Scarecrow,

A reasonable question, and one asked by many.

The general consensus by scientists who study the subject is that global warming is substantiated by the evidence. Further, and this is the contentious issue, the general consensus is also that a significant amount - if not the majority - of the greenhouse gas emissions involved in this global warming are generated by human activity. Of that amount (whether or not that amount does indeed contribute to or augment the measured global warming) the overwhelming fraction or portion of the greenhouse gases produced by human activity is carbon dioxide, and that carbon dioxide is created primarily by internal combustion engines used by automobiles (or the transportation industry for the purists).

While the debate rages about the best way(s) to address this feature of human activity, global warming is not, and while the size of the fraction attributable to human activity might be, that human activity is a contributor is well established. This is not to say that humans are the only cause, or are solely responsible, as some of the unbalanced fruitcakes would have you believe.

If one accepts that the proposition that global warming has been substantiated, that it is driven primarily by the greenhouse gasses in the atmosphere, and we are responsible for the management of the portion of the greenhouse gas emissions which we produce, then it makes sense to focus our initial efforts on the one – carbon dioxide – we produce most.