The governing body of any local unit of government may declare, by ordinance or resolution, an emergency existing within the local unit of government whenever conditions arise by reason of a riot or civil commotion, a disaster, or an imminent threat of a disaster, that impairs transportation, food or fuel supplies, medical care, fire, health or police protection, or other critical systems of the local unit of government. The period of the emergency shall be limited by the ordinance or resolution to the time during which the emergency conditions exist or are likely to exist.