More significantly, Prop. 90 also addresses regulatory takings. It requires compensation whenever a government action significantly decreases property value, subject to some exceptions. One of the exceptions is a grandfather clause for existing rules; another allows regulation for public health or safety.
Prop. 90 is remarkably ambiguous and vague. Thus, if it is adopted, its effect on environmental regulation in California is difficult to predict. If given broad sweep by the California courts, it could make certain kinds of environmental regulation substantially more difficult. At the very least, Prop. 90 will be a fertile source of litigation, keeping lawyers busy and well paid for years to come.
To find out more about Prop. 90, you might want to look at the white paper (here) issued by the California Center for Environmental Law & Policy (CCELP).