At present, without climate change, the Southwest is relying on the unsustainable withdrawal of groundwater reserves to meet today?s demand; those reserves will be drained over the next century as population and incomes grow. With climate change, the Southwest water crisis will grow far worse. Continuing the current trend in global greenhouse-gas emissions will make the cost of the next century?s projected water shortage at least 25 percent higher. Adaptation (conservation and efficiency) measures, however, have the potential to greatly lower water use throughout the region. As climate change exacerbates water woes, some adaptation will be essential to stave off unplanned water shortages and restrictions. Bringing the Southwest?s water use down to sustainable levels will necessitate either very strong residential adaptation measures, or a combination of strong agricultural adaptation measures (including the elimination of some low-value crops) and moderate residential measures.