From the Niels Bohr Institute:
...With completely new research results geophysicist Peter Ditlevsen, Centre for Ice and Climate at the Niels Bohr Institute, has found part of the explanation for the mystery of the sudden change of the ice ages. He has made model calculations of the climate of the past and compared it to the concrete data from seabed cores, which tell us about the climatic fluctuations of the past.
From the results he has been able to construct a diagram over the possible climatic conditions resulting from the variation in solar radiation. It appears that the ice ages and interglacial periods are not a gradual fluctuation between cold and warm climates.
What happened 1 million years ago was that the climate system went from a situation where it fluctuated between two states (cold and warm) with a 40.000 year cycle, corresponding to the dominant change in the Sun's radiation. After this period the dynamic changed so that the climate jumped between 3 states, that is to say between a warm interglacial climate like our present climate, a colder climate and a very cold ice age climate. It is still the 40.000 year variation in solar radiation which controls our current fluctuations, but it results in changing climate periods of 80.000 and 120.000 years.
Chaotic dynamic climate
The climate does not become gradually colder or warmer - it jumps from the one state to the other. That which gets the climate to jump is that when the solar radiation changes and reaches a certain threshold - a 'tipping point', the existing climate state, e.g. an ice age, is no longer viable and so the climate jumps over into another state, e.g. a warm interglacial period. In chaos dynamics this phenomenon is called a bifurcation or a 'catastrophe'.
In addition to the change in solar radiation there can be random changes in the Earth's weather variations, that contribute to triggering the bifurcation or the 'catastrophe'. Such variations are called 'noise', and a theory is, that the atmosphere's CO2 level can be an important noise-factor. This means that there is the possibility that the 'noise' is a decisive factor for very large climate changes, which can therefore be unpredictable.
There is still no explanation for the change in the climate system 1 million years ago, but one theory is that the atmosphere's CO2-level fell to the lowest level ever. If so, the manmade increase in CO2 may result in a return to 40.000 year ice age cycles.
"The new results are an important piece of the puzzle for understanding the ice ages and their climate dynamics. In the manmade climate changes, that we are possibly in the middle of now, one worries a lot about the possible so-called 'tipping points'. The bifurcations that are now identified in the natural climate fluctuations are tipping points, so this is of course an important step in our understanding of climate changes", explains Peter Ditlevsen.
"The bifurcation structure and noise assisted transitions in the Pleistocene glacial cycles": http://www.agu.org/journals/pa/papersinpress.shtml#id2008PA001673