Fairly regularly, for the past 700,000 years, a glacial period has occurred every 100,000 years. These periods are characterized by a significant drop in both average surface temperature and atmospheric CO₂ levels. During glacial periods, surface temperature dropped from about an average of about 15°C to 10°C. Atmospheric CO₂ levels dropped from about 280 ppm to 200 ppm.

Prior to 700,000 years ago, glacial periods actually occurred more frequently. About every 40,000 years, the Earth experienced a glacial, although of less severity than the more recent glacials. There may be multiple causes of this effect, some relating to the Earth's orbit, but they will not be discussed within this presentation.

In the graph below, notice the increased irregularity in temperature towards the more recent past.