The most likely candidate for that climatic variable
force that comes to mind is solar variability (because I can think of no other
force that can change or reverse in a different trend often enough, and quick enough to account for the historical climatic record) and the
primary and secondary effects associated with this solar variability which I feel are a significant player in glacial / inter-glacial cycles, counter climatic trends when taken into consideration with these factors which are, land / ocean arrangements, mean land elevation, mean
magnetic field strength of the earth (
magnetic excursions), the mean state of the climate (average global temperature), the initial state of the earth's climate (how close to interglacial - glacial threshold condition it is) the state of random terrestrial (violent volcanic eruption, or a random atmospheric circulation / oceanic pattern that feeds upon itself possibly) / extra terrestrial events (super-nova in vicinity of earth or a random impact) along with Milankovitch Cycles.