This paper overviews the latest information on the impact of global warming on water cycles and resources, with a focus on links to health science. Many people may think that water issues mainly involve securing safe drinking water for regions lacking this crucial resource. However, of world water withdrawals, 70% is for agricultural water. Therefore, issues of water scarcity are highly connected with agriculture and food production. Global warming is expected to result in decreased water availability in semi-arid regions where major crop regions are located. Crop production in semi-arid regions already requires more water inputs than does agriculture in regions of ample natural rainfall; with global warming, the water situation in semi-arid regions is expected to worsen. In addition to the decrease in water for agriculture, domestic and drinking water supplies will also be threatened in semi-arid areas. In contrast, other regions may face the problem of too much water and flood disasters. Currently, the flood-affected population worldwide ranges from an annual minimum of approximately 30 million to a maximum of 300 million people. Projections for the late 21st century, however, suggest that 300 million will be the minimum number affected by flooding each year. The consequences of water decreases in some areas and increases in others caused by global warming will likely have close links to health. Therefore, health scientists should pay close attention to issues of climate and water resource change.