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I think somebody already covered this, but...Data. Earth is warming at unheard of pace. Science shows that CO2 is a major driver of this (not correlated, but causal). Oceans acidifying, leading to mass extinctions.
You say "Most Americans...", but that's not what most Americans believe, according to the data: http://www.gallup.com/poll/190010/concern-global-warming-eight-year-high.aspx. Most Americans actually are in line with the rest of the world on this, which is fairly remarkable given how many Americans' get their coverage on the issue. E.G. through the only major political party in the world whose "expert political beliefs" are that the world is not warming, and even if it were, humans aren't causing it.