http://www.clearlight.com/~mhieb/WVFossils/ice_ages.html
http://vathena.arc.nasa.gov/curric/land/global/climchng.html
http://junkscience.com/MSU_Temps/historical_CO2.htm
http://www.geocraft.com/WVFossils/Carboniferous_climate.html
These two figures show former temperatures with major periods of glaciation labeled. The dashed lines are the present global average temperature of about 15° C (59° F). Thus the solid curves show small changes from this average; note that the temperature drops only about 5° C during a glaciation. This has occurred about every 100,000 years, with smaller wiggles in between. That is, there has been a 100,000 year glaciation cycle for the past million years or so, and there may be shorter cycles as well.
The most recent glaciation, 20,000 years ago, is called the Laurentide, and Earth is still recovering from it. This map from the The Illinois State Museum exhibit on ice ages shows the extent of that ice.
The most recent small drop in average temperature caused the Little Ice Age of 1500-1700 AD, which history describes. Mountain glaciers advanced in Europe and rivers like the Thames in England froze solid, which doesn't happen now.
Click image for animation
The growth of the ice sheets began about 120,000 years ago as ice built up on the continents in the Northern Hemisphere, especially in Canada and Europe. The largest extent of these ice sheets occurred 18,000 years ago. At that time the largest ice sheets were between 3.5 and 4 km thick. In North America the largest ice sheet was the Laurentide Ice Sheet centered on Hudson Bay with other sheets centered on Greenland and in the Canadian Rocky Mountains. As these ice sheets expanded they grew together, covering Baffin Bay and eventually the Great Lakes and New England. In northwestern Europe the Fennoscandian Ice Sheet began to grow and expand south to cover what is now Norway and Sweden and north to cover the exposed continental shelf. Over time the ice sheet grew to cover Finland and the United Kingdom. This ice sheet extended east to the Ural Mountains where it met the Siberian Ice Sheet. Before the last ice age ice sheets already existed on Antarctica and on Greenland.
Most people seem surprised when we say current levels are relatively low, at least from a long-term perspective - understandable considering the constant media/activist bleat about current levels being allegedly "catastrophically high." Even more express surprise that Earth is currently suffering one of its chilliest episodes in about six hundred million (600,000,000) years. Given that the late Ordovician suffered an ice age (with associated mass extinction) while atmospheric CO2 levels were more than 4,000ppm higher than those of today (yes, that's a full order of magnitude higher), levels at which current 'guesstimations' of climate sensitivity to atmospheric CO2 suggest every last skerrick of ice should have been melted off the planet, we admit significant scepticism over simplistic claims of small increment in atmospheric CO2 equating to toasted planet. Granted, continental configuration now is nothing like it was then, Sol's irradiance differs, as do orbits, obliquity, etc., etc. but there is no obvious correlation between atmospheric CO2 and planetary temperature over the last 600 million years, so why would such relatively tiny amounts suddenly become a critical factor now? 