Researchers associated with the Potsdam Institute for Climate Impact Research have just published a new study in Nature claiming that because of man-made global warming, the next ice age will start 100,000 years rather than just 50,000 years from now. During the last glacial maximum about 21,000 years ago, glaciers covered about 25 percent of the Earth's land area and sea level was more than 400 feet lower than today. Global average temperature was about was 3°C to 5°C cooler than the present. The rhythm of ice ages over the past 3 million years or so is related to how predictable changes in the Earth's orbit affects the amount of sunlight hitting the northern hemisphere. During the last ice age levels of carbon dioxide in the atmosphere fell to below 200 parts per million.
What I found most interesting in the study is that the researchers suggest that we barely missed entering a new ice age a couple of hundred years ago:
Using an ensemble of simulations generated by an Earth system model of intermediate complexity constrained by palaeoclimatic data, we suggest that glacial inception was narrowly missed before the beginning of the Industrial Revolution. The missed inception can be accounted for by the combined effect of relatively high late-Holocene CO2 concentrations and the low orbital eccentricity of the Earth.
For reference, the current level of atmospheric carbon dioxide is around 400 ppm largely thanks to humanity.