1. a The Natural Step Foundation , Slottsbacken 6, S-111 30, Stockholm, Sweden
2. d Institute of Physical Resource Theory, Chalmers University of Technology and Gotebörg University , S-412 96, Gotebörg, Sweden
3. b School of Affairs, University of Maryland , College Park, MD, 20742, USA
4. c The Natural Step , 4000 Bridgeway, Suite 102, Sausalito, CA, 94965, USA