It is indeed.
CO2 concentrations are rising - fact.
We are responsible for most of the rise - fact.
CO2 is a greenhouse gas that, everything else being equal, increases warming on a planet - fact.
On average temperatures have risen as CO2 has risen - fact.
Now, who denies these facts? A great many people for a great many reasons.
1) The ignorant. This is the easiest category to understand. Some people simply do not understand (for example) the greenhouse effect or why radiation matters to temperatures.
2) The misinformed. These people are intentionally misinformed by organizations like FOX News to support specific political agendas. They are indeed ignorant, but not willfully so.
3) People who feel threatened. As Upton Sinclair wisely said, "It is difficult to get a man to understand something, when his salary depends upon his not understanding it." Thus a coal worker might "not believe" in AGW because he feels his job would be threatened if he did, and thus engages in magical thinking; if I don't believe it it will be untrue.
4) People who hew to ideological ideals. Many conservatives disregard science and try to impose dogma in its place. This was seen in the smoking debate; conservatives were the last deniers of the risks of smoking and secondhand smoke. Today we see this conservative dogma in the creationism and the climate change debates. They feel if they can lie/distract/distort the issue sufficiently that they will "win" since science is seen as a "tool" of the left.
And what would you say if someone claimed that you were lying, that the science isn't settled, that no one can prove it works, that it doesn't get 40mpg, or it might but the efficiency doesn't come from the engine, and anyway everyone will die if they drive in 40mpg deathtraps?
