The U.S.-Mexican border is characterized by a semi-arid climate, with higher precipitation toward both coasts. Evaporation exceeds precipitation for many months of the year, causing a soil moisture deficit. Many studies relate climate and hydrology along the U.S. border states (Green and Sellers, 1964; Hastings and Turner, 1965; Norwine, 1978; Cayan and Peterson, 1989) and along the Mexican border states (Mosiño and García, 1973; Schmidt, 1975; Acosta, 1988; Cavazos, 1998). The western border has an annual rainfall of about 250 mm yr-1, with temperatures ranging from 10 to 28°C. The Arizona-Sonora border has a much more extreme and desert climate, with only 80 mm yr-1 of precipitation and maximum temperatures reaching 45°C. Around the Ciudad Juarez (Mexico)-El Paso (Texas) border, annual precipitation is around 220 mm, with maximum temperatures reaching 40°C. Finally, in the easternmost part of the border (Brownsville, Texas-Matamoros, Mexico), annual precipitation is close to 675 mm, with temperatures ranging from 11 to 38°C.
There is large climate variability along the U.S.-Mexican border states. Drought and floods occur frequently (Powell Consortium, 1995), some influenced by the occurrence of El Niño (Cavazos and Hastenrath, 1990; Cayan and Webb, 1992). The Mexican states along the border are highly vulnerable to drought, particularly along the Rio Grande region, areas with low precipitation rates (less then a 100 mm yr-1) (Mundo and Martínez Austria, 1993; Hernández, 1995; Mendoza et al., 1997). Strong El Niño events during the summer result in severe water deficit in northern Mexico (Magaña and Quintanar, 1997), and serious negative impacts in agricultural activities. Such vulnerable conditions during drought periods have resulted in legal disputes between Mexico and the state of Texas with regard to rights to Rio Grande water.
It is unclear what the signs of future changes in precipitation and water availability will be along the border (Mearns et al., 1995; Magaña et al., 1997; Mendoza et al., 1997). Some scenarios suggest increased winter precipitation and decreased summer precipitation (Magaña et al., 1998). Without reliable predictions of precipitation changes across drainage basins, little confidence can be placed in hypothesized effects of global warming on annual runoff (Karl and Riebsame, 1989). Therefore, to examine potential impacts of climate change on water availability, most studies make use of long instrumental records of precipitation and streamflow. Some analyses indicate that in recent decades there is a positive trend in streamflow and even precipitation, corresponding to more water availability along the U.S.-Mexican border (Magaña et al., 1998; Magaña and Conde, 2000). However, demand for water in agriculture, industries, and cities is increasing steadily, surpassing recent increases in water availability (Mundo and Martínez Austria, 1994).
The signal of climate change along the U.S.-Mexican border may be exacerbated along the Mexican side by differences in land use. Along the U.S. side, the presence of gardens and green areas lead to a cooler environment relative to the Mexican side. Currently, maximum temperatures in contiguous border towns may differ by as much as 3°C as a result of different characteristics in vegetation (Balling, 1988). This cross-border contrast is possible because water consumption in U.S. border cities is four times larger than in Mexican border cities. Surface water distribution along the border (Rio Grande and Colorado watersheds) is regulated by the Binational Treaty of 1944. Yet, there are no regulations for subsurface water (Sanchez, 1994).
Some of the U.S.-Caribbean border problems that may increase as a result of climate change are related to:
Considering the high population density in part of the Caribbean islands and coastal areas, human settlements will be highly affected by sea-level rise and saline intrusion (Vincente et al., 1993). Coral reefs in the region may be severely affected by coral bleaching induced by warmer water temperatures (Milliman, 1993).
Tourism is a major economic activity in most of the region. A change in rainfall patterns, tropical storms, and warming of the climate in temperate countries may affect the comparative advantages of this sector in the region (Alm et al., 1993). Narrow beaches combined with projected sea-level rise contribute to the vulnerability of the tourism sector to changes in climate (Gable, 1997).
Potential consequences of changes in extreme events (e.g., hurricanes) are not well-defined. Other trends, such as changing demographic patterns, may exacerbate impacts. Recent large losses of life from rainfall-induced floods, mudflows, and landslides reflect increasing concentrations of residents in high risk areas (Rodriguez, 1997; Pulwarty, 1998).
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