A sustainable, renewable natural resource that is used in place of, and as a substitute for, finite, exhaustible natural resources that are being or have been exhausted. A sustainable resource is one in which the amount used today cannot reduce the amount available tomorrow. The best example is solar energy. No matter how much solar energy we use today, the same amount reaches the planet every day. A backstop resource is then a sustainable resource, like solar energy, that society may use after finite resources, like fossil fuels, have been exhausted. In fact, solar energy is considered THE backstop energy resource. It is THE "safety net" that's available when fossil fuels are depleted.