“The impact on climate of these cars will be zero.”
If one travels north on I-95 in the second half of April, one will notice that the trees can be green around Richmond, nearly bare just south of Fredericksburg, and increasingly green as one heads north through the Washington suburbs towards the Capitol.
One can also notice via the clouds in the sky a heat island roughly centered on downtown DC. This used to extend to the northernmost point in DC in the 1980s, but now extends to the Washington Beltway.
Higher levels of CO2 have an effect on the atmosphere, but it is limited in area.
Urban heat islands have nothing to do with CO2 if that's what you are saying.