The interesting thing about CO2 is that it lets the energy of visible light through.
That's why you can see things through our atmosphere with your eyes. CO2 is pretty transparent for visible light.
But when that visible light energy gets absorbed by the earth's surface, the surface heats up and converts that visible light energy into infrared thermal energy.
And CO2 absorbs that very effectively.
So in that sense, you can think of CO2 as a one way insulator.
Because CO2 lets the visible energy in quickly but once that's converted to heat, CO2 slows the speed that the thermal energy gets back out.
