The substantive issue can be framed several different ways, here are two:
1) Does the EPA have the power to regulate carbon dioxide?
2) Is carbon dioxide a pollutant as defined by the Clean Air Act?
However, the more fundamental issue is whether the parties suing the EPA even have the "standing" (or the right) to sue. I'm not going to bore everyone with a dissertation on Administrative law or Constitutional law...but....To have standing a suing party must be able to show there is a specific harm and they (the suing party) where harmed. The difficult argument that the plaintiffs are trying to argue is that EPA's failure to regulate carbon dioxide emissions has harmed their environment (the environment in which the parties live not the environment generally). Basically, the plaintiff is going to have to show that BUT FOR the failure of the EPA to regulate carbon dioxide some harm would not have happened. I think this is a tall order for the plaintiffs, but not impossible.
Even if the Plaintiffs are able to show standing (like I said hard but not impossible). I think it will be hard for the Supreme Court to make the EPA effectively regulate carbon dioxide. The Court is not in business of making regulations, they can tell the EPA to go regulate but the EPA can then make up what ever regulations its wants.
Personally, I think this law suit is mainly a lobbying tool. The best option is to have Congress change the Clean Air Act to explicitly define carbon dioxide as a pollutant. Also, Congress doesn't like it when the Supreme Court steps on its toes. But again this is just my opinion.
