This graph shows the global average temperature variation for the past 450 thousand years. (Time runs from right to left.) The pulse of the ice ages can be seen, but the signal appears noisy, with numerous irregular jumps. The mechanism which causes this sequence of pseudo-periodic oscillations in the climate is not well understood and is a topic of active research in climate mathematics.
Data from ftp://ftp.ncdc.noaa.gov/pub/data/paleo/contributions_by_author/zachos2001/zachos2001.txt