Pressure of natural physical forces which causes the actor reasonably to believe that his or her act is the only means of preventing imminent public disaster, or imminent death or great bodily harm to the actor or another and which causes him or her so to act, is a defense to a prosecution for any crime based on that act, except that if the prosecution is for first-degree intentional homicide, the degree of the crime is reduced to 2nd-degree intentional homicide.