What is the major issue or concern about nonrenewable energy sources?

Non renewable energy sources are the sources which once used, cannot be used again. Because of this reason, they get depleted and exploited and have very less chance of getting replenished.

These non renewable resources like fossil fuel are in limited supply making it a reason of concern because they are used as a major source of energy. Moreover they also have an adverse effect on the environment after they are used like they cause pollution.