A state, when a resource is available in a finite quantity at a particular point of time, is called scarcity. Shortage implies a situation wherein the supply of a product is lower than its demand.
Scarcity is when something is rare and difficult to reproduce. On the contrary, the shortage is when an item is popular and easy to get, but sometimes supply does not satisfy demand.
Scarcity is ever-lasting whereas shortage is a short-term, i.e. the problem of shortage can be resolved.
Scarcity is a natural phenomenon. As opposed to the shortage, which is created by market forces of demand and supply.
The term scarcity is used in the context of natural resources like time, oil, land, etc. which depletes over time. Unlike shortage, that occurs for man-made goods or services.