The most prevalent greenhouse gas BY FAR is water vapor (something like 95% of all greenhouse gases) but most climate change "experts" always exclude that when sharing their spun numbers. Anyway, that's a rant for another day... most folks refer to CO2 in such discussions and scientists estimate that humans contribute around 3%-4% of total CO2 emissions with the rest coming from natural causes. So the contribution from cars would be some subset of that. Like maxpowers said, it's insignificant.