Peak water is a concept that underlines the growing constraints on the availability, quality, and use of freshwater resources. Peak water was defined in a 2010 peer-reviewed article in the Proceedings of the National Academy of Sciences by Peter Gleick and Meena Palaniappan. They distinguish between peak renewable, peak non-renewable, and peak ecological water in order to demonstrate the fact that although there is a vast amount of water on the planet, sustainably managed water is becoming scarce. There are concerns about impending peak water in several areas around the world: