What is a noun for the word scarce?

There isn't a single perfect noun equivalent for "scarce." The closest you can get depends on the context:

General Scarcity:

* Shortage: This is the most common and general term.

* Deficit: This emphasizes a lack of something needed.

* Paucity: This suggests a small quantity or number.

Specific Scarcity:

* Rarity: This emphasizes something being uncommon or rare.

* Lack: This is a more direct way to say there is not enough.

* Depletion: This suggests something has been used up or consumed.

The best noun to use will depend on what you are trying to describe.