Tragedy of the commons

A very good overview of the issues from The Economist

It is not simply that three-quarters of those living on less than $2 a day still depend in some way on commonly held resources. The concept of the commons is also spreading to new areas. Their essential feature is that they share one characteristic with private property and one with public goods. Like public goods, they are not “excludable”: the common resource is too extensive to keep people out very easily. But they are also “subtractable” (or “rivalrous”), like private property: if one person uses them, another’s access is diminished. (With a classic public good, such as street lighting, one person’s usage does not affect anyone else.) Many things other than rainforests or drylands share these attributes.

The important point for new areas like climate change is that tragedy is not innevitable.