evidence: Several causes have been proposed: cyclical lows in solar radiation, heightened volcanic activity, changes in the ocean circulation, variations in Earth's orbit and axial tilt (orbital forcing), inherent variability in global climate, and decreases in the human population (such as from the massacres by Genghis Khan, the Black Death and the epidemics emerging in the Americas upon European contact[12][13]). [...] - ^ Pollack, Henry N.; Huang, Shaopeng; Smerdon, Jason E. (2006). "Five centuries of climate change in Australia: The view from underground". Journal of Quaternary Science. 21 (7): 701. Bibcode:2006JQS....21..701P. doi:10.1002/jqs.1060.