If I understand it correctly (I am into arts in school and uni),...

If I understand it correctly (I am into arts in school and uni), the ice age ended because the earth tilted differently so the earth started having longer summers (this is what that research says), while CO2 in the atmosphere warms the earth up because, as mentions by shadyjames above: "CO2 is more opaque to infra-red than visible light. Thus it will trap outgoing IR radiation but let in the sun, making things warmer the more there is."