Mostly irrelevant
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1. Many US states are initiating their tough standards

2. The market is 'outlawing' coal anyway through lower prices for natural gas, etc

3. The US public doesn't use the metric system - industry does, but don't say anything - along with Liberia it is the only country that officially uses the Imperial system ( although the USA refers to it as 'standard'). So being only one of 3 countries not in the accord,
 is not a problem for the USA. (speaking of the metric system when will Canada finally get around to adopting metric in a practical sense? Or are you going to try to tell me you don't know how tall you are in feet and inches or how much you weigh in pounds?)
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It will take years for the USA to 'leave' the accord anyway, and by then?