No extra cost to warming [...] Sea level is rising as we warm up from the little ice age, and much land is subsiding.
Whatever the cause, we would need to mitigate sea level rises with measures such as relocation or sea walls, all of which are costly. The best available science points to AGW as the cause of the rise, and therefore it makes sense to pay for the mitigation with AGW sources.
it benefits agriculture and humans do well in warmth, much better than cold.
The problem is that the "warming" is an average of far wilder fluctuations in weather. The earth doesn't just get uniformly a bit warmer, and the localized effects can be devastating. More importantly, even if a bit of warming is beneficial on the average, continuing the trend - especially past a certain threshold into a feedback loop of uncontrollable warming - is obviously foolish. Unless you claim to know exactly how much greenhouse gasses we can release into the atmosphere for best effect, it would be prudent to not find out the hard way.
Pollution from cars--hmm, not much lately since the advent of catalytic converters.
"Today’s on-road vehicles produce over a third of the carbon monoxide and nitrogen oxides in our atmosphere", says the Union of Concerned Scientists. The bottom of that article discusses the pollution's effects on public health.