From Envirowiki

Backcasting is a concept that deals with long term plans. It is designed as an alternative to using forecasting as a method for policy making and design. The basic concept is to set a goal, or a scenario end point, and work back from there, figuring out the steps mostlikely to be successful. The concept is the same, or similar to critical path analysis

Forecasting works on taking past trends, and combining them with current information, and extrapolating a scenario from that.