Ponting, Clive, A New Green History of the World: The Environment and the Collapse of Great Civilizations (New York: Random House, 2011), p. 147.

Lorenz, Edward N., “Predictability: Does the Flap of a Butterfly’s Wings in Brazil Set Off a Tornado in Texas?”, presentation at the 139th meeting of the American Association for the Advancement of Science, Massachusetts Institute of Technology, Cambridge, MA, December 29, 1972, http://eaps4.mit.edu/research/Lorenz/Butterfly_1972.pdf.