During the last couple of decades, much work has been done to try to determine earth's average temperature over a long period of time. One of the ways this is done is by drilling ice samples in Greenland and determining from the layers of ice (much like tree rings) what gases are present and what the average temperature was that year.

Unfortunately, this record over Greenland appears to give valid readings only up until the last inter-glacial period. Luckily enough, the drill records for the Antarctic have recently revealed records over the last 400 thousand years.

Consider this chart of temperatures over a 400 Thousand year time period recently gained from ice cores in the Antarctic. Please read from left to right. It starts at current temperatures and goes back in time. It shows the recent spike that marks the end of the last glacial period. Notice the cold periods extend for about 100k years and there are 4 warm spikes approx. every 100k years that last for an average of 10k years. These correspond with readings taken in Greenland over the period of time that Greenland can be measured and are viewed as reflective of overall climate on Earth.

Any description of global warming will have to address the temperature's cyclical nature over the last 400k years.

Extent of Glaciers

Ice coverage of the northern hemispere 18k years
before present compared to modern day

(source: http://www.magazine.noaa.gov/stories/mag95.htm)

Consider these pictures showing ice levels (white and light blue) towards the end of the last glacial period (about 18k years ago) compared to ice levels now (an inter-glacial period). The world was very different, with the ice covering New York, Toronto, Edmonton, Calgary, ... in the 1-2 kilometer range. The sea level was some 100 meters lower than present and the shore lines were closer to what is called the continental shelf, rather than current ocean shore lines. Notice also the land bridge between Alaska and Russia (light green) that facilitated the movement of humans into the Americas from Asia during the last glacial period.

Any description of global warming will have to address the issue of the last 10k years that shows a dramatic reduction in ice buildup over Canada and northern Europe.