Nobody ever says CO2 emits heat. That's not how global warming works. Heat is held in the atmosphere longer since it radiates in but can not radiate out as easily because greenhouse gases prevent that.

It can pass through because it has higher energy on the way in. It naturally loses some energy in our atmosphere - too much to radiate through and out again. if gases can't hold heat how do you think a hot air balloon works? it doesn't hold it permanently but it does hold it for a while.