  We present a phase-space model that simulates Pleistocene ice volume changes
based on Earth's orbital parameters. Terminations in the model are triggered by
a combination of ice volume and orbital forcing and agree well with age
estimates for Late Pleistocene terminations. The average phase at which model
terminations begin is approximately 90 +/- 90 degrees before the maxima in all
three orbital cycles. The large variability in phase is likely caused by
interactions between the three cycles and ice volume. Unlike previous ice
volume models, this model produces an orbitally driven increase in 100-kyr
power during the mid-Pleistocene transition without any change in model
parameters. This supports the hypothesis that Pleistocene variations in the
100-kyr power of glacial cycles could be caused, at least in part, by changes
in Earth's orbital parameters, such as amplitude modulation of the 100-kyr
eccentricity cycle, rather than changes within the climate system.
