The cause of ice ages

It would give everyone a greater degree of confidence in climate predictions if scientists had cracked once and for all the issue of what triggers ice ages. As this post at Real Climate makes clear, they are still several theories around, and no doubt more to come. (Cyclic orbit changes are a significant part of it, but the exact mechanism seems still very much up for grabs.)

The comments to the post include these one, which I add here just to give some background on the whole history of ice ages:

...the question "What triggers ice ages?" only applies to the late Pleistocene (since about 800,000 years ago). From the onset of northern-hemisphere glaciation (about 3 million years ago) to the "mid-Pleistocene transition" (about 800,000 years ago), glacial advance and retreat follows a strong 41,000-year cycle, which has led to its being called "the 41 ky world" (Raymo & Nisancioglu 2003, Paleoceanography, 18, 1011). This is surely due to the changes of earth's obliquity, since changes in the amplitude of the climate signal correspond to changes in the amplitude of the obliquity cycle (Lisiecki & Raymo 2007, Quaternary Science Reviews, 26, 56).

But since the mid-Pleistocene transition (not precisely since, this happens intermittently before that time) glacial changes are dominated by a 100,000-year cycle. The behavior during the "late Pleistocene" was originally attributed to changes in earth's eccentricity, but that idea has now fallen out of favor. Huybers & Wunsch (2005, Nature, 434, 491) and Huybers (Quaternary Science Reviews, 26, 37) have convincingly shown that even during the late Pleistocene, the timing of deglaciations is strongly correlated to the obliquity cycle. They find no such relationship for the precession cycle or the eccentricity cycle.

(This comment seems to be by someone the scientists who run Real Climate trust.)

And this comment is make by one of the Real Climate authors, in response to the question of when would we be next due for an ice age were it not for global warming:

We've just come out of one of the big every-100KYr glaciations, and the normal course of events is to build up to another biggy through a series of small, short glaciations over the next 100KYr. In the normal course of events, the first try at an ice age would be due sometime in the next 20,000 years but I myself wouldn't try to pin it down more than that. One of the most interesting attempts so far to say what global warming might do to the glacial cycle is in the paper (pdf) by Archer and Ganopolski that appeared in the AGU journal GGG. I'll leave it to David to say whether that has been followed up by more detailed GCM work.

By the way, I don't post this to express scepticism about legitimate concern over CO2 levels, but it is interesting that something as significant as ice ages are not properly understood yet.