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International political borders rarely coincide with natural ecological boundaries. Because neighboring countries often share ecosystems and species, they also share ecosystem services. For example, the United States and Mexico share the provisioning service of groundwater provided by the All‐American Canal in California; the regulating service of agave crop pollination by long‐nosed bats; and the aesthetic value of the North American monarch butterfly, a cultural service. We use the Millennium Ecosystem Assessment (MA) to elucidate how drivers in one country can affect ecosystem services and human well‐being in other countries. We suggest that the concept of ecosystem services, as articulated by the MA, could be used as an organizing principle for transboundary conservation, because it meets many of the criteria for successful transboundary policy. It would frame conservation in terms of mutual interests between countries, consider a diversity of stakeholders, and provide a means for linking multiple services and assessing tradeoffs between uses of services.

The spatial and human dimensions of climate change are brought into relief at international borders where climate change poses particular challenges. This article explores "double exposure" to climatic and globalization processes for the U.S.-Mexico border region, where rapid urbanization, industrialization, and agricultural intensification result in vulnerability to water scarcity as the primary climate change concern. For portions of the western border within the North American monsoon climate regime, the Intergovernmental Panel on Climate Change projects temperature increases of 2 to 4 • C by midcentury and up to 3 to 5 • C by 2100, with possible decreases of 5 to 8 percent in precipitation. Like the climate and water drivers themselves, proposed societal responses can also be regionalized across borders. Nevertheless, binational responses are confronted by a complex institutional landscape. The coproduction of science and policy must be situated in the context of competing institutional jurisdictions and legitimacy claims. Adaptation to climate change is conventionally understood as more difficult at international borders, yet regionalizing adaptive responses could also potentially increase resilience. We assess three cases of transboundary collaboration in the Arizona-Sonora region based on specific indicators that contribute importantly to building adaptive capacity. We conclude that three key factors can increase resilience over the long term: shared social learning, the formation of binational "communities of practice" among water managers or disaster-relief planners, and the coproduction of climate knowledge. Key Words: adaptive capacity, climate change, U.S.-Mexico border, vulnerability, water.Las dimensiones espaciales y humanas del cambio climático se hacen particularmente relevantes en las fronteras internacionales, lugares donde el cambio climático genera retos especiales. Este artículo explora la "doble exposición" de los procesos climáticos y globalizadores para la región fronteriza EE.UU.-México, donde la rápida urbanización, industrialización e intensificación agrícola resultan en vulnerabilidad por escasez de agua, como la preopcupación primaria por el cambio climático. En porciones de la frontera occidental ubicada dentro del régimen climático del monzón norteamericano, el Panel Intergubernamental de Cambio Climático proyecta incrementos de las temperaturas de 2 • a 4 • C para mediados de siglo y de hasta 3 • a 5 • C para el 2100, junto con una posible disminución de la precipitación de 5 al 8 por ciento. De la misma manera que ocurre con lo concerniente a clima y agua, las respuestas sociales que se proponen también pueden regionalizarse a través de las fronteras. Sin embargo, las respuestas binacionales se ven confrontadas con un paisaje institucional complejo. La coproducción de ciencia y políticas debe situarse en el

▪ Abstract  The US-Mexico border region illustrates the challenges of binational environmental management in the context of a harsh physical environment, rapid growth, and economic integration. Transboundary and shared resources and conflicts include limited surface water supplies, depletion of groundwater, air and water pollution, hazardous waste, and conservation of important natural ecosystems. Public policy responses to environmental problems on the border include binational institutions such as the IBWC, BECC and CEC, the latter two established in response to environmental concerns about the North American Free Trade Agreement (NAFTA). Environmental social movements and nongovernmental organizations have also become important agents in the region. These new institutions and social movements are especially interesting on the Mexican side of the border where political and economic conditions have often limited environmental enforcement and conservation, and where recent policy changes also include changes in land and water law, political democratization, and government decentralization.

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