The earth is primarily a water planet. The vast majority of energetic input occurs from infrared light emitted by the sun that ends up being absorbed by the earths oceans and earths atmosphere. What happens on land or in man made vehicles has at best a minor affect on this planetary system. There are also probably time lags in cause and effect. So you cannot look at what is happening in nature now and assume that current energetic input has created the present day situation. For example, we just past the longest day of the summer up here around 32 degress north. The two hottest months of the year here occur after the days start getting shorter.
If memory serves, the total greenhouse effect (which is normal) is about 100 degrees, and most of that is due to water vapor. So the Earth would be much colder without the greenhouse effect. Now CO2 is responsible only for about 1 or 2 percent of this. So if the CO2 increases by 50%, it would be reasonable to conclude that a degree or 2 of warming is possible. These numbers are rough, my memory is porous, but I think this is the basic idea.