One of the outstanding puzzles in paleoclimatology is to determine the cause of ice ages.
Data shows that CO2 (in the red) varies with temperature (black) in the figure below, indicating that CO2 plays a significant role in controlling glaciation, but what controls CO2?
We use simple models to attack this problem.
Output from a 2-equation model on the right shows the major elements of the glacial cycles, including the fact that temperature lags CO2 at the end of ice ages.
In this model, oceanic release of CO2 controls ice ages, which are triggered by variations in the earths orbit.
A. McC. HOGG, (2008).
Glacial Cycles and Carbon Dioxide: A conceptual model.
Geophys. Res. Lett., 35, L01701, doi:10.1029/2007GL032071. [Online copy from GRL]