"About a million years ago, the earth’s ice ages became colder and longer – with a geologically sudden jump from thinner, smaller glaciers that came and went every 41,000 years to thicker, larger ice sheets that persisted for 100,000 years.
Although several hypotheses have been put forward to explain this transition, including a long-term decline in the atmospheric CO2 level, the phenomenon remains a scientific conundrum.
... studies recognize that the prolonged deep freezes of the ice ages are set off partly by perpetual but regular changes in the earth’s orbit around the sun.
That’s the basis of a hypothesis proposed by Serbian engineer and meteorologist Milutin Milankovitch.
As shown in the figure below, the earth orbits the sun in an elliptical path and spins on an axis that is tilted.
The elliptical orbit stretches and contracts over a 100,000-year cycle (top), while the angle of tilt or obliquity oscillates with a 41,000-year period (bottom), and the planet also wobbles on its axis in a 26,000-year cycle (center).
... it had been suspected that the transition from 41,000-year to 100,000-year cycles was due to a long-term decline in the atmospheric CO2 level over both glacial and interglacial epochs.
But that belief held when ice-core data went back only about 800,000 years.
Armed with their new data from 2 million years in the past, the first Princeton team discovered surprisingly that the average CO2 level was unchanged over that time span, even though the minimum level dropped after the transition to longer ice age cycles.
This means that the 100,000-year transition can’t be attributed to CO2 ...
... While it has long been known that the atmospheric CO2 level and global temperatures varied in tandem over glacial cycles, and that CO2 lagged temperature, the causes of the CO2 fluctuations are not well understood."