That it's scientifically impossible to show that these marginal emissions reductions will have an impact.

Of course it's true that it's essentially impossible to identify how exactly emissions cuts would translate to remediation of specific impacts.  But that is not the test for standing, and if it were, no environmental case or toxic tort could ever proceed.

For example, let's say Georgia (my home state) fails to comply with its State Implementation Plan under the Clean Air Act, say by allowing more growth in transportation than the SIP allows.  Can environmental groups sue to force Georgia to implement the SIP?  Of course they can.

But where do they get standing?  Well, the marginal increase in transportation increases air pollutants in the Atlanta area, which in turn increases the risk that particular Sierra Club members get asthma.  

There is no way the enviros can prove that if they win, the marginal decrease in transportation demand will cut emissions sufficiently to improve anyone in particular's health.  So what?  A credible mitigation of a scientifically verifiable risk is enough to satisfy standing, and if it isn't, then all sorts of environmental cases would fall apart.

Chris ..

The reason that there would be no standing issue in the example you cite is that Congress wrote a "citizen enforcement" clause into the Clean Air Act. Essentially, Congress gave everyone standing.

Which leads me to my real point .. the courts are not the correct place for this argument.  The plaintiffs should instead lobby the Executive Branch to do what they wich, or lobby Congress to clarify what is expected of the EPA.  

It is very bad in the long run for the courts to be involved in this kind of thing; look at abortion and Roe v. Wade for an example of what happens when the court makes policy decisions.  

The kind of change that is desired is far more effective when it comes from the elected entities of our government.

Tom - the citizen enforcement provision of the CLean Air Act is not relevant for standing purposes.  The standing requirement is a constitutional, Article III requirement, and the plaintiffs in my hypothetical would need to show injury, causation and redressibility just like Massachusetts does in the current case.

I can understand the argument that this is outside the expertise of the judicial branch.  But here's the problem.  Let's imagine the CLean Air Act unambiguously commanded EPA to regulate carbon dioxide.  EPA simply doesn't do it, in clear contravention of the statute.

If no global-warming related injury can give rise to standing, then no one can get into court to force EPA to act.  EPA is utterly insulated from any check or balance from the judiciary.  I think that would be bad.