Historically, global warming, and cooling occur naturally.
Ice ages occur in about 100,000 year cycles.
Changes in the eccentricity of the earth's orbit are the driving factor.
When the orbit is very eccentric, an ice age occurs.
when the orbit is more circular, we have an inter-glacial.
that's what's happening now, and it will be a few thousand years before we can expect natural causes to drive the climate toward a new ice age.
http://en.wikipedia.org/wiki/Milankovitch_cycles
Here's the bigger version of the graph.
Note how ice age cycles line up with eccentricity cycles.
http://en.wikipedia.org/wiki/File:Milankovitch_Variations.png
This time it's different. And here's why.
http://dels.nas.edu/dels/rpt_briefs/climate_change_2008_final.pdf <== here's the science.
http://fora.tv/2009/08/18/A_REALLY_Inconvenient_Truth_Dan_Miller <== This is an hour long, but is very good.