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342 assessment of climate change in the southwest united statesThis chapter defines the border region and how observed climate trends since 1961and projected climate change conditions have affected or are likely to affect the region.Next, the chapter provides a framework to understand climate-related vulnerability,adaptive capacity, and adaptation, and examines the major drivers (forces) that lead tovulnerability and the evidence of sectoral impacts of climate change resulting in vulnerabilityin the border region.There are three important caveats as to the scope and analysis of this study. First, consistentwith the risk-based vulnerability framework suggested by the National ClimateAssessment, this discussion assesses the sensitivity, exposure, and capacity for responsefor a given population or sector. The evidence for this analysis is based on qualitativeand (to a lesser extent) quantitative studies in specific contexts within the border region.With regard to the climatology, vulnerability, consequences, and impacts, there are nocomprehensive studies that encompass the western portion of the U.S.-Mexico border.The highlighted vulnerabilities presented in the executive summary are those that aresustained by strong evidence from multiple contexts within the region. In most cases (especiallydrawing on qualitative studies), while the evidence of a vulnerability, impact,or consequence may be strong, it is often not sufficiently calibrated to assess degreesof exposure or sensitivity (of populations or sectors) to a climate risk. Thus, to a largeextent, there is an imperfect fit between the kind of evidence available and the requirementto assess precisely the relative exposure and sensitivity. Second, for the purposesof this report, this chapter analyzes and highlights those areas of vulnerability (e.g., urban,agriculture, socioeconomic) judged to be of paramount importance and for whichthere is robust evidence, while excluding other important regional vulnerabilities (suchas health and livelihoods). Third, this chapter focuses on assessing key vulnerabilities. Itis not within the scope of this chapter to fully represent the adaptation activities—ongoingor planned—that may aid in reducing these vulnerabilities, but some are discussedhere. (See also Chapter 18 for a general overview of solutions and choices for respondingto climate change in ways that reduce risks and support sustainable development.)Throughout this chapter, four callout boxes present evidence of successful trans-bordercooperation or collaboration. Collaboration is a significant component for strengtheningthe region’s adaptive capacity and for building resilience.16.2 Definition of the Border RegionThe analysis here focuses on the U.S.-Mexico border—delimited for the purposes of thischapter to the western portion of the border from San Diego-Tijuana on the Pacific coastto the Paso del Norte area i of the Rio Grande—which has been identified as a distinct regionthat serves as the interface between Mexico and the western United States (Gansterand Lorey 2008) (Figure 16.1). The western portion of the border region corresponds tothe definition used in the National Climate Assessment (NCA) for the Southwest region,which includes among the U.S. border states only California, Arizona, and New Mexico(and excludes Texas, which is part of the NCA’s Middle West region). Nevertheless, thisanalysis incorporates the Paso del Norte corridor due to its key importance in the NewMexico portion of the border region.