Cities have the best chance of reducing water use when people actively participate in water conservation, but even that might not be enough in the future.

For example, under a plausible, moderately high emissions scenario, Phoenix’s available surface water supply was forecast to drop below the historical average by 2060. Even when we simulated higher participation in conservation programs, there was no noticeable change in the water availability, suggesting that any savings from reducing demand were counteracted by losses from upstream flow reductions. Encouraging people to use less water is a start, but there is a limit to how much people can conserve.

We found similar results in Denver under a moderate emissions scenario and in Las Vegas under a moderately high emissions scenario, indicating that even moderate climate change could lead to extreme scarcity conditions that are not manageable through demand-side changes alone.