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344 assessment of climate change in the southwest united states16.3 Border Region Climate Variability, Climate Change,and ImpactsThe border region considered here is characterized by high aridity and high temperatures.Typically, about half of the eastern part of the region’s precipitation falls in thesummer months, associated with the North American monsoon, while the majority ofannual precipitation in the Californias falls between November and March. The regionis subject to both significant inter-annual and multi-decadal variability in precipitation. iiiThis variability, associated with ENSO, has driven droughts and floods and challengedhydrological planning in the region. iv Further challenging this understanding is a paucityof data, particularly on the high-altitude mountainous regions in northern Mexico.Differences in the availability of high-quality and continuous meteorological and hydrologicalrecords spanning long periods of time, and relatively poor data sharing complicateunderstanding of the border region’s climate. The scarcity of such data makes itdifficult to verify climate model projections at fine spatial scales.Also, reconciling differences in projected changes in temperature, based on global climatemodel (GCM) studies conducted separately by U.S. and Mexican scientists (Table16.1), v is complicated by the fact that (1) they use different sets of models from the IPCCFourth Assessment archive; (2) they use different methods of downscaling output fromcoarse spatial scale models to finer regional spatial scales; vi (3) in some cases they do notuse the same greenhouse gas (GHG) emissions scenarios; (4) they average future projectionsfor different spans of years; and (5) they use different spans of years for providinga measure of average historical climate. High quality data are essential for statisticallydownscaling GCM output. Thus, issues with meteorological observations add to severalother sources of uncertainty (see discussion in Chapters 2 and 19).Table 16.1 Summary of projected changes in selected climate parametersProjected Change Direction of Change Border Subregion Affected ConfidenceAverage annualtemperatureIncreasingThroughout the border region; lowest magnitude ofincrease is near the coast; greatest is Arizona-Sonoraborder or New Mexico-Chihuahua borderHighAverage summertemperatureIncreasingThroughout the border region; greatest increases inthe Sonoran Desert border regionHighAverage wintertemperatureIncreasingThroughout the border region; greatest increases inthe Sonoran Desert border regionHighAverage annualmaximumtemperatureIncreasingThroughout the border region; greatest increasesin the eastern Chihuahuan Desert; only estimatedsouth of the borderMedium-HighAverage annualminimumtemperatureIncreasingThroughout the border region; greatest increasesin the Sonoran Desert; only estimated south of theborderMedium-High