Its simple...
...reduce greenhouse gasses. We already know these are not good for human health even if they had virtually no effect on climate. And yes, we need to continue to work on reducing automotive emissions in addition to addressing other sources. This, of course, is what the Kyoto Protocal was all about - an international agreement to reduce greenhouse gas emissions worldwide. The Bush administration has abandoned any international effort to curtail the emission of greenhouse gasses and virtually no progress has been made during his tenure. Four years have been wasted while the pace of climate change is increasing.