Necessity is a justification to crimes. Necessity is similar to the duress defense but duress involves a threat from humans. To establish necessity, a defendant presents evidence that he committed a crime because of pressure from natural forces. The defendant must establish that he believed that the conduct was necessary to avoid harm to society that would be greater than the harm caused by his criminal conduct. The physical pressures render the defendant's otherwise criminal conduct justifiable.
There are two exceptions. The defense is not available to a defendant who is responsible for creating the situation that required him to choose between the two evils. Additionally, causing the death of a person to protect property is never justified.
An example might help explain the circumstances under which the defense is appropriate. During a violent storm on a ship, a passenger determines it is necessary to throw some cargo overboard to save the lives of the people on the ship. Such actions would not constitute criminal damage to property. However, under no circumstances could someone throw passengers overboard in order to save cargo.