1. IEA (International Energy Agency), World Energy Outlook (2017)
2. Own research by Winfried Hoffmann
3. E.U. von Weizsäcker, A.B. Lovins, L.H. Lovins, Factor Four: Doubling Wealth–Halving Resource Use: The New Report to The Club of Rome (Earthscan, London, 1998)
4. E.U. von Weizsäcker, K. Hargroves, M. Smith, Factor 5. Transforming the Global Economy through 80% Improvements in Resource Productivity, (Earthscan, London, 2009)
5. UN (United Nations), World Population Prospects: The 2012 Revision, Highlights and Advance Tables (Working Paper No. ESA/WP.228 2013)