The USA has the world's largest national economy. US consumers constitute the most important market for manufactured goods and energy. Per capita emissions of GHG's in the US are among the highest in the world. The US dollar is the currency of choice in world financial markets. The US is the world's only superpower . . .

All of these things, and more, add up to the realization that the USA must lead on climate change if the problem is to be addressed. You might consider, too, that climate change is now a cornerstone of international diplomatic and economic relations, For the US to ignore this is to surrender American primacy on a wide range of different fronts. 

It's also to turn away from the enormous economic potential of emerging technologies in green energy and energy efficiency. You might notice, for instance, that several major nations are announcing the phase out of gasoline powered automobiles . . . in this century. Goodbye overseas markets for US gas-powered cars!