1. Keeling C. C. (1973) Private communication.
2. Minerals Yearbook, (annually) Bureau of Mines, United States Department of the Interior, U.S. Government Printing Office, Washington, D.C.
3. Commentary on and extension of calculative procedures for CO2 production;Rotty;Tellus,1973
4. Statistical Yearbook, (annually) Department of Economic and Social Affairs, United Nations, New York.
5. World Energy Supplies,1972