Ice ages has lower CO2 in the atmosphere. But you need to consider that the last time CO2 emissions were similar to today's was the Eemian Interglacial Period, which if you look it into dropped off quite significantly and into an ice age. Given Interglacial Period trends, we are nearing the end of one. As Ksand24 rightly said, the last 200 years of CO2 ppm are not falling in line with the trends of the last 400,000 years, and it's undeniable this is going to have an effect on how things go moving forward, especially if we dont work to curb it.
In a simple model, the earth has what's known as an albedo value ie it's ability to bounce in coming radiation back into space. To have a higher albedo, you needs things like cloud cover, ice and snow, etc. With carbon emissions causing a rise in GMST, we are seeing a decrease in the planet's albedo value ie melting ice caps, warmer winters, etc (among other things), meaning that the earth and it's atmosphere are growing hotter from the radiation it is having less and less ability to refract back into space.
This being just one effect.