BTW, Orbital forcing (what you mean by glaciation) is indeed substantial but only on the millennial timescale for high-latitudes during the summer season. 

http://www.ncdc.noaa.gov/paleo/pubs/kaufman2009/

Think about it like this, you have a climate signal that's composed of individual forcing factors (orbital forcing, greenhouse gases, volcanoes, the sun, etc....), there's plenty of ways to separate these signals out in fact there's a whole field of engineering and mathematics called signal processing which does exactly that. Once you separate out all the signals from the noise, the strongest and largest contributor to the climate signal now is forcing from CO2. There's other ways to measure CO2's contribution to the climate signal, but I'm most familiar with signal processing and am happy to discuss how it works with you. BTW it's basically the same principal as how your cell phone works, picking up the signal from all the noise.