In this time of ‘global warming’ it may seem strange to refer to ice ages, but if you take the history of the planet earth, there are regarded as 5 main ice ages over this time, the earliest known one happening around 2 billion ago, the last started about 3 million years ago. We may still be within that ice age. Within those period you have interglacial periods of warm and cold, over the last 450,000 years there have been 5 of these and we may be in one at the moment, a time of being warmer than normal for the ice age.
But what seems to be noticeable is the sudden change from one state being an average of 4-5°C to an average of 20-21°C or suddenly back again. The rise and fall seem very quick in geological terms, because we were not there not knowing how fast this happens. We assume it takes tens thousands of years, but could happen in hundreds or less.
The past lows seem to be at about 430,000, 355,000, 270,000, 225,000, 190,000, 150,000, 70,000 and 25,000 years ago. The past highs seem to be 410,000, 335,000, 295,000, 250,000, 210,000, 180,000, 140,000, 95,000, 50,000 and now. Taking the two cycles together they seem to average out at a peak or a low every 52,000 years which is close to the average of 5 whole cycles within 450,000 averaging 45,000 years.
Since the last peak was around about 50,000 years ago we should be close to a peak and possibly to risk maybe 7°C further, but what comes after that; possibly a fall of up to 17°C within 5,000 years. Nobody can be sure of the time period. With a runaway climate in either direction it could happen within a 100 year period. All we can estimate is that the earth has likely a standard average climate range of between 4-21°C spending roughly as much of this time at the bottom as at the top.
It may be that the very characteristics that result from a very hot period promote the characteristics that force a fall in temperature and vice versa. So, a hot period produces a sudden fall when thresholds are exceeded, and a cold period produces a rise when other thresholds are exceeded, quite possibly due to the relective and heat absorbing nature of ice, water and water vapour.