Look at that chart below. It is striking that the USA not only has the largest population of automobiles in the world (highest penetration per unit population), but also contributes significantly to the CO2 emissions (size of the ball). Clearly, so much can be done to reduce the impact of the automotive industry on anthropogenic global warming.
However, what is even more worrisome? Look at China, for example. See how low their automotive penetration is compared to a country like US, and see what their CO2 contributions are. Now imagine if China was to develop an automotive penetration that was as high as the USA? What would be its contribution to the greenhouse gases if cleaner technologies were not adopted? This is a major question, not just for China but for all rapidly developing countries. Of course they deserve the development, the growth, and the improved mobility that is a part of it all, but this world really needs to figure out how to make clean technologies accessible and affordable to these countries. If not, we are headed towards some real trouble.