Without any CO2 in the atmosphere, the Earth would be a frozen planet.Marker 1
CO2 molecules can reflect heat.
Image by UCAR | visit website for more informationMarker 1
This creates a warming blanket around Earth (greenhouse effect).
Our average temperature today is also 14°C. Making it four degrees colder doesn't seem like a big deal.
But 4 degrees is the difference between a very pleasant Berlin and a Berlin at the edge of a monstrous glacier that covered a third of Europe.
This is definitely not completely accurate. The ice is a picture of the Perito Moreno glacier in Argentina. I've done my best to scale it: The Reichstag building is 48 m high, the highest columns of the glacier about 70 m. The real glacier would probably be less steep at its end.
A map of the last ice age from Wikipedia:
A very cool picture by Randall Munroe at https://xkcd.com/
So... how do our human CO2 emissions impact temperatures?