changing climate represent a “new normal” that could exacerbate crop yield declines from unexpected irrigation interruptions and rangeland productivity due to decreased precipitation (Diffenbaugh et al. 2015; Seager et al. 2007). If drought conditions become the “new normal,” then groundwater reserves would likely decline (Taylor et al. 2013) effectively removing the secondary source responsible for buffering severe drought effects.
Theme three: spatial analysis supports informed adaptation, within and outside the Southwest
The articles of this collection indicate that adaptation measures could be targeted at vulnerable locations for particular forest or crop types. Thorne et al. (this issue) report that some forested regions were at risk in all climate scenarios tested whereas some areas were less impacted, indicating locations where planned adaptation is necessary with higher certainty. The most widespread forest type in the SW, pinyon juniper woodland, was less exposed. In contrast, redwood forests and oak woodlands were highly vulnerable in all four analyses. There is a growing need for forest adaptation management that anticipates landscape-scale climate change effects. Spatial analyses provide information to identify suitable sites of higher elevation where sensitive conifers might be replaced by other conifer species via adaptive management (Lenihan et al. 2003).
From a crop perspective, spatial analysis of Elias et al. (this issue) provided an initial means to rank order which regions should be examined more rigorously. For example, the four North Central Valley California counties (Butte, Colusa, Glenn, and Sutter) with large rice cultivation area (76–95%) were less impacted than other counties with similar area under cultivation but composed of different crops. Conversely, Yuma, Arizona, was more impacted than other counties with similar cultivation area due to production of sensitive crops near the upper bounds of optimal temperatures. Thus, spatial analyses provide information for targeted responses.
While it is intuitive to assume that some systems are already detrimentally impacted in the arid SW, regional temperature–yield analysis of this SI supports this assertion. Estimates of yield reduction from heat stress for maize and cotton indicate that heat stress reduced cotton yield by 26% and maize yield by 18% compared with potential yield. Spatial analyses for cotton, alfalfa, and maize highlight how thermally suitable production areas are projected to shift. Some locations will shift outside thermal range by midcentury; however, impacts are crop-dependent. While the potential area for maize cultivation is projected to decrease by 20%, a few regions will shift into the thermal range for maize. Cotton cultivation area expands northward and nearly doubles by midcentury (Elias et al. 2017a,b); so, given adequate soil quality and water resources, other areas could learn from the Arizona and California cotton-growing regions.
Moreover, some locations outside of the Southwestern US will transition to similar climatic conditions in the future. Southwestern agriculture has learned to function adaptively to support resilient communities. The knowledge base that supports Southwestern resilient agriculture can be valuable to producers in other areas possibly facing similar conditions for the first time. Climate analogue analysis (Ramirez-Villegas et al. 2011), which operates on the premise that there are present locations able to represent future climate, though at different geographic locations, could be valuable to producers both within and outside the SW. Therefore, adaptive capacity can be conferred by both experiential knowledge of similar events (i.e. previous droughts) and similar landscapes (i.e. climate analogues).
Producers typically use past experiences of weather and related agricultural impacts to assess and manage future risks (Wilke and Morton 2017). While we may not simply linearly extrapolate future climatic conditions based on historical trends (e.g. non-stationarity; Milly et al. 2008), adaptation and management responses in the Southwest may act as analogues for other producers in areas expected to experience warmer and drier climates. The SW may act as a harbinger of things to come; however, the resilience of SW producers serves as an example to build adaptive capacity here and elsewhere (Wilder et al. 2010).
Theme four: adaptation has long been practiced in the region and will continue
An alternate understanding of adaptation has been noted among rural residents of the Southwest who understand, respond to, and plan for weather and climate (Brugger and Crimmins 2013). Humans are seen to co-evolve with the climate, acquire experiential knowledge, adjust activities to long-term climate, and be motivated by an attachment to place and rural values.
Irrigation is a long-practiced adaptation strategy in the Southwest; the evaporative cooling effect allows crops to persist in locations hotter than reported thermal ranges. Five of the states with the highest total irrigation water use in the US are located in the Southwest. Water was the most frequently mentioned climate- and weather-related topic in an assessment of Arizona producers (Brugger and Crimmins 2013). In the Southwest, most adaptive solutions will involve water. Growers may select drought-tolerant cultivars or, where possible, practice deficit irrigation to ensure enough available water during critical periods, such as flowering. Adaptation and enhancing adaptive capacity to climatic impacts encompasses many options beyond the cursory examples mentioned here.
Regional water availability will be directly and indirectly impacted by elevated temperatures. In addition, elevated temperatures may necessitate earlier planting to adjust for temperature shifts. However, earlier spring planting can increase frost damage risk (Kim et al. 2014); therefore, shifting planting dates to accommodate warming trends may not be a consistently viable adaptation option. Similarly, forest managers will need to decide to manage for resilience, resistance, or allow forests to change (Millar and Stephenson 2015). A full range of active forest management activities will likely occur across the span of southwestern forests (Thorne et al. 2016).
Harvested forages from irrigated croplands are a vital adaptive strategy in times of lower available rangeland forage production. The thermally suitable area for irrigated alfalfa production will decrease by 14% by midcentury in multiple areas of the SW (Elias et al. 2017a,b). The alfalfa cultivation regions impacted by changing future temperatures are California’s Imperial Valley, the lower Colorado River Valley along the California–Arizona border, and the Gila River corridor west of Phoenix. From a rangeland perspective, some areas of diversification to impart resilience include using other sources of water (degraded water) for irrigation, expanded production of drought-tolerant feeds (Joyce et al. 2013), and using cattle well-suited to lower forage production and non-grass forages, such as Raramuri Criollo cattle (Anderson et al. 2015).
Transformative practices to sustain animal agriculture, such as the promise of Criollo cattle, are highlighted by Havstad et al. (this issue). Numerous long-term studies on the grazing effects of large-cattle breeds with genetics from northern Europe, which are typically used in the Southwestern United States (i.e. Angus, Hereford), indicate that these breeds tend to focus their grazing along well-defined cattle trails as well as near water sources, such as wells and tanks (Bailey 2004). From an environmental perspective, these behaviors are associated with perennial grass losses (Nash et al. 1999), erosion, and dust emissions. From an economic perspective, these behaviors translate into requirements for expensive supplemental feed, especially during severe drought. A markedly different type of cattle has been identified based on weight, genetic origin, and adaptation to harsh desert environments, which may be better-suited to a warmer, drier Southwest. Research is underway to describe why the impact of Criollo on arid and semi-arid landscapes seems to be minimal compared to larger breeds (Peinetti et al. 2011). Preliminary results indicate that the Criollo will travel further from water, utilize pasture more uniformly, and use ecological/states that differ markedly from the large-cattle breeds, all of which translate to a smaller environmental impact. Criollo cattle may serve as an alternative to larger breeds to enhance livestock production adaptive capacity on Southwestern rangeland systems.