'Factor four' is a simple yet radical concept which, it has been suggested, holds the key to sustainable development. It refers to a hypothetical fourfold increase in 'resource productivity', brought about by simultaneously doubling wealth and halving resource consumption.
The concept was introduced in 1998, in a book of the same name written by L. Hunter Lovins and Amory Lovins of the Rocky Mountain Institute, and Ernst von Weizsäcker, founder of the Wuppertal Institute for Climate, Environment & Energy. This is illustrated with 50 examples of technologies that could be called upon to deliver the necessary improvement in resource efficiency, including ultra fuel-efficient cars, low-energy homes
Its origins date back to 1972, when a report by the Club of Rome called 'Limits to Growth' issued a stark warning that economic growth was using up resources at a rate that could not be sustained for much longer.
Often mentioned in the same breath as Factor 4 is 'Factor 10', whose proponents argue that in the long term, resource use in developed countries needs to be slashed tenfold if we are to approach sustainability. The reasoning behind this is that globally, consumption needs to be halved, but that the greatest reduction should be borne by those countries that are currently the most profligate in their use of resources.