CO2 does not “trap heat” in the atmosphere and the increase in CO2 is not a buildup of our emissions. In fact these are two pseudoscientific myths that appear frequently in alarmist press reports and teaching materials. Neither one is true.
The science is quite different. It is actually pretty complex, which is why the climate change science is so widely debated.
No heat is trapped by CO2 in the air
CO2 both adds heat to the atmosphere and removes it, so it certainly does not trap it there. CO2 is a secondary greenhouse gas (GHG), with water vapor being the primary GHG. What being a GHG means is two different things, as GHGs both add and remove heat in the atmosphere.
It all begins when the sun’s incoming energy heats the surface of the Earth. Some of that energy is then given off as infrared light, which is usually called “long wave radiation” or LWR. A lot of this LWR simply passes through the atmosphere and goes out into space, where it is gone forever. But some of it is intercepted and absorbed by GHG molecules.
These energized molecules then give off this absorbed LWR energy to the rest of the air as heat. (Heat is not a substance; rather it is just molecular motion.) So at this point we can say that the CO2 has heated the